AES GCM mode requires special handling about open-crypto

[tanner0101]

@vzsg If the cipher/tag pattern is only supported by GCM it's hard to justify the creation of a whole new protocol
I like the idea of appending the tag to the text. Is there some character that we are sure won't appear in the encryption that would be easy to split by as well? If not, just lobbing off the last 12 bytes is easy. We could even make a helper for it.

My vote is for 3. Are there any options you like more than others?

[vzsg]

"Splitting" was a bad choice of words on my part, I actually meant appending without separator and lobbing off by count.

I'm leaning towards option 3 too. To me, this scheme is a workable compromise even if a caller wants to manipulate the tag separately. Which I think shouldn't really happen among the users of this ciphering utility.

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Regarding option 2, there are other authenticated modes of operation, but only GCM is listed in the enum now. CCM also seems to be supported by CLibreSSL.

[tanner0101]

GCM has been removed in #23. This is open for someone to implement.