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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?
  2. General considerations

Then use the 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.

Reference: The PSA Crypto API specification is available online. Mbed TLS implements a large subset of the specification which is documented in the psa/crypto*.h headers.

Additional resources

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 (secure enclave, secure element, HSM, etc.).
  • PSA APIs have consistent 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 an 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 is enabled. (It is enabled in the default configuration.)

You should probably enable MBEDTLS_USE_PSA_CRYPTO as well (it is disabled by default). This option causes the PK, X.509 and TLS modules to use PSA crypto under the hood.

By default, the PSA crypto API offers a similar set of cryptographic mechanisms as those offered by the legacy API (configured by MBEDTLS_XXX macros). The PSA crypto API also has its own configuration mechanism; see “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 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.

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 an 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: PSA_SUCCESS == 0 for success, or a negative value PSA_ERROR_xxx indicating an error.

Memory management

Apart from keys, as described in “Key management” below, APIs that need to preserve state between function calls store this state in a structure allocated by the calling code. For example, multipart operations store state in a multipart operation object.

All PSA operation objects must be zero-initialized (or equivalently, initialized with the provided PSA_XXX_INIT macro or psa_xxx_init() function) before calling any API function.

Functions that output data require an output buffer of sufficient size. For all PSA crypto API functions that have an output buffer, there is a corresponding macro, generally called PSA_XXX_OUTPUT_SIZE, that calculates a sufficient size for the output buffer, given the relevant parameters. In some cases, there may be macros with less precision which can be resolved at compile time. For example, for the size of a buffer containing a hash, you can use PSA_HASH_LENGTH(hash_alg) where hash_alg is a specific hash algorithm, or PSA_HASH_MAX_SIZE for a buffer that is long enough for any supported hash. See the relevant sections of this document and of the reference documentation for more details.

Key management

One of the major differences between the legacy API and the PSA API is that in the PSA API, access to keys is indirect. Operations that require a key take a parameter of type psa_key_id_t, which is an identifier for the key. This allows the API to be used with keys that are not directly accessible to the application, for example because they are stored in a secure environment that does not allow the key material to be exported.

To use a key:

  1. First create a key object with a key creation function. The two most common ones are psa_import_key if you have the key material available and psa_generate_key to create a random key. The key creation function has the key identifier as an output parameter.
  2. Use the key as desired, passing the key identifier obtained during the key creation.
  3. Finally destroy the key object with psa_destroy_key.

See “Cipher key management”, “MAC key management”, “Key lifecycle for asymmetric cryptography”, “Creating keys for asymmetric cryptography” and “Diffie-Hellman key pair management” for more details about key management in specific workflows, including information about choosing the key's attributes.

If you need access to the key material, call psa_export_key. If you need the public key corresponding to a key pair object, call psa_export_public_key.

Note that a key consumes a key store entry, which is distinct from heap memory, until it is destroyed or the application exits. (This is not true for persistent keys, which instead consume disk space. Since persistent keys have no analog in the legacy API, we will not discuss them further in this document.)

Summary of API modules

Header Function prefix PSA equivalent
aes.h mbedtls_aes_ Symmetric encryption
aria.h mbedtls_aria_ Symmetric encryption
asn1.h mbedtls_asn1_ No change (PK support interface)
asn1write.h mbedtls_asn1_write_ No change (PK support interface)
base64.h mbedtls_base64_ No change (PK support interface)
bignum.h mbedtls_mpi_ None (no low-level arithmetic)
build_info.h MBEDTLS_ No change (not a crypto API)
camellia.h mbedtls_camellia_ Symmetric encryption
ccm.h mbedtls_ccm_ Symmetric encryption, Authenticated cipher operations
chacha20.h mbedtls_chacha20_ Symmetric encryption
chachapoly.h mbedtls_chachapoly_ Symmetric encryption, Authenticated cipher operations
check_config.h N/A No public APIs (internal support header)
cipher.h mbedtls_cipher_ Symmetric encryption
cmac.h mbedtls_cipher_cmac_ Hashes and MAC, 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_ct_ Constant-time functions
ctr_drbg.h mbedtls_ctr_drbg_ Random generation interface, Deterministic pseudorandom generation
debug.h mbedtls_debug_ No change (not a crypto API)
des.h mbedtls_des_ Symmetric encryption
dhm.h mbedtls_dhm_ Asymmetric cryptography
ecdh.h mbedtls_ecdh_ Asymmetric cryptography
ecdsa.h mbedtls_ecdsa_ Asymmetric cryptography
ecjpake.h mbedtls_ecjpake_ EC-JPAKE
ecp.h mbedtls_ecp_ Asymmetric cryptography
entropy.h mbedtls_entropy_ Random generation interface, Entropy sources
error.h mbedtls_*err* Error messages
gcm.h mbedtls_gcm_ Symmetric encryption, Authenticated cipher operations
hkdf.h mbedtls_hkdf_ HKDF
hmac_drbg.h mbedtls_hmac_drbg_ Random generation interface, Deterministic pseudorandom generation
lms.h mbedtls_lms_ No change (LMS signatures)
mbedtls_config.h MBEDTLS_ Compile-time configuration
md.h mbedtls_md_ Hashes and MAC
md5.h mbedtls_md5_ 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_ Migration path not yet defined
oid.h mbedtls_oid_ No change (PK support interface)
pem.h mbedtls_pem_ No change (PK support interface)
pk.h mbedtls_pk_ Asymmetric cryptography
pkcs5.h mbedtls_pkcs5_ PKCS#5 module
pkcs7.h mbedtls_pkcs7_ No change (not a crypto API)
pkcs12.h mbedtls_pkcs12_ PKCS#12 module
platform.h mbedtls_platform_ No change (not a crypto API)
platform_time.h mbedtls_*time* No change (not a crypto API)
platform_util.h mbedtls_platform_ No change (not a crypto API)
poly1305.h mbedtls_poly1305_ None (but there is Chacha20-Poly1305 AEAD)
private_access.h N/A No public APIs (internal support header)
psa_util.h N/A No public APIs (internal support header)
ripemd160.h mbedtls_ripemd160_ Hashes and MAC
rsa.h mbedtls_rsa_ Asymmetric cryptography
sha1.h mbedtls_sha1_ Hashes and MAC
sha3.h mbedtls_sha3_ Hashes and MAC
sha256.h mbedtls_sha256_ Hashes and MAC
sha512.h mbedtls_sha512_ 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_ciphersuite_ 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 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, and you can set MBEDTLS_PSA_CRYPTO_USER_CONFIG_FILE 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_ symbol 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_WANT_KEY_TYPE_xxx_KEY_PAIR_BASIC enables basic support for the key type, and in particular support for operations with a key of that type for enabled algorithms. This is automatically enabled if any of the other PSA_WANT_KEY_TYPE_xxx_KEY_PAIR_yyy options are enabled.
    • PSA_WANT_KEY_TYPE_xxx_KEY_PAIR_IMPORT enables support for psa_import_key to import a key of that type.
    • PSA_WANT_KEY_TYPE_xxx_KEY_PAIR_GENERATE enables support for psa_generate_key to randomly generate a key of that type.
    • PSA_WANT_KEY_TYPE_xxx_KEY_PAIR_DERIVE enables support for psa_key_derivation_output_key to deterministically derive a key of that type.
    • PSA_WANT_KEY_TYPE_xxx_KEY_PAIR_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 non-zero 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_BASIC, 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 normally not include it. This allows builds that use few features to have a small code size. 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.

