Re: [jose] Use of AES-HMAC algorithm - Consensus Request

[Dick Hardt <[e-mail]>]

FWIW: I'd strongly prefer a longer key than a different KDF scheme.

On Apr 7, 2013, at 12:32 PM, "Peck, Michael A" <[e-mail]> wrote:

> I prefer using a KDF, as I think the approach of using a long key over-complicates key management by requiring generation and transport of a longer key than necessary, and it may also provide a false sense of the overall security level.
> 
> As previously discussed, the Concat KDF is not appropriate, but there are other NIST KDFs that seem acceptable for this key expansion purpose.
> For instance, the "KDF in Counter Mode" in NIST SP 800-108 Section 5.1, with the PRF algorithm set to the same HMAC algorithm used in the corresponding AEAD algorithm.  (i.e. use HMAC-SHA-256 as PRF with AES-128 + HMAC-SHA-256, and use HMAC-SHA-512 as PRF with AES-256 + HMAC-SHA-512).
> 
> For example, for AES-128 + HMAC-SHA-256, where "key" is an inputted 128-bit "master" key:
> Expanded key = HMAC-SHA-256(key, 0x00 0x00 0x00 0x01 | "A128CBC+HS256" | 0x00 | 0x00 0x00 0x01 0x00)
> AES key = first 128 bits of expanded key
> HMAC key = second 128 bits of expanded key
> 
> If JOSE chooses to use draft-mcgrew-aead-aes-cbc-hmac-sha2, I think it would be worth discussing with the draft authors whether they are interested in performing the KDF in the AEAD algorithm itself (for example, input a 128-bit key into their AEAD_AES_128_CBC_HMAC_SHA_256 and have it be responsible for deriving the 128-bit AES key and 128-bit HMAC key).
> 
> Mike
> 
>> -----Original Message-----
>> From: [e-mail] [mailto:[e-mail]] On Behalf Of
>> Jim Schaad
>> Sent: Thursday, April 04, 2013 7:51 PM
>> To: [e-mail]
>> Subject: Re: [jose] Use of AES-HMAC algorithm - Consensus Request
>> 
>> <chair>
>> 
>> At the request of the editors, this is a formal consensus call on the first
>> item in the list below.  If there are objects to use a single long key
>> rather than a KDF function for the AES-CBC/HMAC algorithm please speak up
>> now.  To date nobody has said that I was wrong to assume the consensus on
>> that item.
>> 
>> Call ends in one week.
>> 
>> Jim
>> 
>> 
>>> -----Original Message-----
>>> From: [e-mail] [mailto:[e-mail]] On Behalf Of
>> Jim
>>> Schaad
>>> Sent: Wednesday, March 27, 2013 3:21 PM
>>> To: [e-mail]
>>> Subject: [jose] Use of AES-HMAC algorithm
>>> 
>>> <chair>
>>> After spending time looking at and thinking about how to resolve this
>> issue
>>> since I was unable to do so at the last F2F meeting.  I have come up with
>> the
>>> following set of issues that might need to be addressed as part of
>> resolving
>>> this issue.
>>> 
>>> 1.  Do we change from using KDC to having a double size key for the
>>> algorithm?  I think that there is probably a consensus that this should be
>> done.
>>> 
>>> 2. Should IVs be prepended to the encrypted body as part of the encoding
>>> steps?  If so then this change should be universal.
>>> 
>>> Doing so would eliminate one field from all of the encoding formats which
>>> should be considered a plus.
>>> Doing so would force code writers to understand how large the IV is for
>> all
>>> algorithms as the IV would no longer be a separate item.
>>> 
>>> 3. Should Authentication Tags be appended to the encrypted body as part of
>>> the encoding steps?
>>> 
>>> Doing so would eliminate one field from all of the encoding formats which
>>> should be considered a plus.
>>> Doing so would force code writers to understand how large the IV is for
>> all
>>> algorithms as the IV would no longer be a separate item.
>>> Doing so would force a re-organization for the multiple recipient case as
>>> either all recipient specific data would need to be excluded from the
>>> authentication step or all of the recipient data would need to be included
>> for
>>> by all recipients.
>>> Changing how the recipient info is process is going to give a performance
>>> benefit for sending encrypted items for multiple recipients.
>>> The current strategy of a single IV and key pair with AES-GCM and
>> different
>>> authentication data needs to have CFRG look at it.  I am worried that it
>> might
>>> be a serious security flaw.
>>> 
>>> 4. Should we reference the McGrew draft and provide text on how things
>> are
>>> changed or should we "copy" the draft into our text?
>>> 
>>> 5.  If we allow for the use of AES-GCM or AES-HMAC for doing key wrapping,
>>> does this change how we think about any of the above questions?
>>> 
>>> Allowing for AES-GCM for key wrapping has a benefit for hardware
>> situations
>>> as only the encrypt and not the decrypt functions need to be placed in
>>> hardware.  However allowing for this key wrapping give a problem as there
>> is
>>> no way to encode the three fields into the encrypted value unless with use
>>> either a JSON structure in this location or we do use the single appended
>>> binary output stream.  The first approach leads to an expansion of the
>> field by
>>> double base64 encoding which is highly undesirable.
>>> 
>>> Jim
>>> 
>>> 
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>>> https://www.ietf.org/mailman/listinfo/jose
>> 
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