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*.html
*.pdf
!PSACryptoDriverModelSpec.pdf

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Mbed Crypto storage specification
=================================
This document specifies how Mbed Crypto uses storage.
Mbed Crypto may be upgraded on an existing device with the storage preserved. Therefore:
1. Any change may break existing installations and may require an upgrade path.
1. This document retains historical information about all past released versions. Do not remove information from this document unless it has always been incorrect or it is about a version that you are sure was never released.
Mbed Crypto 0.1.0
-----------------
Tags: mbedcrypto-0.1.0b, mbedcrypto-0.1.0b2
Released in November 2018. <br>
Integrated in Mbed OS 5.11.
Supported backends:
* [PSA ITS](#file-namespace-on-its-for-0.1.0)
* [C stdio](#file-namespace-on-stdio-for-0.1.0)
Supported features:
* [Persistent transparent keys](#key-file-format-for-0.1.0) designated by a [slot number](#key-names-for-0.1.0).
* [Nonvolatile random seed](#nonvolatile-random-seed-file-format-for-0.1.0) on ITS only.
This is a beta release, and we do not promise backward compatibility, with one exception:
> On Mbed OS, if a device has a nonvolatile random seed file produced with Mbed OS 5.11.x and is upgraded to a later version of Mbed OS, the nonvolatile random seed file is preserved or upgraded.
We do not make any promises regarding key storage, or regarding the nonvolatile random seed file on other platforms.
### Key names for 0.1.0
Information about each key is stored in a dedicated file whose name is constructed from the key identifier. The way in which the file name is constructed depends on the storage backend. The content of the file is described [below](#key-file-format-for-0.1.0).
The valid values for a key identifier are the range from 1 to 0xfffeffff. This limitation on the range is not documented in user-facing documentation: according to the user-facing documentation, arbitrary 32-bit values are valid.
The code uses the following constant in an internal header (note that despite the name, this value is actually one plus the maximum permitted value):
#define PSA_MAX_PERSISTENT_KEY_IDENTIFIER 0xffff0000
There is a shared namespace for all callers.
### Key file format for 0.1.0
All integers are encoded in little-endian order in 8-bit bytes.
The layout of a key file is:
* magic (8 bytes): `"PSA\0KEY\0"`
* version (4 bytes): 0
* type (4 bytes): `psa_key_type_t` value
* policy usage flags (4 bytes): `psa_key_usage_t` value
* policy usage algorithm (4 bytes): `psa_algorithm_t` value
* key material length (4 bytes)
* key material: output of `psa_export_key`
* Any trailing data is rejected on load.
### Nonvolatile random seed file format for 0.1.0
The nonvolatile random seed file contains a seed for the random generator. If present, it is rewritten at each boot as part of the random generator initialization.
The file format is just the seed as a byte string with no metadata or encoding of any kind.
### File namespace on ITS for 0.1.0
Assumption: ITS provides a 32-bit file identifier namespace. The Crypto service can use arbitrary file identifiers and no other part of the system accesses the same file identifier namespace.
* File 0: unused.
* Files 1 through 0xfffeffff: [content](#key-file-format-for-0.1.0) of the [key whose identifier is the file identifier](#key-names-for-0.1.0).
* File 0xffffff52 (`PSA_CRYPTO_ITS_RANDOM_SEED_UID`): [nonvolatile random seed](#nonvolatile-random-seed-file-format-for-0.1.0).
* Files 0xffff0000 through 0xffffff51, 0xffffff53 through 0xffffffff: unused.
### File namespace on stdio for 0.1.0
Assumption: C stdio, allowing names containing lowercase letters, digits and underscores, of length up to 23.
An undocumented build-time configuration value `CRYPTO_STORAGE_FILE_LOCATION` allows storing the key files in a directory other than the current directory. This value is simply prepended to the file name (so it must end with a directory separator to put the keys in a different directory).
* `CRYPTO_STORAGE_FILE_LOCATION "psa_key_slot_0"`: used as a temporary file. Must be writable. May be overwritten or deleted if present.
* `sprintf(CRYPTO_STORAGE_FILE_LOCATION "psa_key_slot_%lu", key_id)` [content](#key-file-format-for-0.1.0) of the [key whose identifier](#key-names-for-0.1.0) is `key_id`.
* Other files: unused.
Mbed Crypto 1.0.0
-----------------
Tags: mbedcrypto-1.0.0d4, mbedcrypto-1.0.0
Released in February 2019. <br>
Integrated in Mbed OS 5.12.
Supported integrations:
* [PSA platform](#file-namespace-on-a-psa-platform-for-1.0.0)
* [library using PSA ITS](#file-namespace-on-its-as-a-library-for-1.0.0)
* [library using C stdio](#file-namespace-on-stdio-for-1.0.0)
Supported features:
* [Persistent transparent keys](#key-file-format-for-1.0.0) designated by a [key identifier and owner](#key-names-for-1.0.0).
* [Nonvolatile random seed](#nonvolatile-random-seed-file-format-for-1.0.0) on ITS only.
Backward compatibility commitments: TBD
### Key names for 1.0.0
Information about each key is stored in a dedicated file designated by the key identifier. In integrations where there is no concept of key owner (in particular, in library integrations), the key identifier is exactly the key identifier as defined in the PSA Cryptography API specification (`psa_key_id_t`). In integrations where there is a concept of key owner (integration into a service for example), the key identifier is made of an owner identifier (its semantics and type are integration specific) and of the key identifier (`psa_key_id_t`) from the key owner point of view.
The way in which the file name is constructed from the key identifier depends on the storage backend. The content of the file is described [below](#key-file-format-for-1.0.0).
* Library integration: the key file name is just the key identifier as defined in the PSA crypto specification. This is a 32-bit value.
* PSA service integration: the key file name is `(uint32_t)owner_uid << 32 | key_id` where `key_id` is the key identifier from the owner point of view and `owner_uid` (of type `int32_t`) is the calling partition identifier provided to the server by the partition manager. This is a 64-bit value.
### Key file format for 1.0.0
The layout is identical to [0.1.0](#key-file-format-for-0.1.0) so far. However note that the encoding of key types, algorithms and key material has changed, therefore the storage format is not compatible (despite using the same value in the version field so far).
### Nonvolatile random seed file format for 1.0.0
[Identical to 0.1.0](#nonvolatile-random-seed-file-format-for-0.1.0).
### File namespace on a PSA platform for 1.0.0
Assumption: ITS provides a 64-bit file identifier namespace. The Crypto service can use arbitrary file identifiers and no other part of the system accesses the same file identifier namespace.
Assumption: the owner identifier is a nonzero value of type `int32_t`.
* Files 0 through 0xffffff51, 0xffffff53 through 0xffffffff: unused, reserved for internal use of the crypto library or crypto service.
* File 0xffffff52 (`PSA_CRYPTO_ITS_RANDOM_SEED_UID`): [nonvolatile random seed](#nonvolatile-random-seed-file-format-for-0.1.0).
* Files 0x100000000 through 0xffffffffffff: [content](#key-file-format-for-1.0.0) of the [key whose identifier is the file identifier](#key-names-for-1.0.0). The upper 32 bits determine the owner.
### File namespace on ITS as a library for 1.0.0
Assumption: ITS provides a 64-bit file identifier namespace. The entity using the crypto library can use arbitrary file identifiers and no other part of the system accesses the same file identifier namespace.
This is a library integration, so there is no owner. The key file identifier is identical to the key identifier.
* File 0: unused.
* Files 1 through 0xfffeffff: [content](#key-file-format-for-1.0.0) of the [key whose identifier is the file identifier](#key-names-for-1.0.0).
* File 0xffffff52 (`PSA_CRYPTO_ITS_RANDOM_SEED_UID`): [nonvolatile random seed](#nonvolatile-random-seed-file-format-for-1.0.0).
* Files 0xffff0000 through 0xffffff51, 0xffffff53 through 0xffffffff, 0x100000000 through 0xffffffffffffffff: unused.
### File namespace on stdio for 1.0.0
This is a library integration, so there is no owner. The key file identifier is identical to the key identifier.
[Identical to 0.1.0](#file-namespace-on-stdio-for-0.1.0).
### Upgrade from 0.1.0 to 1.0.0.
* Delete files 1 through 0xfffeffff, which contain keys in a format that is no longer supported.
### Suggested changes to make before 1.0.0
The library integration and the PSA platform integration use different sets of file names. This is annoyingly non-uniform. For example, if we want to store non-key files, we have room in different ranges (0 through 0xffffffff on a PSA platform, 0xffff0000 through 0xffffffffffffffff in a library integration).
It would simplify things to always have a 32-bit owner, with a nonzero value, and thus reserve the range 0–0xffffffff for internal library use.
Mbed Crypto 1.1.0
-----------------
Tags: mbedcrypto-1.1.0
Released in early June 2019. <br>
Integrated in Mbed OS 5.13.
Identical to [1.0.0](#mbed-crypto-1.0.0) except for some changes in the key file format.
### Key file format for 1.1.0
The key file format is identical to [1.0.0](#key-file-format-for-1.0.0), except for the following changes:
* A new policy field, marked as [NEW:1.1.0] below.
* The encoding of key types, algorithms and key material has changed, therefore the storage format is not compatible (despite using the same value in the version field so far).
A self-contained description of the file layout follows.
All integers are encoded in little-endian order in 8-bit bytes.
The layout of a key file is:
* magic (8 bytes): `"PSA\0KEY\0"`
* version (4 bytes): 0
* type (4 bytes): `psa_key_type_t` value
* policy usage flags (4 bytes): `psa_key_usage_t` value
* policy usage algorithm (4 bytes): `psa_algorithm_t` value
* policy enrollment algorithm (4 bytes): `psa_algorithm_t` value [NEW:1.1.0]
* key material length (4 bytes)
* key material: output of `psa_export_key`
* Any trailing data is rejected on load.
Mbed Crypto TBD
---------------
Tags: TBD
Released in TBD 2019. <br>
Integrated in Mbed OS TBD.
### Changes introduced in TBD
* The layout of a key file now has a lifetime field before the type field.
* Key files can store references to keys in a secure element. In such key files, the key material contains the slot number.
### File namespace on a PSA platform on TBD
Assumption: ITS provides a 64-bit file identifier namespace. The Crypto service can use arbitrary file identifiers and no other part of the system accesses the same file identifier namespace.
Assumption: the owner identifier is a nonzero value of type `int32_t`.
* Files 0 through 0xfffeffff: unused.
* Files 0xffff0000 through 0xffffffff: reserved for internal use of the crypto library or crypto service. See [non-key files](#non-key-files-on-tbd).
* Files 0x100000000 through 0xffffffffffff: [content](#key-file-format-for-1.0.0) of the [key whose identifier is the file identifier](#key-names-for-1.0.0). The upper 32 bits determine the owner.
### File namespace on ITS as a library on TBD
Assumption: ITS provides a 64-bit file identifier namespace. The entity using the crypto library can use arbitrary file identifiers and no other part of the system accesses the same file identifier namespace.
This is a library integration, so there is no owner. The key file identifier is identical to the key identifier.
* File 0: unused.
* Files 1 through 0xfffeffff: [content](#key-file-format-for-1.0.0) of the [key whose identifier is the file identifier](#key-names-for-1.0.0).
* Files 0xffff0000 through 0xffffffff: reserved for internal use of the crypto library or crypto service. See [non-key files](#non-key-files-on-tbd).
* Files 0x100000000 through 0xffffffffffffffff: unused.
### Non-key files on TBD
File identifiers in the range 0xffff0000 through 0xffffffff are reserved for internal use in Mbed Crypto.
* Files 0xfffffe02 through 0xfffffeff (`PSA_CRYPTO_SE_DRIVER_ITS_UID_BASE + lifetime`): secure element driver storage. The content of the file is the secure element driver's persistent data.
* File 0xffffff52 (`PSA_CRYPTO_ITS_RANDOM_SEED_UID`): [nonvolatile random seed](#nonvolatile-random-seed-file-format-for-1.0.0).
* File 0xffffff54 (`PSA_CRYPTO_ITS_TRANSACTION_UID`): [transaction file](#transaction-file-format-for-tbd).
* Other files are unused and reserved for future use.
### Key file format for TBD
All integers are encoded in little-endian order in 8-bit bytes except where otherwise indicated.
The layout of a key file is:
* magic (8 bytes): `"PSA\0KEY\0"`.
* version (4 bytes): 0.
* lifetime (4 bytes): `psa_key_lifetime_t` value.
* type (4 bytes): `psa_key_type_t` value.
* policy usage flags (4 bytes): `psa_key_usage_t` value.
* policy usage algorithm (4 bytes): `psa_algorithm_t` value.
* policy enrollment algorithm (4 bytes): `psa_algorithm_t` value.
* key material length (4 bytes).
* key material:
* For a transparent key: output of `psa_export_key`.
* For an opaque key (unified driver interface): driver-specific opaque key blob.
* For an opaque key (key in a secure element): slot number (8 bytes), in platform endianness.
* Any trailing data is rejected on load.
### Transaction file format for TBD
The transaction file contains data about an ongoing action that cannot be completed atomically. It exists only if there is an ongoing transaction.
All integers are encoded in platform endianness.
All currently existing transactions concern a key in a secure element.
The layout of a transaction file is:
* type (2 bytes): the [transaction type](#transaction-types-on-tbd).
* unused (2 bytes)
* lifetime (4 bytes): `psa_key_lifetime_t` value that corresponds to a key in a secure element.
* slot number (8 bytes): `psa_key_slot_number_t` value. This is the unique designation of the key for the secure element driver.
* key identifier (4 bytes in a library integration, 8 bytes on a PSA platform): the internal representation of the key identifier. On a PSA platform, this encodes the key owner in the same way as [in file identifiers for key files](#file-namespace-on-a-psa-platform-on-tbd)).
#### Transaction types on TBD
* 0x0001: key creation. The following locations may or may not contain data about the key that is being created:
* The slot in the secure element designated by the slot number.
* The file containing the key metadata designated by the key identifier.
* The driver persistent data.
* 0x0002: key destruction. The following locations may or may not still contain data about the key that is being destroyed:
* The slot in the secure element designated by the slot number.
* The file containing the key metadata designated by the key identifier.
* The driver persistent data.
Mbed Crypto TBD
---------------
Tags: TBD
Released in TBD 2020. <br>
Integrated in Mbed OS TBD.
### Changes introduced in TBD
* The type field has been split into a type and a bits field of 2 bytes each.
### Key file format for TBD
All integers are encoded in little-endian order in 8-bit bytes except where otherwise indicated.
The layout of a key file is:
* magic (8 bytes): `"PSA\0KEY\0"`.
* version (4 bytes): 0.
* lifetime (4 bytes): `psa_key_lifetime_t` value.
* type (2 bytes): `psa_key_type_t` value.
* bits (2 bytes): `psa_key_bits_t` value.
* policy usage flags (4 bytes): `psa_key_usage_t` value.
* policy usage algorithm (4 bytes): `psa_algorithm_t` value.
* policy enrollment algorithm (4 bytes): `psa_algorithm_t` value.
* key material length (4 bytes).
* key material:
* For a transparent key: output of `psa_export_key`.
* For an opaque key (unified driver interface): driver-specific opaque key blob.
* For an opaque key (key in a secure element): slot number (8 bytes), in platform endianness.
* Any trailing data is rejected on load.

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TLS 1.3 Experimental Developments
=================================
Overview
--------
Mbed TLS doesn't support the TLS 1.3 protocol yet, but a prototype is in development.
