[cabfpub] OCSP Stapling and Short-Lived Certificates Proposal
Robert Relyea
rrelyea at redhat.com
Mon Mar 25 16:24:40 MST 2013
On 03/25/2013 03:42 AM, Yngve N. Pettersen wrote:
> On Mon, 25 Mar 2013 11:37:15 +0100, Gervase Markham <gerv at mozilla.org>
> wrote:
>
>> On 23/03/13 05:23, Ryan Sleevi wrote:
>>> If the CA has issued a valid, signed OCSP response, then they have no
>>> ability to revoke that certificate for any client that supports
>>> stapling, until that OCSP response expires.
>> And if I were an attacker, the very first thing I'd go, on obtaining my
>> dodgy cert, would be to grab a valid OCSP response for it so I had that
>> in the bank too.
> This is the reason why I would have preferred that OCSP stapled responses
> had a freshness requirement, meaning that they would have to be refetched
> (and regenerated) every few hours, no matter that it is nominally valid
> for days.
>
The only date that the client can rely on for 'how fresh is this' is the
date on the OCSP response. Any OCSP response can come from multiple
sources, and the date that we actually 'fetched' the response is
irrelevant. The client can't tell if the server has refetched the OCSP
response or not unless the OCSP response has a fresher date in the
signed response.
Any plan that assumes the client records or cares when the last time it
fetched a response is irrelevant. This whole matter is in the hands of
the CA issuing the OCSP response. If the response is issued to be valid
for 7 days, it would be extremely difficult for the CA to confidently
revoke the cert before the end of the 7 days. Yes many, even most users
will see the revoked response, but a user under active attack won't see
the response until the end of the 7 days (as rsleevi has clearly pointed
out).
bob
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