Re: [Trans] [trans] #80 (rfc6962-bis): Re-introduce the issuer keyhash into the Precertificate

[Rob Stradling <[e-mail]>]

On 02/07/15 15:00, Stephen Kent wrote:
> Rob,

Hi Steve.

> I agree with your conclusoon that inclusion of the issuer public key hash
> will enable a browser to verify the correspondence between an SCT without
> interacting with a log. Even though we don't yet have a browser behavior
> spec, I agree that this is a good candidate behavior to enable.
>
> Do you propose this as an extension to the current SCT format, or a change
> to the format?

A change to the format.  Proposal here:

https://github.com/google/certificate-transparency-rfcs/pull/58

> If you propose a change, then I'll argue that my proposed
> cert type declaration and checks performed fields also should be included,
> since we'll be making a not backward compatible change to the SCT.

6962-bis has already made backwards-incompatible changes to RFC6962 SCT v1.

> If this is an (optional) extension, then I guess my proposed fields can be
> the second and third extensions defined.

I think that required fields should be in the main part of the SCT 
structure, whilst optional fields should be extensions.

IINM, we agreed a while ago that your proposed fields (cert type 
declaration and checks performed) would be, at most, optional.

(I still think that it makes more sense for monitors, not logs, to 
perform checks relating to the declared cert type).

> Steve
>> #80: Re-introduce the issuer key hash into the Precertificate
>>
>>
>> Comment (by [e-mail]):
>>
>>   This problem affects browser clients and any other client that wants to
>>   verify that an SCT corresponds to a particular cert.  A browser has the
>>   full cert chain (from the TLS handshake) and the corresponding SCTs,
>> but
>>   it is _not_ expected to have the full log entries (i.e. extra_data,
>> etc).
>>
>>   In the current text, the SCT is bound to the Issuer Name (because
>> that's
>>   in the TBSCertificate), but not to the Issuer Key (because that's
>> not in
>>   the TBSCertificate).
>>
>>   There could exist two or more publicly-trusted intermediate CA certs
>> with
>>   the same Name but different Keys.  One of those intermediates might be
>>   logged, while the other(s) are not logged.  Each of them could issue
>> leaf
>>   certs that have an identical TBSCertificate.  Only one of those leaf
>> certs
>>   needs to be logged for there to exist SCT(s) that are valid for all of
>>   those leaf certs.
>>
>>   The logged leaf cert might get revoked, but the other(s) might not get
>>   revoked.  However, the SCT(s) would still work for all of those certs,
>>   including the non-logged ones.
>>
>>   Therefore, we need to fix SCTs so that they're bound to the Issuer Key
>>   Hash as well as the Issuer Name.
>>
>
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-- 
Rob Stradling
Senior Research & Development Scientist
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