Stable parts of this prototype that can be independently tested are being successively
upstreamed under the guard of the following macro:
```
MBEDTLS_SSL_PROTO_TLS1_3_EXPERIMENTAL
```
This macro will likely be renamed to `MBEDTLS_SSL_PROTO_TLS1_3` once a minimal viable
implementation of the TLS 1.3 protocol is available.
See the [documentation of `MBEDTLS_SSL_PROTO_TLS1_3_EXPERIMENTAL`](../../include/mbedtls/config.h)
for more information.
Status
------
The following lists which parts of the TLS 1.3 prototype have already been upstreamed
together with their level of testing:
* TLS 1.3 record protection mechanisms
The record protection routines `mbedtls_ssl_{encrypt|decrypt}_buf()` have been extended
to support the modified TLS 1.3 record protection mechanism, including modified computation
of AAD, IV, and the introduction of a flexible padding.
Those record protection routines have unit tests in `test_suite_ssl` alongside the
tests for the other record protection routines.
TODO: Add some test vectors from RFC 8448.
- The HKDF key derivation function on which the TLS 1.3 key schedule is based,
is already present as an independent module controlled by `MBEDTLS_HKDF_C`
independently of the development of the TLS 1.3 prototype.
- The TLS 1.3-specific HKDF-based key derivation functions (see RFC 8446):
* HKDF-Expand-Label
* Derive-Secret
- Secret evolution
* The traffic {Key,IV} generation from secret
Those functions are implemented in `library/ssl_tls13_keys.c` and
tested in `test_suite_ssl` using test vectors from RFC 8448 and
https://tls13.ulfheim.net/.

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## Getting started with Mbed Crypto
### What is Mbed Crypto?
Mbed Crypto is an open source cryptographic library that supports a wide range of cryptographic operations, including:
* Key management
* Hashing
* Symmetric cryptography
* Asymmetric cryptography
* Message authentication (MAC)
* Key generation and derivation
* Authenticated encryption with associated data (AEAD)
The Mbed Crypto library is a reference implementation of the cryptography interface of the Arm Platform Security Architecture (PSA). It is written in portable C.
The Mbed Crypto library is distributed under the Apache License, version 2.0.
#### Platform Security Architecture (PSA)
Arm's Platform Security Architecture (PSA) is a holistic set of threat models,
security analyses, hardware and firmware architecture specifications, and an open source firmware reference implementation. PSA provides a recipe, based on industry best practice, that enables you to design security into both hardware and firmware consistently. Part of the API provided by PSA is the cryptography interface, which provides access to a set of primitives.
### Using Mbed Crypto
* [Getting the Mbed Crypto library](#getting-the-mbed-crypto-library)
* [Building the Mbed Crypto library](#building-the-mbed-crypto-library)
* [Using the Mbed Crypto library](#using-the-mbed-crypto-library)
* [Importing a key](#importing-a-key)
* [Signing a message using RSA](#signing-a-message-using-RSA)
* [Encrypting or decrypting using symmetric ciphers](#encrypting-or-decrypting-using-symmetric-ciphers)
* [Hashing a message](#hashing-a-message)
* [Deriving a new key from an existing key](#deriving-a-new-key-from-an-existing-key)
* [Generating a random value](#generating-a-random-value)
* [Authenticating and encrypting or decrypting a message](#authenticating-and-encrypting-or-decrypting-a-message)
* [Generating and exporting keys](#generating-and-exporting-keys)
* [More about the Mbed Crypto library](#more-about-the-mbed-crypto-library)
### Getting the Mbed Crypto library
Mbed Crypto releases are available in the [public GitHub repository](https://github.com/ARMmbed/mbed-crypto).
### Building the Mbed Crypto library
**Prerequisites to building the library with the provided makefiles:**
* GNU Make.
* A C toolchain (compiler, linker, archiver).
* Python 2 or Python 3 (either works) to generate the test code.
* Perl to run the tests.
If you have a C compiler such as GCC or Clang, just run `make` in the top-level directory to build the library, a set of unit tests and some sample programs.
To select a different compiler, set the `CC` variable to the name or path of the compiler and linker (default: `cc`) and set `AR` to a compatible archiver (default: `ar`); for example:
```
make CC=arm-linux-gnueabi-gcc AR=arm-linux-gnueabi-ar
```
The provided makefiles pass options to the compiler that assume a GCC-like command line syntax. To use a different compiler, you may need to pass different values for `CFLAGS`, `WARNINGS_CFLAGS` and `LDFLAGS`.
To run the unit tests on the host machine, run `make test` from the top-level directory. If you are cross-compiling, copy the test executable from the `tests` directory to the target machine.
### Using the Mbed Crypto library
To use the Mbed Crypto APIs, call `psa_crypto_init()` before calling any other API. This initializes the library.
### Importing a key
To use a key for cryptography operations in Mbed Crypto, you need to first
import it. The import operation returns the identifier of the key for use
with other function calls.
**Prerequisites to importing keys:**
* Initialize the library with a successful call to `psa_crypto_init()`.
This example shows how to import a key:
```C
void import_a_key(const uint8_t *key, size_t key_len)
{
psa_status_t status;
psa_key_attributes_t attributes = PSA_KEY_ATTRIBUTES_INIT;
psa_key_id_t key;
printf("Import an AES key...\t");
fflush(stdout);
/* Initialize PSA Crypto */
status = psa_crypto_init();
if (status != PSA_SUCCESS) {
printf("Failed to initialize PSA Crypto\n");
return;
}
/* Set key attributes */
psa_set_key_usage_flags(&attributes, 0);
psa_set_key_algorithm(&attributes, 0);
psa_set_key_type(&attributes, PSA_KEY_TYPE_AES);
psa_set_key_bits(&attributes, 128);
/* Import the key */
status = psa_import_key(&attributes, key, key_len, &key);
if (status != PSA_SUCCESS) {
printf("Failed to import key\n");
return;
}
printf("Imported a key\n");
/* Free the attributes */
psa_reset_key_attributes(&attributes);
/* Destroy the key */
psa_destroy_key(key);
mbedtls_psa_crypto_free();
}
```
### Signing a message using RSA
Mbed Crypto supports encrypting, decrypting, signing and verifying messages using public key signature algorithms, such as RSA or ECDSA.
**Prerequisites to performing asymmetric signature operations:**
* Initialize the library with a successful call to `psa_crypto_init()`.
* Have a valid key with appropriate attributes set:
* Usage flag `PSA_KEY_USAGE_SIGN_HASH` to allow signing.
* Usage flag `PSA_KEY_USAGE_VERIFY_HASH` to allow signature verification.
* Algorithm set to the desired signature algorithm.
This example shows how to sign a hash that has already been calculated:
```C
void sign_a_message_using_rsa(const uint8_t *key, size_t key_len)
{
psa_status_t status;
psa_key_attributes_t attributes = PSA_KEY_ATTRIBUTES_INIT;
uint8_t hash[32] = {0x50, 0xd8, 0x58, 0xe0, 0x98, 0x5e, 0xcc, 0x7f,
0x60, 0x41, 0x8a, 0xaf, 0x0c, 0xc5, 0xab, 0x58,
0x7f, 0x42, 0xc2, 0x57, 0x0a, 0x88, 0x40, 0x95,
0xa9, 0xe8, 0xcc, 0xac, 0xd0, 0xf6, 0x54, 0x5c};
uint8_t signature[PSA_SIGNATURE_MAX_SIZE] = {0};
size_t signature_length;
psa_key_id_t key;
printf("Sign a message...\t");
fflush(stdout);
/* Initialize PSA Crypto */
status = psa_crypto_init();
if (status != PSA_SUCCESS) {
printf("Failed to initialize PSA Crypto\n");
return;
}
/* Set key attributes */
psa_set_key_usage_flags(&attributes, PSA_KEY_USAGE_SIGN_HASH);
psa_set_key_algorithm(&attributes, PSA_ALG_RSA_PKCS1V15_SIGN_RAW);
psa_set_key_type(&attributes, PSA_KEY_TYPE_RSA_KEY_PAIR);
psa_set_key_bits(&attributes, 1024);
/* Import the key */
status = psa_import_key(&attributes, key, key_len, &key);
if (status != PSA_SUCCESS) {
printf("Failed to import key\n");
return;
}
/* Sign message using the key */
status = psa_sign_hash(key, PSA_ALG_RSA_PKCS1V15_SIGN_RAW,
hash, sizeof(hash),
signature, sizeof(signature),
&signature_length);
if (status != PSA_SUCCESS) {
printf("Failed to sign\n");
return;
}
printf("Signed a message\n");
/* Free the attributes */
psa_reset_key_attributes(&attributes);
/* Destroy the key */
psa_destroy_key(key);
mbedtls_psa_crypto_free();
}
```
### Using symmetric ciphers
Mbed Crypto supports encrypting and decrypting messages using various symmetric cipher algorithms (both block and stream ciphers).
**Prerequisites to working with the symmetric cipher API:**
* Initialize the library with a successful call to `psa_crypto_init()`.
* Have a symmetric key. This key's usage flags must include `PSA_KEY_USAGE_ENCRYPT` to allow encryption or `PSA_KEY_USAGE_DECRYPT` to allow decryption.
**To encrypt a message with a symmetric cipher:**
1. Allocate an operation (`psa_cipher_operation_t`) structure to pass to the cipher functions.
1. Initialize the operation structure to zero or to `PSA_CIPHER_OPERATION_INIT`.
1. Call `psa_cipher_encrypt_setup()` to specify the algorithm and the key to be used.
1. Call either `psa_cipher_generate_iv()` or `psa_cipher_set_iv()` to generate or set the initialization vector (IV). We recommend calling `psa_cipher_generate_iv()`, unless you require a specific IV value.
1. Call `psa_cipher_update()` with the message to encrypt. You may call this function multiple times, passing successive fragments of the message on successive calls.
1. Call `psa_cipher_finish()` to end the operation and output the encrypted message.
This example shows how to encrypt data using an AES (Advanced Encryption Standard) key in CBC (Cipher Block Chaining) mode with no padding (assuming all prerequisites have been fulfilled):
```c
void encrypt_with_symmetric_ciphers(const uint8_t *key, size_t key_len)
{
enum {
block_size = PSA_BLOCK_CIPHER_BLOCK_SIZE(PSA_KEY_TYPE_AES),
};
psa_status_t status;
psa_key_attributes_t attributes = PSA_KEY_ATTRIBUTES_INIT;
psa_algorithm_t alg = PSA_ALG_CBC_NO_PADDING;
uint8_t plaintext[block_size] = SOME_PLAINTEXT;
uint8_t iv[block_size];
size_t iv_len;
uint8_t output[block_size];
size_t output_len;
psa_key_id_t key;
psa_cipher_operation_t operation = PSA_CIPHER_OPERATION_INIT;
printf("Encrypt with cipher...\t");
fflush(stdout);
/* Initialize PSA Crypto */
status = psa_crypto_init();
if (status != PSA_SUCCESS)
{
printf("Failed to initialize PSA Crypto\n");
return;
}
/* Import a key */
psa_set_key_usage_flags(&attributes, PSA_KEY_USAGE_ENCRYPT);
psa_set_key_algorithm(&attributes, alg);
psa_set_key_type(&attributes, PSA_KEY_TYPE_AES);
psa_set_key_bits(&attributes, 128);
status = psa_import_key(&attributes, key, key_len, &key);
if (status != PSA_SUCCESS) {
printf("Failed to import a key\n");
return;
}
psa_reset_key_attributes(&attributes);
/* Encrypt the plaintext */
status = psa_cipher_encrypt_setup(&operation, key, alg);
if (status != PSA_SUCCESS) {
printf("Failed to begin cipher operation\n");
return;
}
status = psa_cipher_generate_iv(&operation, iv, sizeof(iv), &iv_len);
if (status != PSA_SUCCESS) {
printf("Failed to generate IV\n");
return;
}
status = psa_cipher_update(&operation, plaintext, sizeof(plaintext),
output, sizeof(output), &output_len);
if (status != PSA_SUCCESS) {
printf("Failed to update cipher operation\n");
return;
}
status = psa_cipher_finish(&operation, output + output_len,
sizeof(output) - output_len, &output_len);
if (status != PSA_SUCCESS) {
printf("Failed to finish cipher operation\n");
return;
}
printf("Encrypted plaintext\n");
/* Clean up cipher operation context */
psa_cipher_abort(&operation);
/* Destroy the key */
psa_destroy_key(key);
mbedtls_psa_crypto_free();
}
```
**To decrypt a message with a symmetric cipher:**
1. Allocate an operation (`psa_cipher_operation_t`) structure to pass to the cipher functions.
1. Initialize the operation structure to zero or to `PSA_CIPHER_OPERATION_INIT`.
1. Call `psa_cipher_decrypt_setup()` to specify the algorithm and the key to be used.
1. Call `psa_cipher_set_iv()` with the IV for the decryption.
1. Call `psa_cipher_update()` with the message to encrypt. You may call this function multiple times, passing successive fragments of the message on successive calls.
1. Call `psa_cipher_finish()` to end the operation and output the decrypted message.
This example shows how to decrypt encrypted data using an AES key in CBC mode with no padding
(assuming all prerequisites have been fulfilled):
```c
void decrypt_with_symmetric_ciphers(const uint8_t *key, size_t key_len)
{
enum {
block_size = PSA_BLOCK_CIPHER_BLOCK_SIZE(PSA_KEY_TYPE_AES),
};
psa_status_t status;
psa_key_attributes_t attributes = PSA_KEY_ATTRIBUTES_INIT;
psa_algorithm_t alg = PSA_ALG_CBC_NO_PADDING;
psa_cipher_operation_t operation = PSA_CIPHER_OPERATION_INIT;
uint8_t ciphertext[block_size] = SOME_CIPHERTEXT;
uint8_t iv[block_size] = ENCRYPTED_WITH_IV;
uint8_t output[block_size];
size_t output_len;
psa_key_id_t key;
printf("Decrypt with cipher...\t");
fflush(stdout);
/* Initialize PSA Crypto */
status = psa_crypto_init();
if (status != PSA_SUCCESS)
{
printf("Failed to initialize PSA Crypto\n");
return;
}
/* Import a key */
psa_set_key_usage_flags(&attributes, PSA_KEY_USAGE_DECRYPT);
psa_set_key_algorithm(&attributes, alg);
psa_set_key_type(&attributes, PSA_KEY_TYPE_AES);
psa_set_key_bits(&attributes, 128);
status = psa_import_key(&attributes, key, key_len, &key);
if (status != PSA_SUCCESS) {
printf("Failed to import a key\n");
return;
}
psa_reset_key_attributes(&attributes);
/* Decrypt the ciphertext */
status = psa_cipher_decrypt_setup(&operation, key, alg);
if (status != PSA_SUCCESS) {
printf("Failed to begin cipher operation\n");
return;
}
status = psa_cipher_set_iv(&operation, iv, sizeof(iv));
if (status != PSA_SUCCESS) {
printf("Failed to set IV\n");
return;
}
status = psa_cipher_update(&operation, ciphertext, sizeof(ciphertext),
output, sizeof(output), &output_len);
if (status != PSA_SUCCESS) {
printf("Failed to update cipher operation\n");
return;
}
status = psa_cipher_finish(&operation, output + output_len,
sizeof(output) - output_len, &output_len);
if (status != PSA_SUCCESS) {
printf("Failed to finish cipher operation\n");
return;
}
printf("Decrypted ciphertext\n");
/* Clean up cipher operation context */
psa_cipher_abort(&operation);
/* Destroy the key */
psa_destroy_key(key);
mbedtls_psa_crypto_free();
}
```
#### Handling cipher operation contexts
After you've initialized the operation structure with a successful call to `psa_cipher_encrypt_setup()` or `psa_cipher_decrypt_setup()`, you can terminate the operation at any time by calling `psa_cipher_abort()`.
