docs/psa-transition.md

# Transitioning to the PSA API

> I have code written for `mbedtls_` cryptography APIs. How do I migrate to `psa_` APIs?

## Introduction

Mbed TLS is gradually moving from legacy `mbedtls_xxx` APIs to newer `psa_xxx` APIs for cryptography. Note that this only concerns cryptography APIs, not X.509 or SSL/TLS APIs.

This guide is intended to help migrate existing applications that use Mbed TLS for cryptography. It aims to cover common use cases, but cannot cover all possible scenarios.

### Suggested reading

This document is long, but you probably don't need to read all of it. You should start with the following sections:

1. [Where can I find documentation?](#where-can-i-find-documentation)
2. [General considerations](#general-considerations)

Then use the [summary of API modules](#summary-of-api-modules), the table of contents or a text search to locate the sections that interest you, based on what legacy interfaces your code is currently using.

### Where can I find documentation?

**Tutorial**: See the [getting started guide](https://mbed-tls.readthedocs.io/en/latest/getting_started/psa/).

**Reference**: The [PSA Crypto API specification](https://arm-software.github.io/psa-api/crypto/) is available online. Mbed TLS implements a large subset of the specification which is documented in the [`psa/crypto*.h` headers](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto_8h/).

### Additional resources

* [Mbed TLS open issues](https://github.com/Mbed-TLS/mbedtls/issues)
* [PSA API open issues](https://github.com/ARM-software/psa-api/issues) (not just cryptography APIs)
* [Mbed TLS mailing list](https://lists.trustedfirmware.org/mailman3/lists/mbed-tls.lists.trustedfirmware.org/)

### Why change the API?

* Mbed TLS APIs are traditionally very transparent: the caller can access internal fields of operations. This is less true in the 3.x major version than before, but still the case to some extent. This offers applications some flexibility, but it removes flexibility from the implementation. For example, it is hard to support hardware acceleration, because the API constrains how the data must be represented. PSA APIs were designed to be more opaque, giving more freedom to the implementation.
* Mbed TLS legacy APIs require key material to be present in the application memory. The PSA Crypto API natively supports operations on keys stored in an external [location](https://arm-software.github.io/psa-api/crypto/1.1/api/keys/lifetimes.html#c.psa_key_location_t) (secure enclave, secure element, HSM, etc.).
* PSA APIs have [consistent conventions ](https://arm-software.github.io/psa-api/crypto/1.1/overview/conventions.html#parameter-conventions) which many legacy APIs in Mbed TLS do not follow. For example, many legacy cryptography functions require the caller to know how large an output buffer needs to be based on the selected algorithm, whereas in the PSA API, all buffer arguments have a well-defined size and those sizes are checked.
* Mbed TLS legacy APIs require passing around a random generator argument where needed. This has historically been problematic with functions that were created without a RNG argument but later needed one as part of a security countermeasure. The PSA crypto subsystem maintains a global random generator, resolving this problem.

### Migration timeline

* Mbed TLS 2.15.0 (Nov 2018): first release with a draft implementation of the PSA API.
* Mbed TLS 2.18.0 (Jun 2019): The PSA API is available in the default build.
* Mbed TLS 3.1.0 (Dec 2021): TLS 1.3 support is the first major feature that requires the PSA API.
* Mbed TLS 4.0.0 (2024?): X.509 and TLS require the PSA API. Removal of some legacy crypto APIs.
* Mbed TLS 5.0.0 (??): Removal of the remaining non-PSA crypto APIs.

## General considerations

### Configuration of the PSA subsystem

To make the PSA API available, make sure that the configuration option [`MBEDTLS_PSA_CRYPTO_C`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/mbedtls__config_8h/#c.MBEDTLS_PSA_CRYPTO_C) is enabled (it is enabled in the default configuration).

You should probably enable [`MBEDTLS_USE_PSA_CRYPTO`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/mbedtls__config_8h/#mbedtls__config_8h_1a70fd7b97d5f11170546583f2095942a6) as well (it is disabled by default). This option causes the PK, X.509 and TLS modules to use PSA crypto under the hood. Some functions that facilitate the transition are only available when `MBEDTLS_USE_PSA_CRYPTO` is enabled.

By default, the PSA crypto API offers a similar set of cryptographic mechanisms as those offered by the legacy API. The PSA crypto API also has its own configuration mechanism; see “[Cryptographic mechanism availability](#cryptographic-mechanism-availability)”.

### Header files

Applications only need to include a single header file:
```
#include <psa/crypto.h>
```

### General application layout

Before any cryptographic operation, call [`psa_crypto_init`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__initialization/#group__initialization_1ga2de150803fc2f7dc6101d5af7e921dd9) and check that it succeeds. (A failure indicates an abnormal system state from which most applications cannot recover.)

If you wish to free all resources associated with PSA cryptography, call [`mbedtls_psa_crypto_free`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__extra_8h/#_CPPv423mbedtls_psa_crypto_freev).

The PSA subsystem has an internal random generator. As a consequence, you do not need to instantiate one manually (no need to create an `mbedtls_entropy_context` and a `mbedtls_xxx_drbg_context`).

### Error codes

Mbed TLS functions return a status of type `int`: 0 for success (or, occasionally, a positive value which is the output length), or a negative value `MBEDTLS_ERR_xxx` indicating an error.

PSA functions return a status of type [`psa_status_t`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__error/#group__error_1ga05676e70ba5c6a7565aff3c36677c1f9): `PSA_SUCCESS == 0` for success, or a negative value [`PSA_ERROR_xxx`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__error/) indicating an error.

## Summary of API modules

| Header | Function prefix | PSA equivalent |
| ------ | --------------- | -------------- |
| `aes.h` | `mbedtls_aes_` | [Symmetric encryption](#symmetric-encryption) |
| `aria.h` | `mbedtls_aria_` | [Symmetric encryption](#symmetric-encryption) |
| `asn1.h` | `mbedtls_asn1_` | No change (not a crypto API) |
| `asn1write.h` | `mbedtls_asn1write_` | No change (not a crypto API) |
| `base64.h` | `mbedtls_base64_` | [PK format support interfaces](#pk-format-support-interfaces) |
| `bignum.h` | `mbedtls_bignum_` | None (no low-level arithmetic) |
| `build_info.h` | `MBEDTLS_` | No change (not a crypto API) |
| `camellia.h` | `mbedtls_camellia_` | [Symmetric encryption](#symmetric-encryption) |
| `ccm.h` | `mbedtls_ccm_` | [Symmetric encryption](#symmetric-encryption), [Authenticated cipher operations](#authenticated-cipher-operations) |
| `chacha20.h` | `mbedtls_chacha20_` | [Symmetric encryption](#symmetric-encryption) |
| `chachapoly.h` | `mbedtls_chachapoly_` | [Symmetric encryption](#symmetric-encryption) |
| `check_config.h` | N/A | No public APIs (internal support header) |
| `cipher.h` | `mbedtls_cipher_` | [Symmetric encryption](#symmetric-encryption) |
| `cmac.h` | `mbedtls_cmac_` | [Hashes and MAC](#hashes-and-mac), [MAC calculation](#mac-calculation) |
| `compat-2.x.h` | various | None (transitional APIs) |
| `config_psa.h` | N/A | No public APIs (internal support header) |
| `constant_time.h` | `mbedtls_constant_time_` | [Constant-time functions](#constant-time-functions) |
| `ctr_drbg.h` | `mbedtls_ctr_drbg_` | [Random generation interface](#random-generation-interface), [Deterministic pseudorandom generation](#deterministic-pseudorandom-generation) |
| `debug.h` | `mbedtls_debug_` | No change (not a crypto API) |
| `des.h` | `mbedtls_des_` | [Symmetric encryption](#symmetric-encryption) |
| `dhm.h` | `mbedtls_dhm_` | [Asymmetric cryptography](#asymmetric-cryptography) |
| `ecdh.h` | `mbedtls_ecdh_` | [Asymmetric cryptography](#asymmetric-cryptography) |
| `ecdsa.h` | `mbedtls_ecdsa_` | [Asymmetric cryptography](#asymmetric-cryptography) |
| `ecjpake.h` | `mbedtls_ecjpake_` | [EC-JPAKE](#ec-jpake) |
| `ecp.h` | `mbedtls_ecp_` | [Asymmetric cryptography](#asymmetric-cryptography) |
| `entropy.h` | `mbedtls_entropy_` | [Random generation interface](#random-generation-interface), [Entropy sources](#entropy-sources) |
| `error.h` | `mbedtls_error_` | [Error messages](#error-messages) |
| `gcm.h` | `mbedtls_gcm_` | [Symmetric encryption](#symmetric-encryption), [Authenticated cipher operations](#authenticated-cipher-operations) |
| `hkdf.h` | `mbedtls_hkdf_` | [HKDF](#hkdf) |
| `hmac_drbg.h` | `mbedtls_hmac_drbg_` | [Random generation interface](#random-generation-interface), [Deterministic pseudorandom generation](#deterministic-pseudorandom-generation) |
| `lms.h` | `mbedtls_lms_` | No migration path yet |
| `mbedtls_config.h` | `MBEDTLS_` | [Compile-time configuration](#compile-time-configuration) |
| `md.h` | `mbedtls_md_` | [Hashes and MAC](#hashes-and-mac) |
| `md5.h` | `mbedtls_md5_` | [Hashes and MAC](#hashes-and-mac) |
| `memory_buffer_alloc.h` | `mbedtls_memory_buffer_alloc_` | No change (not a crypto API) |
| `net_sockets.h` | `mbedtls_net_` | No change (not a crypto API) |
| `nist_kw.h` | `mbedtls_nist_kw_` | No migration path yet |
| `oid.h` | `mbedtls_oid_` | [PK format support interfaces](#pk-format-support-interfaces) |
| `pem.h` | `mbedtls_pem_` | [PK format support interfaces](#pk-format-support-interfaces) |
| `pk.h` | `mbedtls_pk_` | [Asymmetric cryptography](#asymmetric-cryptography) |
| `pkcs5.h` | `mbedtls_pkcs5_` | [PKCS#5 module](#pkcs5-module) |
| `pkcs7.h` | `mbedtls_pkcs7_` | No change (not a crypto API) |
| `pkcs12.h` | `mbedtls_pkcs12_` | [PKCS#12 module](#pkcs12-module) |
| `platform.h` | `mbedtls_platform_` | No change (not a crypto API) |
| `platform_time.h` | `mbedtls_platform_time_` | No change (not a crypto API) |
| `platform_util.h` | `mbedtls_platform_util_` | No change (not a crypto API) |
| `poly1305.h` | `mbedtls_poly1305_` | None (but there is Chacha20-Poly1305 [AEAD](#symmetric-encryption)) |
| `private_access.h` | N/A | No public APIs (internal support header) |
| `psa_util.h` | `mbedtls_psa_` | No public APIs (internal support header) |
| `ripemd160.h` | `mbedtls_ripemd160_` | [Hashes and MAC](#hashes-and-mac) |
| `rsa.h` | `mbedtls_rsa_` | [Asymmetric cryptography](#asymmetric-cryptography) |
| `sha1.h` | `mbedtls_sha1_` | [Hashes and MAC](#hashes-and-mac) |
| `sha3.h` | `mbedtls_sha3_` | [Hashes and MAC](#hashes-and-mac) |
| `sha256.h` | `mbedtls_sha256_` | [Hashes and MAC](#hashes-and-mac) |
| `sha512.h` | `mbedtls_sha512_` | [Hashes and MAC](#hashes-and-mac) |
| `ssl.h` | `mbedtls_ssl_` | No change (not a crypto API) |
| `ssl_cache.h` | `mbedtls_ssl_cache_` | No change (not a crypto API) |
| `ssl_ciphersuites.h` | `mbedtls_ssl_ciphersuites_` | No change (not a crypto API) |
| `ssl_cookie.h` | `mbedtls_ssl_cookie_` | No change (not a crypto API) |
| `ssl_ticket.h` | `mbedtls_ssl_ticket_` | No change (not a crypto API) |
| `threading.h` | `mbedtls_threading_` | No change (not a crypto API) |
| `timing.h` | `mbedtls_timing_` | No change (not a crypto API) |
| `version.h` | `mbedtls_version_` | No change (not a crypto API) |
| `x509.h` | `mbedtls_x509` | No change (not a crypto API) |
| `x509_crl.h` | `mbedtls_x509` | No change (not a crypto API) |
| `x509_crt.h` | `mbedtls_x509` | No change (not a crypto API) |
| `x509_csr.h` | `mbedtls_x509` | No change (not a crypto API) |

## Compile-time configuration

### Cryptographic mechanism availability

**This section only applies if `MBEDTLS_PSA_CRYPTO_CONFIG` is enabled.** This option is disabled in the default configuration.

When the configuration option [`MBEDTLS_PSA_CRYPTO_CONFIG`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/mbedtls__config_8h/#mbedtls__config_8h_1a5aca5ddcffb586acad82f9aef26db056) is enabled, the cryptographic mechanisms available through the PSA API are determined by the contents of the header file `"psa/crypto_config.h"`. You can override the file location with the macro [`MBEDTLS_PSA_CRYPTO_CONFIG_FILE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/mbedtls__config_8h/#mbedtls__config_8h_1a25f7e358caa101570cb9519705c2b873), and you can set [`MBEDTLS_PSA_CRYPTO_USER_CONFIG_FILE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/mbedtls__config_8h/#mbedtls__config_8h_1abd1870cc0d2681183a3018a7247cb137) to the path of an additional file (similar to `MBEDTLS_CONFIG_FILE` and `MBEDTLS_USER_CONFIG_FILE` for legacy configuration symbols).

The availability of cryptographic mechanisms in the PSA API is based on a systematic pattern:

* To make `PSA_ALG_aaa` available, enable `PSA_WANT_ALG_aaa`.
  For parametrized algorithms, there is a `PSA_WANT_` symbols both for the main macro and for each argument. For example, to make `PSA_ALG_HMAC(PSA_ALG_SHA_256)` available, enable both `PSA_WANT_ALG_HMAC` and `PSA_WANT_ALG_SHA_256`.

* To make `PSA_KEY_TYPE_ttt` available, enable `PSA_WANT_KEY_TYPE_ttt`.

