[Cfrg] One Key -> RE: considering new topics for CFRG

[Paul Lambert <[e-mail]>]

More on this single topic:

> > >> >   - Public key based methods that can be readily be used
> > >> >     to both sign data and be used to develop encryption keys

Here are the NIST guidelines that generated my wish:
   http://csrc.nist.gov/groups/ST/toolkit/documents/SP800-56Arev1_3-8-07.pdf

"A static key pair may be used in more than
one key establishment scheme. However, one
static public/private key pair shall not
be used for different purposes (for example, a
digital signature key pair is not to be used
for key establishment or vice versa) with the
following possible exception: when requesting
the (initial) certificate for a public static
key establishment key, the key establishment 
private key associated with the public key
may be used to sign the certificate request
. See SP 800-57, Part 1 on Key Usage for further information"

This immediately divides any PK architecture in half with
separate signature and key establishment PKs.

It's odd that the guidelines allow the key 
establishment key to sign once, but not after the
certificate request.

Questions for the group:
1) Are there any demonstrable attacks on the 
combined usage of key establishment and signature algorithms?

2) What are the known safe combinations?
(I've seen at least one Edwards curve paper on combined usage)



Paul