The call to `psa_cipher_abort()` frees any resources associated with the operation, except for the operation structure itself.
Mbed Crypto implicitly calls `psa_cipher_abort()` when:
* A call to `psa_cipher_generate_iv()`, `psa_cipher_set_iv()` or `psa_cipher_update()` fails (returning any status other than `PSA_SUCCESS`).
* A call to `psa_cipher_finish()` succeeds or fails.
After an implicit or explicit call to `psa_cipher_abort()`, the operation structure is invalidated; in other words, you cannot reuse the operation structure for the same operation. You can, however, reuse the operation structure for a different operation by calling either `psa_cipher_encrypt_setup()` or `psa_cipher_decrypt_setup()` again.
You must call `psa_cipher_abort()` at some point for any operation that is initialized successfully (by a successful call to `psa_cipher_encrypt_setup()` or `psa_cipher_decrypt_setup()`).
Making multiple sequential calls to `psa_cipher_abort()` on an operation that is terminated (either implicitly or explicitly) is safe and has no effect.
### Hashing a message
Mbed Crypto lets you compute and verify hashes using various hashing
algorithms.
**Prerequisites to working with the hash APIs:**
* Initialize the library with a successful call to `psa_crypto_init()`.
**To calculate a hash:**
1. Allocate an operation structure (`psa_hash_operation_t`) to pass to the hash functions.
1. Initialize the operation structure to zero or to `PSA_HASH_OPERATION_INIT`.
1. Call `psa_hash_setup()` to specify the hash algorithm.
1. Call `psa_hash_update()` with the message to encrypt. You may call this function multiple times, passing successive fragments of the message on successive calls.
1. Call `psa_hash_finish()` to calculate the hash, or `psa_hash_verify()` to compare the computed hash with an expected hash value.
This example shows how to calculate the SHA-256 hash of a message:
```c
psa_status_t status;
psa_algorithm_t alg = PSA_ALG_SHA_256;
psa_hash_operation_t operation = PSA_HASH_OPERATION_INIT;
unsigned char input[] = { 'a', 'b', 'c' };
unsigned char actual_hash[PSA_HASH_MAX_SIZE];
size_t actual_hash_len;
printf("Hash a message...\t");
fflush(stdout);
/* Initialize PSA Crypto */
status = psa_crypto_init();
if (status != PSA_SUCCESS) {
printf("Failed to initialize PSA Crypto\n");
return;
}
/* Compute hash of message */
status = psa_hash_setup(&operation, alg);
if (status != PSA_SUCCESS) {
printf("Failed to begin hash operation\n");
return;
}
status = psa_hash_update(&operation, input, sizeof(input));
if (status != PSA_SUCCESS) {
printf("Failed to update hash operation\n");
return;
}
status = psa_hash_finish(&operation, actual_hash, sizeof(actual_hash),
&actual_hash_len);
if (status != PSA_SUCCESS) {
printf("Failed to finish hash operation\n");
return;
}
printf("Hashed a message\n");
/* Clean up hash operation context */
psa_hash_abort(&operation);
mbedtls_psa_crypto_free();
```
This example shows how to verify the SHA-256 hash of a message:
```c
psa_status_t status;
psa_algorithm_t alg = PSA_ALG_SHA_256;
psa_hash_operation_t operation = PSA_HASH_OPERATION_INIT;
unsigned char input[] = { 'a', 'b', 'c' };
unsigned char expected_hash[] = {
0xba, 0x78, 0x16, 0xbf, 0x8f, 0x01, 0xcf, 0xea, 0x41, 0x41, 0x40, 0xde,
0x5d, 0xae, 0x22, 0x23, 0xb0, 0x03, 0x61, 0xa3, 0x96, 0x17, 0x7a, 0x9c,
0xb4, 0x10, 0xff, 0x61, 0xf2, 0x00, 0x15, 0xad
};
size_t expected_hash_len = PSA_HASH_SIZE(alg);
printf("Verify a hash...\t");
fflush(stdout);
/* Initialize PSA Crypto */
status = psa_crypto_init();
if (status != PSA_SUCCESS) {
printf("Failed to initialize PSA Crypto\n");
return;
}
/* Verify message hash */
status = psa_hash_setup(&operation, alg);
if (status != PSA_SUCCESS) {
printf("Failed to begin hash operation\n");
return;
}
status = psa_hash_update(&operation, input, sizeof(input));
if (status != PSA_SUCCESS) {
printf("Failed to update hash operation\n");
return;
}
status = psa_hash_verify(&operation, expected_hash, expected_hash_len);
if (status != PSA_SUCCESS) {
printf("Failed to verify hash\n");
return;
}
printf("Verified a hash\n");
/* Clean up hash operation context */
psa_hash_abort(&operation);
mbedtls_psa_crypto_free();
```
The API provides the macro `PSA_HASH_SIZE`, which returns the expected hash length (in bytes) for the specified algorithm.
#### Handling hash operation contexts
After a successful call to `psa_hash_setup()`, you can terminate the operation at any time by calling `psa_hash_abort()`. The call to `psa_hash_abort()` frees any resources associated with the operation, except for the operation structure itself.
Mbed Crypto implicitly calls `psa_hash_abort()` when:
1. A call to `psa_hash_update()` fails (returning any status other than `PSA_SUCCESS`).
1. A call to `psa_hash_finish()` succeeds or fails.
1. A call to `psa_hash_verify()` succeeds or fails.
After an implicit or explicit call to `psa_hash_abort()`, the operation structure is invalidated; in other words, you cannot reuse the operation structure for the same operation. You can, however, reuse the operation structure for a different operation by calling `psa_hash_setup()` again.
You must call `psa_hash_abort()` at some point for any operation that is initialized successfully (by a successful call to `psa_hash_setup()`) .
Making multiple sequential calls to `psa_hash_abort()` on an operation that has already been terminated (either implicitly or explicitly) is safe and has no effect.
### Generating a random value
Mbed Crypto can generate random data.
**Prerequisites to generating random data:**
* Initialize the library with a successful call to `psa_crypto_init()`.
<span class="notes">**Note:** To generate a random key, use `psa_generate_key()` instead of `psa_generate_random()`.</span>
This example shows how to generate ten bytes of random data by calling `psa_generate_random()`:
```C
psa_status_t status;
uint8_t random[10] = { 0 };
printf("Generate random...\t");
fflush(stdout);
/* Initialize PSA Crypto */
status = psa_crypto_init();
if (status != PSA_SUCCESS) {
printf("Failed to initialize PSA Crypto\n");
return;
}
status = psa_generate_random(random, sizeof(random));
if (status != PSA_SUCCESS) {
printf("Failed to generate a random value\n");
return;
}
printf("Generated random data\n");
/* Clean up */
mbedtls_psa_crypto_free();
```
### Deriving a new key from an existing key
Mbed Crypto provides a key derivation API that lets you derive new keys from
existing ones. The key derivation API has functions to take inputs, including
other keys and data, and functions to generate outputs, such as new keys or
other data.
You must first initialize and set up a key derivation context,
provided with a key and, optionally, other data. Then, use the key derivation context to either read derived data to a buffer or send derived data directly to a key slot.
See the documentation for the particular algorithm (such as HKDF or the TLS1.2 PRF) for
information about which inputs to pass when, and when you can obtain which outputs.
**Prerequisites to working with the key derivation APIs:**
* Initialize the library with a successful call to `psa_crypto_init()`.
* Use a key with the appropriate attributes set:
* Usage flags set for key derivation (`PSA_KEY_USAGE_DERIVE`)
* Key type set to `PSA_KEY_TYPE_DERIVE`.
* Algorithm set to a key derivation algorithm
(for example, `PSA_ALG_HKDF(PSA_ALG_SHA_256)`).
**To derive a new AES-CTR 128-bit encryption key into a given key slot using HKDF
with a given key, salt and info:**
1. Set up the key derivation context using the `psa_key_derivation_setup()`
function, specifying the derivation algorithm `PSA_ALG_HKDF(PSA_ALG_SHA_256)`.
1. Provide an optional salt with `psa_key_derivation_input_bytes()`.
1. Provide info with `psa_key_derivation_input_bytes()`.
1. Provide a secret with `psa_key_derivation_input_key()`, referencing a key that
can be used for key derivation.
1. Set the key attributes desired for the new derived key. We'll set
the `PSA_KEY_USAGE_ENCRYPT` usage flag and the `PSA_ALG_CTR` algorithm for this
example.
1. Derive the key by calling `psa_key_derivation_output_key()`.
1. Clean up the key derivation context.
At this point, the derived key slot holds a new 128-bit AES-CTR encryption key
derived from the key, salt and info provided:
```C
psa_status_t status;
psa_key_attributes_t attributes = PSA_KEY_ATTRIBUTES_INIT;
static const unsigned char key[] = {
0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b,
0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b,
0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b,
0x0b };
static const unsigned char salt[] = {
0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06,
0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c };
static const unsigned char info[] = {
0xf0, 0xf1, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6,
0xf7, 0xf8, 0xf9 };
psa_algorithm_t alg = PSA_ALG_HKDF(PSA_ALG_SHA_256);
psa_key_derivation_operation_t operation =
PSA_KEY_DERIVATION_OPERATION_INIT;
size_t derived_bits = 128;
size_t capacity = PSA_BITS_TO_BYTES(derived_bits);
psa_key_id_t base_key;
psa_key_id_t derived_key;
printf("Derive a key (HKDF)...\t");
fflush(stdout);
/* Initialize PSA Crypto */
status = psa_crypto_init();
if (status != PSA_SUCCESS) {
printf("Failed to initialize PSA Crypto\n");
return;
}
/* Import a key for use in key derivation. If such a key has already been
* generated or imported, you can skip this part. */
psa_set_key_usage_flags(&attributes, PSA_KEY_USAGE_DERIVE);
psa_set_key_algorithm(&attributes, alg);
psa_set_key_type(&attributes, PSA_KEY_TYPE_DERIVE);
status = psa_import_key(&attributes, key, sizeof(key), &base_key);
if (status != PSA_SUCCESS) {
printf("Failed to import a key\n");
return;
}
psa_reset_key_attributes(&attributes);
/* Derive a key */
status = psa_key_derivation_setup(&operation, alg);
if (status != PSA_SUCCESS) {
printf("Failed to begin key derivation\n");
return;
}
status = psa_key_derivation_set_capacity(&operation, capacity);
if (status != PSA_SUCCESS) {
printf("Failed to set capacity\n");
return;
}
status = psa_key_derivation_input_bytes(&operation,
PSA_KEY_DERIVATION_INPUT_SALT,
salt, sizeof(salt));
if (status != PSA_SUCCESS) {
printf("Failed to input salt (extract)\n");
return;
}
status = psa_key_derivation_input_key(&operation,
PSA_KEY_DERIVATION_INPUT_SECRET,
base_key);
if (status != PSA_SUCCESS) {
printf("Failed to input key (extract)\n");
return;
}
status = psa_key_derivation_input_bytes(&operation,
PSA_KEY_DERIVATION_INPUT_INFO,
info, sizeof(info));
if (status != PSA_SUCCESS) {
printf("Failed to input info (expand)\n");
return;
}
psa_set_key_usage_flags(&attributes, PSA_KEY_USAGE_ENCRYPT);
psa_set_key_algorithm(&attributes, PSA_ALG_CTR);
psa_set_key_type(&attributes, PSA_KEY_TYPE_AES);
psa_set_key_bits(&attributes, 128);
status = psa_key_derivation_output_key(&attributes, &operation,
&derived_key);
if (status != PSA_SUCCESS) {
printf("Failed to derive key\n");
return;
}
psa_reset_key_attributes(&attributes);
printf("Derived key\n");
/* Clean up key derivation operation */
psa_key_derivation_abort(&operation);
/* Destroy the keys */
psa_destroy_key(derived_key);
psa_destroy_key(base_key);
mbedtls_psa_crypto_free();
```
### Authenticating and encrypting or decrypting a message
Mbed Crypto provides a simple way to authenticate and encrypt with associated data (AEAD), supporting the `PSA_ALG_CCM` algorithm.
**Prerequisites to working with the AEAD cipher APIs:**
* Initialize the library with a successful call to `psa_crypto_init()`.
* The key attributes for the key used for derivation must have the `PSA_KEY_USAGE_ENCRYPT` or `PSA_KEY_USAGE_DECRYPT` usage flags.
This example shows how to authenticate and encrypt a message:
```C
psa_status_t status;
static const uint8_t key[] = {
0xC0, 0xC1, 0xC2, 0xC3, 0xC4, 0xC5, 0xC6, 0xC7,
0xC8, 0xC9, 0xCA, 0xCB, 0xCC, 0xCD, 0xCE, 0xCF };
static const uint8_t nonce[] = {
0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
0x08, 0x09, 0x0A, 0x0B };
static const uint8_t additional_data[] = {
0xEC, 0x46, 0xBB, 0x63, 0xB0, 0x25,
0x20, 0xC3, 0x3C, 0x49, 0xFD, 0x70 };
static const uint8_t input_data[] = {
0xB9, 0x6B, 0x49, 0xE2, 0x1D, 0x62, 0x17, 0x41,
0x63, 0x28, 0x75, 0xDB, 0x7F, 0x6C, 0x92, 0x43,
0xD2, 0xD7, 0xC2 };
uint8_t *output_data = NULL;
size_t output_size = 0;
size_t output_length = 0;
size_t tag_length = 16;
psa_key_attributes_t attributes = PSA_KEY_ATTRIBUTES_INIT;
psa_key_id_t key;
printf("Authenticate encrypt...\t");
fflush(stdout);
/* Initialize PSA Crypto */
status = psa_crypto_init();
if (status != PSA_SUCCESS) {
printf("Failed to initialize PSA Crypto\n");
return;
}
output_size = sizeof(input_data) + tag_length;
output_data = (uint8_t *)malloc(output_size);
if (!output_data) {
printf("Out of memory\n");
return;
}
/* Import a key */
psa_set_key_usage_flags(&attributes, PSA_KEY_USAGE_ENCRYPT);
psa_set_key_algorithm(&attributes, PSA_ALG_CCM);
psa_set_key_type(&attributes, PSA_KEY_TYPE_AES);
psa_set_key_bits(&attributes, 128);
status = psa_import_key(&attributes, key, sizeof(key), &key);
psa_reset_key_attributes(&attributes);
/* Authenticate and encrypt */
status = psa_aead_encrypt(key, PSA_ALG_CCM,
nonce, sizeof(nonce),
additional_data, sizeof(additional_data),
input_data, sizeof(input_data),
output_data, output_size,
&output_length);
if (status != PSA_SUCCESS) {
printf("Failed to authenticate and encrypt\n");
return;
}
printf("Authenticated and encrypted\n");
/* Clean up */
free(output_data);
/* Destroy the key */
psa_destroy_key(key);
mbedtls_psa_crypto_free();
```
This example shows how to authenticate and decrypt a message:
```C
psa_status_t status;
static const uint8_t key_data[] = {
0xC0, 0xC1, 0xC2, 0xC3, 0xC4, 0xC5, 0xC6, 0xC7,
0xC8, 0xC9, 0xCA, 0xCB, 0xCC, 0xCD, 0xCE, 0xCF };
static const uint8_t nonce[] = {
0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
0x08, 0x09, 0x0A, 0x0B };
static const uint8_t additional_data[] = {
0xEC, 0x46, 0xBB, 0x63, 0xB0, 0x25,
0x20, 0xC3, 0x3C, 0x49, 0xFD, 0x70 };
static const uint8_t input_data[] = {
0x20, 0x30, 0xE0, 0x36, 0xED, 0x09, 0xA0, 0x45, 0xAF, 0x3C, 0xBA, 0xEE,
0x0F, 0xC8, 0x48, 0xAF, 0xCD, 0x89, 0x54, 0xF4, 0xF6, 0x3F, 0x28, 0x9A,
0xA1, 0xDD, 0xB2, 0xB8, 0x09, 0xCD, 0x7C, 0xE1, 0x46, 0xE9, 0x98 };
uint8_t *output_data = NULL;
size_t output_size = 0;
size_t output_length = 0;
psa_key_attributes_t attributes = PSA_KEY_ATTRIBUTES_INIT;
psa_key_id_t key;
printf("Authenticate decrypt...\t");
fflush(stdout);
/* Initialize PSA Crypto */
status = psa_crypto_init();
if (status != PSA_SUCCESS) {
printf("Failed to initialize PSA Crypto\n");
return;
}
output_size = sizeof(input_data);
output_data = (uint8_t *)malloc(output_size);
if (!output_data) {
printf("Out of memory\n");
return;
}
/* Import a key */
psa_set_key_usage_flags(&attributes, PSA_KEY_USAGE_DECRYPT);
psa_set_key_algorithm(&attributes, PSA_ALG_CCM);
psa_set_key_type(&attributes, PSA_KEY_TYPE_AES);
psa_set_key_bits(&attributes, 128);
status = psa_import_key(&attributes, key_data, sizeof(key_data), &key);
if (status != PSA_SUCCESS) {
printf("Failed to import a key\n");
return;
}
psa_reset_key_attributes(&attributes);
/* Authenticate and decrypt */
status = psa_aead_decrypt(key, PSA_ALG_CCM,
nonce, sizeof(nonce),
additional_data, sizeof(additional_data),
input_data, sizeof(input_data),
output_data, output_size,
&output_length);
if (status != PSA_SUCCESS) {
printf("Failed to authenticate and decrypt %ld\n", status);
return;
}
printf("Authenticated and decrypted\n");
/* Clean up */
free(output_data);
/* Destroy the key */
psa_destroy_key(key);
mbedtls_psa_crypto_free();
```
### Generating and exporting keys
Mbed Crypto provides a simple way to generate a key or key pair.