    As an exception, starting in Mbed TLS 3.5.0, for key pair types, the feature selection is more fine-grained, with an additional suffix:
    * `PSA_KEY_TYPE_xxx_USE` enables support for operations with a key of that type (for enabled algorithms). This is automatically enabled if any key creation method (`IMPORT`, `GENERATE` or `DERIVE`) is enabled.
    * `PSA_KEY_TYPE_xxx_IMPORT` enables support for `psa_import_key` to import a key of that type.
    * `PSA_KEY_TYPE_xxx_GENERATE` enables support for `psa_generate_key` to randomly generate a key of that type.
    * `PSA_KEY_TYPE_xxx_DERIVE` enables support for `psa_key_derivation_output_key` to deterministically derive a key of that type.
    * `PSA_KEY_TYPE_xxx_EXPORT` enables support for `psa_export_key` to export a key of that type.

    Enabling any support for a key pair type automatically enables support for the corresponding public key type, as well as support for `psa_export_public_key` on the private key.

* To make `PSA_ECC_FAMILY_fff` available for size sss, enable `PSA_WANT_ECC_fff_sss`.

Note that all `PSA_WANT_xxx` symbols must be set to a nonzero value. In particular, setting `PSA_WANT_xxx` to an empty value may not be handled consistently.

For example, the following configuration enables hashing with SHA-256, AEAD with AES-GCM, signature with deterministic ECDSA using SHA-256 on the curve secp256r1 using a randomly generated key as well as the corresponding verification, and ECDH key exchange on secp256r1 and Curve25519.

```
#define PSA_WANT_ALG_SHA_256 1

#define PSA_WANT_KEY_TYPE_AES 1
#define PSA_WANT_ALG_GCM 1

#define PSA_WANT_KEY_TYPE_ECC_KEY_PAIR_GENERATE 1
// ^^ In Mbed TLS <= 3.4, enable PSA_WANT_KEY_TYPE_ECC_KEY_PAIR instead
// ^^ implicitly enables PSA_WANT_KEY_TYPE_ECC_KEY_PAIR_USE, PSA_WANT_KEY_TYPE_ECC_PUBLIC_KEY
#define PSA_WANT_ECC_SECP_R1_256 1 // secp256r1 (suitable for ECDSA and ECDH)
#define PSA_WANT_ECC_MONTGOMERY_255 1 // Curve25519 (suitable for ECDH)
#define PSA_WANT_ALG_DETERMINISTIC_ECDSA 1
#define PSA_WANT_ALG_ECDH
```

If a mechanism is not enabled by `PSA_WANT_xxx`, Mbed TLS will often not include it, to reduce the size of the compiled library. However, this is not guaranteed: a mechanism that is not explicitly requested can be enabled because it is a dependency of another configuration option, because it is used internally, or because the granularity is not fine enough to distinguish between it and another mechanism that is requested.

Under the hood, `PSA_WANT_xxx` enables the necessary legacy modules. Note that if a mechanism has a PSA accelerator driver, the corresponding legacy module is typically not needed. Thus applications that use a cryptographic mechanism both through the legacy API and through the PSA API need to explicitly enable both the `PSA_WANT_xxx` symbols and the `MBEDTLS_xxx` symbols.

## Miscellaneous support modules

### Error messages

At the time of writing, there is no equivalent to the error messages provided by `mbedtls_strerror`. However, you can use the companion program `programs/psa/psa_constant_names` to convert various numbers (`psa_status_t`, `psa_algorithm_t`, `psa_key_type_t`, `psa_ecc_family_t`, `psa_dh_family_t`, `psa_key_usage_t`) to their input representation. The conversion doesn't depend on the library configuration or the target platform, so you can use a native build of this program even if you cross-compile your application.

```
$ programs/psa/psa_constant_names error -138
PSA_ERROR_BUFFER_TOO_SMALL
$ programs/psa/psa_constant_names type 0x7112
PSA_KEY_TYPE_ECC_KEY_PAIR(PSA_ECC_FAMILY_SECP_R1)
$ programs/psa/psa_constant_names alg 0x06000609
PSA_ALG_ECDSA(PSA_ALG_SHA_256)
```

The other functions in `error.h` are specific to the construction of Mbed TLS error code and are not relevant to the PSA API. PSA error codes are never the combination of multiple codes.

### Constant-time functions

The PSA API does not have an equivalent to the timing-side-channel-resistance utility functions in `constant_time.h`. Continue using `constant_time.h` as needed.

Note that the PSA API does include features that reduce the need for `mbedtls_ct_memcmp`:

* To compare a MAC with a reference value, use `psa_mac_verify` rather of `psa_mac_compute` followed by `mbedtls_ct_memcmp`, or use `psa_mac_verify_setup` and `psa_mac_verify_finish` in the multi-part case. See “[MAC calculation](#mac-calculation)”.
* The AEAD decryption functions take care of verifying the tag. See “[Authenticated cipher operations](#authenticated-cipher-operations)”.

## Symmetric encryption

All PSA APIs have algorithm agility, where the functions depend only on the nature of the operation and the choice of a specific algorithm comes from an argument. There is no special API for a particular block cipher (`aes.h`, `aria.h`, `camellia.h`, `des.h`), a particular block cipher mode (`ccm.h`, `gcm.h`) or a particular stream cipher (`chacha20.h`, `chachapoly.h`). To migrate code using those low-level modules, please follow the recommendations in the following sections, using the same principles as the corresponding `cipher.h` API.

### Cipher mechanism selection

Instead of `mbedtls_cipher_id_t` (`MBEDTLS_CIPHER_ID_xxx` constants), `mbedtls_cipher_type_t` (`MBEDTLS_CIPHER_base_size_mode` constants), `mbedtls_cipher_mode_t` (`MBEDTLS_CIPHER_MODE_xxx` constants) and `mbedtls_cipher_padding_t` (`MBEDTLS_CIPHER_PADDING_xxx` constants), use the [`PSA_KEY_TYPE_xxx` and `PSA_ALG_xxx` constants](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/).

For modes that are based on a block cipher, the key type encodes the choice of block cipher:
[`PSA_KEY_TYPE_AES`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga6ee54579dcf278c677eda4bb1a29575e),
[`PSA_KEY_TYPE_ARIA`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#c.PSA_KEY_TYPE_ARIA),
[`PSA_KEY_TYPE_CAMELLIA`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1gad8e5da742343fd5519f9d8a630c2ed81),
[`PSA_KEY_TYPE_DES`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga577562bfbbc691c820d55ec308333138).
The algorithm encodes the mode and if relevant the padding type:

* Unauthenticated cipher modes:
  [`PSA_ALG_CTR`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1gad318309706a769cffdc64e4c7e06b2e9),
  [`PSA_ALG_CFB`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga0088c933e01d671f263a9a1f177cb5bc),
  [`PSA_ALG_OFB`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1gae96bb421fa634c6fa8f571f0112f1ddb),
  [`PSA_ALG_XTS`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1gaa722c0e426a797fd6d99623f59748125),
  [`PSA_ALG_ECB_NO_PADDING`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1gab8f0609cd0f12cccc9c950fd5a81a0e3),
  [`PSA_ALG_CBC_NO_PADDING`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1gacb332d72716958880ee7f97d8365ae66),
  [`PSA_ALG_CBC_PKCS7`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1gaef50d2e9716eb6d476046608e4e0c78c),
  [`PSA_ALG_CCM_STAR_NO_TAG`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga89627bb27ec3ce642853ab8554a88572).
* Other padding modes, which are obsolete, are not available in the PSA API. If you need them, handle the padding in your application code and use the `NO_PADDING` algorithm.
* AEAD modes:
  [`PSA_ALG_CCM`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1gac2c0e7d21f1b2df5e76bcb4a8f84273c),
  [`PSA_ALG_GCM`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga0d7d02b15aaae490d38277d99f1c637c).
* KW/KWP modes are not available in the PSA API at the time of writing.

For the ChaCha20 unauthenticated cipher, use [`PSA_KEY_TYPE_CHACHA20`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga901548883b3bce56cc21c3a22cf8d93c) with [`PSA_ALG_STREAM_CIPHER`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1gad98c105198f7428f7d1dffcb2cd398cd).
For the Chacha20+Poly1305 AEAD, use [`PSA_KEY_TYPE_CHACHA20`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga901548883b3bce56cc21c3a22cf8d93c) with [`PSA_ALG_CHACHA20_POLY1305`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga1fec55093541640a71bdd022d4adfb9c)

### Cipher mechanism availability

For each key type value `PSA_KEY_TYPE_xxx`, the symbol `PSA_WANT_KEY_TYPE_xxx` is defined with a nonzero value if the library is built with support for that key type. For each algorithm value `PSA_ALG_yyy`, the symbol `PSA_WANT_ALG_yyy` is defined with a nonzero value if the library is built with support for that algorithm. Note that for a mechanism to be supported, both the key type and the algorithm must be supported.

For example, to test if AES-CBC-PKCS7 is supported, in the legacy API, you could write:
```
#if defined(MBEDTLS_AES_C) && \
    defined(MBEDTLS_CIPHER_MODE_CBC) && defined(MBEDTLS_CIPHER_PADDING_PKCS7)
```
The equivalent in the PSA API is
```
#if PSA_WANT_KEY_TYPE_AES && PSA_WANT_ALG_CBC_PKCS7
```

### Cipher metadata

Both APIs express key sizes in bits. Note however that in the PSA API, the size of a _buffer_ is always expressed in bytes, even if that buffer contains a key.

The following table lists corresponding PSA macros for maximum-size macros that take all supported algorithms into account.

| Legacy macro | PSA macro |
| ------------ | --------- |
| `MBEDTLS_MAX_IV_LENGTH` | [`PSA_CIPHER_IV_MAX_SIZE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#c.PSA_CIPHER_IV_MAX_SIZE), [`PSA_AEAD_NONCE_MAX_SIZE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#crypto__sizes_8h_1ac2a332765ba4ccfc24935d6f7f48fcc7) |
| `MBEDTLS_MAX_BLOCK_LENGTH` | [`PSA_BLOCK_CIPHER_BLOCK_MAX_SIZE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#c.PSA_BLOCK_CIPHER_BLOCK_MAX_SIZE) |
| `MBEDTLS_MAX_KEY_LENGTH` | no equivalent|

There is no equivalent to the type `mbedtls_cipher_info_t` and the functions `mbedtls_cipher_info_from_type` and `mbedtls_cipher_info_from_values` in the PSA API because it is unnecessary. All macros and functions operate directly on key type values (`psa_key_type_t`, `PSA_KEY_TYPE_xxx` constants) and algorithm values (`psa_algorithm_t`, `PSA_ALG_xxx` constants).

| Legacy function | PSA macro |
| --------------- | --------- |
| `mbedtls_cipher_info_get_iv_size` | [`PSA_CIPHER_IV_LENGTH`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#c.PSA_CIPHER_IV_LENGTH), [`PSA_AEAD_NONCE_LENGTH`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#c.PSA_AEAD_NONCE_LENGTH) |
| `mbedtls_cipher_info_get_block_size` | not available (use specific macros for the IV, nonce or tag length) |

The following features have no PSA equivalent:

* `mbedtls_cipher_list`: the PSA API does not currently have a discovery mechanism for cryptographic mechanisms, but one may be added in the future.
* `mbedtls_cipher_info_has_variable_key_bitlen`, `mbedtls_cipher_info_has_variable_iv_size`: the PSA API does not currently have such mechanism for high-level metadata information.
* `mbedtls_cipher_info_from_string`: there is no equivalent of Mbed TLS's lookup based on a (nonstandard) name.

### Cipher key management

The legacy API and the PSA API have a different organization of operations in several respects:

* In the legacy API, each operation object contains the necessary key material. In the PSA API, an operation object contains a reference to a key object. To perform a cryptographic operation, you must create a key object first. However, for a one-shot operation, you do not need an operation object, just a single function call.
* The legacy API uses the same interface for authenticated and non-authenticated ciphers, while the PSA API has separate functions.
* The legacy API uses the same functions for encryption and decryption, while the PSA API has separate functions where applicable.

Here is an overview of the lifecycle of a key object.

1. First define the attributes of the key by filling a [`psa_key_attributes_t` structure](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__attributes/#group__attributes_1ga0ec645e1fdafe59d591104451ebf5680). You need to set the following parameters:
    * Call [`psa_set_key_type`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__attributes/#group__attributes_1ga6857ef0ecb3fa844d4536939d9c64025) to set the key type to the desired `PSA_KEY_TYPE_xxx` value (see “[Cipher mechanism selection](#cipher-mechanism-selection)”).
    * Call [`psa_set_key_bits`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__attributes/#group__attributes_1gaf61683ac87f87687a40262b5afbfa018) to set the key's size in bits. This is optional with `psa_import_key`, which determines the key size from the length of the key material.
    * Call [`psa_set_key_algorithm`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__attributes/#group__attributes_1gaeb8341ca52baa0279475ea3fd3bcdc98) to set the algorithm to the desired `PSA_ALG_xxx` value (see “[Cipher mechanism selection](#cipher-mechanism-selection)”). By design, the same key cannot be used with multiple algorithms.
    * Call [`psa_set_key_usage_flags`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__attributes/#group__attributes_1ga42a65b3c4522ce9b67ea5ea7720e17de) to enable at least [`PSA_KEY_USAGE_ENCRYPT`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__policy/#c.PSA_KEY_USAGE_ENCRYPT) or [`PSA_KEY_USAGE_DECRYPT`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__policy/#c.PSA_KEY_USAGE_DECRYPT), depending on which direction you want to use the key in. To allow both directions, use the flag mask `PSA_KEY_USAGE_DECRYPT | PSA_KEY_USAGE_ENCRYPT`. The same policy flags cover authenticated and non-authenticated encryption/decryption.
2. Call one of the key creation functions, passing the attributes defined in the previous step, to get an identifier of type [`psa_key_id_t`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__types_8h/#_CPPv412psa_key_id_t) to the key object.
    * Use [`psa_import_key`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__import__export/#group__import__export_1ga0336ea76bf30587ab204a8296462327b) to directly import key material.
    * If the key is randomly generated, use [`psa_generate_key`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__random/#group__random_1ga1985eae417dfbccedf50d5fff54ea8c5).
    * If the key is derived from other material (for example from a key exchange), use the [key derivation interface](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__key__derivation/) and create the key with [`psa_key_derivation_output_key`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__key__derivation/#group__key__derivation_1gada7a6e17222ea9e7a6be6864a00316e1).
3. Call the functions in the following sections to perform operations on the key. The same key object can be used in multiple operations.
4. To free the resources used by the key object, call [`psa_destroy_key`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__key__management/#group__key__management_1ga5f52644312291335682fbc0292c43cd2) after all operations with that key are finished.