Optimization options

When PSA Crypto mechanisms are implemented by the built-in code from Mbed TLS, the legacy optimization options (e.g. MBEDTLS_SHA256_SMALLER, MBEDTLS_ECP_WINDOW_SIZE, etc.) apply to the PSA implementation as well (they invoke the same code under the hood).

The PSA Crypto API may use accelerator drivers. In this case any options controlling the driver behavior are driver-specific.

Alternative implementations (MBEDTLS_xxx_ALT options)

In the Mbed TLS legacy interface, you can replace some cryptographic primitives and modes by an alternative implementation, by enabling configuration options of the form MBEDTLS_xxx_ALT and linking with your own implementation of the affected function or module. Alternative implementations remain supported in Mbed TLS 3.x even if the application code uses the PSA API. However, they will be removed from the next version of the library.

The corresponding PSA feature is accelerator drivers. To implement an accelerator driver, see the PSA cryptoprocessor driver example and guide. In an application that uses both the legacy interface and the PSA interface for the same mechanism, only some algorithms support calling a PSA driver from the legacy interface. See the Guide to driver-only builds for more information.

Self-tests

There is currently no PSA equivalent to the self-tests enabled by MBEDTLS_SELF_TEST.

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 a programmer-friendly 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 than 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”.
  • The AEAD decryption functions take care of verifying the tag. See “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.

For modes that are based on a block cipher, the key type encodes the choice of block cipher: PSA_KEY_TYPE_AES, PSA_KEY_TYPE_ARIA, PSA_KEY_TYPE_CAMELLIA, PSA_KEY_TYPE_DES. The algorithm encodes the mode and if relevant the padding type:

For the ChaCha20 unauthenticated cipher, use PSA_KEY_TYPE_CHACHA20 with PSA_ALG_STREAM_CIPHER. For the Chacha20+Poly1305 AEAD, use PSA_KEY_TYPE_CHACHA20 with PSA_ALG_CHACHA20_POLY1305

Cipher mechanism availability

For each key type value PSA_KEY_TYPE_xxx, the symbol PSA_WANT_KEY_TYPE_xxx is defined with a non-zero 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 non-zero 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, PSA_AEAD_NONCE_MAX_SIZE
MBEDTLS_MAX_BLOCK_LENGTH 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, 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. You need to set the following parameters:
  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 to the key object.
  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 after all operations with that key are finished.

Unauthenticated cipher operations

Recall the workflow 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 workflow with the PSA API is to call a single function:

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

  1. Create an operation object of type psa_cipher_operation_t and zero-initialize it (or use the corresponding INIT macro).
  2. Select the key and algorithm with psa_cipher_encrypt_setup or psa_cipher_decrypt_setup depending on the desired direction.
  3. When encrypting with a random IV, use psa_cipher_generate_iv. When encrypting with a chosen IV, or when decrypting, set the IV with psa_cipher_set_iv. Skip this step with ECB since it does not use an IV.
  4. Call psa_cipher_update as many times as needed. You can use PSA_CIPHER_UPDATE_OUTPUT_SIZE or PSA_CIPHER_UPDATE_OUTPUT_MAX_SIZE to determine a sufficient size for the output buffer.
  5. Call psa_cipher_finish to obtain the last part of the output. You can use PSA_CIPHER_FINISH_OUTPUT_SIZE or PSA_CIPHER_FINISH_OUTPUT_MAX_SIZE to determine a sufficient size for the output buffer.

If you need to interrupt the operation after calling the setup function without calling the finish function, call psa_cipher_abort.

Authenticated cipher operations

Recall the workflow of an authenticated cipher operation in the legacy Mbed TLS cipher API (or similar workflows 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:

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

  1. Create an operation object of type psa_aead_operation_t and zero-initialize it (or use the corresponding INIT macro).
  2. Select the key and algorithm with psa_aead_encrypt_setup or psa_aead_decrypt_setup depending on the desired direction.
  3. When encrypting with a random nonce, use psa_aead_generate_nonce. When encrypting with a chosen nonce, or when decrypting, set the nonce with psa_aead_set_nonce. If the algorithm is CCM, you must also call psa_aead_set_lengths before or after setting the nonce (for other algorithms, this is permitted but not needed).
  4. Call psa_aead_update_ad as many times as needed.
  5. Call psa_aead_update as many times as needed. You can use PSA_AEAD_UPDATE_OUTPUT_SIZE or PSA_AEAD_UPDATE_OUTPUT_MAX_SIZE to determine a sufficient size for the output buffer.
  6. Finally:

If you need to interrupt the operation after calling the setup function without calling the finish or verify function, call psa_aead_abort.

Miscellaneous cipher operation management

The equivalent of mbedtls_cipher_reset is to call psa_cipher_abort or psa_aead_abort. 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 not have a direct interface for the AES-CMAC-PRF-128 algorithm from RFC 4615 calculated by mbedtls_aes_cmac_prf_128 at the time of writing. You can implement it using the MAC interface with an AES key and the CMAC algorithm.

