mbedtls/docs/use-psa-crypto.md

This document describes the compile-time configuration option MBEDTLS_USE_PSA_CRYPTO from a user's perspective.

This option makes the X.509 and TLS library use PSA for cryptographic operations, and enables new APIs for using keys handled by PSA Crypto.

General considerations

Application code: when this option is enabled, you need to call psa_crypto_init() before calling any function from the SSL/TLS, X.509 or PK module.

Scope: MBEDTLS_USE_PSA_CRYPTO has no effect on the most of the TLS 1.3 code, which always uses PSA crypto. The parts of the TLS 1.3 code that will use PSA Crypto or not depending on the value of this option are:

• record protection;
• running handshake hash;
• asymmetric signature verification & generation;
• X.509 certificate chain verification. You need to enable MBEDTLS_USE_PSA_CRYPTO if you want TLS 1.3 to use PSA everywhere.

Important note: Even with this option disabled, some modules may still use PSA Crypto. However, it is then their responsibility to make sure it's safe to do so; in particular those modules do not require psa_crypto_init() to be called. So, enabling MBEDTLS_USE_PSA_CRYPTO basically means:

• as a user, you promise to call psa_crypto_init() before using any function from PK, X.509 or TLS;
• in return, those modules will use PSA Crypto as much as possible (see exceptions belos). Conversely, not enabling this option means you have no obligation to call psa_crypto_init() (unless as documented by other options such as TLS 1.3), but modules can still decide to use PSA if they can determine it is available and initialized.

New APIs / API extensions

PSA-held (opaque) keys in the PK layer

New API function: mbedtls_pk_setup_opaque() - can be used to wrap a PSA key pair into a PK context. The key can be used for private-key operations and its public part can be exported.

Benefits: isolation of long-term secrets, use of PSA Crypto drivers.

Limitations: can only wrap a key pair, can only use it for private key operations. (That is, signature generation, and for RSA decryption too.) Note: for ECDSA, currently this uses randomized ECDSA while Mbed TLS uses deterministic ECDSA by default. The following operations are not supported with a context set this way, while they would be available with a normal context: mbedtls_pk_check_pair(), mbedtls_pk_debug(), all public key operations.

Use in X.509 and TLS: opt-in. The application needs to construct the PK context using the new API in order to get the benefits; it can then pass the resulting context to the following existing APIs:

• mbedtls_ssl_conf_own_cert() or mbedtls_ssl_set_hs_own_cert() to use the key together with a certificate for certificate-based key exchanges;
• mbedtls_x509write_csr_set_key() to generate a CSR (certificate signature request);
• mbedtls_x509write_crt_set_issuer_key() to generate a certificate.

PSA-held (opaque) keys for TLS pre-shared keys (PSK)

New API functions: mbedtls_ssl_conf_psk_opaque() and mbedtls_ssl_set_hs_psk_opaque(). Call one of these from an application to register a PSA key for use with a PSK key exchange.

Benefits: isolation of long-term secrets.

Limitations: none.

Use in TLS: opt-in. The application needs to register the key using one of the new APIs to get the benefits.

PSA-based operations in the Cipher layer

There is a new API function mbedtls_cipher_setup_psa() to set up a context that will call PSA to store the key and perform the operations.

This function only worked for a small number of ciphers. It is now deprecated and it is recommended to use psa_cipher_xxx() or psa_aead_xxx() functions directly instead.

Warning: This function will be removed in a future version of Mbed TLS. If you are using it and would like us to keep it, please let us know about your use case.

Internal changes

All of these internal changes are active as soon as MBEDTLS_USE_PSA_CRYPTO is enabled, no change required on the application side.

TLS: most crypto operations based on PSA

Current exceptions:

• Finite-field (non-EC) Diffie-Hellman (used in key exchanges: DHE-RSA, DHE-PSK).
• Restartable operations when MBEDTLS_ECP_RESTARTABLE is also enabled (see the documentation of that option).

Other than the above exceptions, all crypto operations are based on PSA when MBEDTLS_USE_PSA_CRYPTO is enabled.

X.509: most crypto operations based on PSA

Current exceptions:

• Restartable operations when MBEDTLS_ECP_RESTARTABLE is also enabled (see the documentation of that option).

Other than the above exception, all crypto operations are based on PSA when MBEDTLS_USE_PSA_CRYPTO is enabled.

PK layer: most crypto operations based on PSA

Current exceptions:

• Verification of RSA-PSS signatures with an MGF hash that's different from the message hash.
• Restartable operations when MBEDTLS_ECP_RESTARTABLE is also enabled (see the documentation of that option).

Other than the above exceptions, all crypto operations are based on PSA when MBEDTLS_USE_PSA_CRYPTO is enabled.