Re: [lamps] Ben Campbell's Yes on draft-ietf-lamps-rfc5751-bis-10: (with COMMENT)

Russ Housley <housley@vigilsec.com> Tue, 03 July 2018 21:11 UTC

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From: Russ Housley <housley@vigilsec.com>
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Date: Tue, 03 Jul 2018 17:05:47 -0400
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To: Jim Schaad <ietf@augustcellars.com>, Ben Campbell <ben@nostrum.com>
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Subject: Re: [lamps] Ben Campbell's Yes on draft-ietf-lamps-rfc5751-bis-10: (with COMMENT)
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Ben and Jim:

>> Substantive Comments:
>> 
>> §2.3, 2nd to last paragraph: I don't understand what it means to say
>> recipients MAY enforce a "MUST be supported" requirement. Am I correct to
>> assume the "MUST use the weaker" only applies if the sender used both
>> key-wrap algorithms?
> 
> No, it is possible that one could use a 128-bit content encryption algorithm, but use a 256-bit key wrap algorithm (or the reverse).  The text would apply in this case and allows for a recipient to avoid the MUST be the same length requirement if they wish.

It would be safe to allow the key for the key wrap algorithm to be larger  (e.g., AES-256  key wrap algorithm with AES-128 content encryption algorithm).

That said, a long time ago, we settled on this language because it was very difficult to come up with rules if the key wrap algorithm and the content encryption algorithm were using very different underlying ciphers.

Russ