There's been some discussion of revocation services deep in other threads.
I think understanding these is important, and I suspect my knowledge is too limited; does anyone have a link to a primer?
One place to start is the OpenValidation site (which I think is run by a vendor of validation software, so may not be totally disinterested):
http://www.openvalidation.org/en/info/index.html
Other validation tools vendors have overviews as well (which of course may have their own vendor-specific bias:
http://www.corestreet.com/whitepapers/w02_01v2_certificate_validation_choices.pdf http://www.tumbleweed.com/products/validationauthority/va_wp_cv_in_pki.html
(The latter requires registration.) And here's an academic paper on OCSP that Google turned up:
http://www.guanotronic.com/~serge/paper.pdf
Finally, there's lots of PKI-related references at
http://www.pki-page.org/
but I didn't have time to wade through all the reference to try to find anything specifically on cert validation.
What proportion of CAs run a revocation service?
All commercial CAs support at least CRLs AFAIK, as do pretty much all other CAs that we'd want to consider including in our default list.
Incidentally, note that there's actually multiple levels of CRLs we have to worry about: There are the CRLs for the root CAs, but often root CAs often have subordinate CAs that are the ones actually issuing the certs. In those cases it's the subordinate CA CRLs that are of most interest for checking end entity certs (e.g., for SSL-enabled web sites), while the root CRLs are there for the (hopefully) rare case when the root has the revoke a cert for a subordinate CA.
What proportion of them use OCSP?
It's fairly low, at least as evidenced by those CAs in my list applying to be included:
http://www.hecker.org/mozilla/ca-certificate-list
However VeriSign and Thawte run OCSP responders (at ocsp.verisign.com and ocsp.thawte.com respectively), so this accounts for a significant fraction of SSL certs in use.
Also note that OCSP responders can be run by organizations other than CAs; this is what the folks at openvalidation.org do. However usually such services are based on the data in published CRLs, and hence aren't any more up to date than those CRLs.
Can someone summarise the issues with turning on OCSP in Firefox by default?
I'll leave this question to the developers. However I think the most likely approach would be to default Firefox to the option "Use OCSP to validate only certificates that specify an OCSP service URL". (See Preferences... -> Advanced -> Validation).
Does Firefox support CRLs? Can it get them automatically? Why doesn't it? Are they too big?
Firefox does support CRLs. You can download CRLs from my page listed above; try it on a CA that's already in Firefox 1.0.1, like one of the two Comodo CAs. Note that Firefox can retrieve CRLs automatically, but you have to enable this.
As to why Firefox can't just enable CRL checking and downloading of CRLs by default, that question is better directed to the NSS developers; IIRC the reasons are not trivial.
Note that for a few CAs the CRLs are rather big; the extreme case of this is the DoD PKI with millions of certs issued. For that reason DoD is moving to online validation; see the following Government Computer News story:
http://www.gcn.com/24_6/dodcomputing/35282-1.html
If CRLs are a pain to fetch, could we have a scheme where being suspicious of an SSL site (according to some sort of phishing detector) triggered a CRL download?
One key issue is knowing where to download the CRL from. As I understand it the certificate can contain information about this (just as there's a certificate field for OCSP info) but I don't think NSS yet understands it. Again I'll defer to the developers for the complete answer.
Frank
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