KEY(n) -> KEYMAT in Security Associations (06-369r4)

[Black_David at emc.com]

* From the T10 Reflector (t10 at t10.org), posted by:
* Black_David at emc.com
*
Ralph,
There are essentially no cryptographic limits on the size of KEYMAT
- NIST bounds it to (2^32 - 1) * hash output size, which even for
SHA-1 is far more keying material than can reasonably be used at once.
The IPsec examples that need a lot of keys (initial IKE SA and ESP/AH
nesting) need 6 or 7 keys at once.  Limiting this to 16 keys (for one
invocation of the KDF) ought to be sufficient, so 
16 * 512 / 8 = 1,024 bytes, which is a nice round number.
Thanks,
--David
> What would be the maximum size of KEYMAT?
> 
> For example, can I assume that one pass through a KDF cannot
> generate more than 10 keys? Thus, 10 * (largest key size)
> would be the max KEYMAT bit string (i.e., (10 * 512) / 8
> equals a max KEYMAT byte size of 640).
> 
> All the best,
> 
> .Ralph
> 
> Black_David at emc.com wrote:
> > * From the T10 Reflector (t10 at t10.org), posted by:
> > * Black_David at emc.com
> > *
> > The original work that lead to the KEY(n) element in security
> > associations was based on a design assumption that all keys and
> > the hash output size would be the same, namely 256 bits.  Based
> > on comments in the CAP meeting, that's now clearly an overly
> > restrictive design assumption, so the array of keys [KEY(n)
> > element] in a security association should be changed to a
> > keying material bitstring [KEYMAT].  The consumer/user of the
> > keying material would specify how much is needed, and the KDF
> > produces the result (rounded up according to what the KDF can
> > do).  This is the approach used in IKEv2 (RFC 4306, Section
> > 2.17) and is consistent with the NIST KDF that is specified in
> > 06-369r4.  This change should be made.
> >
> > Thanks,
> > --David
> > ----------------------------------------------------
> > David L. Black, Senior Technologist
> > EMC Corporation, 176 South St., Hopkinton, MA  01748
> > +1 (508) 293-7953		  FAX: +1 (508) 293-7786
> > black_david at emc.com        Mobile: +1 (978) 394-7754
> > ----------------------------------------------------
> >
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