Hello,

This is a request for feedback about the next major release of Mbed TLS (Mbed TLS 4.0). (Mbed TLS 3.6 LTS will remain supported with its current feature set until at least Q2 2027.) Please reply to this thread or on the GitHub issue linked below. If you wish to leave feedback privately, you can reply privately and your feedback will be anonymized before sharing outside Arm.

https://github.com/Mbed-TLS/mbedtls/issues/9202

In 2025 (by the time Mbed TLS 4.0 is released), are CBC-based cipher suites still relevant for Mbed TLS? If you still need support for CBC-based cipher suites (as opposed to cipher suites using AEAD: CCM, GCM or ChaChaPoly, or null cipher suites), please let us know.

Removing them would allow us to significantly simplify some parts of the TLS code. They are difficult to implement securely due to being very sensitive to side channels; we think we got it right, but at the expense of performance, code size and maintainability.

One option we're considering is to keep CBC cipher suites, but only when the encrypt-then-MAC (EtM) extension is enabled. However, this is problematic because the TLS protocol does not allow a client to indicate that it requires EtM support, which could lead to a failed connection even when the server also have an AEAD cipher suite in common.

Best regards,