Use authenticated encryption?

[henrycg]

https://github.com/edauterman/hsm-impl/blob/66e816ca1900977520e4d34886c488f2c793af29/agent/punc_enc.c#L21

Should we be using some authenticated encryption mode (e.g., GCM) instead of ECB mode here?

[edauterman]

The GCM support on the solo key is a little weird, which is why we're using HMAC on line 26.