Dear MbedTLS team,
I maintain the Mbed TLS integration for our embedded products. We are still migrating from Mbed TLS 2.x to 3.6.x and need guidance on replacing the legacy SSL key export behavior. One of our legacy products utilizes handshake key material from mbedtls_ssl_conf_export_keys_cb in an accelerator. For compatibility with existing deployments, we must preserve this key export so the other side can decode.
Section “SSL key export interface change” in 3.0-migration-guide.md (as referenced for the 3.6.x line) states that mbedtls_ssl_conf_export_keys_cb() and mbedtls_ssl_conf_export_keys_ext_cb() were replaced with mbedtls_ssl_set_export_keys_cb(), and that the new callback no longer exports raw keys and IV. The guide suggests that applications requiring raw traffic keys may derive them from the master secret and handshake transcript hashes from the on-wire data. I'm not sure how this can be coded. What is the recommended way to obtain or derive the raw keys and IV (or equivalent data) under the 3.6.x API? Could you point to documentation, examples of code would be very beneficial.
We would also like to align with the 4.1.x in future. If there are changes in this area, we are interested in a common and supported approach to avoid repeated breaking migrations.
Thank you for your time.
Kind regards,
Piotr