Hash mechanism selection

The equivalent to mbedtls_md_type_t and MBEDTLS_MD_XXX 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

The following helper functions can be used to convert between the 2 types:

  • mbedtls_md_psa_alg_from_type() converts from legacy mbedtls_md_type_t to PSA's psa_algorithm_t.
  • mbedtls_md_type_from_psa_alg() converts from PSA's psa_algorithm_t to legacy mbedtls_md_type_t.

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 and a key type value of type psa_key_type_t.

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 non-zero 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 non-zero 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
MBEDTLS_MD_MAX_BLOCK_SIZE PSA_HMAC_MAX_HASH_BLOCK_SIZE
mbedtls_md_get_size PSA_HASH_LENGTH
mbedtls_md_get_size_from_type 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. In addition, to compare the hash of a message with an expected value, you can call psa_hash_compare 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 and zero-initialize it (or use the corresponding INIT macro).
  2. Call psa_hash_setup to specify the algorithm.
  3. Call psa_hash_update as many times as necessary.
  4. To obtain the hash, call psa_hash_finish. Alternatively, to verify the hash against an expected value, call psa_hash_verify.

If you need to interrupt the operation after calling the setup function without calling the finish or verify function, call psa_hash_abort.

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. You need to set the following parameters:
  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 to the key object.
  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 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 and zero-initialize it (or use the corresponding INIT macro).
  2. Call psa_mac_sign_setup to specify the algorithm and the key. See “MAC key management” for how to obtain a key identifier.
  3. Call psa_mac_update as many times as necessary.
  4. To obtain the MAC, call psa_mac_sign_finish.

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

  1. Create an operation object of type psa_mac_operation_t and zero-initialize it (or use the corresponding INIT macro).
  2. Call psa_mac_verify_setup to specify the algorithm and the key. See “MAC key management” for how to obtain a key identifier.
  3. Call psa_mac_update as many times as necessary.
  4. To verify the MAC against an expected value, call psa_mac_verify_finish.

If you need to interrupt the operation after calling the setup function without calling the finish function, call psa_mac_abort.

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

In both cases, see “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 or psa_mac_abort. 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. 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. 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 and zero-initialize it (or use the corresponding INIT macro).
  2. Call psa_key_derivation_setup to select the algorithm, which is a value of type psa_algorithm_t. For HKDF and variants, use one of the macros PSA_ALG_HKDF, PSA_ALG_HKDF_EXTRACT or PSA_ALG_HKDF_EXPAND with the hash algorithm passed as an argument. For example PSA_ALG_HKDF(PSA_ALG_SHA_256) selects HKDF-SHA-256.
  3. Call psa_key_derivation_input_bytes on each of the inputs in the order listed below. (Use psa_key_derivation_input_key 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 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 for the secret input.
    3. PSA_KEY_DERIVATION_INPUT_INFO for the info string used during the expansion step. Omit this step for HKDF-Extract.
  4. Call psa_key_derivation_output_bytes 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 instead if you want to use a chunk as a PSA key.
  5. Call psa_key_derivation_abort 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. 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 and zero-initialize it (or use the corresponding INIT macro).
  2. Call psa_key_derivation_setup to select the algorithm, which is a value of type psa_algorithm_t. For PBKDF2-HMAC, select PSA_ALG_PBKDF2_HMAC(hash) where hash is the underlying hash algorithm (see “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 on each of the inputs in the order listed below. (Use psa_key_derivation_input_key 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 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 for the password.
  5. Call psa_key_derivation_output_bytes 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 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. (Note that the verify functions are not yet present in the 3.5 release of Mbed TLS. They are expected to be released in version 3.6.0.)
  6. Call psa_key_derivation_abort to free the resources associated with the key derivation object.

The function mbedtls_pkcs5_pbes2 is only intended 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_pbe are only intended as support functions 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.h and hmac_drbg.h. See the next sections for remaining use cases for entropy and DRBG.

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.

If your application mixes uses of the PSA crypto API and the mbedtls API and you need to pass an RNG argument to a legacy or X.509/TLS function, include the header file <mbedtls/psa_util.h> and use:

You can remove the Mbed TLS RNG boilerplate (mbedtls_entropy_init, mbedtls_ctr_drbg_init, mbedtls_ctr_drbg_seed, mbedtls_ctr_drbg_random, mbedtls_ctr_drbg_free, mbedtls_entropy_free — or hmac_drbg equivalents of the ctr_drbg functions) once you have finished replacing the references to mbedtls_ctr_drbg_random (or mbedtls_hmac_drbg_random) by mbedtls_psa_get_random.

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 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”), elliptic curve cryptography (see “ECC mechanism selection” and “EC-JPAKE”) and finite-field Diffie-Hellman (see “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. (Some documentation references mbedtls_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 an 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. You need to set the following parameters:
  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 to the key object.
  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 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 those flags (combined with bitwise-or) to psa_set_key_usage_flags.

Usage Flag
export public key 0 (always permitted)
export private key PSA_KEY_USAGE_EXPORT
Sign a message directly PSA_KEY_USAGE_SIGN_MESSAGE
Sign an already-calculated hash at least one of PSA_KEY_USAGE_SIGN_MESSAGE or PSA_KEY_USAGE_SIGN_HASH
Verify a message directly PSA_KEY_USAGE_VERIFY_MESSAGE
Verify an already-calculated hash at least one of PSA_KEY_USAGE_VERIFY_MESSAGE or PSA_KEY_USAGE_VERIFY_HASH
Encryption PSA_KEY_USAGE_ENCRYPT
Decryption PSA_KEY_USAGE_DECRYPT
Key agreement PSA_KEY_USAGE_DERIVE

The sections “RSA mechanism selection”, “Elliptic curve mechanism selection” and “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 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 algorithms in each operation.
  • In addition to the algorithm (or wildcard) selected with psa_set_key_algorithm, you can use psa_set_key_enrollment_algorithm 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” for more information about MBEDTLS_PK_RSA_ALT.

The PSA API uses policies and algorithm parameters rather than key types to distinguish between RSA-based mechanisms. The PSA algorithm selection corresponds to the mbedtls_pk_type_t value passed to mbedtls_pk_{sign,verify}_ext. It 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.

