Check authentication tag in penumbra

[bencmbrook]

Right now we're skipping the authtag verification step, which is a tiny security flaw, because it doesn't verify the signature (see step 3 below), meaning Transcend could tamper with the encrypted bytes passed through (which, worst case, just breaks the file).

All we need to do is:

1. set the authtag on the decipher object. This is done.
2. decrypt all the data This is done.
3. then verify the signature with decipher.final. This has to happen after decryption. I think new TransformStream takes a close argument where this may be possible.

This is good to do, because...

• in the future we might add AAD like the User ID to federate files with coreIdentifiers.
• people using this package for data that has more serious implications of tampering (i.e. where they're somehow vulnerable to a chosen ciphertext attack)

[dmattia]

Could we add a severity to this issue @bencmbrook or @michaelfarrell76 so we can track it under an SLA? It would be good to assign someone as well for all open security Issues