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 limitations

Compile-time: enabling MBEDTLS_USE_PSA_CRYPTO requires MBEDTLS_ECP_RESTARTABLE to be disabled.

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 parts of the code that are specific to TLS 1.3; those parts always use PSA Crypto. The parts of the TLS 1.3 code that are common with TLS 1.2, however, follow this option (this is currently just the record protection code, and X.509). You need to enable MBEDTLS_USE_PSA_CRYPTO if you want TLS 1.3 to use PSA everywhere.

New APIs / API extensions

PSA-held (opaque) keys in the PK layer

There is a new API function mbedtls_pk_setup_opaque() that can be used to wrap a PSA keypair 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 keypair, 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 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 ECDSA-based key exchanges (note: while this is supported on both sides, it's currently only tested client-side);
• 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)

There are two 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: only TLS 1.2 for now.

Use in TLS: opt-in. The application needs to register the key using 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.

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:

• EC J-PAKE (when MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED)
• finite-field (non-EC) Diffie-Hellman (use in key exchanges: DHE-RSA, DHE-PSK)

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 exception:

• verification of RSA-PSS signatures with a salt length that is different from the hash length.

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

PK layer: most crypto operations based on PSA

Current exception:

• verification of RSA-PSS signatures with a salt length that is different from the hash length.

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