An RSA key pair has the type PSA_KEY_TYPE_RSA_KEY_PAIR. 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:

Elliptic curve mechanism selection

The PK types MBEDTLS_PK_ECKEY, MBEDTLS_PK_ECKEY_DH and MBEDTLS_PK_ECDSA correspond to elliptic-curve 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) where curve is a curve family identifier.

An ECC key pair has the type PSA_KEY_TYPE_ECC_KEY_PAIR(curve) 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. You can use the following functions to convert between the PSA and legacy elliptic curve designations:

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 192
MBEDTLS_ECP_DP_SECP224R1 PSA_ECC_FAMILY_SECP_R1 224
MBEDTLS_ECP_DP_SECP256R1 PSA_ECC_FAMILY_SECP_R1 256
MBEDTLS_ECP_DP_SECP384R1 PSA_ECC_FAMILY_SECP_R1 384
MBEDTLS_ECP_DP_SECP521R1 PSA_ECC_FAMILY_SECP_R1 521
MBEDTLS_ECP_DP_BP256R1 PSA_ECC_FAMILY_BRAINPOOL_P_R1 256
MBEDTLS_ECP_DP_BP384R1 PSA_ECC_FAMILY_BRAINPOOL_P_R1 384
MBEDTLS_ECP_DP_BP512R1 PSA_ECC_FAMILY_BRAINPOOL_P_R1 512
MBEDTLS_ECP_DP_CURVE25519 PSA_ECC_FAMILY_MONTGOMERY 255
MBEDTLS_ECP_DP_SECP192K1 PSA_ECC_FAMILY_SECP_K1 192
MBEDTLS_ECP_DP_SECP224K1 not supported N/A
MBEDTLS_ECP_DP_SECP256K1 PSA_ECC_FAMILY_SECP_K1 256
MBEDTLS_ECP_DP_CURVE448 PSA_ECC_FAMILY_MONTGOMERY 448

The following cryptographic algorithms work with ECC keys:

Diffie-Hellman mechanism selection

A finite-field Diffie-Hellman key pair has the type PSA_KEY_TYPE_DH_KEY_PAIR(group) where group is a group family as explained below.

A finite-field Diffie-Hellman public key has the type PSA_KEY_TYPE_DH_PUBLIC_KEY(group) where group is a group family as explained below. Due to the design of the API, there is rarely a need to use Diffie-Hellman public key objects.

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 2048
MBEDTLS_DHM_RFC7919_FFDHE3072_P_BIN PSA_DH_FAMILY_RFC7919 3072
MBEDTLS_DHM_RFC7919_FFDHE4096_P_BIN PSA_DH_FAMILY_RFC7919 4096
MBEDTLS_DHM_RFC7919_FFDHE6144_P_BIN PSA_DH_FAMILY_RFC7919 6144
MBEDTLS_DHM_RFC7919_FFDHE8192_P_BIN PSA_DH_FAMILY_RFC7919 8192

A finite-field Diffie-Hellman key can be used for key agreement with the algorithm PSA_ALG_FFDH.

Creating keys for asymmetric cryptography

The easiest way to create a key pair object is by randomly generating it with psa_generate_key. 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 to psa_set_key_usage_flags; exporting the public key with psa_export_public_key is always permitted.

For RSA keys, psa_generate_key uses 65537 as the public exponent. You can use psa_generate_key_custom to select a different public exponent. As of Mbed TLS 3.6.1, selecting a different public exponent is only supported with the built-in RSA implementation, not with PSA drivers.

To create a key object from existing material, use psa_import_key. 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), but only supports a single format that just contains the number(s) that make up the key, with very little metadata. The table below summarizes the PSA import/export format for key pairs and public keys; see the documentation of psa_export_key and psa_export_public_key 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 subsection for how to use the PK module for key parsing and construct a PSA key object from the PK object.

Creating a PSA key via PK

You can use the PK module as an intermediate step to create an RSA or ECC key for use with PSA. 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.
  • Parsing a key in a format that the PK module supports, but psa_import_key doesn't.
  • 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 a compressed format, an RSA private key without the private exponent, or an RSA private key without the CRT parameters.

For such use cases:

  1. First create a PK object with the desired key material.
  2. Call mbedtls_pk_get_psa_attributes to fill PSA attributes corresponding to the PK key. Pass one of the following values as the usage parameter:
    • PSA_KEY_USAGE_SIGN_HASH or PSA_KEY_USAGE_SIGN_MESSAGE for a key pair used for signing.
    • PSA_KEY_USAGE_DECRYPT for a key pair used for decryption.
    • PSA_KEY_USAGE_DERIVE for a key pair used for key agreement.
    • PSA_KEY_USAGE_VERIFY_HASH or PSA_KEY_USAGE_VERIFY_MESSAGE for a public key pair used for signature verification.
    • PSA_KEY_USAGE_ENCRYPT for a key pair used for encryption.
  3. Optionally, tweak the attributes (this is rarely necessary). For example:
  4. Call mbedtls_pk_import_into_psa to import the key into the PSA key store.
  5. You can now free the PK object with mbedtls_pk_free.

Here is some sample code illustrating the above process, with error checking omitted.

mbedtls_pk_context pk;
mbedtls_pk_init(&pk);
mbedtls_pk_parse_key(&pk, key_buffer, key_buffer_length, NULL, 0,
                     mbedtls_psa_get_random, MBEDTLS_PSA_RANDOM_STATE);
psa_key_attributes_t attributes = PSA_KEY_ATTRIBUTES_INIT;
mbedtls_pk_get_psa_attributes(&pk, PSA_KEY_USAGE_SIGN_HASH, &attributes);
psa_key_id_t key_id;
mbedtls_pk_import_into_psa(&pk, &attributes, &key_id);
mbedtls_pk_free(&pk);
psa_sign_hash(key_id, ...);

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. (This includes mbedtls_ecdsa_keypair objects since that is just a type alias.) Error checks are omitted for simplicity. A future version of Mbed TLS will provide a function to calculate the curve family.