### Unauthenticated cipher operations

Recall the flow of an unauthenticated cipher operation in the legacy Mbed TLS cipher API:

1. Create a cipher context of type `mbedtls_cipher_context_t` and initialize it with `mbedtls_cipher_init`.
2. Establish the operation parameters (algorithm, key, mode) with `mbedtls_cipher_setup`, `mbedtls_cipher_setkey` (or `mbedtls_cipher_setup_psa`), `mbedtls_cipher_set_padding_mode` if applicable.
3. Set the IV with `mbedtls_cipher_set_iv` (except for ECB which does not use an IV).
4. For a one-shot operation, call `mbedtls_cipher_crypt`. To pass the input in multiple parts, call `mbedtls_cipher_update` as many times as necessary followed by `mbedtls_cipher_finish`.
5. Finally free the resources associated with the operation object by calling `mbedtls_cipher_free`.

For a one-shot operation (where the whole plaintext or ciphertext is passed as a single input), the equivalent flow with the PSA API is to call a single function:

* [`psa_cipher_encrypt`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__cipher/#group__cipher_1ga61f02fbfa681c2659546eca52277dbf1) to perform encryption with a random IV of the default size (indicated by [`PSA_CIPHER_IV_LENGTH`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#c.PSA_CIPHER_IV_LENGTH)). (To encrypt with a specified IV, use the multi-part API described below.)  You can use the macro [`PSA_CIPHER_ENCRYPT_OUTPUT_SIZE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#c.PSA_CIPHER_ENCRYPT_OUTPUT_SIZE) or [`PSA_CIPHER_ENCRYPT_OUTPUT_MAX_SIZE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#c.PSA_CIPHER_ENCRYPT_OUTPUT_MAX_SIZE) to determine the size of the output buffer.
* [`psa_cipher_decrypt`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__cipher/#group__cipher_1gab3593f5f14d8c0431dd306d80929215e) to perform decryption with a specified IV. You can use the macro [`PSA_CIPHER_DECRYPT_OUTPUT_SIZE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#c.PSA_CIPHER_DECRYPT_OUTPUT_SIZE) or [`PSA_CIPHER_DECRYPT_OUTPUT_MAX_SIZE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#c.PSA_CIPHER_DECRYPT_OUTPUT_MAX_SIZE) to determine the size of the output buffer.

For a multi-part operation, the equivalent flow with the PSA API is as follows:

1. Create an operation object of type [`psa_cipher_operation_t`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__cipher/#group__cipher_1ga1399de29db657e3737bb09927aae51fa) and zero-initialize it (or use the corresponding `INIT` macro).
2. Select the key and algorithm with [`psa_cipher_encrypt_setup`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__cipher/#group__cipher_1ga587374c0eb8137a572f8e2fc409bb2b4) or [`psa_cipher_decrypt_setup`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__cipher/#group__cipher_1gaa4ba3a167066eaef2ea49abc5dcd1d4b) depending on the desired direction.
3. When encrypting with a random IV, use [`psa_cipher_generate_iv`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__cipher/#group__cipher_1ga29fd7d32a5729226a2f73e7b6487bd8a). When encrypting with a chosen IV, or when decrypting, set the IV with [`psa_cipher_set_iv`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__cipher/#group__cipher_1ga9caddac1a429a5032d6d4a907fb70ba1). Skip this step with ECB since it does not use an IV.
4. Call [`psa_cipher_update`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__cipher/#group__cipher_1gac3ca27ac6682917c48247d01fd96cd0f) as many times as needed. You can use [`PSA_CIPHER_UPDATE_OUTPUT_SIZE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#c.PSA_CIPHER_UPDATE_OUTPUT_SIZE) or [`PSA_CIPHER_UPDATE_OUTPUT_MAX_SIZE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#crypto__sizes_8h_1ab1f6598efd6a7dc56e7ad7e34719eb32) to determine the size of the output buffer.
5. Call [`psa_cipher_finish`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__cipher/#group__cipher_1ga1dcb58b8befe23f8a4d7a1d49c99249b) to obtain the last part of the output. You can use [`PSA_CIPHER_FINISH_OUTPUT_SIZE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#c.PSA_CIPHER_FINISH_OUTPUT_SIZE) or [`PSA_CIPHER_FINISH_OUTPUT_MAX_SIZE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#c.PSA_CIPHER_FINISH_OUTPUT_MAX_SIZE) to determine the size of the output buffer.

If you need to interrupt the operation after calling the setup function without calling the finish function, call [`psa_cipher_abort`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__cipher/#group__cipher_1gaad482cdca2098bca0620596aaa02eaa4).

### Authenticated cipher operations

Recall the flow of an authenticated cipher operation in the legacy Mbed TLS cipher API (or similar flows in the `chachapoly`, `ccm` and `gcm` modules):

1. Create a cipher context of type `mbedtls_cipher_context_t` and initialize it with `mbedtls_cipher_init`.
2. Establish the operation parameters (algorithm, key, mode) with `mbedtls_cipher_setup`, `mbedtls_cipher_setkey` (or `mbedtls_cipher_setup_psa`), `mbedtls_cipher_set_padding_mode` if applicable.
3. Set the nonce with `mbedtls_cipher_set_iv` (or the `starts` function for low-level modules). For CCM, which requires direct use of the `ccm` module, also call `mbedtls_ccm_set_lengths` to set the length of the additional data and of the plaintext.
4. Call `mbedtls_cipher_update_ad` to pass the unencrypted additional data.
5. Call `mbedtls_cipher_update` as many times as necessary to pass the input plaintext or ciphertext.
6. Call `mbedtls_cipher_finish` to obtain the last part of the output. Then call `mbedtls_cipher_write_tag` (when encrypting) or `mbedtls_cipher_check_tag` (when decrypting) to process the authentication tag.
7. Finally free the resources associated with the operation object by calling `mbedtls_cipher_free`.

Steps 3–6 can be replaced by a single call to `mbedtls_cipher_auth_encrypt_ext` or `mbedtls_cipher_auth_decrypt_ext` for a one-shot operation (where the whole plaintext or ciphertext is passed as a single input).

For a one-shot operation, the PSA API allows you to call a single function:

* [`psa_aead_encrypt`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__aead/#group__aead_1gae72e1eb3c2da3ebd843bb9c8db8df509) to perform authenticated encryption with a random nonce of the default size (indicated by [`PSA_AEAD_NONCE_LENGTH`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#c.PSA_AEAD_NONCE_LENGTH)), with the authentication tag written at the end of the output. (To encrypt with a specified nonce, or to separate the tag from the rest of the ciphertext, use the multi-part API described below.)  You can use the macro [`PSA_AEAD_ENCRYPT_OUTPUT_SIZE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#c.PSA_AEAD_ENCRYPT_OUTPUT_SIZE) or [`PSA_AEAD_ENCRYPT_OUTPUT_MAX_SIZE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#c.PSA_AEAD_ENCRYPT_OUTPUT_MAX_SIZE) to determine the size of the output buffer.
* [`psa_aead_decrypt`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__aead/#group__aead_1gae799f6196a22d50c216c947e0320d3ba) to perform authenticated decryption of a ciphertext with the authentication tag at the end. (If the tag is separate, use the multi-part API described below.) You can use the macro [`PSA_AEAD_DECRYPT_OUTPUT_SIZE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#c.PSA_CIPHER_DECRYPT_OUTPUT_SIZE) or [`PSA_AEAD_DECRYPT_OUTPUT_MAX_SIZE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#c.PSA_CIPHER_DECRYPT_OUTPUT_MAX_SIZE) to determine the size of the output buffer.

For a multi-part operation, the equivalent flow with the PSA API is as follows:

1. Create an operation object of type [`psa_aead_operation_t`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__aead/#group__aead_1ga14f6a01afbaa8c5b3d8c5d345cbaa3ed) and zero-initialize it (or use the corresponding `INIT` macro).
2. Select the key and algorithm with [`psa_aead_encrypt_setup`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__aead/#group__aead_1ga2732c40ce8f3619d41359a329e9b46c4) or [`psa_aead_decrypt_setup`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__aead/#group__aead_1gaaa5c5018e67a7a6514b7e76b9a14de26) depending on the desired direction.
3. When encrypting with a random nonce, use [`psa_aead_generate_nonce`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__aead/#group__aead_1ga5799df1c555efd35970b65be51cb07d1). When encrypting with a chosen nonce, or when decrypting, set the nonce with [`psa_aead_set_nonce`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__aead/#group__aead_1ga59132751a6f843d038924cb217b5e13b). If the algorithm is CCM, you must also call [`psa_aead_set_lengths`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__aead/#group__aead_1gad3431e28d05002c2a7b0760610176050) before or after setting the nonce (for other algorithms, this is permitted but not needed).
4. Call [`psa_aead_update_ad`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__aead/#group__aead_1ga6d0eed03f832e5c9c91cb8adf2882569) as many times as needed.
5. Call [`psa_aead_update`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__aead/#group__aead_1gaf6d49864951ca42136b4a9b71ea26e5c) as many times as needed. You can use [`PSA_AEAD_UPDATE_OUTPUT_SIZE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#c.PSA_AEAD_UPDATE_OUTPUT_SIZE) or [`PSA_AEAD_UPDATE_OUTPUT_MAX_SIZE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#c.PSA_AEAD_UPDATE_OUTPUT_MAX_SIZE) to determine the size of the output buffer.
6. Finally:
    * When encrypting, call [`psa_aead_finish`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__aead/#group__aead_1ga759791bbe1763b377c3b5447641f1fc8) to obtain the last part of the ciphertext and the authentication tag. You can use [`PSA_AEAD_FINISH_OUTPUT_SIZE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#c.PSA_AEAD_FINISH_OUTPUT_SIZE) or [`PSA_AEAD_FINISH_OUTPUT_MAX_SIZE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#c.PSA_AEAD_FINISH_OUTPUT_MAX_SIZE) to determine the size of the output buffer.
    * When decrypting, call [`psa_aead_verify`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__aead/#group__aead_1gae0280e2e61a185b893c36d858453f0d0) to obtain the last part of the plaintext and check the authentication tag. You can use [`PSA_AEAD_VERIFY_OUTPUT_SIZE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#c.PSA_AEAD_VERIFY_OUTPUT_SIZE) or [`PSA_AEAD_VERIFY_OUTPUT_MAX_SIZE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#c.PSA_AEAD_VERIFY_OUTPUT_MAX_SIZE) to determine the size of the output buffer.

If you need to interrupt the operation after calling the setup function without calling the finish or verify function, call [`psa_aead_abort`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__aead/#group__aead_1gae8a5f93d92318c8f592ee9fbb9d36ba0).

### Miscellaneous cipher operation management

The equivalent of `mbedtls_cipher_reset` is to call [`psa_cipher_abort`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__cipher/#group__cipher_1gaad482cdca2098bca0620596aaa02eaa4) or [`psa_aead_abort`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__aead/#group__aead_1gae8a5f93d92318c8f592ee9fbb9d36ba0). Note that you must set the key again with a setup function: the PSA API does not have a special way to reuse an operation object with the same key.

There is no equivalent for the `mbedtls_cipher_get_xxx` functions to extract information from an ongoing PSA cipher or AEAD operation. Applications that need this information will need to save it from the key and operation parameters.

## Hashes and MAC

The PSA API groups functions by purpose rather than by underlying primitive: there is a MAC API (equivalent to `md.h` for HMAC, and `cmac.h` for CMAC) and a hash API (equivalent to `md.h` for hashing). There is no special API for a particular hash algorithm (`md5.h`, `sha1.h`, `sha256.h`, `sha512.h`, `sha3.h`). To migrate code using those low-level modules, please follow the recommendations in the following section, using the same principles as the corresponding `md.h` API.

The PSA API does have a direct interface for the AES-CMAC-PRF-128 from RFC 4615 at the time of writing. You can calculate it using the interface to AES-CMAC.

### Hash mechanism selection

The equivalent to `mbedtls_md_type_t` and `MBEDTLS_MD` constants is the type `psa_algorithm_t` and `PSA_ALG_xxx` constants (the type encompasses all categories of cryptographic algorithms, not just hashes). PSA offers a similar selection of algorithms, but note that SHA-1 and SHA-2 are spelled slightly differently.

| Mbed TLS constant      | PSA constant        |
| ---------------------- | ------------------- |
| `MBEDTLS_MD_MD5`       | `PSA_ALG_MD5`       |
| `MBEDTLS_MD_SHA1`      | `PSA_ALG_SHA_1`     |
| `MBEDTLS_MD_SHA224`    | `PSA_ALG_SHA_224`   |
| `MBEDTLS_MD_SHA256`    | `PSA_ALG_SHA_256`   |
| `MBEDTLS_MD_SHA384`    | `PSA_ALG_SHA_384`   |
| `MBEDTLS_MD_SHA512`    | `PSA_ALG_SHA_512`   |
| `MBEDTLS_MD_RIPEMD160` | `PSA_ALG_RIPEMD160` |
| `MBEDTLS_MD_SHA3_224`  | `PSA_ALG_SHA3_224`  |
| `MBEDTLS_MD_SHA3_256`  | `PSA_ALG_SHA3_256`  |
| `MBEDTLS_MD_SHA3_384`  | `PSA_ALG_SHA3_384`  |
| `MBEDTLS_MD_SHA3_512`  | `PSA_ALG_SHA3_512`  |

### MAC mechanism selection

PSA Crypto has a generic API with the same functions for all MAC mechanisms. The mechanism is determined by a combination of an algorithm value of type [`psa_algorithm_t`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1gac2e4d47f1300d73c2f829a6d99252d69) and a key type value of type [`psa_key_type_t`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga63fce6880ca5933b5d6baa257febf1f6).