**Prerequisites to using key generation and export APIs:**
* Initialize the library with a successful call to `psa_crypto_init()`.
**To generate an ECDSA key:**
1. Set the desired key attributes for key generation by calling
`psa_set_key_algorithm()` with the chosen ECDSA algorithm (such as
`PSA_ALG_DETERMINISTIC_ECDSA(PSA_ALG_SHA_256)`). You only want to export the public key, not the key pair (or private key); therefore, do not set `PSA_KEY_USAGE_EXPORT`.
1. Generate a key by calling `psa_generate_key()`.
1. Export the generated public key by calling `psa_export_public_key()`:
```C
enum {
key_bits = 256,
};
psa_status_t status;
size_t exported_length = 0;
static uint8_t exported[PSA_KEY_EXPORT_ECC_PUBLIC_KEY_MAX_SIZE(key_bits)];
psa_key_attributes_t attributes = PSA_KEY_ATTRIBUTES_INIT;
psa_key_id_t key;
printf("Generate a key pair...\t");
fflush(stdout);
/* Initialize PSA Crypto */
status = psa_crypto_init();
if (status != PSA_SUCCESS) {
printf("Failed to initialize PSA Crypto\n");
return;
}
/* Generate a key */
psa_set_key_usage_flags(&attributes, PSA_KEY_USAGE_SIGN_HASH);
psa_set_key_algorithm(&attributes,
PSA_ALG_DETERMINISTIC_ECDSA(PSA_ALG_SHA_256));
psa_set_key_type(&attributes,
PSA_KEY_TYPE_ECC_KEY_PAIR(PSA_ECC_FAMILY_SECP_R1));
psa_set_key_bits(&attributes, key_bits);
status = psa_generate_key(&attributes, &key);
if (status != PSA_SUCCESS) {
printf("Failed to generate key\n");
return;
}
psa_reset_key_attributes(&attributes);
status = psa_export_public_key(key, exported, sizeof(exported),
&exported_length);
if (status != PSA_SUCCESS) {
printf("Failed to export public key %ld\n", status);
return;
}
printf("Exported a public key\n");
/* Destroy the key */
psa_destroy_key(key);
mbedtls_psa_crypto_free();
```
### More about the PSA Crypto API
For more information about the PSA Crypto API, please see the [PSA Cryptography API Specification](https://armmbed.github.io/mbed-crypto/html/index.html).

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The documents in this directory are proposed specifications for Mbed
TLS features. They are not implemented yet, or only partially
implemented. Please follow activity on the `development` branch of
Mbed TLS if you are interested in these features.

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Conditional inclusion of cryptographic mechanism through the PSA API in Mbed TLS
================================================================================
This document is a proposed interface for deciding at build time which cryptographic mechanisms to include in the PSA Cryptography interface.
This is currently a proposal for Mbed TLS. It is not currently on track for standardization in PSA.
Time-stamp: "2020/11/26 09:30:50 GMT"
## Introduction
### Purpose of this specification
The [PSA Cryptography API specification](https://armmbed.github.io/mbed-crypto/psa/#application-programming-interface) specifies the interface between a PSA Cryptography implementation and an application. The interface defines a number of categories of cryptographic algorithms (hashes, MAC, signatures, etc.). In each category, a typical implementation offers many algorithms (e.g. for signatures: RSA-PKCS#1v1.5, RSA-PSS, ECDSA). When building the implementation for a specific use case, it is often desirable to include only a subset of the available cryptographic mechanisms, primarily in order to reduce the code footprint of the compiled system.
The present document proposes a way for an application using the PSA cryptography interface to declare which mechanisms it requires.
### Conditional inclusion of legacy cryptography modules
Mbed TLS offers a way to select which cryptographic mechanisms are included in a build through its configuration file (`config.h`). This mechanism is based on two main sets of symbols: `MBEDTLS_xxx_C` controls the availability of the mechanism to the application, and `MBEDTLS_xxx_ALT` controls the availability of an alternative implementation, so the software implementation is only included if `MBEDTLS_xxx_C` is defined but not `MBEDTLS_xxx_ALT`.
### PSA evolution
In the PSA cryptography interface, the **core** (built-in implementations of cryptographic mechanisms) can be augmented with drivers. **Transparent drivers** replace the built-in implementation of a cryptographic mechanism (or, with **fallback**, the built-in implementation is tried if the driver only has partial support for the mechanism). **Opaque drivers** implement cryptographic mechanisms on keys which are stored in a separate domain such as a secure element, for which the core only does key management and dispatch using wrapped key blobs or key identifiers.
The current model is difficult to adapt to the PSA interface for several reasons. The `MBEDTLS_xxx_ALT` symbols are somewhat inconsistent, and in particular do not work well for asymmetric cryptography. For example, many parts of the ECC code have no `MBEDTLS_xxx_ALT` symbol, so a platform with ECC acceleration that can perform all ECDSA and ECDH operations in the accelerator would still embark the `bignum` module and large parts of the `ecp_curves`, `ecp` and `ecdsa` modules. Also the availability of a transparent driver for a mechanism does not translate directly to `MBEDTLS_xxx` symbols.
### Requirements
[Req.interface] The application can declare which cryptographic mechanisms it needs.
[Req.inclusion] If the application does not require a mechanism, a suitably configured Mbed TLS build must not include it. The granularity of mechanisms must work for typical use cases and has [acceptable limitations](#acceptable-limitations).
[Req.drivers] If a PSA driver is available in the build, a suitably configured Mbed TLS build must not include the corresponding software code (unless a software fallback is needed).
[Req.c] The configuration mechanism consists of C preprocessor definitions, and the build does not require tools other than a C compiler. This is necessary to allow building an application and Mbed TLS in development environments that do not allow third-party tools.
[Req.adaptability] The implementation of the mechanism must be adaptable with future evolution of the PSA cryptography specifications and Mbed TLS. Therefore the interface must remain sufficiently simple and abstract.
### Acceptable limitations
[Limitation.matrix] If a mechanism is defined by a combination of algorithms and key types, for example a block cipher mode (CBC, CTR, CFB, …) and a block permutation (AES, CAMELLIA, ARIA, …), there is no requirement to include only specific combinations.
[Limitation.direction] For mechanisms that have multiple directions (for example encrypt/decrypt, sign/verify), there is no requirement to include only one direction.
[Limitation.size] There is no requirement to include only support for certain key sizes.
[Limitation.multipart] Where there are multiple ways to perform an operation, for example single-part and multi-part, there is no mechanism to select only one or a subset of the possible ways.
## Interface
### PSA Crypto configuration file
The PSA Crypto configuration file `psa/crypto_config.h` defines a series of symbols of the form `PSA_WANT_xxx` where `xxx` describes the feature that the symbol enables. The symbols are documented in the section [“PSA Crypto configuration symbols”](#psa-crypto-configuration-symbols) below.
The symbol `MBEDTLS_PSA_CRYPTO_CONFIG` in `mbedtls/config.h` determines whether `psa/crypto_config.h` is used.
* If `MBEDTLS_PSA_CRYPTO_CONFIG` is unset, which is the default at least in Mbed TLS 2.x versions, things are as they are today: the PSA subsystem includes generic code unconditionally, and includes support for specific mechanisms conditionally based on the existing `MBEDTLS_xxx_` symbols.
* If `MBEDTLS_PSA_CRYPTO_CONFIG` is set, the necessary software implementations of cryptographic algorithms are included based on both the content of the PSA Crypto configuration file and the Mbed TLS configuration file. For example, the code in `aes.c` is enabled if either `mbedtls/config.h` contains `MBEDTLS_AES_C` or `psa/crypto_config.h` contains `PSA_WANT_KEY_TYPE_AES`.
### PSA Crypto configuration symbols
#### Configuration symbol syntax
A PSA Crypto configuration symbol is a C preprocessor symbol whose name starts with `PSA_WANT_`.
* If the symbol is not defined, the corresponding feature is not included.
* If the symbol is defined to a preprocessor expression with the value `1`, the corresponding feature is included.
* If the symbol is defined with a different value, the behavior is currently undefined and reserved for future use.
#### Configuration symbol usage
The presence of a symbol `PSA_WANT_xxx` in the Mbed TLS configuration determines whether a feature is available through the PSA API. These symbols should be used in any place that requires conditional compilation based on the availability of a cryptographic mechanism through the PSA API, including:
* In Mbed TLS test code.
* In Mbed TLS library code using `MBEDTLS_USE_PSA_CRYPTO`, for example in TLS to determine which cipher suites to enable.
* In application code that provides additional features based on cryptographic capabilities, for example additional key parsing and formatting functions, or cipher suite availability for network protocols.
#### Configuration symbol semantics
If a feature is not requested for inclusion in the PSA Crypto configuration file, it may still be included in the build, either because the feature has been requested in some other way, or because the library does not support the exclusion of this feature. Mbed TLS should make a best effort to support the exclusion of all features, but in some cases this may be judged too much effort for too little benefit.
#### Configuration symbols for key types
For each constant or constructor macro of the form `PSA_KEY_TYPE_xxx`, the symbol **`PSA_WANT_KEY_TYPE_xxx`** indicates that support for this key type is desired.
For asymmetric cryptography, `PSA_WANT_KEY_TYPE_xxx_KEY_PAIR` determines whether private-key operations are desired, and `PSA_WANT_KEY_TYPE_xxx_PUBLIC_KEY` determines whether public-key operations are desired. `PSA_WANT_KEY_TYPE_xxx_KEY_PAIR` implicitly enables `PSA_WANT_KEY_TYPE_xxx_PUBLIC_KEY`: there is no way to only include private-key operations (which typically saves little code).
#### Configuration symbols for curves
For elliptic curve key types, only the specified curves are included. To include a curve, include a symbol of the form **`PSA_WANT_ECC_family_size`**. For example: `PSA_WANT_ECC_SECP_R1_256` for secp256r1, `PSA_WANT_ECC_MONTGOMERY_CURVE25519`. It is an error to require an ECC key type but no curve, and Mbed TLS will reject this at compile time.
#### Configuration symbols for algorithms
For each constant or constructor macro of the form `PSA_ALG_xxx`, the symbol **`PSA_WANT_ALG_xxx`** indicates that support for this algorithm is desired.
For parametrized algorithms, the `PSA_WANT_ALG_xxx` symbol indicates whether the base mechanism is supported. Parameters must themselves be included through their own `PSA_WANT_ALG_xxx` symbols. It is an error to include a base mechanism without at least one possible parameter, and Mbed TLS will reject this at compile time. For example, `PSA_WANT_ALG_ECDSA` requires the inclusion of randomized ECDSA for all hash algorithms whose corresponding symbol `PSA_WANT_ALG_xxx` is enabled.
## Implementation
### Additional non-public symbols
#### Accounting for transparent drivers
In addition to the [configuration symbols](#psa-crypto-configuration-symbols), we need two parallel or mostly parallel sets of symbols:
* **`MBEDTLS_PSA_ACCEL_xxx`** indicates whether a fully-featured, fallback-free transparent driver is available.
* **`MBEDTLS_PSA_BUILTIN_xxx`** indicates whether the software implementation is needed.
`MBEDTLS_PSA_ACCEL_xxx` is one of the outputs of the transpilation of a driver description, alongside the glue code for calling the drivers.
`MBEDTLS_PSA_BUILTIN_xxx` is enabled when `PSA_WANT_xxx` is enabled and `MBEDTLS_PSA_ACCEL_xxx` is disabled.
These symbols are not part of the public interface of Mbed TLS towards applications or to drivers, regardless of whether the symbols are actually visible.
### Architecture of symbol definitions
#### New-style definition of configuration symbols
When `MBEDTLS_PSA_CRYPTO_CONFIG` is set, the header file `mbedtls/config.h` needs to define all the `MBEDTLS_xxx_C` configuration symbols, including the ones deduced from the PSA Crypto configuration. It does this by including the new header file **`mbedtls/config_psa.h`**, which defines the `MBEDTLS_PSA_BUILTIN_xxx` symbols and deduces the corresponding `MBEDTLS_xxx_C` (and other) symbols.
`mbedtls/config_psa.h` includes `psa/crypto_config.h`, the user-editable file that defines application requirements.
#### Old-style definition of configuration symbols
When `MBEDTLS_PSA_CRYPTO_CONFIG` is not set, the configuration of Mbed TLS works as before, and the inclusion of non-PSA code only depends on `MBEDTLS_xxx` symbols defined (or not) in `mbedtls/config.h`. Furthermore, the new header file **`mbedtls/config_psa.h`** deduces PSA configuration symbols (`PSA_WANT_xxx`, `MBEDTLS_PSA_BUILTIN_xxx`) from classic configuration symbols (`MBEDTLS_xxx`).
The `PSA_WANT_xxx` definitions in `mbedtls/config_psa.h` are needed not only to build the PSA parts of the library, but also to build code that uses these parts. This includes structure definitions in `psa/crypto_struct.h`, size calculations in `psa/crypto_sizes.h`, and application code that's specific to a given cryptographic mechanism. In Mbed TLS itself, code under `MBEDTLS_USE_PSA_CRYPTO` and conditional compilation guards in tests and sample programs need `PSA_WANT_xxx`.
Since some existing applications use a handwritten `mbedtls/config.h` or an edited copy of `mbedtls/config.h` from an earlier version of Mbed TLS, `mbedtls/config_psa.h` must be included via an already existing header that is not `mbedtls/config.h`, so it is included via `psa/crypto.h` (for example from `psa/crypto_platform.h`).
