Sorry I was only thinking of the KDF issue not the other differences. You far correct there are still non trivia differences between the two.
The two are closer but not the same. I caught the flue on my way home from IETF and probably need to get off the cold medication before answering email. John On 2012-11-11, at 10:30 PM, Michael Jones <[email protected]> wrote: > Using draft-mcgrew-aead-aes-cbc-hmac-sha2 is not the same thing as (1). For > instance, as was discussed after David's presentation at IETF 84, > draft-mcgrew-aead-aes-cbc-hmac-sha2 does not follow the pattern of AEAD > algorithms such as AES GCM, which have two inputs (plaintext, "additional > authenticated data"), and two outputs (ciphertext, "authentication tag"). > Instead, it adds a step combining the ciphertext and "authentication tag" > outputs. > > If you read the draft, implementation of draft-mcgrew-aead-aes-cbc-hmac-sha2 > has a lot more steps than what we have for A128CBC+HS256 and A256CBC+HS512. > It requires generating and adding specific padding bytes. It prefixes the > ciphertext with the IV. It includes the length of the "additional > authenticated data" in the MAC calculation. It combines the two outputs into > one. For decryption, likewise, the two outputs must be split apart, the IV > must be split apart, etc. > > All of these are steps that implementations could get wrong, resulting in > interoperability problems. By keeping all the parameters separate, our > current A128CBC+HS256 and A256CBC+HS512 algorithms eliminate those steps. > > I'm sorry for the apparent confusion between (1) and > draft-mcgrew-aead-aes-cbc-hmac-sha2. While they both explicitly represent > the CMK and CEK, and use the same underlying crypto operations, the details > differ in ways that are likely to matter to implementers. If there was a > version of draft-mcgrew-aead-aes-cbc-hmac-sha2 that kept all the inputs and > outputs separate, I agree that it would be a reasonable candidate for JOSE to > consider. But unlike AES GCM, that's not what it does. > > -- Mike > > > From: [email protected] > > To: [email protected]; [email protected] > > Date: Mon, 12 Nov 2012 09:23:37 +1100 > > Subject: RE: [jose] Choice for WG: Use a KDF with AES CBC or use a longer > > key > > > > > So I’d like to explicitly ask the working group. Do you want us to: > > > > > > (1) Use the concatenation of random CEK and CIK values as the CMK for AES > > > CBC, resulting in a longer CMK? > > > (2) Continue to use a KDF to generate the CEK and CIK from a shorter CMK? > > > > > > 1. Use draft-mcgrew-aead-aes-cbc-hmac-sha2 > > > > -- > > James Manger > _______________________________________________ > jose mailing list > [email protected] > https://www.ietf.org/mailman/listinfo/jose
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