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.

include/mbedtls/check_config.h

@@ -77,6 +77,10 @@
 #error "MBEDTLS_DHM_C defined, but not all prerequisites"
 #endif
 
+#if defined(MBEDTLS_SSL_TRUNCATED_HMAC_COMPAT) && !defined(MBEDTLS_SSL_TRUNCATED_HMAC)
+#error "MBEDTLS_SSL_TRUNCATED_HMAC_COMPAT defined, but not all prerequisites"
+#endif
+
 #if defined(MBEDTLS_CMAC_C) && \
     !defined(MBEDTLS_AES_C) && !defined(MBEDTLS_DES_C)
 #error "MBEDTLS_CMAC_C defined, but not all prerequisites"

include/mbedtls/config.h

@@ -1363,6 +1363,22 @@
  */
 #define MBEDTLS_SSL_TRUNCATED_HMAC
 
+/**
+ * \def MBEDTLS_SSL_TRUNCATED_HMAC_COMPAT
+ *
+ * Fallback to old, non-conforming implementation of the truncated
+ * HMAC extension which also truncates the HMAC key.
+ *
+ * \warning This should only be enabled temporarily when the use
+ *          of truncated HMAC is mandatory *and* the peer is an Mbed TLS
+ *          stack that doesn't use the fixed implementation yet.
+ *
+ * Uncomment to fallback to old, non-compliant truncated HMAC implementation.
+ *
+ * Requires: MBEDTLS_SSL_TRUNCATED_HMAC
+ */
+//#define MBEDTLS_SSL_TRUNCATED_HMAC_COMPAT
+
 /**
  * \def MBEDTLS_THREADING_ALT
  *

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 */