* For HMAC, the algorithm is [`PSA_ALG_HMAC`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga70f397425684b3efcde1e0e34c28261f)`(hash)` where `hash` is the underlying hash algorithm (see “[Hash mechanism selection](#hash-mechanism-selection)”),
  for example `PSA_ALG_HMAC(PSA_ALG_SHA_256)` for HMAC-SHA-256.
  The key type is [`PSA_KEY_TYPE_HMAC`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__values_8h/#c.PSA_KEY_TYPE_HMAC) regardless of the hash algorithm.
* For CMAC, the algorithm is [`PSA_ALG_CMAC`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__values_8h/#c.PSA_ALG_CMAC) regardless of the underlying block cipher. The key type determines the block cipher:
  [`PSA_KEY_TYPE_AES`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga6ee54579dcf278c677eda4bb1a29575e),
  [`PSA_KEY_TYPE_ARIA`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#c.PSA_KEY_TYPE_ARIA),
  [`PSA_KEY_TYPE_CAMELLIA`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1gad8e5da742343fd5519f9d8a630c2ed81) or
  [`PSA_KEY_TYPE_DES`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga577562bfbbc691c820d55ec308333138).

### Hash and MAC mechanism availability

For each key type value `PSA_KEY_TYPE_xxx`, the symbol `PSA_WANT_KEY_TYPE_xxx` is defined with a nonzero value if the library is built with support for that key type. For each algorithm value `PSA_ALG_yyy`, the symbol `PSA_WANT_ALG_yyy` is defined with a nonzero value if the library is built with support for that algorithm. For a compound mechanism, all parts must be supported. In particular, for HMAC, all three of `PSA_WANT_KEY_TYPE_HMAC`, `PSA_WANT_ALG_HMAC` and the underlying hash must be enabled. (A configuration with only one of `PSA_WANT_KEY_TYPE_HMAC` and `PSA_WANT_ALG_HMAC` is technically possible but not useful.)

For example, to test if HMAC-SHA-256 is supported, in the legacy API, you could write:
```
#if defined(MBEDTLS_MD_C) && defined(MBEDTLS_SHA256_C)
```
The equivalent in the PSA API is
```
#if PSA_WANT_KEY_TYPE_HMAC && PSA_WANT_ALG_HMAC && PSA_WANT_ALG_SHA_256
```

To test if AES-CMAC is supported, in the legacy API, you could write:
```
if defined(MBEDTLS_AES_C) && defined(MBEDTLS_CMAC_C)
```
The equivalent in the PSA API is
```
#if PSA_WANT_KEY_TYPE_AES && PSA_WANT_ALG_CMAC
```

### Hash algorithm metadata

There is no equivalent to the type `mbedtls_md_info_t` and the functions `mbedtls_md_info_from_type` and `mbedtls_md_get_type` in the PSA API because it is unnecessary. All macros and functions operate directly on algorithm (`psa_algorithm_t`, `PSA_ALG_xxx` constants).

| Legacy macro | PSA macro |
| ------------ | --------- |
| `MBEDTLS_MD_MAX_SIZE` | [`PSA_HASH_MAX_SIZE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#c.PSA_HASH_MAX_SIZE) |
| `MBEDTLS_MD_MAX_BLOCK_SIZE` | [`PSA_HMAC_MAX_HASH_BLOCK_SIZE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#c.PSA_HMAC_MAX_HASH_BLOCK_SIZE) |
| `mbedtls_md_get_size` | [`PSA_HASH_LENGTH`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#c.PSA_HASH_LENGTH) |
| `mbedtls_md_get_size_from_type` | [`PSA_HASH_LENGTH`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#c.PSA_HASH_LENGTH) |

The following features have no PSA equivalent:

* `mbedtls_md_list`: the PSA API does not currently have a discovery mechanism for cryptographic mechanisms, but one may be added in the future.
* `mbedtls_md_info_from_ctx`
* `mbedtls_cipher_info_from_string`, `mbedtls_md_get_name`: there is no equivalent of Mbed TLS's lookup based on a (nonstandard) name.

### Hash calculation

The equivalent of `mbedtls_md` for a one-shot hash calculation is [`psa_hash_compute`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__hash/#group__hash_1gac69f7f19d96a56c28cf3799d11b12156). In addition, to compare the hash of a message with an expected value, you can call [`psa_hash_compare`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__hash/#group__hash_1ga0c08f4797bec96b886c8c8d7acc2a553) instead of `mbedtls_md` followed by `memcmp` or a constant-time equivalent.

For a multi-part hash calculation, the legacy process is as follows:

1. Create a digest context of type `mbedtls_md_context_t` and initialize it with `mbedtls_md_init`.
2. Call `mbedtls_md_setup` to select the hash algorithm, with `hmac=0`. Then call `mbedtls_md_starts` to start the hash operation.
3. Call `mbedtls_md_update` as many times as necessary.
4. Call `mbedtls_md_finish`. If verifying the hash against an expected value, compare the result with the expected value.
5. Finally free the resources associated with the operation object by calling `mbedtls_md_free`.

The equivalent process in the PSA API is as follows:

1. Create an operation object of type [`psa_hash_operation_t`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__hash/#group__hash_1ga3c4205d2ce66c4095fc5c78c25273fab) and zero-initialize it (or use the corresponding `INIT` macro).
2. Call [`psa_hash_setup`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__hash/#group__hash_1ga8d72896cf70fc4d514c5c6b978912515) to specify the algorithm.
3. Call [`psa_hash_update`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__hash/#group__hash_1ga65b16ef97d7f650899b7db4b7d1112ff) as many times as necessary.
4. To obtain the hash, call [`psa_hash_finish`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__hash/#group__hash_1ga4795fd06a0067b0adcd92e9627b8c97e). Alternatively, to verify the hash against an expected value, call [`psa_hash_verify`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__hash/#group__hash_1ga7be923c5700c9c70ef77ee9b76d1a5c0).

If you need to interrupt the operation after calling the setup function without calling the finish or verify function, call [`psa_hash_abort`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__hash/#group__hash_1gab0b4d5f9912a615559497a467b532928).

There is no equivalent to `mbedtls_md_file` in the PSA API. Load the file data and calculate its hash.

### MAC key management

The legacy API and the PSA API have a different organization of operations in several respects:

* In the legacy API, each operation object contains the necessary key material. In the PSA API, an operation object contains a reference to a key object. To perform a cryptographic operation, you must create a key object first. However, for a one-shot operation, you do not need an operation object, just a single function call.
* The legacy API uses the same interface for authenticated and non-authenticated ciphers, while the PSA API has separate functions.
* The legacy API uses the same functions for encryption and decryption, while the PSA API has separate functions where applicable.

Here is an overview of the lifecycle of a key object.

1. First define the attributes of the key by filling a [`psa_key_attributes_t` structure](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__attributes/#group__attributes_1ga0ec645e1fdafe59d591104451ebf5680). You need to set the following parameters:
    * Call [`psa_set_key_type`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__attributes/#group__attributes_1ga6857ef0ecb3fa844d4536939d9c64025) to set the key type to the desired `PSA_KEY_TYPE_xxx` value (see “[Cipher mechanism selection](#cipher-mechanism-selection)”).
    * Call [`psa_set_key_bits`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__attributes/#group__attributes_1gaf61683ac87f87687a40262b5afbfa018) to set the key's size in bits. This is optional with `psa_import_key`, which determines the key size from the length of the key material.
    * Call [`psa_set_key_algorithm`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__attributes/#group__attributes_1gaeb8341ca52baa0279475ea3fd3bcdc98) to set the algorithm to the desired `PSA_ALG_xxx` value (see “[Cipher mechanism selection](#cipher-mechanism-selection)”). By design, the same key cannot be used with multiple algorithms.
    * Call [`psa_set_key_usage_flags`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__attributes/#group__attributes_1ga42a65b3c4522ce9b67ea5ea7720e17de) to enable at least [`PSA_KEY_USAGE_SIGN_MESSAGE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__policy/#c.PSA_KEY_USAGE_SIGN_MESSAGE) to calculate a MAC or [`PSA_KEY_USAGE_VERIFY_MESSAGE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__policy/#c.PSA_KEY_USAGE_VERIFY_MESSAGE) to verify the MAC of a message. To allow both directions, use the flag mask `PSA_KEY_USAGE_SIGN_MESSAGE | PSA_KEY_USAGE_VERIFY_MESSAGE`.
2. Call one of the key creation functions, passing the attributes defined in the previous step, to get an identifier of type [`psa_key_id_t`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__types_8h/#_CPPv412psa_key_id_t) to the key object.
    * Use [`psa_import_key`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__import__export/#group__import__export_1ga0336ea76bf30587ab204a8296462327b) to directly import key material.
    * If the key is randomly generated, use [`psa_generate_key`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__random/#group__random_1ga1985eae417dfbccedf50d5fff54ea8c5).
    * If the key is derived from other material (for example from a key exchange), use the [key derivation interface](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__key__derivation/) and create the key with [`psa_key_derivation_output_key`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__key__derivation/#group__key__derivation_1gada7a6e17222ea9e7a6be6864a00316e1).
3. Call the functions in the following sections to perform operations on the key. The same key object can be used in multiple operations.
4. To free the resources used by the key object, call [`psa_destroy_key`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__key__management/#group__key__management_1ga5f52644312291335682fbc0292c43cd2) after all operations with that key are finished.

### MAC calculation

The process for a HMAC operation in the legacy API is as follows:

1. Create a digest context of type `mbedtls_md_context_t` and initialize it with `mbedtls_md_init`.
2. Call `mbedtls_md_setup` to select the hash algorithm, with `hmac=1`. Then call `mbedtls_md_hmac_starts` to set the key.
3. Call `mbedtls_md_hmac_update` as many times as necessary.
4. Call `mbedtls_md_hmac_finish`. If verifying the MAC against an expected value, compare the result with the expected value. Note that this comparison should be in constant time to avoid a side channel vulnerability, for example using `mbedtls_ct_memcmp`.
5. Finally free the resources associated with the operation object by calling `mbedtls_md_free`.

The process for a CMAC operation in the legacy API is as follows:

1. Create a cipher context of type `mbedtls_cipher_context_t` and initialize it with `mbedtls_cipher_init`.
2. Call `mbedtls_cipher_setup` to select the block cipher. Then call `mbedtls_md_cmac_starts` to set the key.
3. Call `mbedtls_cipher_cmac_update` as many times as necessary.
4. Call `mbedtls_cipher_cmac_finish`. If verifying the MAC against an expected value, compare the result with the expected value. Note that this comparison should be in constant time to avoid a side channel vulnerability, for example using `mbedtls_ct_memcmp`.
5. Finally free the resources associated with the operation object by calling `mbedtls_cipher_free`.

The process in the PSA API to calculate a MAC is as follows:

1. Create an operation object of type [`psa_mac_operation_t`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group___m_a_c/#group___m_a_c_1ga78f0838b0c4e3db28b26355624d4bd37) and zero-initialize it (or use the corresponding `INIT` macro).
2. Call [`psa_mac_sign_setup`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group___m_a_c/#group___m_a_c_1ga03bc3e3c0b7e55b20d2a238e418d46cd) to specify the algorithm and the key. See “[MAC key management](#mac-key-management)” for how to obtain a key identifier.
3. Call [`psa_mac_update`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group___m_a_c/#group___m_a_c_1ga5560af371497babefe03c9da4e8a1c05) as many times as necessary.
4. To obtain the MAC, call [`psa_mac_sign_finish`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group___m_a_c/#group___m_a_c_1gac22bc0125580c96724a09226cfbc97f2).

To verify a MAC against an expected value, use the following process instead:

1. Create an operation object of type [`psa_mac_operation_t`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group___m_a_c/#group___m_a_c_1ga78f0838b0c4e3db28b26355624d4bd37) and zero-initialize it (or use the corresponding `INIT` macro).
2. Call [`psa_mac_verify_setup`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group___m_a_c/#group___m_a_c_1ga08ae327fcbc5f8e201172fe11e536984) to specify the algorithm and the key. See “[MAC key management](#mac-key-management)” for how to obtain a key identifier.
3. Call [`psa_mac_update`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group___m_a_c/#group___m_a_c_1ga5560af371497babefe03c9da4e8a1c05) as many times as necessary.
4. To verify the MAC against an expected value, call [`psa_mac_verify_finish`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group___m_a_c/#group___m_a_c_1gac92b2930d6728e1be4d011c05d485822).

If you need to interrupt the operation after calling the setup function without calling the finish function, call [`psa_mac_abort`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group___m_a_c/#group___m_a_c_1gacd8dd54855ba1bc0a03f104f252884fd).

The PSA API also offers functions for a one-shot MAC calculation, similar to `mbedtls_cipher_cmac`:

* [`psa_mac_compute`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group___m_a_c/#group___m_a_c_1gabf02ebd3595ea15436967092b5d52878) to calculate the MAC of a buffer in memory.
* [`psa_mac_verify`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group___m_a_c/#group___m_a_c_1gaf6988545df5d5e2466c34d753443b15a) to verify the MAC of a buffer in memory against an expected value.

In both cases, see “[MAC key management](#mac-key-management)” for how to obtain a key identifier.

### Miscellaneous hash or MAC operation management

The equivalent of `mbedtls_md_reset`, `mbedtls_md_hmac_reset` or `mbedtls_cmac_reset` is to call [`psa_hash_abort`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__hash/#group__hash_1gab0b4d5f9912a615559497a467b532928) or [`psa_mac_abort`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group___m_a_c/#group___m_a_c_1gacd8dd54855ba1bc0a03f104f252884fd). Note that you must call a setup function to specify the algorithm and the key (for MAC) again, and they can be different ones.

The equivalent of `mbedtls_md_clone` to clone a hash operation is [`psa_hash_clone`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__hash/#group__hash_1ga39673348f3302b4646bd780034a5aeda). A PSA MAC operation cannot be cloned.

## Key derivation

### HKDF

PSA Crypto provides access to HKDF, HKDF-Extract and HKDF-Expand via its [key derivation interface](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__key__derivation/). This is a generic interface using an operation object with one function call for each input and one function call for each output.