#### Summary of definitions of configuration symbols
Whether `MBEDTLS_PSA_CRYPTO_CONFIG` is set or not, `mbedtls/config_psa.h` includes `mbedtls/crypto_drivers.h`, a header file generated by the transpilation of the driver descriptions. It defines `MBEDTLS_PSA_ACCEL_xxx` symbols according to the availability of transparent drivers without fallback.
The following table summarizes where symbols are defined depending on the configuration mode.
* (U) indicates a symbol that is defined by the user (application).
* (D) indicates a symbol that is deduced from other symbols by code that ships with Mbed TLS.
* (G) indicates a symbol that is generated from driver descriptions.
| Symbols | With `MBEDTLS_PSA_CRYPTO_CONFIG` | Without `MBEDTLS_PSA_CRYPTO_CONFIG` |
| ------------------------- | -------------------------------- | ----------------------------------- |
| `MBEDTLS_xxx_C` | `mbedtls/config.h` (U) or | `mbedtls/config.h` (U) |
| | `mbedtls/config_psa.h` (D) | |
| `PSA_WANT_xxx` | `psa/crypto_config.h` (U) | `mbedtls/config_psa.h` (D) |
| `MBEDTLS_PSA_BUILTIN_xxx` | `mbedtls/config_psa.h` (D) | `mbedtls/config_psa.h` (D) |
| `MBEDTLS_PSA_ACCEL_xxx` | `mbedtls/crypto_drivers.h` (G) | N/A |
#### Visibility of internal symbols
Ideally, the `MBEDTLS_PSA_ACCEL_xxx` and `MBEDTLS_PSA_BUILTIN_xxx` symbols should not be visible to application code or driver code, since they are not part of the public interface of the library. However these symbols are needed to deduce whether to include library modules (for example `MBEDTLS_AES_C` has to be enabled if `MBEDTLS_PSA_BUILTIN_KEY_TYPE_AES` is enabled), which makes it difficult to keep them private.
#### Compile-time checks
The header file **`library/psa_check_config.h`** applies sanity checks to the configuration, throwing `#error` if something is wrong.
A mechanism similar to `mbedtls/check_config.h` detects errors such as enabling ECDSA but no curve.
Since configuration symbols must be undefined or 1, any other value should trigger an `#error`.
#### Automatic generation of preprocessor symbol manipulations
A lot of the preprocessor symbol manipulation is systematic calculations that analyze the configuration. `mbedtls/config_psa.h` and `library/psa_check_config.h` should be generated automatically, in the same manner as `version_features.c`.
### Structure of PSA Crypto library code
#### Conditional inclusion of library entry points
An entry point can be eliminated entirely if no algorithm requires it.
#### Conditional inclusion of mechanism-specific code
Code that is specific to certain key types or to certain algorithms must be guarded by the applicable symbols: `PSA_WANT_xxx` for code that is independent of the application, and `MBEDTLS_PSA_BUILTIN_xxx` for code that calls an Mbed TLS software implementation.
## PSA standardization
### JSON configuration mechanism
At the time of writing, the preferred configuration mechanism for a PSA service is in JSON syntax. The translation from JSON to build instructions is not specified by PSA.
For PSA Crypto, the preferred configuration mechanism would be similar to capability specifications of transparent drivers. The same JSON properties that are used to mean “this driver can perform that mechanism” in a driver description would be used to mean “the application wants to perform that mechanism” in the application configuration.
### From JSON to C
The JSON capability language allows a more fine-grained selection than the C mechanism proposed here. For example, it allows requesting only single-part mechanisms, only certain key sizes, or only certain combinations of algorithms and key types.
The JSON capability language can be translated approximately to the boolean symbol mechanism proposed here. The approximation considers a feature to be enabled if any part of it is enabled. For example, if there is a capability for AES-CTR and one for CAMELLIA-GCM, the translation to boolean symbols will also include AES-GCM and CAMELLIA-CTR. If there is a capability for AES-128, the translation will also include AES-192 and AES-256.
The boolean symbol mechanism proposed here can be translated to a list of JSON capabilities: for each included algorithm, include a capability with that algorithm, the key types that apply to that algorithm, no size restriction, and all the entry points that apply to that algorithm.
## Open questions
### Open questions about the interface
#### Naming of symbols
The names of [elliptic curve symbols](#configuration-symbols-for-curves) are a bit weird: `SECP_R1_256` instead of `SECP256R1`. Should we make them more classical, but less systematic?
#### Impossible combinations
What does it mean to have `PSA_WANT_ALG_ECDSA` enabled but with only Curve25519? Is it a mandatory error?
#### Diffie-Hellman
Way to request only specific groups? Not a priority: constrained devices don't do FFDH. Specify it as may change in future versions.
#### Coexistence with the current Mbed TLS configuration
The two mechanisms have very different designs. Is there serious potential for confusion? Do we understand how the combinations work?
### Open questions about the design
#### Algorithms without a key type or vice versa
Is it realistic to mandate a compile-time error if a key type is required, but no matching algorithm, or vice versa? Is it always the right thing, for example if there is an opaque driver that manipulates this key type?
#### Opaque-only mechanisms
If a mechanism should only be supported in an opaque driver, what does the core need to know about it? Do we have all the information we need?
This is especially relevant to suppress a mechanism completely if there is no matching algorithm. For example, if there is no transparent implementation of RSA or ECDSA, `psa_sign_hash` and `psa_verify_hash` may still be needed if there is an opaque signature driver.
### Open questions about the implementation
#### Testability
Is this proposal decently testable? There are a lot of combinations. What combinations should we test?
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PSA Cryptoprocessor driver developer's guide
============================================
**This is a specification of work in progress. The implementation is not yet merged into Mbed TLS.**
This document describes how to write drivers of cryptoprocessors such as accelerators and secure elements for the PSA cryptography subsystem of Mbed TLS.
This document focuses on behavior that is specific to Mbed TLS. For a reference of the interface between Mbed TLS and drivers, refer to the [PSA Cryptoprocessor Driver Interface specification](psa-driver-interface.html).
The interface is not fully implemented in Mbed TLS yet and is disabled by default. You can enable the experimental work in progress by setting `MBEDTLS_PSA_CRYPTO_DRIVERS` in the compile-time configuration. Please note that the interface may still change: until further notice, we do not guarantee backward compatibility with existing driver code when `MBEDTLS_PSA_CRYPTO_DRIVERS` is enabled.
## Introduction
### Purpose
The PSA cryptography driver interface provides a way to build Mbed TLS with additional code that implements certain cryptographic primitives. This is primarily intended to support platform-specific hardware.
There are two types of drivers:
* **Transparent** drivers implement cryptographic operations on keys that are provided in cleartext at the beginning of each operation. They are typically used for hardware **accelerators**. When a transparent driver is available for a particular combination of parameters (cryptographic algorithm, key type and size, etc.), it is used instead of the default software implementation. Transparent drivers can also be pure software implementations that are distributed as plug-ins to a PSA Crypto implementation.
* **Opaque** drivers implement cryptographic operations on keys that can only be used inside a protected environment such as a **secure element**, a hardware security module, a smartcard, a secure enclave, etc. An opaque driver is invoked for the specific key location that the driver is registered for: the dispatch is based on the key's lifetime.
### Deliverables for a driver
To write a driver, you need to implement some functions with C linkage, and to declare these functions in a **driver description file**. The driver description file declares which functions the driver implements and what cryptographic mechanisms they support. Depending on the driver type, you may also need to define some C types and macros in a header file.
The concrete syntax for a driver description file is JSON. The structure of this JSON file is specified in the section [“Driver description syntax”](psa-driver-interface.html#driver-description-syntax) of the PSA cryptography driver interface specification.
A driver therefore consists of:
* A driver description file (in JSON format).
* C header files defining the types required by the driver description. The names of these header files is declared in the driver description file.
* An object file compiled for the target platform defining the functions required by the driver description. Implementations may allow drivers to be provided as source files and compiled with the core instead of being pre-compiled.
## Driver C interfaces
Mbed TLS calls driver entry points [as specified in the PSA Cryptography Driver Interface specification](psa-driver-interface.html#driver-entry-points) except as otherwise indicated in this section.
## Building and testing your driver
<!-- TODO -->
## Dependencies on the Mbed TLS configuration
<!-- TODO -->

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Building Mbed TLS with PSA cryptoprocessor drivers
==================================================
**This is a specification of work in progress. The implementation is not yet merged into Mbed TLS.**
This document describes how to build Mbed TLS with additional cryptoprocessor drivers that follow the PSA cryptoprocessor driver interface.
The interface is not fully implemented in Mbed TLS yet and is disabled by default. You can enable the experimental work in progress by setting `MBEDTLS_PSA_CRYPTO_DRIVERS` in the compile-time configuration. Please note that the interface may still change: until further notice, we do not guarantee backward compatibility with existing driver code when `MBEDTLS_PSA_CRYPTO_DRIVERS` is enabled.
## Introduction
The PSA cryptography driver interface provides a way to build Mbed TLS with additional code that implements certain cryptographic primitives. This is primarily intended to support platform-specific hardware.
Note that such drivers are only available through the PSA cryptography API (crypto functions beginning with `psa_`, and X.509 and TLS interfaces that reference PSA types).
Concretely speaking, a driver consists of one or more **driver description files** in JSON format and some code to include in the build. The driver code can either be provided in binary form as additional object file to link, or in source form.
## How to build Mbed TLS with drivers
To build Mbed TLS with drivers:
1. Activate `MBEDTLS_PSA_CRYPTO_DRIVERS` in the library configuration.
```
cd /path/to/mbedtls
scripts/config.py set MBEDTLS_PSA_CRYPTO_DRIVERS
```
2. Pass the driver description files through the Make variable `PSA_DRIVERS` when building the library.
```
cd /path/to/mbedtls
make PSA_DRIVERS="/path/to/acme/driver.json /path/to/nadir/driver.json" lib
```
3. Link your application with the implementation of the driver functions.
```
cd /path/to/application
ld myapp.o -L/path/to/acme -lacmedriver -L/path/to/nadir -lnadirdriver -L/path/to/mbedtls -lmbedcrypto
```
<!-- TODO: what if the driver is provided as C source code? -->
<!-- TODO: what about additional include files? -->

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PSA Cryptoprocessor Driver Interface
====================================
This document describes an interface for cryptoprocessor drivers in the PSA cryptography API. This interface complements the [PSA Cryptography API specification](https://armmbed.github.io/mbed-crypto/psa/#application-programming-interface), which describes the interface between a PSA Cryptography implementation and an application.
This specification is work in progress and should be considered to be in a beta stage. There is ongoing work to implement this interface in Mbed TLS, which is the reference implementation of the PSA Cryptography API. At this stage, Arm does not expect major changes, but minor changes are expected based on experience from the first implementation and on external feedback.
Time-stamp: "2020/11/24 11:03:32 GMT"
## Introduction
### Purpose of the driver interface
The PSA Cryptography API defines an interface that allows applications to perform cryptographic operations in a uniform way regardless of how the operations are performed. Under the hood, different keys may be stored and used in different hardware or in different logical partitions, and different algorithms may involve different hardware or software components.
The driver interface allows implementations of the PSA Cryptography API to be built compositionally. An implementation of the PSA Cryptography API is composed of a **core** and zero or more **drivers**. The core handles key management, enforces key usage policies, and dispatches cryptographic operations either to the applicable driver or to built-in code.
Functions in the PSA Cryptography API invoke functions in the core. Code from the core calls drivers as described in the present document.
### Types of drivers
The PSA Cryptography driver interface supports two types of cryptoprocessors, and accordingly two types of drivers.
* **Transparent** drivers implement cryptographic operations on keys that are provided in cleartext at the beginning of each operation. They are typically used for hardware **accelerators**. When a transparent driver is available for a particular combination of parameters (cryptographic algorithm, key type and size, etc.), it is used instead of the default software implementation. Transparent drivers can also be pure software implementations that are distributed as plug-ins to a PSA Cryptography implementation (for example, an alternative implementation with different performance characteristics, or a certified implementation).
* **Opaque** drivers implement cryptographic operations on keys that can only be used inside a protected environment such as a **secure element**, a hardware security module, a smartcard, a secure enclave, etc. An opaque driver is invoked for the specific [key location](#lifetimes-and-locations) that the driver is registered for: the dispatch is based on the key's lifetime.
### Requirements
The present specification was designed to fulfill the following high-level requirements.
[Req.plugins] It is possible to combine multiple drivers from different providers into the same implementation, without any prior arrangement other than choosing certain names and values from disjoint namespaces.
[Req.compile] It is possible to compile the code of each driver and of the core separately, and link them together. A small amount of glue code may need to be compiled once the list of drivers is available.
[Req.types] Support drivers for the following types of hardware: accelerators that operate on keys in cleartext; cryptoprocessors that can wrap keys with a built-in keys but not store user keys; and cryptoprocessors that store key material.
[Req.portable] The interface between drivers and the core does not involve any platform-specific consideration. Driver calls are simple C function calls. Interactions with platform-specific hardware happen only inside the driver (and in fact a driver need not involve any hardware at all).
[Req.location] Applications can tell which location values correspond to which secure element drivers.
[Req.fallback] Accelerator drivers can specify that they do not fully support a cryptographic mechanism and that a fallback to core code may be necessary. Conversely, if an accelerator fully supports cryptographic mechanism, the core must be able to omit code for this mechanism.
[Req.mechanisms] Drivers can specify which mechanisms they support. A driver's code will not be invoked for cryptographic mechanisms that it does not support.
## Overview of drivers
### Deliverables for a driver
To write a driver, you need to implement some functions with C linkage, and to declare these functions in a **driver description file**. The driver description file declares which functions the driver implements and what cryptographic mechanisms they support. If the driver description references custom types, macros or constants, you also need to provide C header files defining those elements.
The concrete syntax for a driver description file is JSON. The structure of this JSON file is specified in the section [“Driver description syntax”](#driver-description-syntax).
A driver therefore consists of:
* A driver description file (in JSON format).
* C header files defining the types required by the driver description. The names of these header files are declared in the driver description file.
* An object file compiled for the target platform defining the entry point functions specified by the driver description. Implementations may allow drivers to be provided as source files and compiled with the core instead of being pre-compiled.
How to provide the driver description file, the C header files and the object code is implementation-dependent.
### Driver description syntax
The concrete syntax for a driver description file is JSON.
#### Driver description list
PSA Cryptography core implementations should support multiple drivers. The driver description files are passed to the implementation as an ordered list in an unspecified manner. This may be, for example, a list of file names passed on a command line, or a JSON list whose elements are individual driver descriptions.
#### Driver description top-level element
A driver description is a JSON object containing the following properties:
* `"prefix"` (mandatory, string). This must be a valid prefix for a C identifier. All the types and functions provided by the driver have a name that starts with this prefix unless overridden with a `"name"` element in the applicable capability as described below.
* `"type"` (mandatory, string). One of `"transparent"` or `"opaque"`.
* `"headers"` (optional, array of strings). A list of header files. These header files must define the types, macros and constants referenced by the driver description. They may declare the entry point functions, but this is not required. They may include other PSA headers and standard headers of the platform. Whether they may include other headers is implementation-specific. If omitted, the list of headers is empty. The header files must be present at the specified location relative to a directory on the compiler's include path when compiling glue code between the core and the drivers.
* `"capabilities"` (mandatory, array of [capabilities](#driver-description-capability)).
A list of **capabilities**. Each capability describes a family of functions that the driver implements for a certain class of cryptographic mechanisms.
