Reduce size of ssl_transform if no MAC ciphersuite is enabled

The hash contexts `ssl_transform->md_ctx_{enc/dec}` are not used if
only AEAD ciphersuites are enabled. This commit removes them from the
`ssl_transform` struct in this case, saving a few bytes.

include/mbedtls/ssl_internal.h

@@ -569,6 +569,8 @@ struct mbedtls_ssl_transform
     unsigned char iv_enc[16];           /*!<  IV (encryption)         */
     unsigned char iv_dec[16];           /*!<  IV (decryption)         */
 
+#if defined(MBEDTLS_SSL_SOME_MODES_USE_MAC)
+
 #if defined(MBEDTLS_SSL_PROTO_SSL3)
     /* Needed only for SSL v3.0 secret */
     unsigned char mac_enc[20];          /*!<  SSL v3.0 secret (enc)   */
@@ -578,13 +580,14 @@ struct mbedtls_ssl_transform
     mbedtls_md_context_t md_ctx_enc;            /*!<  MAC (encryption)        */
     mbedtls_md_context_t md_ctx_dec;            /*!<  MAC (decryption)        */
 
-    mbedtls_cipher_context_t cipher_ctx_enc;    /*!<  encryption context      */
-    mbedtls_cipher_context_t cipher_ctx_dec;    /*!<  decryption context      */
-
 #if defined(MBEDTLS_SSL_ENCRYPT_THEN_MAC)
     int encrypt_then_mac;       /*!< flag for EtM activation                */
 #endif
 
+#endif /* MBEDTLS_SSL_SOME_MODES_USE_MAC */
+
+    mbedtls_cipher_context_t cipher_ctx_enc;    /*!<  encryption context      */
+    mbedtls_cipher_context_t cipher_ctx_dec;    /*!<  decryption context      */
     int minor_ver;
 
     /*

library/ssl_tls.c

@@ -1171,10 +1171,11 @@ int mbedtls_ssl_derive_keys( mbedtls_ssl_context *ssl )
         return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
     }
 
+#if defined(MBEDTLS_SSL_SOME_MODES_USE_MAC)
 #if defined(MBEDTLS_SSL_PROTO_SSL3)
     if( ssl->minor_ver == MBEDTLS_SSL_MINOR_VERSION_0 )
     {
-        if( mac_key_len > sizeof transform->mac_enc )
+        if( mac_key_len > sizeof( transform->mac_enc ) )
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "should never happen" ) );
             return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
@@ -1203,6 +1204,7 @@ int mbedtls_ssl_derive_keys( mbedtls_ssl_context *ssl )
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "should never happen" ) );
         return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
     }
+#endif /* MBEDTLS_SSL_SOME_MODES_USE_MAC */
 
 #if defined(MBEDTLS_SSL_HW_RECORD_ACCEL)
     if( mbedtls_ssl_hw_record_init != NULL )
@@ -1221,6 +1223,9 @@ int mbedtls_ssl_derive_keys( mbedtls_ssl_context *ssl )
             return( MBEDTLS_ERR_SSL_HW_ACCEL_FAILED );
         }
     }
+#else
+    ((void) mac_dec);
+    ((void) mac_enc);
 #endif /* MBEDTLS_SSL_HW_RECORD_ACCEL */
 
 #if defined(MBEDTLS_SSL_EXPORT_KEYS)
@@ -7424,8 +7429,10 @@ static void ssl_transform_init( mbedtls_ssl_transform *transform )
     mbedtls_cipher_init( &transform->cipher_ctx_enc );
     mbedtls_cipher_init( &transform->cipher_ctx_dec );
 
+#if defined(MBEDTLS_SSL_SOME_MODES_USE_MAC)
     mbedtls_md_init( &transform->md_ctx_enc );
     mbedtls_md_init( &transform->md_ctx_dec );
+#endif
 }
 
 void mbedtls_ssl_session_init( mbedtls_ssl_session *session )
@@ -9647,8 +9654,10 @@ void mbedtls_ssl_transform_free( mbedtls_ssl_transform *transform )
     mbedtls_cipher_free( &transform->cipher_ctx_enc );
     mbedtls_cipher_free( &transform->cipher_ctx_dec );
 
+#if defined(MBEDTLS_SSL_SOME_MODES_USE_MAC)
     mbedtls_md_free( &transform->md_ctx_enc );
     mbedtls_md_free( &transform->md_ctx_dec );
+#endif
 
     mbedtls_platform_zeroize( transform, sizeof( mbedtls_ssl_transform ) );
 }