Re: [TLS] rfc4366-bis-03 Discuss #2: hash alg. agility for TrustedCA?

[Dean Anderson <dean at av8.com>]

On Wed, 22 Oct 2008, Simon Josefsson wrote:

> Alfred "=?hp-roman8?B?SM5uZXM=?=" <ah at tr-sys.de> writes:
> 
> > (B)  Non-uniqueness of x509_name in Trusted CA Keys TLS extension
> >
> > The 3rd-to-last paragraph of Section 6 in the -03 version
> > of the rfc4366-bis draft states:
> >
> > !  Note also that it is possible that a key hash or a Distinguished Name
> > !  alone may not uniquely identify a certificate issuer (for example, if
> > !  a particular CA has multiple key pairs). However, here we assume this
> > !  is the case following the use of Distinguished Names to identify
> > !  certificate issuers in TLS.
> >
> > It seems rather unlikely that this non-uniqueness happens for
> > a key hash.
> 
> Can't you have multiple CA certificates for the same key?

You _can_ have multiple key pairs, but knowledge of any key pair allows
one to calculate P and Q, so having multiple key pairs for the same
modulus is a "bad idea", because with one pair one can calculate any
other pair given one of the other pair.  I believe that both Schnier's
Applied Cryptography, 2nd and the CRC Handbook of Applied Cryptography
recommend against this practice.  The CRC Handbook gives the method to 
calculate P and Q given the public and private keys.

		--Dean


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