Fix for #441 - crypt and hash gcm (#546)

* Fix crypt_and_hash to support decrypting GCM encrypted files

* Fix documentation in crypt_and_hash for the generic case

* Remove unused lastn from crypt_and_hash

lastn is not used with the cipher layer as it already provides padding
and understanding of length of the original data.

Backport of fix by Paul Bakker.
1 file changed