Cipher Mode : Process and Finish are Confusing about botan

[randombit]

Sorry, I know this is confusing. The cipher API is kind of weirdly designed. I didn't realize our documentation was that non-specific though, thanks for pointing this out.

You should always call finish, either when encrypting or decrypting.

In fact you can get away with never calling process, it exists just for efficiency when dealing with larger messages. It's perfectly fine/correct/easy to just call start with the nonce then immediately finish with a buffer containing the entire message.

For your decryption example, things maybe "work" but probably not quite as you're thinking. What's happening is the Poly1305 authentication tag (final 16 bytes) is being decrypted and emitted as if it was part of the message, and so you're both not checking the tag (which allows arbitrary forgeries) but also ending up with 16 bytes of extra junk at the end. Maybe if you're only looking at the first bytes of the plaintext this was not obvious.

[TxMat]

Thanks for clearing things up !