* `"key_context"` (not permitted for transparent drivers, mandatory for opaque drivers): information about the [representation of keys](#key-format-for-opaque-drivers).
* `"persistent_state_size"` (not permitted for transparent drivers, optional for opaque drivers, integer or string). The size in bytes of the [persistent state of the driver](#opaque-driver-persistent-state). This may be either a non-negative integer or a C constant expression of type `size_t`.
* `"location"` (not permitted for transparent drivers, optional for opaque drivers, integer or string). The [location value](#lifetimes-and-locations) for which this driver is invoked. In other words, this determines the lifetimes for which the driver is invoked. This may be either a non-negative integer or a C constant expression of type `psa_key_location_t`.
### Driver description capability
#### Capability syntax
A capability declares a family of functions that the driver implements for a certain class of cryptographic mechanisms. The capability specifies which key types and algorithms are covered and the names of the types and functions that implement it.
A capability is a JSON object containing the following properties:
* `"entry_points"` (mandatory, list of strings). Each element is the name of a [driver entry point](#driver-entry-points) or driver entry point family. An entry point is a function defined by the driver. If specified, the core will invoke this capability of the driver only when performing one of the specified operations. The driver must implement all the specified entry points, as well as the types if applicable.
* `"algorithms"` (optional, list of strings). Each element is an [algorithm specification](#algorithm-specifications). If specified, the core will invoke this capability of the driver only when performing one of the specified algorithms. If omitted, the core will invoke this capability for all applicable algorithms.
* `"key_types"` (optional, list of strings). Each element is a [key type specification](#key-type-specifications). If specified, the core will invoke this capability of the driver only for operations involving a key with one of the specified key types. If omitted, the core will invoke this capability of the driver for all applicable key types.
* `"key_sizes"` (optional, list of integers). If specified, the core will invoke this capability of the driver only for operations involving a key with one of the specified key sizes. If omitted, the core will invoke this capability of the driver for all applicable key sizes. Key sizes are expressed in bits.
* `"names"` (optional, object). A mapping from entry point names described by the `"entry_points"` property, to the name of the C function in the driver that implements the corresponding function. If a function is not listed here, name of the driver function that implements it is the driver's prefix followed by an underscore (`_`) followed by the function name. If this property is omitted, it is equivalent to an empty object (so each entry point *suffix* is implemented by a function called *prefix*`_`*suffix*).
* `"fallback"` (optional for transparent drivers, not permitted for opaque drivers, boolean). If present and true, the driver may return `PSA_ERROR_NOT_SUPPORTED`, in which case the core should call another driver or use built-in code to perform this operation. If absent or false, the driver is expected to fully support the mechanisms described by this capability. See the section “[Fallback](#fallback)” for more information.
#### Capability semantics
When the PSA Cryptography implementation performs a cryptographic mechanism, it invokes available driver entry points as described in the section [“Driver entry points”](#driver-entry-points).
A driver is considered available for a cryptographic mechanism that invokes a given entry point if all of the following conditions are met:
* The driver specification includes a capability whose `"entry_points"` list either includes the entry point or includes an entry point family that includes the entry point.
* If the mechanism involves an algorithm:
* either the capability does not have an `"algorithms"` property;
* or the value of the capability's `"algorithms"` property includes an [algorithm specification](#algorithm-specifications) that matches this algorithm.
* If the mechanism involves a key:
* either the key is transparent (its location is `PSA_KEY_LOCATION_LOCAL_STORAGE`) and the driver is transparent;
* or the key is opaque (its location is not `PSA_KEY_LOCATION_LOCAL_STORAGE`) and the driver is an opaque driver whose location is the key's location.
* If the mechanism involves a key:
* either the capability does not have a `"key_types"` property;
* or the value of the capability's `"key_types"` property includes a [key type specification](#key-type-specifications) that matches this algorithm.
* If the mechanism involves a key:
* either the capability does not have a `"key_sizes"` property;
* or the value of the capability's `"key_sizes"` property includes the key's size.
If a driver includes multiple applicable capabilities for a given combination of entry point, algorithm, key type and key size, and all the capabilities map the entry point to the same function name, the driver is considered available for this cryptographic mechanism. If a driver includes multiple applicable capabilities for a given combination of entry point, algorithm, key type and key size, and at least two of these capabilities map the entry point to the different function names, the driver specification is invalid.
If multiple transparent drivers have applicable capabilities for a given combination of entry point, algorithm, key type and key size, the first matching driver in the [specification list](#driver-description-list) is invoked. If the capability has [fallback](#fallback) enabled and the first driver returns `PSA_ERROR_NOT_SUPPORTED`, the next matching driver is invoked, and so on.
If multiple opaque drivers have the same location, the list of driver specifications is invalid.
#### Capability examples
Example 1: the following capability declares that the driver can perform deterministic ECDSA signatures (but not signature verification) using any hash algorithm and any curve that the core supports. If the prefix of this driver is `"acme"`, the function that performs the signature is called `acme_sign_hash`.
```
{
"entry_points": ["sign_hash"],
"algorithms": ["PSA_ALG_DETERMINISTIC_ECDSA(PSA_ALG_ANY_HASH)"],
}
```
Example 2: the following capability declares that the driver can perform deterministic ECDSA signatures using SHA-256 or SHA-384 with a SECP256R1 or SECP384R1 private key (with either hash being possible in combination with either curve). If the prefix of this driver is `"acme"`, the function that performs the signature is called `acme_sign_hash`.
```
{
"entry_points": ["sign_hash"],
"algorithms": ["PSA_ALG_DETERMINISTIC_ECDSA(PSA_ALG_SHA_256)",
"PSA_ALG_DETERMINISTIC_ECDSA(PSA_ALG_SHA_384)"],
"key_types": ["PSA_KEY_TYPE_ECC_KEY_PAIR(PSA_ECC_CURVE_SECP_R1)"],
"key_sizes": [256, 384]
}
```
### Algorithm and key specifications
#### Algorithm specifications
An algorithm specification is a string consisting of a `PSA_ALG_xxx` macro that specifies a cryptographic algorithm or an algorithm wildcard policy defined by the PSA Cryptography API. If the macro takes arguments, the string must have the syntax of a C macro call and each argument must be an algorithm specification or a decimal or hexadecimal literal with no suffix, depending on the expected type of argument.
Spaces are optional after commas. Whether other whitespace is permitted is implementation-specific.
Valid examples:
```
PSA_ALG_SHA_256
PSA_ALG_HMAC(PSA_ALG_SHA_256)
PSA_ALG_KEY_AGREEMENT(PSA_ALG_ECDH, PSA_ALG_HKDF(PSA_ALG_SHA_256))
PSA_ALG_RSA_PSS(PSA_ALG_ANY_HASH)
```
#### Key type specifications
An algorithm specification is a string consisting of a `PSA_KEY_TYPE_xxx` macro that specifies a key type defined by the PSA Cryptography API. If the macro takes an argument, the string must have the syntax of a C macro call and each argument must be the name of a constant of suitable type (curve or group).
The name `_` may be used instead of a curve or group to indicate that the capability concerns all curves or groups.
Valid examples:
```
PSA_KEY_TYPE_AES
PSA_KEY_TYPE_ECC_KEY_PAIR(PSA_ECC_CURVE_SECP_R1)
PSA_KEY_TYPE_ECC_KEY_PAIR(_)
```
### Driver entry points
#### Overview of driver entry points
Drivers define functions, each of which implements an aspect of a capability of a driver, such as a cryptographic operation, a part of a cryptographic operation, or a key management action. These functions are called the **entry points** of the driver. Most driver entry points correspond to a particular function in the PSA Cryptography API. For example, if a call to `psa_sign_hash()` is dispatched to a driver, it invokes the driver's `sign_hash` function.
All driver entry points return a status of type `psa_status_t` which should use the status codes documented for PSA services in general and for PSA Cryptography in particular: `PSA_SUCCESS` indicates that the function succeeded, and `PSA_ERROR_xxx` values indicate that an error occurred.
The signature of a driver entry point generally looks like the signature of the PSA Cryptography API that it implements, with some modifications. This section gives an overview of modifications that apply to whole classes of entry points. Refer to the reference section for each entry point or entry point family for details.
* For entry points that operate on an existing key, the `psa_key_id_t` parameter is replaced by a sequence of three parameters that describe the key:
1. `const psa_key_attributes_t *attributes`: the key attributes.
2. `const uint8_t *key_buffer`: a key material or key context buffer.
3. `size_t key_buffer_size`: the size of the key buffer in bytes.
For transparent drivers, the key buffer contains the key material, in the same format as defined for `psa_export_key()` and `psa_export_public_key()` in the PSA Cryptography API. For opaque drivers, the content of the key buffer is entirely up to the driver.
* For entry points that involve a multi-part operation, the operation state type (`psa_XXX_operation_t`) is replaced by a driver-specific operation state type (*prefix*`_XXX_operation_t`).
* For entry points that are involved in key creation, the `psa_key_id_t *` output parameter is replaced by a sequence of parameters that convey the key context:
1. `uint8_t *key_buffer`: a buffer for the key material or key context.
2. `size_t key_buffer_size`: the size of the key buffer in bytes.
2. `size_t *key_buffer_length`: the length of the data written to the key buffer in bytes.
Some entry points are grouped in families that must be implemented as a whole. If a driver supports an entry point family, it must provide all the entry points in the family.
#### General considerations on driver entry point parameters
Buffer parameters for driver entry points obey the following conventions:
* An input buffer has the type `const uint8_t *` and is immediately followed by a parameter of type `size_t` that indicates the buffer size.
* An output buffer has the type `uint8_t *` and is immediately followed by a parameter of type `size_t` that indicates the buffer size. A third parameter of type `size_t *` is provided to report the actual length of the data written in the buffer if the function succeeds.
* An in-out buffer has the type `uint8_t *` and is immediately followed by a parameter of type `size_t` that indicates the buffer size. In-out buffers are only used when the input and the output have the same length.
Buffers of size 0 may be represented with either a null pointer or a non-null pointer.
Input buffers and other input-only parameters (`const` pointers) may be in read-only memory. Overlap is possible between input buffers, and between an input buffer and an output buffer, but not between two output buffers or between a non-buffer parameter and another parameter.
#### Driver entry points for single-part cryptographic operations
The following driver entry points perform a cryptographic operation in one shot (single-part operation):
* `"hash_compute"` (transparent drivers only): calculation of a hash. Called by `psa_hash_compute()` and `psa_hash_compare()`. To verify a hash with `psa_hash_compare()`, the core calls the driver's `"hash_compute"` entry point and compares the result with the reference hash value.
* `"mac_compute"`: calculation of a MAC. Called by `psa_mac_compute()` and possibly `psa_mac_verify()`. To verify a mac with `psa_mac_verify()`, the core calls an applicable driver's `"mac_verify"` entry point if there is one, otherwise the core calls an applicable driver's `"mac_compute"` entry point and compares the result with the reference MAC value.
* `"mac_verify"`: verification of a MAC. Called by `psa_mac_verify()`. This entry point is mainly useful for drivers of secure elements that verify a MAC without revealing the correct MAC. Although transparent drivers may implement this entry point in addition to `"mac_compute"`, it is generally not useful because the core can call the `"mac_compute"` entry point and compare with the expected MAC value.
* `"cipher_encrypt"`: unauthenticated symmetric cipher encryption. Called by `psa_cipher_encrypt()`.
* `"cipher_decrypt"`: unauthenticated symmetric cipher decryption. Called by `psa_cipher_decrypt()`.
* `"aead_encrypt"`: authenticated encryption with associated data. Called by `psa_aead_encrypt()`.
* `"aead_decrypt"`: authenticated decryption with associated data. Called by `psa_aead_decrypt()`.
* `"asymmetric_encrypt"`: asymmetric encryption. Called by `psa_asymmetric_encrypt()`.
* `"asymmetric_decrypt"`: asymmetric decryption. Called by `psa_asymmetric_decrypt()`.
* `"sign_hash"`: signature of an already calculated hash. Called by `psa_sign_hash()` and possibly `psa_sign_message()`. To sign a message with `psa_sign_message()`, the core calls an applicable driver's `"sign_message"` entry point if there is one, otherwise the core calls an applicable driver's `"hash_compute"` entry point followed by an applicable driver's `"sign_hash"` entry point.
* `"verify_hash"`: verification of an already calculated hash. Called by `psa_verify_hash()` and possibly `psa_verify_message()`. To verify a message with `psa_verify_message()`, the core calls an applicable driver's `"verify_message"` entry point if there is one, otherwise the core calls an applicable driver's `"hash_compute"` entry point followed by an applicable driver's `"verify_hash"` entry point.
* `"sign_message"`: signature of a message. Called by `psa_sign_message()`.
* `"verify_message"`: verification of a message. Called by `psa_verify_message()`.
* `"key_agreement"`: key agreement without a subsequent key derivation. Called by `psa_raw_key_agreement()` and possibly `psa_key_derivation_key_agreement()`.
### Driver entry points for multi-part operations
#### General considerations on multi-part operations
The entry points that implement each step of a multi-part operation are grouped into a family. A driver that implements a multi-part operation must define all of the entry points in this family as well as a type that represents the operation context. The lifecycle of a driver operation context is similar to the lifecycle of an API operation context:
1. The core initializes operation context objects to either all-bits-zero or to logical zero (`{0}`), at its discretion.
1. The core calls the `xxx_setup` entry point for this operation family. If this fails, the core destroys the operation context object without calling any other driver entry point on it.
1. The core calls other entry points that manipulate the operation context object, respecting the constraints.
1. If any entry point fails, the core calls the driver's `xxx_abort` entry point for this operation family, then destroys the operation context object without calling any other driver entry point on it.
1. If a “finish” entry point fails, the core destroys the operation context object without calling any other driver entry point on it. The finish entry points are: *prefix*`_mac_sign_finish`, *prefix*`_mac_verify_finish`, *prefix*`_cipher_fnish`, *prefix*`_aead_finish`, *prefix*`_aead_verify`.
If a driver implements a multi-part operation but not the corresponding single-part operation, the core calls the driver's multipart operation entry points to perform the single-part operation.
#### Multi-part operation entry point family `"hash_multipart"`
This family corresponds to the calculation of a hash in multiple steps.
This family applies to transparent drivers only.
This family requires the following type and entry points:
* Type `"hash_operation_t"`: the type of a hash operation context. It must be possible to copy a hash operation context byte by byte, therefore hash operation contexts must not contain any embedded pointers (except pointers to global data that do not change after the setup step).
* `"hash_setup"`: called by `psa_hash_setup()`.
* `"hash_update"`: called by `psa_hash_update()`.
* `"hash_finish"`: called by `psa_hash_finish()` and `psa_hash_verify()`.
* `"hash_abort"`: called by all multi-part hash functions of the PSA Cryptography API.
To verify a hash with `psa_hash_verify()`, the core calls the driver's *prefix*`_hash_finish` entry point and compares the result with the reference hash value.
For example, a driver with the prefix `"acme"` that implements the `"hash_multipart"` entry point family must define the following type and entry points (assuming that the capability does not use the `"names"` property to declare different type and entry point names):
```
typedef ... acme_hash_operation_t;
psa_status_t acme_hash_setup(acme_hash_operation_t *operation,
psa_algorithm_t alg);
psa_status_t acme_hash_update(acme_hash_operation_t *operation,
const uint8_t *input,
size_t input_length);
psa_status_t acme_hash_finish(acme_hash_operation_t *operation,
uint8_t *hash,
size_t hash_size,
size_t *hash_length);
psa_status_t acme_hash_abort(acme_hash_operation_t *operation);
```
#### Operation family `"mac_multipart"`
TODO
#### Operation family `"mac_verify_multipart"`
TODO
#### Operation family `"cipher_encrypt_multipart"`
TODO
#### Operation family `"cipher_decrypt_multipart"`
TODO
#### Operation family `"aead_encrypt_multipart"`
TODO
#### Operation family `"aead_decrypt_multipart"`
TODO
#### Operation family `"key_derivation"`
This family requires the following type and entry points:
* Type `"key_derivation_operation_t"`: the type of a key derivation operation context.