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;
mbedtls_ecp_write_key_ext(&ec, &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_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 to populate a structure with the attributes of a key, then functions such as psa_get_key_type and psa_get_key_bits 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 mbedtls_pk_get_len or mbedtls_rsa_get_len do, 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 code that calls mbedtls_pk_get_type or mbedtls_pk_can_do only requires the key's type as reported by psa_get_key_type. For code that uses both mbedtls_pk_context objects and PSA metadata encoding, mbedtls_pk_can_do_ext checks the compatibility between a key object and a mechanism. If needed, you can also access a key's policy from its attributes with psa_get_key_usage_flags, psa_get_key_algorithm and psa_get_key_enrollment_algorithm. 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. If the key is a key pair, its policy must allow PSA_KEY_USAGE_EXPORT (see “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. 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” above.

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

Exposing a PSA key via PK

This section discusses how to use a PSA key in a context that requires a PK object, such as PK formatting functions (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), Mbed TLS X.509 functions, Mbed TLS SSL functions, or another API that involves mbedtls_pk_context objects. The PSA key must be an RSA or ECC key since the PK module does not support DH keys. Three functions from pk.h help with that:

  • mbedtls_pk_copy_from_psa copies a PSA key into a PK object. The PSA key must be exportable. The PK object remains valid even if the PSA key is destroyed.
  • mbedtls_pk_copy_public_from_psa copies the public part of a PSA key into a PK object. The PK object remains valid even if the PSA key is destroyed.
  • mbedtls_pk_setup_opaque sets up a PK object that wraps the PSA key. This functionality is only available when MBEDTLS_USE_PSA_CRYPTO is enabled. The PK object has the type MBEDTLS_PK_OPAQUE regardless of whether the key is an RSA or ECC key. The PK object can only be used as permitted by the PSA key's policy. The PK object contains a reference to the PSA key identifier, therefore PSA key must not be destroyed as long as the PK object remains alive.

Here is some sample code illustrating how to use the PK module to format a PSA public key or the public key of a PSA key pair.

int write_psa_pubkey(psa_key_id_t key_id,
                     unsigned char *buf, size_t size, size_t *len) {
    mbedtls_pk_context pk;
    mbedtls_pk_init(&pk);
    int ret = mbedtls_pk_copy_public_from_psa(key_id, &pk);
    if (ret != 0) goto exit;
    ret = mbedtls_pk_write_pubkey_der(&pk, buf, size);
    if (ret < 0) goto exit;
    *len = ret;
    memmove(buf, buf + size - ret, ret);
    ret = 0;
exit:
    mbedtls_pk_free(&pk);
}

Signature operations

The equivalent of mbedtls_pk_sign or mbedtls_pk_sign_ext to sign an already calculated hash is psa_sign_hash. The key must be a key pair allowing the usage PSA_KEY_USAGE_SIGN_HASH (see “Public-key cryptography policies”). Use PSA_SIGN_OUTPUT_SIZE or PSA_SIGNATURE_MAX_SIZE (similar to MBEDTLS_PK_SIGNATURE_MAX_SIZE) to determine a sufficient size for the output buffer. This is also the equivalent of the type-specific functions mbedtls_rsa_pkcs1_sign, mbedtls_rsa_rsassa_pkcs1_v15_sign, mbedtls_rsa_rsassa_pss_sign, mbedtls_rsa_rsassa_pss_sign_ext, mbedtls_ecdsa_sign, mbedtls_ecdsa_sign_det_ext and mbedtls_ecdsa_write_signature. Note that the PSA API uses the raw format for ECDSA signatures, not the ASN.1 format; see “ECDSA signature” for more details.

The equivalent of mbedtls_pk_verify or mbedtls_pk_verify_ext to verify an already calculated hash is psa_verify_hash. The key must be a public key (or a key pair) allowing the usage PSA_KEY_USAGE_VERIFY_HASH (see “Public-key cryptography policies”). This is also the equivalent of the type-specific functions mbedtls_rsa_pkcs1_verify, mbedtls_rsa_rsassa_pkcs1_v15_verify, mbedtls_rsa_rsassa_pss_verify, mbedtls_rsa_rsassa_pss_verify_ext, mbedtls_ecdsa_verify and mbedtls_ecdsa_read_signature. Note that the PSA API uses the raw format for ECDSA signatures, not the ASN.1 format; see “ECDSA signature” for more details.

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” for a restartable variant of this API.

The PSA API also has functions psa_sign_message and psa_verify_message. 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”.

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

ECDSA signature

Note: 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. To convert between the two formats, use mbedtls_ecdsa_raw_to_der or mbedtls_ecdsa_der_to_raw.

ECDSA is the mechanism provided by mbedtls_pk_sign and mbedtls_pk_verify for ECDSA keys, as well as by mbedtls_ecdsa_sign, mbedtls_ecdsa_sign_det_ext, mbedtls_ecdsa_write_signature, mbedtls_ecdsa_verify and mbedtls_ecdsa_read_signature.

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) is a randomized ECDSA signature of a hash calculated with the algorithm hash.
  • PSA_ALG_ECDSA_ANY 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) 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

The legacy API includes an API for “restartable” ECC operations: the operation returns after doing partial computation, and can be resumed. This is intended for highly constrained devices where long cryptographic calculations need to be broken up to poll some inputs, where interrupt-based scheduling is not desired. The legacy API consists of the functions 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 well as several configuration and data manipulation functions.

The PSA API offers similar functionality via “interruptible” public-key operations. 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 and zero-initialize it (or use the corresponding INIT macro).
  2. Call psa_sign_hash_start with the private key object and the hash to verify.
  3. Call psa_sign_hash_complete 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 and zero-initialize it (or use the corresponding INIT macro).
  2. Call psa_verify_hash_start with the private key object and the hash and signature to verify.
  3. Call psa_verify_hash_complete repeatedly until it returns a status other than PSA_OPERATION_INCOMPLETE.

If you need to cancel the operation after calling the start function without waiting for the loop calling the complete function to finish, call psa_sign_hash_abort or psa_verify_hash_abort.

Call psa_interruptible_set_max_ops to set the number of basic operations per call. This is the same unit as mbedtls_ecp_set_max_ops. You can retrieve the current value with psa_interruptible_get_max_ops. The value is PSA_INTERRUPTIBLE_MAX_OPS_UNLIMITED if operations are not restartable, which corresponds to mbedtls_ecp_restart_is_enabled() being false.

