Re: [Trans] Proving certificate freshness

[Ben Laurie <[e-mail]>]

On 1 October 2014 14:45, Daniel Kahn Gillmor <[e-mail]> wrote:
> On 09/30/2014 11:56 PM, David Leon Gil wrote:
>> Something that is rather difficult to prove, at present, is that a
>> certificate has been used after it has expired or been revoked.
>
> To be clear, this is a concern because it's otherwise possible to claim
> abuse of a revoked/expired cert without being able to substantiate the
> claim.
>
> That makes reports of cert misuse less trustworthy, which makes it
> harder to act on them.
>
> However, CT is designed to detect (and hopefully deter) misissuance, not
> misuse.  Is this scope creep?

It doesn't require anything from CT, though, so not really scope creep
for CT itself.

That said, clearly an append-only log can be used for timestamping
directly, so if people think this is a good idea, then it could be
implemented using a log that, say, issues a timestamp for certs
periodically - and you require from the server a reasonably fresh
timestamp.

However, if you're prepared to modify servers, then you might as well
go the whole hog and implement revocation transparency, which would
make it impossible to use a revoked certificate from some short-ish
time after revocation.

Outline protocol: server periodically goes to an RT server and gets
proof of non-inclusion in the list of currently revoked certificates
(this proof would use a verifiable map, as documented in
http://www.links.org/files/RevocationTransparency.pdf - note that our
current thinking on verifying maps is you have a CT-like log of state
changes, which the verifiable map is required to be consistent with,
rather than maintaining a log of root hashes as suggested in that
paper).