* `"key_derivation_setup"`: called by `psa_key_derivation_setup()`.
* `"key_derivation_set_capacity"`: called by `psa_key_derivation_set_capacity()`. The core will always enforce the capacity, therefore this function does not need to do anything for algorithms where the output stream only depends on the effective generated length and not on the capacity.
* `"key_derivation_input_bytes"`: called by `psa_key_derivation_input_bytes()` and `psa_key_derivation_input_key()`. For transparent drivers, when processing a call to `psa_key_derivation_input_key()`, the core always calls the applicable driver's `"key_derivation_input_bytes"` entry point.
* `"key_derivation_input_key"` (opaque drivers only)
* `"key_derivation_output_bytes"`: called by `psa_key_derivation_output_bytes()`; also by `psa_key_derivation_output_key()` for transparent drivers.
* `"key_derivation_output_key"`: called by `psa_key_derivation_output_key()` for transparent drivers when deriving an asymmetric key pair, and also for opaque drivers.
* `"key_derivation_abort"`: called by all key derivation functions of the PSA Cryptography API.
TODO: key input and output for opaque drivers; deterministic key generation for transparent drivers
TODO
### Driver entry points for key management
The driver entry points for key management differ significantly between [transparent drivers](#key-management-with-transparent-drivers) and [opaque drivers](#key-management-with-opaque-drivers). This section describes common elements. Refer to the applicable section for each driver type for more information.
The entry points that create or format key data have the following prototypes for a driver with the prefix `"acme"`:
```
psa_status_t acme_import_key(const psa_key_attributes_t *attributes,
const uint8_t *data,
size_t data_length,
uint8_t *key_buffer,
size_t key_buffer_size,
size_t *key_buffer_length,
size_t *bits); // additional parameter, see below
psa_status_t acme_generate_key(const psa_key_attributes_t *attributes,
uint8_t *key_buffer,
size_t key_buffer_size,
size_t *key_buffer_length);
```
TODO: derivation, copy
* The key attributes (`attributes`) have the same semantics as in the PSA Cryptography application interface.
* For the `"import_key"` entry point, the input in the `data` buffer is either the export format or an implementation-specific format that the core documents as an acceptable input format for `psa_import_key()`.
* The size of the key data buffer `key_buffer` is sufficient for the internal representation of the key. For a transparent driver, this is the key's [export format](#key-format-for-transparent-drivers). For an opaque driver, this is the size determined from the driver description and the key attributes, as specified in the section [“Key format for opaque drivers”](#key-format-for-opaque-drivers).
* For an opaque driver with an `"allocate_key"` entry point, the content of the key data buffer on entry is the output of that entry point.
* The `"import_key"` entry point must determine or validate the key size and set `*bits` as described in the section [“Key size determination on import”](#key-size-determination-on-import) below.
All key creation entry points must ensure that the resulting key is valid as specified in the section [“Key validation”](#key-validation) below. This is primarily important for import entry points since the key data comes from the application.
#### Key size determination on import
The `"import_key"` entry point must determine or validate the key size.
The PSA Cryptography API exposes the key size as part of the key attributes.
When importing a key, the key size recorded in the key attributes can be either a size specified by the caller of the API (who may not be trusted), or `0` which indicates that the size must be calculated from the data.
When the core calls the `"import_key"` entry point to process a call to `psa_import_key`, it passes an `attributes` structure such that `psa_get_key_bits(attributes)` is the size passed by the caller of `psa_import_key`. If this size is `0`, the `"import_key"` entry point must set the `bits` input-output parameter to the correct key size. The semantics of `bits` is as follows:
* The core sets `*bits` to `psa_get_key_bits(attributes)` before calling the `"import_key"` entry point.
* If `*bits == 0`, the driver must determine the key size from the data and set `*bits` to this size. If the key size cannot be determined from the data, the driver must return `PSA_ERROR_INVALID_ARGUMENT` (as of version 1.0 of the PSA Cryptography API specification, it is possible to determine the key size for all standard key types).
* If `*bits != 0`, the driver must check the value of `*bits` against the data and return `PSA_ERROR_INVALID_ARGUMENT` if it does not match. If the driver entry point changes `*bits` to a different value but returns `PSA_SUCCESS`, the core will consider the key as invalid and the import will fail.
#### Key validation
Key creation entry points must produce valid key data. Key data is _valid_ if operations involving the key are guaranteed to work functionally and not to cause indirect security loss. Operation functions are supposed to receive valid keys, and should not have to check and report invalid keys. For example:
* If a cryptographic mechanism is defined as having keying material of a certain size, or if the keying material involves integers that have to be in a certain range, key creation must ensure that the keying material has an appropriate size and falls within an appropriate range.
* If a cryptographic operation involves a division by an integer which is provided as part of a key, key creation must ensure that this integer is nonzero.
* If a cryptographic operation involves two keys A and B (or more), then the creation of A must ensure that using it does not risk compromising B. This applies even if A's policy does not explicitly allow a problematic operation, but A is exportable. In particular, public keys that can potentially be used for key agreement are considered invalid and must not be created if they risk compromising the private key.
* On the other hand, it is acceptable for import to accept a key that cannot be verified as valid if using this key would at most compromise the key itself and material that is secured with this key. For example, RSA key import does not need to verify that the primes are actually prime. Key import may accept an insecure key if the consequences of the insecurity are no worse than a leak of the key prior to its import.
With opaque drivers, the key context can only be used by code from the same driver, so key validity is primarily intended to report key creation errors at creation time rather than during an operation. With transparent drivers, the key context can potentially be used by code from a different provider, so key validity is critical for interoperability.
This section describes some minimal validity requirements for standard key types.
* For symmetric key types, check that the key size is suitable for the type.
* For DES (`PSA_KEY_TYPE_DES`), additionally verify the parity bits.
* For RSA (`PSA_KEY_TYPE_RSA_PUBLIC_KEY`, `PSA_KEY_TYPE_RSA_KEY_PAIR`), check the syntax of the key and make sanity checks on its components. TODO: what sanity checks? Value ranges (e.g. p < n), sanity checks such as parity, minimum and maximum size, what else?
* For elliptic curve private keys (`PSA_KEY_TYPE_ECC_KEY_PAIR`), check the size and range. TODO: what else?
* For elliptic curve public keys (`PSA_KEY_TYPE_ECC_PUBLIC_KEY`), check the size and range, and that the point is on the curve. TODO: what else?
### Miscellaneous driver entry points
#### Driver initialization
A driver may declare an `"init"` entry point in a capability with no algorithm, key type or key size. If so, the core calls this entry point once during the initialization of the PSA Cryptography subsystem. If the init entry point of any driver fails, the initialization of the PSA Cryptography subsystem fails.
When multiple drivers have an init entry point, the order in which they are called is unspecified. It is also unspecified whether other drivers' `"init"` entry points are called if one or more init entry point fails.
On platforms where the PSA Cryptography implementation is a subsystem of a single application, the initialization of the PSA Cryptography subsystem takes place during the call to `psa_crypto_init()`. On platforms where the PSA Cryptography implementation is separate from the application or applications, the initialization of the PSA Cryptography subsystem takes place before or during the first time an application calls `psa_crypto_init()`.
The init entry point does not take any parameter.
### Combining multiple drivers
To declare a cryptoprocessor can handle both cleartext and wrapped keys, you need to provide two driver descriptions, one for a transparent driver and one for an opaque driver. You can use the mapping in capabilities' `"names"` property to arrange for multiple driver entry points to map to the same C function.
## Transparent drivers
### Key format for transparent drivers
The format of a key for transparent drivers is the same as in applications. Refer to the documentation of [`psa_export_key()`](https://armmbed.github.io/mbed-crypto/html/api/keys/management.html#c.psa_export_key) and [`psa_export_public_key()`](https://armmbed.github.io/mbed-crypto/html/api/keys/management.html#c.psa_export_public_key) in the PSA Cryptography API specification. For custom key types defined by an implementation, refer to the documentation of that implementation.
### Key management with transparent drivers
Transparent drivers may provide the following key management entry points:
* [`"import_key"`](#key-import-with-transparent-drivers): called by `psa_import_key()`, only when importing a key pair or a public key (key such that `PSA_KEY_TYPE_IS_ASYMMETRIC` is true).
* `"generate_key"`: called by `psa_generate_key()`, only when generating a key pair (key such that `PSA_KEY_TYPE_IS_KEY_PAIR` is true).
* `"key_derivation_output_key"`: called by `psa_key_derivation_output_key()`, only when deriving a key pair (key such that `PSA_KEY_TYPE_IS_KEY_PAIR` is true).
* `"export_public_key"`: called by the core to obtain the public key of a key pair. The core may call this function at any time to obtain the public key, which can be for `psa_export_public_key()` but also at other times, including during a cryptographic operation that requires the public key such as a call to `psa_verify_message()` on a key pair object.
Transparent drivers are not involved when exporting, copying or destroying keys, or when importing, generating or deriving symmetric keys.
#### Key import with transparent drivers
As discussed in [the general section about key management entry points](#driver-entry-points-for-key-management), the key import entry points has the following prototype for a driver with the prefix `"acme"`:
```
psa_status_t acme_import_key(const psa_key_attributes_t *attributes,
const uint8_t *data,
size_t data_length,
uint8_t *key_buffer,
size_t key_buffer_size,
size_t *key_buffer_length,
size_t *bits);
```
This entry point has several roles:
1. Parse the key data in the input buffer `data`. The driver must support the export format for the key types that the entry point is declared for. It may support additional formats as specified in the description of [`psa_import_key()`](https://armmbed.github.io/mbed-crypto/html/api/keys/management.html#c.psa_export_key) in the PSA Cryptography API specification.
2. Validate the key data. The necessary validation is described in the section [“Key validation with transparent drivers”](#key-validation-with-transparent-drivers) above.
3. [Determine the key size](#key-size-determination-on-import) and output it through `*bits`.
4. Copy the validated key data from `data` to `key_buffer`. The output must be in the canonical format documented for [`psa_export_key()`](https://armmbed.github.io/mbed-crypto/html/api/keys/management.html#c.psa_export_key) or [`psa_export_public_key()`](https://armmbed.github.io/mbed-crypto/html/api/keys/management.html#c.psa_export_public_key), so if the input is not in this format, the entry point must convert it.
### Fallback
Sometimes cryptographic accelerators only support certain cryptographic mechanisms partially. The capability description language allows specifying some restrictions, including restrictions on key sizes, but it cannot cover all the possibilities that may arise in practice. Furthermore, it may be desirable to deploy the same binary image on different devices, only some of which have a cryptographic accelerators.
For these purposes, a transparent driver can declare that it only supports a [capability](#driver-description-capability) partially, by setting the capability's `"fallback"` property to true.
If a transparent driver entry point is part of a capability which has a true `"fallback"` property and returns `PSA_ERROR_NOT_SUPPORTED`, the core will call the next transparent driver that supports the mechanism, if there is one. The core considers drivers in the order given by the [driver description list](#driver-description-list).
If all the available drivers have fallback enabled and return `PSA_ERROR_NOT_SUPPORTED`, the core will perform the operation using built-in code.
As soon as a driver returns any value other than `PSA_ERROR_NOT_SUPPORTED` (`PSA_SUCCESS` or a different error code), this value is returned to the application, without attempting to call any other driver or built-in code.
If a transparent driver entry point is part of a capability where the `"fallback"` property is false or omitted, the core should not include any other code for this capability, whether built in or in another transparent driver.
## Opaque drivers
Opaque drivers allow a PSA Cryptography implementation to delegate cryptographic operations to a separate environment that might not allow exporting key material in cleartext. The opaque driver interface is designed so that the core never inspects the representation of a key. The opaque driver interface is designed to support two subtypes of cryptoprocessors:
* Some cryptoprocessors do not have persistent storage for individual keys. The representation of a key is the key material wrapped with a master key which is located in the cryptoprocessor and never exported from it. The core stores this wrapped key material on behalf of the cryptoprocessor.
* Some cryptoprocessors have persistent storage for individual keys. The representation of a key is an identifier such as label or slot number. The core stores this identifier.
### Key format for opaque drivers
The format of a key for opaque drivers is an opaque blob. The content of this blob is fully up to the driver. The core merely stores this blob.
Note that since the core stores the key context blob as it is in memory, it must only contain data that is meaningful after a reboot. In particular, it must not contain any pointers or transient handles.
The `"key_context"` property in the [driver description](#driver-description-top-level-element) specifies how to calculate the size of the key context as a function of the key type and size. This is an object with the following properties:
* `"base_size"` (integer or string, optional): this many bytes are included in every key context. If omitted, this value defaults to 0.
* `"key_pair_size"` (integer or string, optional): this many bytes are included in every key context for a key pair. If omitted, this value defaults to 0.
* `"public_key_size"` (integer or string, optional): this many bytes are included in every key context for a public key. If omitted, this value defaults to 0.
* `"symmetric_factor"` (integer or string, optional): every key context for a symmetric key includes this many times the key size. If omitted, this value defaults to 0.
* `"store_public_key"` (boolean, optional): If specified and true, for a key pair, the key context includes space for the public key. If omitted or false, no additional space is added for the public key.
* `"size_function"` (string, optional): the name of a function that returns the number of bytes that the driver needs in a key context for a key. This may be a pointer to function. This must be a C identifier; more complex expressions are not permitted. If the core uses this function, it supersedes all the other properties.
The integer properties must be C language constants. A typical value for `"base_size"` is `sizeof(acme_key_context_t)` where `acme_key_context_t` is a type defined in a driver header file.
#### Size of a dynamically allocated key context
If the core supports dynamic allocation for the key context and chooses to use it, and the driver specification includes the `"size_function"` property, the size of the key context is at least
```
size_function(key_type, key_bits)
```
where `size_function` is the function named in the `"size_function"` property, `key_type` is the key type and `key_bits` is the key size in bits. The prototype of the size function is
```
size_t size_function(psa_key_type_t key_type, size_t key_bits);
```
#### Size of a statically allocated key context
If the core does not support dynamic allocation for the key context or chooses not to use it, or if the driver specification does not include the `"size_function"` property, the size of the key context for a key of type `key_type` and of size `key_bits` bits is:
* For a key pair (`PSA_KEY_TYPE_IS_KEY_PAIR(key_type)` is true):
```
base_size + key_pair_size + public_key_overhead
```
where `public_key_overhead = PSA_EXPORT_PUBLIC_KEY_MAX_SIZE(key_type, key_bits)` if the `"store_public_key"` property is true and `public_key_overhead = 0` otherwise.
* For a public key (`PSA_KEY_TYPE_IS_PUBLIC_KEY(key_type)` is true):
```
base_size + public_key_size
```
* For a symmetric key (not a key pair or public key):
```
base_size + symmetric_factor * key_bytes
```
where `key_bytes = ((key_bits + 7) / 8)` is the key size in bytes.
#### Key context size for a secure element with storage
If the key is stored in the secure element and the driver only needs to store a label for the key, use `"base_size"` as the size of the label plus any other metadata that the driver needs to store, and omit the other properties.