1. Create an operation object of type [`psa_key_derivation_operation_t`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__key__derivation/#group__key__derivation_1ga5f099b63799a0959c3d46718c86c2609) and zero-initialize it (or use the corresponding `INIT` macro).
2. Call [`psa_key_derivation_setup`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__key__derivation/#group__key__derivation_1gac0b6a76e45cceb1862752bf041701859) to select the algorithm, which is a value of type [`psa_algorithm_t`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1gac2e4d47f1300d73c2f829a6d99252d69). For HKDF and variants, use one of the macros [`PSA_ALG_HKDF`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__values_8h/#c.PSA_ALG_HKDF), [`PSA_ALG_HKDF_EXTRACT`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__values_8h/#c.PSA_ALG_HKDF_EXTRACT) or [`PSA_ALG_HKDF_EXPAND`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__values_8h/#c.PSA_ALG_HKDF_EXPAND) with the [hash algorithm](#hash-mechanism-selection) passed as an argument. For example `PSA_ALG_HKDF(PSA_ALG_SHA_256)` selects HKDF-SHA-256.
3. Call [`psa_key_derivation_input_bytes`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__key__derivation/#group__key__derivation_1ga8fd934dfb0ca45cbf89542ef2a5494c2) on each of the inputs in the order listed below. (Use [`psa_key_derivation_input_key`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__key__derivation/#group__key__derivation_1gab2d7ce8705dd8e4a093f4b8a21a0c15a) instead for an input that is a PSA key object.) The input step value for each step is as follows:
    1. [`PSA_KEY_DERIVATION_INPUT_SALT`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__derivation/#group__derivation_1gab62757fb125243562c3947a752470d4a) for the salt used during the extraction step. Omit this step for HKDF-Expand. For HKDF, you may omit this step if the salt is empty.
    2. [`PSA_KEY_DERIVATION_INPUT_SECRET`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__derivation/#group__derivation_1ga0ddfbe764baba995c402b1b0ef59392e) for the secret input.
    3. [`PSA_KEY_DERIVATION_INPUT_INFO`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__derivation/#group__derivation_1gacef8df989e09c769233f4b779acb5b7d) for the info string used during the expansion step. Omit this step for HKDF-Extract.
4. Call [`psa_key_derivation_output_bytes`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__key__derivation/#group__key__derivation_1ga06b7eb34a2fa88965f68e3d023fa12b9) to obtain the output of the derivation. You may call this function more than once to retrieve the output in successive chunks. Use [`psa_key_derivation_output_key`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__key__derivation/#group__key__derivation_1gada7a6e17222ea9e7a6be6864a00316e1) instead if you want to use a chunk as a PSA key.
5. Call [`psa_key_derivation_abort`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__key__derivation/#group__key__derivation_1ga90fdd2716124d0bd258826184824675f) to free the resources associated with the key derivation object.

### PKCS#5 module

Applications currently using `mbedtls_pkcs5_pbkdf2_hmac` or `mbedtls_pkcs5_pbkdf2_hmac_ext` can switch to the PSA key derivation API for PBKDF2 (not yet implemented at the time of writing, scheduled to be released in Mbed TLS 3.5). This is a generic interface using an operation object with one function call for each input and one function call for each output.

1. Create an operation object of type [`psa_key_derivation_operation_t`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__key__derivation/#group__key__derivation_1ga5f099b63799a0959c3d46718c86c2609) and zero-initialize it (or use the corresponding `INIT` macro).
2. Call [`psa_key_derivation_setup`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__key__derivation/#group__key__derivation_1gac0b6a76e45cceb1862752bf041701859) to select the algorithm, which is a value of type [`psa_algorithm_t`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1gac2e4d47f1300d73c2f829a6d99252d69). For PBKDF2-HMAC, select `PSA_ALG_PBKDF2_HMAC(hash)` where `hash` is the underlying hash algorithm (see “[Hash mechanism selection](#hash-mechanism-selection)”).
3. Call `psa_key_derivation_input_cost` with the step `PSA_KEY_DERIVATION_INPUT_COST` to select the iteration count.
4. Call [`psa_key_derivation_input_bytes`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__key__derivation/#group__key__derivation_1ga8fd934dfb0ca45cbf89542ef2a5494c2) on each of the inputs in the order listed below. (Use [`psa_key_derivation_input_key`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__key__derivation/#group__key__derivation_1gab2d7ce8705dd8e4a093f4b8a21a0c15a) instead for an input that is a PSA key object.) The input step value for each step is as follows:
    1. [`PSA_KEY_DERIVATION_INPUT_SALT`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__derivation/#group__derivation_1gab62757fb125243562c3947a752470d4a) for the salt used during the extraction step. You may repeat this step to pass the salt in pieces (for example a salt and a pepper).
    2. [`PSA_KEY_DERIVATION_INPUT_SECRET`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__derivation/#group__derivation_1ga0ddfbe764baba995c402b1b0ef59392e) for the password.
5. Call [`psa_key_derivation_output_bytes`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__key__derivation/#group__key__derivation_1ga06b7eb34a2fa88965f68e3d023fa12b9) to obtain the output of the derivation. You may call this function more than once to retrieve the output in successive chunks.
  Use [`psa_key_derivation_output_key`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__key__derivation/#group__key__derivation_1gada7a6e17222ea9e7a6be6864a00316e1) instead if you want to use a chunk as a PSA key.  
  If you want to verify the output against an expected value (for authentication, rather than to derive key material), call `psa_key_derivation_verify_bytes` or `psa_key_derivation_verify_key` instead of `psa_key_derivation_output_bytes`.
6. Call [`psa_key_derivation_abort`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__key__derivation/#group__key__derivation_1ga90fdd2716124d0bd258826184824675f) to free the resources associated with the key derivation object.

The function `mbedtls_pkcs5_pbes2` is only inteded as a support function to parse encrypted private keys in the PK module. It has no PSA equivalent.

### PKCS#12 module

The functions `mbedtls_pkcs12_derivation` and `mbedtls_pkcs12_pbes2` are only intended as supports function to parse encrypted private keys in the PK module. They have no PSA equivalent.

## Random generation

### Random generation interface

The PSA subsystem has an internal random generator. As a consequence, you do not need to instantiate one manually, so most applications using PSA crypto do not need the interfaces from `entropy.h`, `ctr_drbg.` and `hmac_drbg.h`.

The PSA API uses its internal random generator to generate keys (`psa_generate_key`), nonces for encryption (`psa_cipher_generate_iv`, `psa_cipher_encrypt`, `psa_aead_generate_nonce`, `psa_aead_encrypt`, `psa_asymmetric_encrypt`), and other random material as needed. If you need random data for some other purposes, call [`psa_generate_random`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__random/#group__random_1ga1985eae417dfbccedf50d5fff54ea8c5).

### Entropy sources

Unless explicitly configured otherwise, the PSA random generator uses the default entropy sources configured through the legacy interface (`MBEDTLS_ENTROPY_xxx` symbols). Its set of sources is equivalent to an entropy object configured with `mbedtls_entropy_init`.

A future version of Mbed TLS will include a PSA interface for configuring entropy sources. This is likely to replace the legacy interface in Mbed TLS 4.0.

### Deterministic pseudorandom generation

The PSA API does not have a dedicated interface for pseudorandom generation. The [key derivation interface](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__key__derivation/) can serve a similar purpose in some applications, but it does not offer CTR\_DRBG or HMAC\_DRBG. If you need these algorithms, keep using `ctr_drbg.h` and `hmac_drbg.h`, but note that they may be removed from the public API in Mbed TLS 4.0.

## Asymmetric cryptography

The PSA API supports RSA (see “[RSA mechanism selection](#rsa-mechanism-selection)”), elliptic curve cryptography (see “[ECC mechanism selection](#elliptic-curve-mechanism-selection)” and “[EC-JPAKE](#ec-jpake)”) and finite-field Diffie-Hellman (see “[Diffie-Hellman mechanism selection](#diffie-hellman-mechanism-selection)”).

### Key lifecycle for asymmetric cryptography

In the PSA API, keys are referenced by an identifier of type [`psa_key_id_t`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__types_8h/#_CPPv412psa_key_id_t).
(Some documentation references [`mbedtls_svc_key_id_t`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__types_8h/#_CPPv420mbedtls_svc_key_id_t); the two types are identical except when the library is configured for use in a multi-client cryptography service.)
The PSA key identifier tends to play the same role as a `mbedtls_pk_context`, `mbedtls_rsa_context` or `mbedtls_ecp_keypair` structure in the legacy API; however there are major differences in the way the two APIs can be used to create keys or to obtain information about a key.

Here is an overview of the lifecycle of a PSA key object.

1. First define the attributes of the key by filling a [`psa_key_attributes_t` structure](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__attributes/#group__attributes_1ga0ec645e1fdafe59d591104451ebf5680). You need to set the following parameters:
    * Call [`psa_set_key_type`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__attributes/#group__attributes_1ga6857ef0ecb3fa844d4536939d9c64025) to set the key type to the desired `PSA_KEY_TYPE_xxx` value (see “[RSA mechanism selection](#rsa-mechanism-selection)”, “[Elliptic curve mechanism selection](#elliptic-curve-mechanism-selection)” and “[Diffie-Hellman mechanism selection](#diffie-hellman-mechanism-selection)”).
    * Call [`psa_set_key_bits`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__attributes/#group__attributes_1gaf61683ac87f87687a40262b5afbfa018) to set the key's conceptual size in bits. This is optional with `psa_import_key`, which determines the key size from the length of the key material.
    * Call [`psa_set_key_algorithm`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__attributes/#group__attributes_1gaeb8341ca52baa0279475ea3fd3bcdc98) to set the algorithm to the desired `PSA_ALG_xxx` value (see “[RSA mechanism selection](#rsa-mechanism-selection)”, “[Elliptic curve mechanism selection](#elliptic-curve-mechanism-selection)” and “[Diffie-Hellman mechanism selection](#diffie-hellman-mechanism-selection)” as well as “[Public-key cryptography policies](#public-key-cryptography-policies)”).
    * Call [`psa_set_key_usage_flags`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__attributes/#group__attributes_1ga42a65b3c4522ce9b67ea5ea7720e17de) to enable the desired usage types (see “[Public-key cryptography policies](#public-key-cryptography-policies)”).
2. Call one of the key creation functions, passing the attributes defined in the previous step, to get an identifier of type [`psa_key_id_t`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__types_8h/#_CPPv412psa_key_id_t) to the key object.
    * Use [`psa_import_key`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__import__export/#group__import__export_1ga0336ea76bf30587ab204a8296462327b) to directly import key material.
    * If the key is randomly generated, use [`psa_generate_key`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__random/#group__random_1ga1985eae417dfbccedf50d5fff54ea8c5).
    * If the key is derived from other material (for example from a key exchange), use the [key derivation interface](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__key__derivation/) and create the key with [`psa_key_derivation_output_key`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__key__derivation/#group__key__derivation_1gada7a6e17222ea9e7a6be6864a00316e1).
3. Call the functions in the following sections to perform operations on the key. The same key object can be used in multiple operations.
4. To free the resources used by the key object, call [`psa_destroy_key`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__key__management/#group__key__management_1ga5f52644312291335682fbc0292c43cd2) after all operations with that key are finished.

### Public-key cryptography policies

A key's policy indicates what algorithm(s) it can be used with (usage algorithm policy) and what operations are permitted (usage flags).

The following table lists the relevant usage flags for asymmetric cryptography. You can pass an bitwise-or of those flags to [`psa_set_key_usage_flags`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__attributes/#group__attributes_1ga42a65b3c4522ce9b67ea5ea7720e17de).

| Usage | Flag |
| ----- | ---- |
| export public key | 0 (always permitted) |
| export private key | [`PSA_KEY_USAGE_EXPORT`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__policy/#group__policy_1ga7dddccdd1303176e87a4d20c87b589ed) |
| Sign a message directly | [`PSA_KEY_USAGE_SIGN_MESSAGE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__policy/#group__policy_1ga552117ac92b79500cae87d4e65a85c54) |
| Sign an already-calculated hash | at least one of [`PSA_KEY_USAGE_SIGN_MESSAGE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__policy/#group__policy_1ga552117ac92b79500cae87d4e65a85c54) or [`PSA_KEY_USAGE_SIGN_HASH`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__policy/#group__policy_1ga552117ac92b79500cae87d4e65a85c54) |
| Verify a message directly | [`PSA_KEY_USAGE_VERIFY_MESSAGE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__policy/#group__policy_1gabea7ec4173f4f943110329ac2953b2b1) |
| Verify an already-calculated hash | at least one of [`PSA_KEY_USAGE_VERIFY_MESSAGE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__policy/#group__policy_1gabea7ec4173f4f943110329ac2953b2b1) or [`PSA_KEY_USAGE_VERIFY_HASH`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__policy/#group__policy_1gafadf131ef2182045e3483d03aadaa1bd) |
| Encryption | [`PSA_KEY_USAGE_`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__policy/#group__policy_1ga75153b296d045d529d97203a6a995dad) |
| Decryption | [`PSA_KEY_USAGE_`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__policy/#group__policy_1gac3f2d2e5983db1edde9f142ca9bf8e6a) |
| Key agreement | [`PSA_KEY_USAGE_DERIVE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__policy/#group__policy_1gaf19022acc5ef23cf12477f632b48a0b2) |

The sections “[RSA mechanism selection](#rsa-mechanism-selection)”, “[Elliptic curve mechanism selection](#elliptic-curve-mechanism-selection)” and “[Diffie-Hellman mechanism selection](#diffie-hellman-mechanism-selection)” cover the available algorithm values for each key type. Normally, a key can only be used with a single algorithm, following standard good practice. However, there are two ways to relax this requirement.

* Many signature algorithms encode a hash algorithm. Sometimes the same key may need to be used to sign messages with multiple different hashes. In an algorithm policy, you can use [`PSA_ALG_ANY_HASH`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__values_8h/#c.PSA_ALG_ANY_HASH) instead of a hash algorithm value to allow the key to be used with any hash. For example, `psa_set_key_algorithm(&attributes, PSA_ALG_RSA_PSS(PSA_ALG_ANY_HASH))` allows the key to be used with RSASSA-PSS, with different hash algorithhms in each operation.
* In addition to the algorithm (or wildcard) selected with [`psa_set_key_algorithm`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__attributes/#group__attributes_1gaeb8341ca52baa0279475ea3fd3bcdc98), you can use [`psa_set_key_enrollment_algorithm`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__attributes/#group__attributes_1gaffa134b74aa52aa3ed9397fcab4005aa) to permit a second algorithm (or wildcard). This is intended for scenarios where a key is normally used with a single algorithm, but needs to be used with a different algorithm for enrollment (such as an ECDH key for which an ECDSA proof-of-possession is also required).

