Add fallback to non-compliant truncated HMAC for compatibiltiy

In case truncated HMAC must be used but the Mbed TLS peer hasn't been updated
yet, one can use the compile-time option MBEDTLS_SSL_TRUNCATED_HMAC_COMPAT to
temporarily fall back to the old, non-compliant implementation of the truncated
HMAC extension.

library/ssl_tls.c

@@ -713,7 +713,15 @@ int mbedtls_ssl_derive_keys( mbedtls_ssl_context *ssl )
          * so we only need to adjust the length here.
          */
         if( session->trunc_hmac == MBEDTLS_SSL_TRUNC_HMAC_ENABLED )
+        {
             transform->maclen = MBEDTLS_SSL_TRUNCATED_HMAC_LEN;
+
+#if defined(MBEDTLS_SSL_TRUNCATED_HMAC_COMPAT)
+            /* Fall back to old, non-compliant version of the truncated
+             * HMAC implementation which also truncates the key. */
+            mac_key_len = transform->maclen;
+#endif
+        }
 #endif /* MBEDTLS_SSL_TRUNCATED_HMAC */
 
         /* IV length */