[COSE] Authentication tag

[Samuel Erdtman <[e-mail]>]

Hi

I´m working on a JavaScript implementation of the COSE msg specification,
currently working on the GCM encryption.

In the nodejs crypto environment the authentication tag is set separately
i.e. a specific setAuthTag call. I looked into openssl and could see that
that was the case there too.

In the examples provided with the COSE specification I could find out that
the auth tag is appends to the end of the ciphertext.

I tried to find this described in the COSE specification but could not find
it. It might be described in some refereed specification but it was not
obvious to me at least.

If it is not to late I would suggest that authentication tag is lifted out
from the ciphertext and into the unprotected header similar to IV. Or that
it is explicitly described that the authentication tag should be appended
to the ciphertext.

Cheers
Samuel Erdtman