### Asymmetric cryptographic mechanisms

#### RSA mechanism selection

The PK types `MBEDTLS_PK_RSA`, `MBEDTLS_PK_RSASSA_PSS` and `MBEDTLS_PK_RSA_ALT` correspond to RSA key types in the PSA API. In the PSA API, key pairs and public keys are separate object types.
See “[RSA-ALT interface](#rsa-alt-interface)” for more information about `MBEDTLS_PK_RSA_ALT`.

The PSA API uses policies and algorithm parameters rather than key types to distinguish between `MBEDTLS_PK_RSA` and `MBEDTLS_PK_RSASSA_PSS`. The algorithm selection also replaces the use of `mbedtls_rsa_set_padding` on an `mbedtls_rsa_context` object. See the list of algorithms below and the signature and encryption sections for more information.

An RSA public key has the type [`PSA_KEY_TYPE_RSA_PUBLIC_KEY`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga9ba0878f56c8bcd1995ac017a74f513b).

An RSA key pair has the type [`PSA_KEY_TYPE_RSA_KEY_PAIR`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga11745b110166e927e2abeabc7d532051). A key with this type can be used both for private-key and public-key operations (there is no separate key type for a private key without the corresponding public key).
You can always use a private key for operations on the corresponding public key (as long as the policy permits it).

The following cryptographic algorithms work with RSA keys:

* PKCS#1v1.5 RSA signature: [`PSA_ALG_RSA_PKCS1V15_SIGN`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga702ff75385a6ae7d4247033f479439af), [`PSA_ALG_RSA_PKCS1V15_SIGN_RAW`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga4215e2a78dcf834e9a625927faa2a817).
* PKCS#1v1.5 RSA encryption: [`PSA_ALG_RSA_PKCS1V15_CRYPT`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga4c540d3abe43fb9abcb94f2bc51acef9).
* PKCS#1 RSASSA-PSS signature: [`PSA_ALG_RSA_PSS`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga62152bf4cb4bf6aace5e1be8f143564d), [`PSA_ALG_RSA_PSS_ANY_SALT`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga9b7355a2cd6bde88177634d539127f2b).
* PKCS#1 RSAES-OAEP encryption: [`PSA_ALG_RSA_OAEP`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1gaa1235dc3fdd9839c6c1b1a9857344c76).

#### Elliptic curve mechanism selection

The PK types `MBEDTLS_PK_ECKEY`, `MBEDTLS_PK_ECKEY_DH` and `MBEDTLS_PK_ECDSA` correspond to RSA key types in the PSA API. In the PSA API, key pairs and public keys are separate object types. The PSA API uses policies and algorithm parameters rather than key types to distinguish between the PK EC types.

An ECC public key has the type [`PSA_KEY_TYPE_ECC_PUBLIC_KEY(curve)`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1gad54c03d3b47020e571a72cd01d978cf2) where `curve` is a curve family identifier.

An ECC key pair has the type [`PSA_KEY_TYPE_ECC_KEY_PAIR(curve)`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga0b6f5d4d5037c54ffa850d8059c32df0) where `curve` is a curve family identifier. A key with this type can be used both for private-key and public-key operations (there is no separate key type for a private key without the corresponding public key).
You can always use a private key for operations on the corresponding public key (as long as the policy permits it).

A curve is fully determined by a curve family identifier and the private key size in bits. The following table gives the correspondence between legacy and PSA elliptic curve designations.

| Mbed TLS legacy curve identifier | PSA curve family | Curve bit-size |
| -------------------------------- | ---------------- | -------------- |
| `MBEDTLS_ECP_DP_SECP192R1` | [`PSA_ECC_FAMILY_SECP_R1`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga48bb340b5544ba617b0f5b89542665a7) | 192 |
| `MBEDTLS_ECP_DP_SECP224R1` | [`PSA_ECC_FAMILY_SECP_R1`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga48bb340b5544ba617b0f5b89542665a7) | 224 |
| `MBEDTLS_ECP_DP_SECP256R1` | [`PSA_ECC_FAMILY_SECP_R1`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga48bb340b5544ba617b0f5b89542665a7) | 256 |
| `MBEDTLS_ECP_DP_SECP384R1` | [`PSA_ECC_FAMILY_SECP_R1`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga48bb340b5544ba617b0f5b89542665a7) | 384 |
| `MBEDTLS_ECP_DP_SECP521R1` | [`PSA_ECC_FAMILY_SECP_R1`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga48bb340b5544ba617b0f5b89542665a7) | 521 |
| `MBEDTLS_ECP_DP_BP256R1` | [`PSA_ECC_FAMILY_BRAINPOOL_P_R1`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1gac1643f1baf38b30d07c20a6eac697f15) | 256 |
| `MBEDTLS_ECP_DP_BP384R1` | [`PSA_ECC_FAMILY_BRAINPOOL_P_R1`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1gac1643f1baf38b30d07c20a6eac697f15) | 384 |
| `MBEDTLS_ECP_DP_BP512R1` | [`PSA_ECC_FAMILY_BRAINPOOL_P_R1`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1gac1643f1baf38b30d07c20a6eac697f15) | 512 |
| `MBEDTLS_ECP_DP_CURVE25519` | [`PSA_ECC_FAMILY_MONTGOMERY`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga1f624c5cdaf25b21287af33024e1aff8) | 255 |
| `MBEDTLS_ECP_DP_SECP192K1` | [`PSA_ECC_FAMILY_SECP_K1`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga48bb340b5544ba617b0f5b89542665a7) | 192 |
| `MBEDTLS_ECP_DP_SECP224K1` | not supported | 224 |
| `MBEDTLS_ECP_DP_SECP256K1` | [`PSA_ECC_FAMILY_SECP_K1`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga48bb340b5544ba617b0f5b89542665a7) | 256 |
| `MBEDTLS_ECP_DP_CURVE448` | [`PSA_ECC_FAMILY_MONTGOMERY`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga1f624c5cdaf25b21287af33024e1aff8) | 448 |

The following cryptographic algorithms work with ECC keys:

* ECDH key agreement (including X25519 and X448): [`PSA_ALG_ECDH`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1gab2dbcf71b63785e7dd7b54a100edee43).
* ECDSA: [`PSA_ALG_ECDSA`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga7e3ce9f514a227d5ba5d8318870452e3), [`PSA_ALG_ECDSA_ANY`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga51d6b6044a62e33cae0cf64bfc3b22a4), [`PSA_ALG_DETERMINISTIC_ECDSA`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga11da566bcd341661c8de921e2ca5ed03).

#### Diffie-Hellman mechanism selection

A finite-field Diffie-Hellman public key has the type [`PSA_KEY_TYPE_DH_PUBLIC_KEY`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1gaa22f0f2ea89b929f2fadc19890cc5d5c).

A finite-field Diffie-Hellman key pair has the type [`PSA_KEY_TYPE_DH_KEY_PAIR`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1gab4f857c4cd56f5fe65ded421e61bcc8c). A key with this type can be used both for private-key and public-key operations (there is no separate key type for a private key without the corresponding public key).

The PSA API only supports Diffie-Hellman with predefined groups. A group is fully determined by a group family identifier and the public key size in bits.

| Mbed TLS DH group P value | PSA DH group family | Bit-size |
| ------------------------- | ------------------- | -------- |
| `MBEDTLS_DHM_RFC7919_FFDHE2048_P_BIN` | [`PSA_DH_FAMILY_RFC7919`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga7be917e67fe4a567fb36864035822ff7) | 2048 |
| `MBEDTLS_DHM_RFC7919_FFDHE3072_P_BIN` | [`PSA_DH_FAMILY_RFC7919`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga7be917e67fe4a567fb36864035822ff7) | 3072 |
| `MBEDTLS_DHM_RFC7919_FFDHE4096_P_BIN` | [`PSA_DH_FAMILY_RFC7919`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga7be917e67fe4a567fb36864035822ff7) | 4096 |
| `MBEDTLS_DHM_RFC7919_FFDHE6144_P_BIN` | [`PSA_DH_FAMILY_RFC7919`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga7be917e67fe4a567fb36864035822ff7) | 6144 |
| `MBEDTLS_DHM_RFC7919_FFDHE8192_P_BIN` | [`PSA_DH_FAMILY_RFC7919`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga7be917e67fe4a567fb36864035822ff7) | 8192 |

A finite-field Diffie-Hellman key can be used for key agreement with the algorithm [`PSA_ALG_FFDH`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga0ebbb6f93a05b6511e6f108ffd2d1eb4).

### Creating keys for asymmetric cryptography

The easiest way to create a key pair object is by randomly generating it with [`psa_generate_key`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__random/#group__random_1ga1985eae417dfbccedf50d5fff54ea8c5). Compared with the low-level functions from the legacy API (`mbedtls_rsa_gen_key`, `mbedtls_ecp_gen_privkey`, `mbedtls_ecp_gen_keypair`, `mbedtls_ecp_gen_keypair_base`, `mbedtls_ecdsa_genkey`), this directly creates an object that can be used with high-level APIs, but removes some of the flexibility. Note that if you want to export the generated private key, you must pass the flag [`PSA_KEY_USAGE_EXPORT`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__policy/#group__policy_1ga7dddccdd1303176e87a4d20c87b589ed) to [`psa_set_key_usage_flags`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__attributes/#group__attributes_1ga42a65b3c4522ce9b67ea5ea7720e17de); exporting the public key with [`psa_export_public_key`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__import__export/#group__import__export_1gaf22ae73312217aaede2ea02cdebb6062) is always permitted.

For RSA keys, `psa_generate_key` always uses 65537 as the public exponent. If you need a different public exponent, use the legacy interface to create the key then import it as described in “[Importing legacy keys via the PK module](#importing-legacy-keys-via-the-pk-module)”.

To create a key object from existing material, use [`psa_import_key`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__import__export/#group__import__export_1ga0336ea76bf30587ab204a8296462327b). While this function has the same basic goal as the PK parse functions (`mbedtls_pk_parse_key`, `mbedtls_pk_parse_public_key`, `mbedtls_pk_parse_subpubkey`), it is limited to a single format that just contains the number(s) that make up the key, with very little metadata. This format is a substring of the formats accepted by the PK functions (except for finite-field Diffie-Hellman which the PK module does not support). The table below summarizes the PSA import/export format for key pairs and public keys; see the documentation of [`psa_export_key`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__import__export/#group__import__export_1ga668e35be8d2852ad3feeef74ac6f75bf) and [`psa_export_public_key`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__import__export/#group__import__export_1gaf22ae73312217aaede2ea02cdebb6062) for more details.

| Key type | PSA import/export format |
| -------- | ------------------------ |
| RSA key pair | PKCS#1 RSAPrivateKey DER encoding (including both private exponent and CRT parameters) |
| RSA public key | PKCS#1 RSAPublicKey DER encoding |
| ECC key pair | Fixed-length private value (not containing the public key) |
| ECC public key (Weierstrass curve) | Fixed-length uncompressed point |
| ECC public key (Montgomery curve) | Fixed-length public value |
| FFDH key pair | Fixed-length private value (not containing the public key) |
| FFDH public key | Fixed-length public value |

There is no equivalent of `mbedtls_pk_parse_keyfile` and `mbedtls_pk_parse_public_keyfile`. Either call the legacy function or load the file data manually.

A future extension of the PSA API will support other import formats. Until those are implemented, see the following subsections for ways to use the PK module for key parsing and construct a PSA key object from the PK object.

#### Importing legacy keys via the PK module

You can use glue functions in the PK module to create a key object using the legacy API, then import that object into the PSA subsystem. This is useful for use cases that the PSA API does not currently cover, such as:

* Parsing a key in a format with metadata without knowing its type ahead of time.
* Importing a key which you have in the form of a list of numbers, rather than the binary encoding required by `psa_import_key`.
* Importing a key with less information than what the PSA API needs, for example an ECC public key in compressed format, an RSA private key without the private exponent, or an RSA private key without the CRT parameters.
* Generating an RSA key with $e \ne 65537$.

#### Importing a PK key by wrapping

If you have a PK object, you can call `mbedtls_pk_wrap_as_opaque` to create a PSA key object with the same key material. (This function is only present in builds with `MBEDTLS_USE_PSA_CRYPTO` enabled. It is experimental and [will likely be replaced by a slightly different interface in a future version of Mbed TLS](https://github.com/Mbed-TLS/mbedtls/issues/7760)) This function automatically determines the PSA key type, and lets you specify the usage policy (see “[Public-key cryptography policies](#public-key-cryptography-policies)”). Once you've called this function, you can destroy the PK object. This function calls `psa_import_key` internally; call [`psa_destroy_key`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__key__management/#group__key__management_1ga5f52644312291335682fbc0292c43cd2) to destroy the PSA key object once your application no longer needs it. Common scenarios where this flow is useful are:

* You have working code that's calling `mbedtls_pk_parse_key`, `mbedtls_pk_parse_public_key`, `mbedtls_pk_parse_subpubkey`, `mbedtls_pk_parse_keyfile` or `mbedtls_pk_parse_public_keyfile` to create a PK object.
* You have working code that's using the `rsa.h` or `ecp.h` API to create a key object, and there is no PSA equivalent.

You can use this flow to import an RSA key via a `mbedtls_rsa_context` object or an ECC key via a `mbedtls_ecp_keypair` object:

1. Call `mbedtls_pk_init` then `mbedtls_pk_setup` to set up a PK context for the desired key type (`MBEDTLS_PK_RSA` or `MBEDTLS_PK_ECKEY`).
2. Call `mbedtls_pk_rsa` or `mbedtls_pk_ec` to obtain the underlying low-level context.
3. Call `mbedtls_rsa_xxx` or `mbedtls_ecp_xxx` functions to construct the desired key. For example:
    * `mbedtls_rsa_import` or `mbedtls_rsa_import_raw` followed by `mbedtls_rsa_complete` to create an RSA private key without all the parameters required by the PSA API.
    * `mbedtls_rsa_gen_key` to generate an RSA private key with a custom public exponent.
4. Call `mbedtls_pk_wrap_as_opaque` as described above to create a corresponding PSA key object.
5. Call `mbedtls_pk_free` to free the resources associated with the PK object.

