I am the assigned Gen-ART reviewer for this draft. For background on
Gen-ART, please see the FAQ at
<http://wiki.tools.ietf.org/area/gen/trac/wiki/GenArtfaq>.
Please resolve these comments along with any other Last Call comments
you may receive.
Document: draft-gutmann-cms-hmac-enc-05
Reviewer: Alexey Melnikov
Review Date: 2011-07-08
IETF LC End Date: 2011-07-20
IESG Telechat date: 2011-08-11
Summary: This draft is almost ready for publication as a standard track
RFC.
Major issues: none
Minor issues:
3. CMS Encrypt-and-Authenticate Overview
Conventional CMS encryption uses a content encryption key (CEK) to
encrypt a message payload. Authenticated encryption requires two
keys, one for encryption and a second one for authentication. Like
other mechanisms that use authenticated encryption, this document
employs a pseudorandom function (PRF) to convert a single block of
keying material into the two keys required for encryption and
authentication. This converts the standard CMS encryption operation:
KEK( CEK ) || CEK( data )
It would be good to expand KEK on the first use.
Also, it would have been nice to specify all parameters here and below,
so that it is clear where MAC-K and CEK-K are used.
into:
KEK( master_secret ) || MAC( CEK( data ) )
where the MAC and encryption keys are derived from the master_secret
via:
MAC-K := PRF( master_secret, "authentication" );
CEK-K := PRF( master_secret, "encryption" );
4.2. Rationale
Using a fixed-length key rather than making it a user-selectable
parameter is done for the same reason as AES' quantised key lengths:
there's no benefit to allowing, say, 137-bit keys over basic 128- and
256-bit lengths, it adds unnecessary complexity, and if the lengths
are user-defined then there'll always be someone who wants keys that
go up to 12.
Excuse my ignorance, but what does "go up to 12" (and "go to 11"
elsewhere) mean?
Nits/editorial comments: none
(id-nits reports one Downref, but it was called out in the IETF LC
announcement)
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