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Re: [OAUTH-WG] JWE with A128CBC-HS256

Antonio Sanso <asanso@adobe.com> Mon, 31 March 2014 06:43 UTC

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From: Antonio Sanso <asanso@adobe.com>
To: John Bradley <ve7jtb@ve7jtb.com>
Thread-Topic: [OAUTH-WG] JWE with A128CBC-HS256
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Date: Mon, 31 Mar 2014 06:43:30 +0000
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Subject: Re: [OAUTH-WG] JWE with A128CBC-HS256
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thanks a lot John,

On Mar 28, 2014, at 5:09 PM, John Bradley <ve7jtb@ve7jtb.com<mailto:ve7jtb@ve7jtb.com>> wrote:

This reference may be useful to you. http://tools.ietf.org/html/draft-mcgrew-aead-aes-cbc-hmac-sha2

The part of the spec you need is  http://tools.ietf.org/html/draft-ietf-jose-json-web-algorithms-24#page-23

We originally used a KDF as you mention.  In order to simplify the alg and align with draft-mcgrew-aead-aes-cbc-hmac-sha2.

K is the concatenation of the AES key and teh HMAC Key.

question,  are the examples in the spec already updated to use the new mechanism?
There are some obsolete references in the JWE spec. E.g. in [2] says:


as described where this algorithm is
   defined in Sections 4.8<http://tools.ietf.org/html/draft-ietf-jose-json-web-encryption-24#section-4.8> and 4.8.3<http://tools.ietf.org/html/draft-ietf-jose-json-web-encryption-24#section-4.8.3> of JWA,

These sections seems to point to on old version of the spec (Section 4.8.3 doesn’t even exist anymore in JWA)

regards

antonio

[2] http://tools.ietf.org/html/draft-ietf-jose-json-web-encryption-24#appendix-B


John B.


On Mar 28, 2014, at 11:19 AM, Antonio Sanso <asanso@adobe.com<mailto:asanso@adobe.com>> wrote:

hi *,

in the JWT specification [0] there is an example of a JWE that use A128CBC-HS256 for content encrpyption.
Now I am not a cryptographer my self but IIUC the same CEK is used for encrypting with AES and authentication HMAC.

AFAIK is better to use two different keys for those 2 different primitives (this will not obviously apply to AES_GCM).

Unless I am missing something... :)

regards

antonio

[0] http://tools.ietf.org/html/draft-ietf-oauth-json-web-token-19#appendix-A.1
[1] http://tools.ietf.org/html/draft-ietf-jose-json-web-encryption-24#appendix-A.2
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