Hi,

We'd like to announce a change we intend to make: starting with Mbed TLS 3.1, it will no longer be possible to build with TLS, X.509, or PK without support for PSA Crypto.

Details: currently, use of PSA Crypto APIs from the TLS, X.509 and PK layers is controlled by the option MBEDTLS_USE_PSA_CRYPTO. When this option is disabled (which is the default), it's possible to build without MBEDTLS_PSA_CRYPTO_C. Starting with 3.1, we intend to start making TLS, X.509 and PK use PSA Crypto unconditionally, so MBEDTLS_PSA_CRYPTO_C will be automatically enabled in the build as soon as TLS, X.509 or PK is enabled.

Impact: for users who already build with PSA Crypto enabled (the default), no impact. For users who currently disable MBEDTLS_PSA_CRYPTO_C in their configuration, starting with 3.1 there will be no functional changes, but the size of the built library will increase due to the additional features enabled. The increase depends on the configuration, application, platform, and toolchain, but the order of magnitude currently ranges from about 9 KB for a minimal TLS configuration with LTO (link-time optimisation) to about 30 KB for a full configuration without link-time garbage collection (though size-constrained devices are very unlikely to use such a configuration); we aim to reduce this overhead in the future.

Rationale: maintaining two versions of every cryptographic operation in upper layers (one PSA, one non-PSA) imposes a significant burden on new developments. By removing that burden, we hope to progress faster on things that are more important in the long run, including better integration of PSA Crypto in TLS and X.509, and future code size optimisation of PSA Crypto. We realise the impact on users who were excluding PSA Crypto is significant. We're going to release Mbed TLS 2.28 in the coming months, which is not affected by this change and will receive bug fixes and security fixes for at least 3 years; we hope it provides an acceptable fall-back solution to affected users.

In the long run, PSA Crypto will become the only Crypto API we offer; we want to make its footprint as small as possible considering its feature set and we hope this change will enable us to make faster progress towards that goal.

Best regards, Manuel Pégourié-Gonnard for the Mbed TLS team.