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) 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 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) and PSA_ALG_RSA_PSS_ANY_SALT(hash). They differ only for verification, and have exactly the same behavior for signature. 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. The key must be a public key (or a key pair) allowing the usage PSA_KEY_USAGE_ENCRYPT (see “Public-key cryptography policies”). Use the macro PSA_ASYMMETRIC_ENCRYPT_OUTPUT_SIZE or 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. The key must be a key pair allowing the usage PSA_KEY_USAGE_DECRYPT (see “Public-key cryptography policies”). Use the macro PSA_ASYMMETRIC_DECRYPT_OUTPUT_SIZE or 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.

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) where hash is a hash algorithm value (PSA_ALG_xxx, see “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.

Private-public key consistency

There is no direct equivalent of the functions mbedtls_rsa_check_privkey, mbedtls_rsa_check_pubkey,mbedtls_ecp_check_privkey, mbedtls_ecp_check_pubkey. The PSA API performs some basic checks when it imports a key, and may perform additional checks before performing an operation if needed, so it will never perform an operation on a key that does not satisfy these checks, but the details of when the check is performed may change between versions of the library.

The legacy API provides functions mbedtls_pk_check_pair, mbedtls_rsa_check_pub_priv and mbedtls_ecp_check_pub_priv, which can be used to check the consistency between a private key and a public key. To perform such a check with the PSA API, you can export the public keys; this works because the PSA representation of public keys is canonical.

  • Prepare a key object containing the private key, for example with psa_import_key.
  • Prepare a key object containing the public key, for example with psa_import_key.
  • Export both public keys with psa_export_public_key (this is possible regardless of the usage policies on the keys) and compare the output. 
    // Error checking omitted
unsigned char pub1[PSA_EXPORT_PUBLIC_KEY_MAX_SIZE];
unsigned char pub2[PSA_EXPORT_PUBLIC_KEY_MAX_SIZE];
size_t length1, length2;
psa_export_public_key(key1, pub1, sizeof(pub1), &length1);
psa_export_public_key(key2, pub2, sizeof(pub2), &length2);
if (length1 == length2 && !memcmp(pub1, pub2, length1))
    puts("The keys match");
else
    puts("The keys do not match");

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.

Key agreement

The PSA API has a generic interface for key agreement, covering the main use of both ecdh.h and dhm.h.

Diffie-Hellman key pair management

The PSA API manipulates keys as such, rather than via an operation context. Thus, to use Diffie-Hellman, you need to create a key object, then perform the key exchange, then destroy the key. There is no equivalent to the types mbedtls_ecdh_context and mbedtls_dhm_context.

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. You need to set the following parameters:
  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 to the key object.
  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 after all operations with that key are finished.

Performing a key agreement

Call psa_export_public_key to obtain the public key that needs to be sent to the other party. Use the macros PSA_EXPORT_PUBLIC_KEY_OUTPUT_SIZE or PSA_EXPORT_PUBLIC_KEY_MAX_SIZE to determine a sufficient size for the output buffer.

Call psa_raw_key_agreement to calculate the shared secret from your private key and the other party's public key. Use the macros PSA_RAW_KEY_AGREEMENT_OUTPUT_SIZE or PSA_RAW_KEY_AGREEMENT_OUTPUT_MAX_SIZE to determine a sufficient size for the output buffer.

Call psa_key_derivation_key_agreement instead of psa_raw_key_agreement to use the resulting shared secret as the secret input to a key derivation. See “HKDF” for an example of the key derivation interface.

Translating a legacy key agreement contextless workflow

A typical workflow for ECDH using the legacy API without a context object is:

  1. Initialize objects:
    • mbedtls_ecp_group grp for the curve;
    • mbedtls_mpi our_priv for our private key;
    • mbedtls_ecp_point our_pub for our public key;
    • mbedtls_ecp_point their_pub for their public key (this may be the same variable as our_pub if the application does not need to hold both at the same time);
    • mbedtls_mpi z for the shared secret (this may be the same variable as our_priv when doing ephemeral ECDH).
  2. Call mbedtls_ecp_group_load on grp to select the curve.
  3. Call mbedtls_ecdh_gen_public on grp, our_priv (output) and our_pub (output) to generate a key pair and retrieve the corresponding public key.
  4. Send our_pub to the peer. Retrieve the peer's public key and import it into their_pub. These two actions may be performed in either order.
  5. Call mbedtls_ecdh_compute_shared on grp, z (output), their_pub and our_priv. Use the raw shared secret z, typically, to construct a shared key.
  6. Free grp, our_priv, our_pub, their_pub and z.

The corresponding workflow with the PSA API is as follows:

  1. Initialize objects:
  2. Prepare an attribute structure as described in “Diffie-Hellman key pair management”, in particular selecting the curve with psa_set_key_type.
  3. Call psa_generate_key on attributes and our_key (output) to generate a key pair, then psa_export_public_key on our_key and our_pub (output) to obtain our public key.
  4. Send our_pub to the peer. Retrieve the peer's public key and import it into their_pub. These two actions may be performed in either order.
  5. Call psa_raw_key_agreement on our_key, their_pub and shared_secret (output).
    Alternatively, call psa_key_derivation_key_agreement to use the shared secret directly in a key derivation operation (see “Performing a key agreement”).
  6. Call psa_destroy_key on key_id, and free the memory buffers.

Steps 4–6 are only performed once for a "true" ephemeral Diffie-Hellman. They may be repeated multiple times for a "fake ephemeral" Diffie-Hellman where the same private key is used for multiple key exchanges, but it not saved.

Translating a legacy ephemeral key agreement TLS server workflow

The legacy API offers the following workflow for an ephemeral Diffie-Hellman key agreement in a TLS 1.2 server. The PSA version of this workflow can also be used with other protocols, on the side of the party that selects the curve or group and sends its public key first.

  1. Setup phase:
    1. Initialize a context of type mbedtls_ecdh_context or mbedtls_dhm_context with mbedtls_ecdh_init or mbedtls_dhm_init.
    2. Call mbedtls_ecdh_setup or mbedtls_dhm_set_group to select the curve or group.
    3. Call mbedtls_ecdh_make_params or mbedtls_dhm_make_params to generate our key pair and obtain a TLS ServerKeyExchange message encoding the selected curve/group and our public key.
  2. Send the ServerKeyExchange message to the peer.
  3. Retrieve the peer's public key.
  4. Call mbedtls_ecdh_read_public or mbedtls_dhm_read_public on the peer's public key, then call mbedtls_ecdh_calc_secret or mbedtls_dhm_calc_secret to calculate the shared secret.
  5. Free the context with mbedtls_ecdh_free or mbedtls_dhm_free.