#### Importing a PK key by export-import

This section explains how to export a PK object in the PSA import format. The process depends on the key type. You can use `mbedtls_pk_get_type` or `mbedtls_pk_can_do` to distinguish between RSA and ECC keys. The snippets below assume that the key is in an `mbedtls_pk_context pk`, and omit error checking.

For an RSA private key:

```
unsigned char buf[PSA_EXPORT_KEY_PAIR_MAX_SIZE];
size_t length = mbedtls_pk_write_key_der(&pk, buf, sizeof(buf));
psa_key_attributes_t attributes = PSA_KEY_ATTRIBUTES_INIT;
psa_set_key_attributes(&attributes, PSA_KEY_TYPE_RSA_KEY_PAIR);
psa_set_key_usage_flags(&attributes, PSA_KEY_USAGE_... | ...);
psa_set_key_algorithm(&attributes, PSA_ALGORITHM_...);
psa_key_id_t key_id = 0;
psa_import_key(&attributes, buf + sizeof(buf) - length, length, &key_id);
mbedtls_pk_free(&pk);
```

For an ECC private key (a future version of Mbed TLS [will provide a function to calculate the curve family](https://github.com/Mbed-TLS/mbedtls/issues/7764)):

```
unsigned char buf[PSA_BITS_TO_BYTES(PSA_VENDOR_ECC_MAX_CURVE_BITS)];
size_t length = PSA_BITS_TO_BYTES(mbedtls_pk_bitlen(&pk));
mbedtls_ecp_keypair *ec = mbedtls_pk_ec(&pk);
mbedtls_ecp_write_key(ec, buf, length);
psa_ecc_curve_t curve = ...; // need to determine the curve family manually
psa_key_attributes_t attributes = PSA_KEY_ATTRIBUTES_INIT;
psa_set_key_attributes(&attributes, PSA_KEY_TYPE_ECC_KEY_PAIR(curve));
psa_set_key_usage_flags(&attributes, PSA_KEY_USAGE_... | ...);
psa_set_key_algorithm(&attributes, PSA_ALGORITHM_...);
psa_key_id_t key_id = 0;
psa_import_key(&attributes, buf, length, &key_id);
mbedtls_pk_free(&pk);
```

For an RSA or ECC public key:

```
unsigned char buf[PSA_EXPORT_PUBLIC_KEY_MAX_SIZE];
size_t length = mbedtls_pk_write_pubkey(&pk, buf, sizeof(buf));
psa_key_attributes_t attributes = PSA_KEY_ATTRIBUTES_INIT;
psa_set_key_attributes(&attributes, ...); // need to determine the type manually
psa_set_key_usage_flags(&attributes, PSA_KEY_USAGE_... | ...);
psa_set_key_algorithm(&attributes, PSA_ALGORITHM_...);
psa_key_id_t key_id = 0;
psa_import_key(&attributes, buf + sizeof(buf) - length, length, &key_id);
mbedtls_pk_free(&pk);
```

#### Importing an elliptic curve key from ECP

This section explains how to use the `ecp.h` API to create an elliptic curve key in a format suitable for `psa_import_key`.

You can use this, for example, to import an ECC key in the form of a compressed point by calling `mbedtls_ecp_point_read_binary` then following the process below.

The following code snippet illustrates how to import a private key which is initially in an `mbedtls_ecp_keypair` object. Error checks are omitted for simplicity. A future version of Mbed TLS [will provide a function to calculate the curve family](https://github.com/Mbed-TLS/mbedtls/issues/7764).

```
mbedtls_ecp_keypair ec;
mbedtls_ecp_keypair_init(&ec);
// Omitted: fill ec with key material
// (the public key will not be used and does not need to be set)
unsigned char buf[PSA_BITS_TO_BYTES(PSA_VENDOR_ECC_MAX_CURVE_BITS)];
size_t length = PSA_BITS_TO_BYTES(mbedtls_pk_bitlen(&pk));
mbedtls_ecp_write_key(&ec, buf, length);
psa_ecc_curve_t curve = ...; // need to determine the curve family manually
psa_key_attributes_t attributes = PSA_KEY_ATTRIBUTES_INIT;
psa_set_key_attributes(&attributes, PSA_KEY_TYPE_ECC_KEY_PAIR(curve));
psa_set_key_usage_flags(&attributes, PSA_KEY_USAGE_... | ...);
psa_set_key_algorithm(&attributes, PSA_ALGORITHM_...);
psa_key_id_t key_id = 0;
psa_import_key(&attributes, buf, length, &key_id);
mbedtls_ecp_keypair_free(&ec);
```
The following code snippet illustrates how to import a private key which is initially in an `mbedtls_ecp_keypair` object. Error checks are omitted for simplicity.

```
mbedtls_ecp_group grp;
mbedtls_ecp_group_init(&grp);
mbedtls_ecp_group_load(&grp, MBEDTLS_ECP_DP_...);
mbedtls_ecp_point pt;
mbedtls_ecp_point_init(&pt);
// Omitted: fill pt with key material
unsigned char buf[PSA_BITS_TO_BYTES(PSA_VENDOR_ECC_PUBLIC_KEY_MAX_SIZE)];
size_t length;
mbedtls_ecp_point_write_binary(&grp, &pt, &length, buf, sizeof(buf));
psa_ecc_curve_t curve = ...; // need to determine the curve family manually
psa_key_attributes_t attributes = PSA_KEY_ATTRIBUTES_INIT;
psa_set_key_attributes(&attributes, PSA_KEY_TYPE_ECC_PUBLIC_KEY(curve));
psa_set_key_usage_flags(&attributes, PSA_KEY_USAGE_... | ...);
psa_set_key_algorithm(&attributes, PSA_ALGORITHM_...);
psa_key_id_t key_id = 0;
psa_import_key(&attributes, buf, length, &key_id);
mbedtls_ecp_point_free(&pt);
mbedtls_ecp_group_free(&grp);
```

### Key pair and public key metadata

There is no equivalent to the type `mbedtls_pk_info_t` and the functions `mbedtls_pk_info_from_type` in the PSA API because it is unnecessary. All macros and functions operate directly on key type values (`psa_key_type_t`, `PSA_KEY_TYPE_xxx` constants) and algorithm values (`psa_algorithm_t`, `PSA_ALG_xxx` constants).

You can call [`psa_get_key_attributes`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__attributes/#group__attributes_1gacbbf5c11eac6cd70c87ffb936e1b9be2) to populate a structure with the attributes of a key, then functions such as [`psa_get_key_type`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__attributes/#group__attributes_1gae4fb812af4f57aa1ad85e335a865b918) and [`psa_get_key_bits`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__attributes/#group__attributes_1ga5bee85c2164ad3d4c0d42501241eeb06) to obtain a key's type (`PSA_KEY_TYPE_xxx` value) and size (nominal size in bits).

The bit-size from `psa_get_key_bits` is the same as the one from `mbedtls_pk_get_bitlen`. To convert to bytes as with `mbedtls_pk_get_len` or `mbedtls_rsa_get_len`, you can use the macro `PSA_BITS_TO_BYTES`; however note that the PSA API has generic macros for each related buffer size (export, signature size, etc.), so you should generally use those instead. The present document lists those macros where it explains the usage of the corresponding function.

Most uses of `mbedtls_pk_get_type` and `mbedtls_pk_can_do` only require knowing a key's type as reported by [`psa_get_key_type`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__attributes/#group__attributes_1gae4fb812af4f57aa1ad85e335a865b918). If needed, you can also access a key's policy from its attributes, with [`psa_get_key_usage_flags`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__attributes/#group__attributes_1gaa1af20f142ca722222c6d98678a0c448), [`psa_get_key_algorithm`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__attributes/#group__attributes_1gac255da850a00bbed925390044f016b34) and [`psa_get_key_enrollment_algorithm`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__attributes/#group__attributes_1ga39803b62a97198cf630854db9b53c588). The algorithm policy also conveys the padding and hash information provided by `mbedtls_rsa_get_padding_mode` and `mbedtls_rsa_get_md_alg`.

### Exporting a public key or a key pair

To export a PSA key pair or public key, call [`psa_export_key`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__import__export/#group__import__export_1ga668e35be8d2852ad3feeef74ac6f75bf). If the key is a key pair, its policy must allow `PSA_KEY_USAGE_EXPORT` (see “[Public-key cryptography policies](#public-key-cryptography-policies)”).

To export a PSA public key or to export the public key of a PSA key pair object, call [`psa_export_public_key`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__import__export/#group__import__export_1gaf22ae73312217aaede2ea02cdebb6062). This is always permitted regardless of the key's policy.

The export format is the same format used for `psa_import_key`, described in “[Creating keys for asymmetric cryptography](#creating-keys-for-asymmetric-cryptography)” above.

A future extension of the PSA API will support other export formats. Until those are implemented, see the following subsections for ways to use the PK module to format a PSA key.

#### Exporting a PK key by wrapping

You can wrap a PSA key object in a PK key context with `mbedtls_pk_setup_opaque`. This allows you to call functions such as `mbedtls_pk_write_key_der`, `mbedtls_pk_write_pubkey_der`, `mbedtls_pk_write_pubkey_pem`, `mbedtls_pk_write_key_pem` or `mbedtls_pk_write_pubkey` to export the key data in various formats.

### Signature operations

The equivalent of `mbedtls_pk_sign`, `mbedtls_rsa_pkcs1_sign`, `mbedtls_rsa_rsassa_pkcs1_v15_sign`, `mbedtls_rsa_rsassa_pss_sign` or `mbedtls_rsa_rsassa_pss_sign_ext` to sign an already calculated hash is [`psa_sign_hash`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__asymmetric/#group__asymmetric_1ga785e746a31a7b2a35ae5175c5ace3c5c).
The key must be a key pair allowing the usage `PSA_KEY_USAGE_SIGN_HASH` (see “[Public-key cryptography policies](#public-key-cryptography-policies)”).
Use [`PSA_SIGN_OUTPUT_SIZE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#c.PSA_SIGN_OUTPUT_SIZE) or [`PSA_SIGNATURE_MAX_SIZE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#c.PSA_SIGNATURE_MAX_SIZE) (similar to `MBEDTLS_PK_SIGNATURE_MAX_SIZE`) to determine the size of the output buffer.

The equivalent of `mbedtls_pk_verify` or `mbedtls_pk_verify_ext` to verify an already calculated hash is [`psa_verify_hash`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__asymmetric/#group__asymmetric_1gae2ffbf01e5266391aff22b101a49f5f5).
The key must be a public key or a key pair allowing the usage `PSA_KEY_USAGE_VERIFY_HASH` (see “[Public-key cryptography policies](#public-key-cryptography-policies)”).

Generally, `psa_sign_hash` and `psa_verify_hash` require the input to have the correct length for the hash (this has historically not always been enforced in the corresponding legacy APIs).

See also “[Restartable ECDSA signature](#restartable-ecdsa-signature)” for a restartable variant of this API.

The PSA API also has functions [`psa_sign_message`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__asymmetric/#group__asymmetric_1ga963ecadae9c38c85826f9a13cf1529b9) and [`psa_verify_message`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__asymmetric/#group__asymmetric_1ga01c11f480b185a4268bebd013df7c14c). These functions combine the hash calculation with the signature calculation or verification.
For `psa_sign_message`, either the usage flag `PSA_KEY_USAGE_SIGN_MESSAGE` or `PSA_KEY_USAGE_SIGN_HASH` is sufficient.
For `psa_verify_message`, either the usage flag `PSA_KEY_USAGE_VERIFY_MESSAGE` or `PSA_KEY_USAGE_VERIFY_HASH` is sufficient.

Most signature algorithms involve a hash algorithm. See “[Hash mechanism selection](#hash-mechanism-selection)”.

The following subsections describe the PSA signature mechanisms that correspond to legacy Mbed TLS mechanisms.

#### ECDSA signature

In the PSA API, **the format of an ECDSA signature is the raw fixed-size format. This is different from the legacy API** which uses the ASN.1 DER format for ECDSA signatures. A future version of Mbed TLS [will provide a way to convert between the two formats](https://github.com/Mbed-TLS/mbedtls/issues/7765).
<!-- No equivalent because related to the DER format: MBEDTLS_ECDSA_MAX_SIG_LEN, MBEDTLS_ECDSA_MAX_LEN -->

This is the mechanism provided by `mbedtls_pk_sign` and `mbedtls_pk_verify` for ECDSA keys, and by `mbedtls_ecdsa_sign`, `mbedtls_ecdsa_sign_det_ext`, `mbedtls_ecdsa_write_signature`, `mbedtls_ecdsa_write_signature` and `mbedtls_ecdsa_verify`, .

The PSA API offers three algorithm constructors for ECDSA. They differ only for signature, and have exactly the same behavior for verification.

* [`PSA_ALG_ECDSA(hash)`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga7e3ce9f514a227d5ba5d8318870452e3) is a randomized ECDSA signature of a hash calculated with the algorithm `hash`.
* [`PSA_ALG_ECDSA_ANY`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga51d6b6044a62e33cae0cf64bfc3b22a4) is equivalent to `PSA_ALG_ECDSA`, but does not require specifying a hash as part of the algorithm. It can only be used with `psa_sign_hash` and `psa_verify_hash`, with no constraint on the length of the hash.
* [`PSA_ALG_DETERMINISTIC_ECDSA(hash)`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga11da566bcd341661c8de921e2ca5ed03) is a deterministic ECDSA signature of a hash calculated with the algorithm `hash`. This is the same as the functionality offered by `MBEDTLS_ECDSA_DETERMINISTIC` in the legacy API.
    * For `psa_sign_message` with `PSA_ALG_DETERMINISTIC_ECDSA`, the same hash algorithm is used to hash the message and to parametrize the deterministic signature generation.

