Bug in calculating authentication tag

[apasquale]

Hi, I found an issue while trying to decrypt something with enc: A256CBC-HS512 which was encrypted by another library. I was getting 'Authentication tag verification failed' with this library but not on others.

After hunting down the differences on how the tag was calculated I narrowed it down to this line

According to RFC 7516 JSON Web Encryption (JWE)

The octet string AL, which is the number of bits in AAD expressed as a big-endian 64-bit unsigned integer

I made the changes to both decrypt and encrypt in my fork, but was unable to figure out how to fix all the failing tests in spec/jwe/enc_spec.rbso haven't raised a PR.

[francescobbo]

Good catch. Working on it right now!

👍