The corresponding workflow with the PSA API is as follows:

  1. Setup phase:
    1. Generate an ECDH or DHM key pair with psa_generate_key as described in “Diffie-Hellman key pair management”.
    2. Call psa_export_public_key to obtain our public key.
    3. Format a ServerKeyExchange message containing the curve/group selection and our public key.
  2. Send the ServerKeyExchange message to the peer.
  3. Retrieve the peer's public key.
  4. Call psa_raw_key_agreement on our_key, their_pub and shared_secret (output).
    Alternatively, call psa_key_derivation_key_agreement to use the shared secret directly in a key derivation operation (see “Performing a key agreement”).
  5. Call psa_destroy_key to free the resources associated with our key pair.

Translating a legacy ephemeral key agreement TLS client workflow

The legacy API offers the following workflow for an ephemeral Diffie-Hellman key agreement in a TLS 1.2 client. The PSA version of this workflow can also be used with other protocols, on the side of the party that receives a message indicating both the choice of curve or group, and the peer's public key.

  1. Upon reception of a TLS ServerKeyExchange message received from the peer, which encodes the selected curve/group and the peer's public key:
    1. Initialize a context of type mbedtls_ecdh_context or mbedtls_dhm_context with mbedtls_ecdh_init or mbedtls_dhm_init.
    2. Call mbedtls_ecdh_read_params or mbedtls_dhm_read_params to input the data from the ServerKeyExchange message.
  2. Call mbedtls_ecdh_make_public or mbedtls_dh_make_public to generate our private key and export our public key.
  3. Send our public key to the peer.
  4. Call mbedtls_ecdh_calc_secret or mbedtls_dhm_calc_secret to calculate the shared secret.
  5. Free the context with mbedtls_ecdh_free or mbedtls_dhm_free.

The corresponding workflow with the PSA API is as follows:

  1. Upon reception of a TLS ServerKeyExchange message received from the peer, which encodes the selected curve/group and the peer's public key:
    1. Decode the selected curve/group and use this to determine a PSA key type (PSA_KEY_TYPE_ECC_KEY_PAIR(curve) or PSA_KEY_TYPE_DH_KEY_PAIR(group)), a key size and an algorithm.
  2. Generate an ECDH or DHM key pair with psa_generate_key as described in “Diffie-Hellman key pair management”. Call psa_export_public_key to obtain our public key.
  3. Send our public key to the peer.
  4. Call psa_raw_key_agreement on our_key, their_pub and shared_secret (output).
    Alternatively, call psa_key_derivation_key_agreement to use the shared secret directly in a key derivation operation (see “Performing a key agreement”).
  5. Call psa_destroy_key to free the resources associated with our key pair.

ECDH and DHM metadata functions

You can obtain data and metadata from an ECDH key agreement through the PSA API as follows:

  • With either side, accessing the group: call psa_get_key_attributes on the key identifier, then psa_get_key_type and psa_get_key_bits to obtain metadata about the key.
  • Accessing our public key: call psa_export_public_key on the PSA key identifier.
  • Accessing our private key: call psa_export_key on the key identifier. Note that the key policy must allow PSA_KEY_USAGE_EXPORT (see “Public-key cryptography policies”).
  • Accessing the peer's public key: there is no PSA equivalent since the PSA API only uses the peer's public key to immediately calculate the shared secret. If your application needs the peer's public key for some other purpose, store it separately.

The functions mbedtls_dhm_get_bitlen, mbedtls_dhm_get_len and mbedtls_dhm_get_value allow the caller to obtain metadata about the keys used for the key exchange. The PSA equivalents access the key identifier:

  • mbedtls_dhm_get_bitlen, mbedtls_dhm_get_len: call psa_get_key_attributes on the PSA key identifier, then psa_get_key_bits.
  • mbedtls_dhm_get_value for MBEDTLS_DHM_PARAM_X (our private key): call psa_export_key on the key identifier. Note that the key policy must allow PSA_KEY_USAGE_EXPORT (see “Public-key cryptography policies”).
  • mbedtls_dhm_get_value for MBEDTLS_DHM_PARAM_GX (our public key): call psa_export_public_key on the PSA key identifier.
  • mbedtls_dhm_get_value for MBEDTLS_DHM_PARAM_GY (peer's public key): the there is no PSA equivalent since the PSA API only uses the peer's public key to immediately calculate the shared secret. If your application needs the peer's public key for some other purpose, store it separately.
  • mbedtls_dhm_get_value for MBEDTLS_DHM_PARAM_K (shared secret): this is the value calculated by psa_raw_key_agreement or psa_key_derivation_key_agreement. If you need to use it multiple times (for example to derive multiple values independently), call psa_raw_key_agreement and make a copy.
  • mbedtls_dhm_get_value for MBEDTLS_DHM_PARAM_P or MBEDTLS_DHM_PARAM_G (group parameters): there is no PSA API to retrieve these values.

The PSA API for finite-field Diffie-Hellman only supports predefined groups. Therefore there is no equivalent to mbedtls_dhm_parse_dhm, mbedtls_dhm_parse_dhmfile, and the MBEDTLS_DHM_xxx_BIN macros.

Restartable key agreement

Restartable key agreement (enabled by mbedtls_ecdh_enable_restart) is not yet available through the PSA API. It will be added under the name “interruptible key agreement” in a future version of the library, with an interface that's similar to the interruptible signature interface described in “Restartable ECDSA signature”.

Additional information about Elliptic-curve cryptography

Information about a curve

The legacy API identifies a curve by an MBEDTLS_ECP_DP_xxx value of type mbedtls_ecp_group_id. The PSA API identifies a curve by a PSA_ECC_FAMILY_xxx value and the private value's bit-size. See “Elliptic curve mechanism selection” for the correspondence between the two sets of values.