If the key is stored in the secure element, but the secure element does not store the public part of a key pair and cannot recompute it on demand, additionally use the `"store_public_key"` property with the value `true`. Note that this only influences the size of the key context: the driver code must copy the public key to the key context and retrieve it on demand in its `export_public_key` entry point.
#### Key context size for a secure element without storage
If the key is stored in wrapped form outside the secure element, and the wrapped form of the key plus any metadata has up to *N* bytes of overhead, use *N* as the value of the `"base_size"` property and set the `"symmetric_factor"` property to 1. Set the `"key_pair_size"` and `"public_key_size"` properties appropriately for the largest supported key pair and the largest supported public key respectively.
### Key management with opaque drivers
Opaque drivers may provide the following key management entry points:
* `"export_key"`: called by `psa_export_key()`, or by `psa_copy_key()` when copying a key from or to a different [location](#lifetimes-and-locations).
* `"export_public_key"`: called by the core to obtain the public key of a key pair. The core may call this entry point at any time to obtain the public key, which can be for `psa_export_public_key()` but also at other times, including during a cryptographic operation that requires the public key such as a call to `psa_verify_message()` on a key pair object.
* `"import_key"`: called by `psa_import_key()`, or by `psa_copy_key()` when copying a key from another location.
* `"generate_key"`: called by `psa_generate_key()`.
* `"key_derivation_output_key"`: called by `psa_key_derivation_output_key()`.
* `"copy_key"`: called by `psa_copy_key()` when copying a key within the same [location](#lifetimes-and-locations).
In addition, secure elements that store the key material internally must provide the following two entry points:
* `"allocate_key"`: called by `psa_import_key()`, `psa_generate_key()`, `psa_key_derivation_output_key()` or `psa_copy_key()` before creating a key in the location of this driver.
* `"destroy_key"`: called by `psa_destroy_key()`.
#### Key creation in a secure element without storage
This section describes the key creation process for secure elements that do not store the key material. The driver must obtain a wrapped form of the key material which the core will store. A driver for such a secure element has no `"allocate_key"` or `"destroy_key"` entry point.
When creating a key with an opaque driver which does not have an `"allocate_key"` or `"destroy_key"` entry point:
1. The core allocates memory for the key context.
2. The core calls the driver's import, generate, derive or copy entry point.
3. The core saves the resulting wrapped key material and any other data that the key context may contain.
To destroy a key, the core simply destroys the wrapped key material, without invoking driver code.
#### Key management in a secure element with storage
This section describes the key creation and key destruction processes for secure elements that have persistent storage for the key material. A driver for such a secure element has two mandatory entry points:
* `"allocate_key"`: this function obtains an internal identifier for the key. This may be, for example, a unique label or a slot number.
* `"destroy_key"`: this function invalidates the internal identifier and destroys the associated key material.
These functions have the following prototypes for a driver with the prefix `"acme"`:
```
psa_status_t acme_allocate_key(const psa_key_attributes_t *attributes,
uint8_t *key_buffer,
size_t key_buffer_size);
psa_status_t acme_destroy_key(const psa_key_attributes_t *attributes,
const uint8_t *key_buffer,
size_t key_buffer_size);
```
When creating a persistent key with an opaque driver which has an `"allocate_key"` entry point:
1. The core calls the driver's `"allocate_key"` entry point. This function typically allocates an internal identifier for the key without modifying the state of the secure element and stores the identifier in the key context. This function should not modify the state of the secure element. It may modify the copy of the persistent state of the driver in memory.
1. The core saves the key context to persistent storage.
1. The core calls the driver's key creation entry point.
1. The core saves the updated key context to persistent storage.
If a failure occurs after the `"allocate_key"` step but before the call to the second driver entry point, the core will do one of the following:
* Fail the creation of the key without indicating this to the driver. This can happen, in particular, if the device loses power immediately after the key allocation entry point returns.
* Call the driver's `"destroy_key"` entry point.
To destroy a key, the core calls the driver's `"destroy_key"` entry point.
Note that the key allocation and destruction entry points must not rely solely on the key identifier in the key attributes to identify a key. Some implementations of the PSA Cryptography API store keys on behalf of multiple clients, and different clients may use the same key identifier to designate different keys. The manner in which the core distinguishes keys that have the same identifier but are part of the key namespace for different clients is implementation-dependent and is not accessible to drivers. Some typical strategies to allocate an internal key identifier are:
* Maintain a set of free slot numbers which is stored either in the secure element or in the driver's persistent storage. To allocate a key slot, find a free slot number, mark it as occupied and store the number in the key context. When the key is destroyed, mark the slot number as free.
* Maintain a monotonic counter with a practically unbounded range in the secure element or in the driver's persistent storage. To allocate a key slot, increment the counter and store the current value in the key context. Destroying a key does not change the counter.
TODO: explain constraints on how the driver updates its persistent state for resilience
TODO: some of the above doesn't apply to volatile keys
#### Key creation entry points in opaque drivers
The key creation entry points have the following prototypes for a driver with the prefix `"acme"`:
```
psa_status_t acme_import_key(const psa_key_attributes_t *attributes,
const uint8_t *data,
size_t data_length,
uint8_t *key_buffer,
size_t key_buffer_size,
size_t *key_buffer_length,
size_t *bits);
psa_status_t acme_generate_key(const psa_key_attributes_t *attributes,
uint8_t *key_buffer,
size_t key_buffer_size,
size_t *key_buffer_length);
```
If the driver has an [`"allocate_key"` entry point](#key-management-in-a-secure-element-with-storage), the core calls the `"allocate_key"` entry point with the same attributes on the same key buffer before calling the key creation entry point.
TODO: derivation, copy
#### Key export entry points in opaque drivers
The key export entry points have the following prototypes for a driver with the prefix `"acme"`:
```
psa_status_t acme_export_key(const psa_key_attributes_t *attributes,
const uint8_t *key_buffer,
size_t key_buffer_size,
uint8_t *data,
size_t data_size,
size_t *data_length);
psa_status_t acme_export_public_key(const psa_key_attributes_t *attributes,
const uint8_t *key_buffer,
size_t key_buffer_size,
uint8_t *data,
size_t data_size,
size_t *data_length);
```
The core will only call `acme_export_public_key` on a private key. Drivers implementers may choose to store the public key in the key context buffer or to recalculate it on demand. If the key context includes the public key, it needs to have an adequate size; see [“Key format for opaque drivers”](#key-format-for-opaque-drivers).
The core guarantees that the size of the output buffer (`data_size`) is sufficient to export any key with the given attributes. The driver must set `*data_length` to the exact size of the exported key.
### Opaque driver persistent state
The core maintains persistent state on behalf of an opaque driver. This persistent state consists of a single byte array whose size is given by the `"persistent_state_size"` property in the [driver description](#driver-description-top-level-element).
The core loads the persistent state in memory before it calls the driver's [init entry point](#driver-initialization). It is adjusted to match the size declared by the driver, in case a driver upgrade changes the size:
* The first time the driver is loaded on a system, the persistent state is all-bits-zero.
* If the stored persistent state is smaller than the declared size, the core pads the persistent state with all-bits-zero at the end.
* If the stored persistent state is larger than the declared size, the core truncates the persistent state to the declared size.
The core provides the following callback functions, which an opaque driver may call while it is processing a call from the driver:
```
psa_status_t psa_crypto_driver_get_persistent_state(uint_8_t **persistent_state_ptr);
psa_status_t psa_crypto_driver_commit_persistent_state(size_t from, size_t length);
```
`psa_crypto_driver_get_persistent_state` sets `*persistent_state_ptr` to a pointer to the first byte of the persistent state. This pointer remains valid during a call to a driver entry point. Once the entry point returns, the pointer is no longer valid. The core guarantees that calls to `psa_crypto_driver_get_persistent_state` within the same entry point return the same address for the persistent state, but this address may change between calls to an entry point.
`psa_crypto_driver_commit_persistent_state` updates the persistent state in persistent storage. Only the portion at byte offsets `from` inclusive to `from + length` exclusive is guaranteed to be updated; it is unspecified whether changes made to other parts of the state are taken into account. The driver must call this function after updating the persistent state in memory and before returning from the entry point, otherwise it is unspecified whether the persistent state is updated.
The core will not update the persistent state in storage while an entry point is running except when the entry point calls `psa_crypto_driver_commit_persistent_state`. It may update the persistent state in storage after an entry point returns.
In a multithreaded environment, the driver may only call these two functions from the thread that is executing the entry point.
## How to use drivers from an application
### Using transparent drivers
Transparent drivers linked into the library are automatically used for the mechanisms that they implement.
### Using opaque drivers
Each opaque driver is assigned a [location](#lifetimes-and-locations). The driver is invoked for all actions that use a key in that location. A key's location is indicated by its lifetime. The application chooses the key's lifetime when it creates the key.
For example, the following snippet creates an AES-GCM key which is only accessible inside the secure element designated by the location `PSA_KEY_LOCATION_acme`.
```
psa_key_attributes_t attributes = PSA_KEY_ATTRIBUTES_INIT;
psa_set_key_lifetime(&attributes, PSA_KEY_LIFETIME_FROM_PERSISTENCE_AND_LOCATION(
PSA_KEY_PERSISTENCE_DEFAULT, PSA_KEY_LOCATION_acme));
psa_set_key_identifer(&attributes, 42);
psa_set_key_type(&attributes, PSA_KEY_TYPE_AES);
psa_set_key_size(&attributes, 128);
psa_set_key_algorithm(&attributes, PSA_ALG_GCM);
psa_set_key_usage_flags(&attributes, PSA_KEY_USAGE_ENCRYPT | PSA_KEY_USAGE_DECRYPT);
psa_key_id_t key;
psa_generate_key(&attributes, &key);
```
## Using opaque drivers from an application
### Lifetimes and locations
The PSA Cryptography API, version 1.0.0, defines [lifetimes](https://armmbed.github.io/mbed-crypto/html/api/keys/attributes.html?highlight=psa_key_lifetime_t#c.psa_key_lifetime_t) as an attribute of a key that indicates where the key is stored and which application and system actions will create and destroy it. The lifetime is expressed as a 32-bit value (`typedef uint32_t psa_key_lifetime_t`). An upcoming version of the PSA Cryptography API defines more structure for lifetime values to separate these two aspects of the lifetime:
* Bits 0–7 are a _persistence level_. This value indicates what device management actions can cause it to be destroyed. In particular, it indicates whether the key is volatile or persistent.
* Bits 8–31 are a _location indicator_. This value indicates where the key material is stored and where operations on the key are performed. Location values can be stored in a variable of type `psa_key_location_t`.
An opaque driver is attached to a specific location. Keys in the default location (`PSA_KEY_LOCATION_LOCAL_STORAGE = 0`) are transparent: the core has direct access to the key material. For keys in a location that is managed by an opaque driver, only the secure element has access to the key material and can perform operations on the key, while the core only manipulates a wrapped form of the key or an identifier of the key.
### Creating a key in a secure element
The core defines a compile-time constant for each opaque driver indicating its location called `PSA_KEY_LOCATION_`*prefix* where *prefix* is the value of the `"prefix"` property in the driver description. For convenience, Mbed TLS also declares a compile-time constant for the corresponding lifetime with the default persistence called `PSA_KEY_LIFETIME_`*prefix*. Therefore, to declare an opaque key in the location with the prefix `foo` with the default persistence, call `psa_set_key_lifetime` during the key creation as follows:
```
psa_set_key_lifetime(&attributes, PSA_KEY_LIFETIME_foo);
```
To declare a volatile key:
```
psa_set_key_lifetime(&attributes, PSA_KEY_LIFETIME_FROM_PERSISTENCE_AND_LOCATION(
PSA_KEY_LOCATION_foo,
PSA_KEY_PERSISTENCE_VOLATILE));
```
Generally speaking, to declare a key with a specified persistence:
```
psa_set_key_lifetime(&attributes, PSA_KEY_LIFETIME_FROM_PERSISTENCE_AND_LOCATION(
PSA_KEY_LOCATION_foo,
persistence));
```
## Open questions
### Driver declarations
#### Declaring driver entry points
The core may want to provide declarations for the driver entry points so that it can compile code using them. At the time of writing this paragraph, the driver headers must define types but there is no obligation for them to declare functions. The core knows what the function names and argument types are, so it can generate prototypes.
It should be ok for driver functions to be function-like macros or function pointers.
#### Driver location values
How does a driver author decide which location values to use? It should be possible to combine drivers from different sources. Use the same vendor assignment as for PSA services?
Can the driver assembly process generate distinct location values as needed? This can be convenient, but it's also risky: if you upgrade a device, you need the location values to be the same between builds.
The current plan is for Arm to maintain a registry of vendors and assign a location namespace to each vendor. Parts of the namespace would be reserved for implementations and integrators.
#### Multiple transparent drivers
When multiple transparent drivers implement the same mechanism, which one is called? The first one? The last one? Unspecified? Or is this an error (excluding capabilities with fallback enabled)?
The current choice is that the first one is used, which allows having a preference order on drivers, but may mask integration errors.
### Driver function interfaces
#### Driver function parameter conventions
Should 0-size buffers be guaranteed to have a non-null pointers?
Should drivers really have to cope with overlap?
Should the core guarantee that the output buffer size has the size indicated by the applicable buffer size macro (which may be an overestimation)?
### Partial computations in drivers
#### Substitution points
Earlier drafts of the driver interface had a concept of _substitution points_: places in the calculation where a driver may be called. Some hardware doesn't do the whole calculation, but only the “main” part. This goes both for transparent and opaque drivers. Some common examples:
* A processor that performs the RSA exponentiation, but not the padding. The driver should be able to leverage the padding code in the core.
* A processor that performs a block cipher operation only for a single block, or only in ECB mode, or only in CTR mode. The core would perform the block mode (CBC, CTR, CCM, ...).
This concept, or some other way to reuse portable code such as specifying inner functions like `psa_rsa_pad` in the core, should be added to the specification.
### Key management
#### Mixing drivers in key derivation
How does `psa_key_derivation_output_key` work when the extraction part and the expansion part use different drivers?
#### Public key calculation
ECC key pairs are represented as the private key value only. The public key needs to be calculated from that. Both transparent drivers and opaque drivers provide a function to calculate the public key (`"export_public_key"`).
The specification doesn't mention when the public key might be calculated. The core may calculate it on creation, on demand, or anything in between. Opaque drivers have a choice of storing the public key in the key context or calculating it on demand and can convey whether the core should store the public key with the `"store_public_key"` property. Is this good enough or should the specification include non-functional requirements?
#### Symmetric key validation with transparent drivers
Should the entry point be called for symmetric keys as well?
#### Support for custom import formats
[“Driver entry points for key management”](#driver-entry-points-for-key-management) states that the input to `"import_key"` can be an implementation-defined format. Is this a good idea? It reduces driver portability, since a core that accepts a custom format would not work with a driver that doesn't accept this format. On the other hand, if a driver accepts a custom format, the core should let it through because the driver presumably handles it more efficiently (in terms of speed and code size) than the core could.
Allowing custom formats also causes a problem with import: the core can't know the size of the key representation until it knows the bit-size of the key, but determining the bit-size of the key is part of the job of the `"import_key"` entry point. For standard key types, this could plausibly be an issue for RSA private keys, where an implementation might accept a custom format that omits the CRT parameters (or that omits *d*).
### Opaque drivers
#### Opaque driver persistent state
The driver is allowed to update the state at any time. Is this ok?
An example use case for updating the persistent state at arbitrary times is to renew a key that is used to encrypt communications between the application processor and the secure element.
`psa_crypto_driver_get_persistent_state` does not identify the calling driver, so the driver needs to remember which driver it's calling. This may require a thread-local variable in a multithreaded core. Is this ok?
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