Unlike the legacy API, where `mbedtls_pk_sign` and `mbedtls_ecdsa_write_signature` automatically select deterministic ECDSA if both are available, the PSA API requires the application to select the preferred variant. ECDSA verification cannot distinguish between randomized and deterministic ECDSA (except in so far as if the same message is signed twice and the signatures are different, then at least one of the signatures is not the determinstic variant), so in most cases switching between the two is a compatible change.

#### Restartable ECDSA signature

There is a PSA API for interruptible public-key operations, offering similar functionality to the legacy restartable API (`mbedtls_pk_sign_restartable`, `mbedtls_pk_verify_restartable`, `mbedtls_ecdsa_sign_restartable`, `mbedtls_ecdsa_verify_restartable`, `mbedtls_ecdsa_write_signature_restartable`, `mbedtls_ecdsa_read_signature_restartable`).

As of Mbed TLS 3.5, it is only implemented for ECDSA, for the same curves as the legacy API; this will likely be extended to ECDH in the short term. At the time of writing, no extension is planned to other curves or other algorithms.

The flow of operations for an interruptible signature operation is as follows:

1. Create an operation object of type [`psa_sign_hash_interruptible_operation_t`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__interruptible__hash/#group__interruptible__hash_1ga6948d4653175b1b530a265540066a7e7) and zero-initialize it (or use the corresponding `INIT` macro).
2. Call [`psa_sign_hash_start`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__interruptible__hash/#group__interruptible__hash_1ga441988da830205182b3e791352537fac) with the private key object and the hash to verify.
3. Call [`psa_sign_hash_complete`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__interruptible__hash/#group__interruptible__hash_1ga79849aaa7004a85d2ffbc4b658a333dd) repeatedly until it returns a status other than `PSA_OPERATION_INCOMPLETE`.

The flow of operations for an interruptible signature verification operation is as follows:

1. Create an operation object of type [`psa_verify_hash_interruptible_operation_t`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__interruptible__hash/#group__interruptible__hash_1ga537054cf4909ad1426331ae4ce7148bb) and zero-initialize it (or use the corresponding `INIT` macro).
2. Call [`psa_verify_hash_start`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__interruptible__hash/#group__interruptible__hash_1ga912eb51fb94056858f451f276ee289cb) with the private key object and the hash and signature to verify.
3. Call [`psa_verify_hash_complete`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__interruptible__hash/#group__interruptible__hash_1ga67fe82352bc2f8c0343e231a70a5bc7d) repeatedly until it returns a status other than `PSA_OPERATION_INCOMPLETE`.

If you need to interrupt the operation after calling the start function without waiting for the complete function to return a success or failure status, call [`psa_sign_hash_abort`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__interruptible__hash/#group__interruptible__hash_1gae893a4813aa8e03bd201fe4f1bbbb403) or [`psa_verify_hash_abort`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__interruptible__hash/#group__interruptible__hash_1ga18dc9c0cc27d590c5e3b186094d90f88).

Call [`psa_interruptible_set_max_ops`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__interruptible__hash/#group__interruptible__hash_1ga6d86790b31657c13705214f373af869e) to set the number of basic operations per call. This is the same unit as `mbedtls_ecp_set_max_ops`.

#### PKCS#1 v1.5 RSA signature

This mechanism corresponds to `mbedtls_pk_sign`, `mbedtls_pk_verify`, `mbedtls_rsa_pkcs1_sign` and `mbedtls_rsa_pkcs1_verify` for an RSA key, unless PSS has been selected with `mbedtls_rsa_set_padding` on the underlying RSA key context. This mechanism also corresponds to `mbedtls_rsa_rsassa_pkcs1_v15_sign` and `mbedtls_rsa_rsassa_pkcs1_v15_verify`.

The PSA API has two algorithm constructors:

* [`PSA_ALG_RSA_PKCS1V15_SIGN(hash)`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga702ff75385a6ae7d4247033f479439af) formats the hash as specified in PKCS#1. The hash algorithm corresponds to the `md_alg` parameter of the legacy functions.
* [`PSA_ALG_RSA_PKCS1V15_SIGN_RAW`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga4215e2a78dcf834e9a625927faa2a817) uses the “hash” input in lieu of a DigestInfo structure. This is the same as calling the legacy functions with `md_alg=MBEDTLS_MD_NONE`.

#### PKCS#1 RSASSA-PSS signature

This mechanism corresponds to `mbedtls_pk_sign_ext` and `mbedtls_pk_verify_ext` for an RSA key, as well as `mbedtls_pk_sign`, `mbedtls_pk_verify`, `mbedtls_rsa_pkcs1_sign` and `mbedtls_rsa_pkcs1_verify` if PSS has been selected on the underlying RSA context with `mbedlts_rsa_set_padding`.
It also corresponds to `mbedtls_rsa_rsassa_pss_sign` and `mbedtls_rsa_rsassa_pss_sign_ext`, `mbedtls_rsa_rsassa_pss_verify` and `mbedtls_rsa_rsassa_pss_verify_ext`.

The PSA API has two algorithm constructors: [`PSA_ALG_RSA_PSS(hash)`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga62152bf4cb4bf6aace5e1be8f143564d) and [`PSA_ALG_RSA_PSS_ANY_SALT(hash)`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga9b7355a2cd6bde88177634d539127f2b). The hash algorithm `hash` corresponds to the `md_alg` parameter passed to the legacy API. It is used to hash the message, to create the salted hash, and for the mask generation with MGF1. The PSA API does not support using different hash algorithms for these different purposes.

With respect to the salt length:

* When signing, the salt is random, and the salt length is the largest possible salt length up to the hash length. This is the same as passing `MBEDTLS_RSA_SALT_LEN_ANY` as the salt length to `xxx_ext` legacy functions or using a legacy function that does not have a `saltlen` argument.
* When verifying, `PSA_ALG_RSA_PSS` requires the the salt length to the largest possible salt length up to the hash length (i.e. the same that would be used for signing).
* When verifying, `PSA_ALG_RSA_PSS_ANY_SALT` accepts any salt length. This is the same as passing `MBEDTLS_RSA_SALT_LEN_ANY` as the salt length to `xxx_ext` legacy functions or using a legacy function that does not have a `saltlen` argument.

### Asymmetric encryption and decryption

The equivalent of `mbedtls_pk_encrypt`, `mbedtls_rsa_pkcs1_encrypt`, `mbedtls_rsa_rsaes_pkcs1_v15_encrypt` or `mbedtls_rsa_rsaes_oaep_encrypt` to encrypt a short message (typically a symmetric key) is [`psa_asymmetric_encrypt`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__asymmetric/#group__asymmetric_1gaa17f61e4ddafd1823d2c834b3706c290).
The key must be a public key allowing the usage `PSA_KEY_USAGE_ENCRYPT` (see “[Public-key cryptography policies](#public-key-cryptography-policies)”).
Use the macro [`PSA_ASYMMETRIC_ENCRYPT_OUTPUT_SIZE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#crypto__sizes_8h_1a66ba3bd93e5ec52870ccc3848778bad8) or [`PSA_ASYMMETRIC_ENCRYPT_OUTPUT_MAX_SIZE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#c.PSA_ASYMMETRIC_ENCRYPT_OUTPUT_MAX_SIZE) to determine the output buffer size.

The equivalent of `mbedtls_pk_decrypt`, `mbedtls_rsa_pkcs1_decrypt`, `mbedtls_rsa_rsaes_pkcs1_v15_decrypt` or `mbedtls_rsa_rsaes_oaep_decrypt` to decrypt a short message (typically a symmetric key) is [`psa_asymmetric_decrypt`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__asymmetric/#group__asymmetric_1ga4f968756f6b22aab362b598b202d83d7).
The key must be a key pair allowing the usage `PSA_KEY_USAGE_DECRYPT` (see “[Public-key cryptography policies](#public-key-cryptography-policies)”).
Use the macro [`PSA_ASYMMETRIC_DECRYPT_OUTPUT_SIZE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#crypto__sizes_8h_1a61a246f3eac41989821d982e56fea6c1) or [`PSA_ASYMMETRIC_DECRYPT_OUTPUT_MAX_SIZE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__sizes_8h/#c.PSA_ASYMMETRIC_DECRYPT_OUTPUT_MAX_SIZE) to determine the output buffer size.

The following subsections describe the PSA asymmetric encryption mechanisms that correspond to legacy Mbed TLS mechanisms.

#### RSA PKCS#1v1.5 encryption

This is the mechanism used by the PK functions and by `mbedtls_rsa_pkcs1_{encrypt,decrypt}` unless `mbedtls_rsa_set_padding` has been called on the underlying RSA key context.
This is also the mechanism used by `mbedtls_rsa_rsaes_pkcs1_v15_{encrypt,decrypt}`.

The PSA algorithm is [`PSA_ALG_RSA_PKCS1V15_CRYPT`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1ga4c540d3abe43fb9abcb94f2bc51acef9).

Beware that PKCS#1v1.5 decryption is subject to padding oracle attacks. Revealing when `psa_asymmetric_decrypt` returns `PSA_ERROR_INVALID_PADDING` may allow an adversary to decrypt arbitrary ciphertexts.

#### RSA RSAES-OAEP

This is the mechanism used by `mbedtls_rsa_rsaes_oaep_{encrypt,decrypt}`.

The PSA algorithm is [`PSA_ALG_RSA_OAEP(hash)`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__crypto__types/#group__crypto__types_1gaa1235dc3fdd9839c6c1b1a9857344c76) where `hash` is a hash algorithm value (`PSA_ALG_xxx`, see “[Hash mechanism selection](#hash-mechanism-selection)”).

As with the PK API, the mask generation is MGF1, the label is empty, and the same hash algorithm is used for MGF1 and to hash the label. The PSA API does not offer a way to choose a different label or a different hash algorithm for the label.

### PK functionality with no PSA equivalent

There is no PSA equivalent of the debug functionality provided by `mbedtls_pk_debug`. Use `psa_export_key` to export the key if desired.

There is no PSA equivalent to Mbed TLS's custom key type names exposed by `mbedtls_pk_get_name`.

The PSA API does not expose partially constructed key objects. This makes the following functions unnecessary:

* `mbedtls_rsa_copy`, `mbedtls_ecp_copy`: a PSA key object is immutable, so the copy would have to be identical.
* `mbedtls_pk_check_pair`, `mbedtls_rsa_check_privkey`, `mbedtls_rsa_check_pubkey`, `mbedtls_rsa_check_pub_priv`,`mbedtls_ecp_check_privkey`, `mbedtls_ecp_check_pubkey`, `mbedtls_ecp_check_pub_priv`: if a key has been constructed successfully, it is guaranteed to be valid.

### Key agreement

_(Section not written yet)_

<!-- TODO: ecdh.h, dhm.h -->

### Additional information about Elliptic-curve cryptography

_(Section not written yet)_

<!-- TODO: ecp.h -->

#### ECC functionality with no PSA equivalent

There is no PSA equivalent of `mbedtls_ecdsa_can_do` to query the capabilities of a curve at runtime. Check the documentation of each curve family to see what algorithms it supports.

There is no PSA equivalent to the types `mbedtls_ecdsa_context` and `mbedtls_ecdsa_restart_ctx`, and to basic ECDSA context manipulation functions including `mbedtls_ecdsa_from_keypair`, because they are not needed: the PSA API does not have ECDSA-specific context types.

### Additional information about RSA

#### RSA-ALT interface

Implementers of the RSA-ALT interface (`MBEDTLS_PK_RSA_ALT` pk type, `mbedtls_pk_setup_rsa_alt` setup function) should migrate to the [PSA cryptoprocessor driver interface](https://github.com/Mbed-TLS/mbedtls/blob/development/docs/psa-driver-example-and-guide.md).

* If the purpose of the ALT interface is acceleration only: use the accelerator driver interface. This is fully transparent to application code.
* If the purpose of the ALT interface is to isolate the private key in a high-security environment: use the opaque driver interface. This is mostly transparent to user code. Code that uses a key via its key identifier does not need to know whether the key is transparent (equivalent of `MBEDTLS_PK_RSA`) or opaque (equivalent of `MBEDTLS_PK_RSA_ALT`). When creating a key, it will be transparent by default; to create an opaque key, call [`psa_set_key_lifetime`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__attributes/#group__attributes_1gac03ccf09ca6d36cc3d5b43f8303db6f7) to set the key's location to the chosen location value for the driver, e.g.
    ```
    psa_set_key_lifetime(&attributes, PSA_KEY_LIFETIME_FROM_PERSISTENCE_AND_LOCATION(
        PSA_KEY_PERSISTENCE_VOLATILE, MY_RSA_DRIVER_LOCATION));
    ```

The PSA subsystem uses its internal random generator both for randomized algorithms and to generate blinding value. As a consequence, none of the API functions take an RNG parameter.

#### RSA functionality with no PSA equivalent

The PSA API does not provide direct access to the exponentiation primitive as with `mbedtls_rsa_public` and `mbedtls_rsa_private`. If you need an RSA-based mechanism that is not supported by the PSA API, please contact us so that we can extend the API to support it.

The PSA API does not support constructing RSA keys progressively from numbers with `mbedtls_rsa_import` or `mbedtls_rsa_import_raw` followed by `mbedtls_rsa_complete`. See “[Importing a PK key by wrapping](#importing-a-pk-key-by-wrapping)”.

There is no direct equivalent of `mbedtls_rsa_export`, `mbedtls_rsa_export_raw` and `mbedtls_rsa_export_crt` to export some of the numbers in a key. You can export the whole key with `psa_export_key`, or with `psa_export_public_key` to export the public key from a key pair object. See also “[Exporting a public key or a key pair](#exporting-a-public-key-or-a-key-pair)”.

### PK format support interfaces

The interfaces in `base64.h`, `asn1.h`, `asn1write.h`, `oid.h` and `pem.h` are intended to support X.509 and key file formats. They have no PSA equivalent since they are not directly about cryptography. They remain unchanged in Mbed TLS 3.x. In the future, they are likely to move out of the cryptography part of Mbed TLS and into the public-key/X.509 part.

## EC-JPAKE

The PSA API exposes EC-JPAKE via the algorithm [`PSA_ALG_JPAKE`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/crypto__extra_8h/#c.PSA_ALG_JPAKE) and the PAKE API functions. At the time of writing, the PAKE API is still experimental, but it should offer the same functionality as the legacy `ecjpake.h`. Please consult the documentation of your version of Mbed TLS for more information.