Re: [Trans] path validation

[Rob Stradling <[e-mail]>]

On 01/10/14 15:34, Stephen Kent wrote:
<snip>
> I thought the CT design  makes a counter argument, i.e., that CAs are
> motivated to log certs because, over time, TLS clients will reject
> connections to servers when there is no evidence of an SCT.

Over time, yes, we hope that this will happen.  But we obviously can't 
guarantee that, in N months/years from now, 100% of TLS clients will 
support CT.

> if this argument is true, then having logs check for syntactic
> mis-issuance is a good thing.

I disagree.  There is a gap between what is syntactically properly 
issued (according to CABF guidelines, etc) and what a typical TLS client 
actually accepts.  If logs checked for syntactic mis-issuance, a rogue 
CA could exploit this gap by maliciously mis-issuing certs containing 
"syntax errors".  CT would not reveal the attack (because the logs would 
refuse to issue SCTs), but TLS clients that don't reject connections 
when no SCT is provided would be vulnerable.

We want to provide as much "herd immunity" as possible to TLS clients 
that don't support CT.  This means that we need all certs to be publicly 
logged, to maximize the chances that any (maliciously) mis-issued cert 
will be discovered quickly.

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