[cabfpub] For discussion: Restricting the use of file-based demonstrations of control

Rob Stradling rob.stradling at comodo.com
Mon Jun 2 21:04:36 UTC 2014

On 02/06/14 15:30, Ryan Sleevi wrote:
> On Jun 2, 2014 7:15 AM, Rob Stradling wrote:
> > Ryan, how about if the authorized CA was permitted to use each CSR
> > hash value as a demonstration of control a maximum of _once_ ?
> 1) Doesn't solve the CA A / CA B confusion I mentioned

By "authorized CA" I was assuming that the demonstration-of-control file 
would contain both the CSR Hash and the CA's domain name (as Comodo does 
today), and that if a CA supporting this scheme finds that it's not 
their domain name, then they MUST NOT issue.

> 2) Would prohibit issuance of a new cert for the same key, since CSRs
> don't contain a nonce, and the request would potentially be identical
> (thus same hash value, thus impossible to know if this is an old or new
> request).

Yeah OK, that makes it awkward.

Oh, BTW, our current scheme requires the attacker to generate a CSR that 
collides with both the SHA-1 _and_ MD5 hashes of the legitimate CSR.

I thought the N-bits-of-randomness-in-serial-numbers requirement was 
added specifically because SHA-1 is on its last legs.  Can we not say 
that SHA-256 (and, hopefully, SHA-1-plus-MD5) has a large enough 
security margin that the randomness requirement isn't actually necessary?

> I realize that in the case of "re-issuance", CA A likely has all the
> information necessary to issue without a new CSR, I just don't know if
> that is actually practiced. It would seem to induce yet another issuance
> path, which I understand CAs are not fond of.

Rob Stradling
Senior Research & Development Scientist
COMODO - Creating Trust Online
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