Fix wrong length limit in GCM

See for example page 8 of
http://csrc.nist.gov/publications/nistpubs/800-38D/SP-800-38D.pdf

The previous constant probably came from a typo as it was 2^26 - 2^5 instead
of 2^36 - 2^5. Clearly the intention was to allow for a constant bigger than
2^32 as the ull suffix and cast to uint64_t show.

fixes #362

ChangeLog

@@ -1,5 +1,10 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
+= mbed TLS 2.2.1 released 2015-12-xx
+
+Bugfix
+   * Fix over-restricive length limit in GCM. Found by Andreas-N. #362
+
 = mbed TLS 2.2.0 released 2015-11-04
 
 Security

library/gcm.c

@@ -362,7 +362,7 @@ int mbedtls_gcm_update( mbedtls_gcm_context *ctx,
     /* Total length is restricted to 2^39 - 256 bits, ie 2^36 - 2^5 bytes
      * Also check for possible overflow */
     if( ctx->len + length < ctx->len ||
-        (uint64_t) ctx->len + length > 0x03FFFFE0ull )
+        (uint64_t) ctx->len + length > 0xFFFFFFFE0ull )
     {
         return( MBEDTLS_ERR_GCM_BAD_INPUT );
     }