There is no PSA equivalent of the mbedtls_ecp_group data structure (and so no equivalent to mbedtls_ecp_group_init, mbedtls_ecp_group_load, mbedtls_ecp_group_copy and mbedtls_ecp_group_free) or of the mbedtls_ecp_curve_info data structure (and so no equivalent to mbedtls_ecp_curve_info_from_grp_id) because they are not needed. All API elements identify the curve directly by its family and size.

The bit-size used by the PSA API is the size of the private key. For most curves, the PSA bit-size, the bit_size field in mbedtls_ecp_curve_info, the nbits field in mbedtls_ecp_group and the pbits field in mbedtls_ecp_group are the same. The following table lists curves for which they are different.

Curve grp->nbits grp->pbits curve_info->bit_size PSA bit-size
secp224k1 225 224 224 not supported
Curve25519 253 255 256 255
Curve448 446 448 448 448

There is no exact PSA equivalent of the type mbedtls_ecp_curve_type and the function mbedtls_ecp_get_type, but the curve family encodes the same information. PSA_ECC_FAMILY_MONTGOMERY is the only Montgomery family. All other families supported in Mbed TLS 3.4.0 are short Weierstrass families.

There is no PSA equivalent for the following functionality:

  • The name field of mbedtls_ecp_curve_info, and the function mbedtls_ecp_curve_info_from_name. There is no equivalent of Mbed TLS's lookup based on the name used for the curve in TLS specifications.
  • The tls_id field of mbedtls_ecp_curve_info, the constant MBEDTLS_ECP_TLS_NAMED_CURVE, and the functions mbedtls_ecp_curve_info_from_tls_id, mbedtls_ecp_tls_read_group, mbedtls_ecp_tls_read_group_id and mbedtls_ecp_tls_write_group. The PSA crypto API does not have this dedicated support for the TLS protocol.
  • Retrieving the parameters of a curve from the fields of an mbedtls_ecp_group structure.

Information about supported curves

The PSA API does not currently have a discovery mechanism for cryptographic mechanisms (although one may be added in the future). Thus there is no equivalent for MBEDTLS_ECP_DP_MAX and the functions mbedtls_ecp_curve_list and mbedtls_ecp_grp_id_list.

The API provides macros that give the maximum supported sizes for various kinds of objects. The following table lists equivalents for MBEDTLS_ECP_MAX_xxx macros.

Legacy macro PSA equivalent
MBEDTLS_ECP_MAX_BITS PSA_VENDOR_ECC_MAX_CURVE_BITS
MBEDTLS_ECP_MAX_BYTES PSA_BITS_TO_BYTES(PSA_VENDOR_ECC_MAX_CURVE_BITS)
MBEDTLS_ECP_MAX_PT_LEN PSA_KEY_EXPORT_ECC_PUBLIC_KEY_MAX_SIZE(PSA_VENDOR_ECC_MAX_CURVE_BITS)

Restartable ECC

The PSA API supports the equivalent of restartable operations, but only for signatures at the time of writing. See “Restartable ECDSA signature”.

There is no PSA API for elliptic curve arithmetic as such, and therefore no equivalent of mbedtls_ecp_restart_ctx and functions that operate on it.

There is PSA no equivalent of the MBEDTLS_ECP_OPS_xxx constants.

ECC functionality with no PSA equivalent

There is no PSA equivalent of mbedtls_ecdsa_can_do and mbedtls_ecdh_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.

No curve arithmetic

The PSA API is a cryptography API, not an arithmetic API. As a consequence, there is no PSA equivalent for the ECC arithmetic functionality exposed by ecp.h:

  • Manipulation of point objects and input-output: the type mbedtls_ecp_point and functions operating on it (mbedtls_ecp_point_xxx, mbedtls_ecp_copy, mbedtls_ecp_{set,is}_zero, mbedtls_ecp_tls_{read,write}_point). Note that the PSA export format for public keys corresponds to the uncompressed point format (MBEDTLS_ECP_PF_UNCOMPRESSED), so psa_import_key, psa_export_key and psa_export_public_key are equivalent to mbedtls_ecp_point_read_binary and mbedtls_ecp_point_write_binary for uncompressed points. The PSA API does not currently support compressed points, but it is likely that such support will be added in the future.
  • Manipulation of key pairs as such, with a bridge to bignum arithmetic (mbedtls_ecp_keypair type, mbedtls_ecp_export). However, the PSA export format for ECC private keys used by psa_import_key, psa_export_key is the same as the format used by mbedtls_ecp_read_key and mbedtls_ecp_write_key_ext.
  • Elliptic curve arithmetic (mbedtls_ecp_mul, mbedtls_ecp_muladd and their restartable variants).

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.

  • 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 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 values. 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 submit an issue on GitHub 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”.

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”.

A PSA key object is immutable, so there is no need for an equivalent of mbedtls_rsa_copy. (There is a function psa_copy_key, but it is only useful to make a copy of a key with a different policy of ownership; both concepts are out of scope of this document since they have no equivalent in the legacy API.)

LMS signatures

A future version of Mbed TLS will support LMS keys and signatures through the PSA API (psa_generate_key, psa_export_public_key, psa_import_key, psa_sign_hash, psa_verify_hash, etc.). However, this is likely to happen after Mbed TLS 4.0, therefore the next major version of Mbed TLS will likely keep the existing lms.h interface.

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.

In Mbed TLS 4.0, we are planning to keep the ASN.1 interfaces mostly unchanged. The evolution of Base64, OID and PEM as separate interfaces is still undecided at the time of writing.

EC-JPAKE

The PSA API exposes EC-JPAKE via the algorithm 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.

Please note a few differences between the two APIs: the legacy API is geared towards the use of EC-JPAKE in TLS 1.2, whereas the PSA API is protocol-agnostic.

  • The PSA API is finer-grained and offers more flexibility in message ordering. Where the legacy API makes a single function call, the PSA API may require multiple calls.
  • The legacy API uses the TLS 1.2 wire format in the input or output format of several functions. In particular, one of the messages embeds the curve identifier in the TLS protocol. The PSA API uses protocol-agnostic formats.
  • The legacy API always applies the key derivation specified by TLS 1.2 to the shared secret. With the PSA API, use a key derivation with PSA_ALG_TLS12_ECJPAKE_TO_PMS for the same calculation.