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Just a quick question. Does this mean that if your OCSP servers were to go down, a lot of SSL enabled websites and applications will stop working? Seems like a serious single point of failure for modern day internet. I was always under the assumption that clients do not have to contact the CA (every time?) before a TLS handshake takes place.

OCSP Stapling seems to be the way to mitigate this problem, but not all web servers implement it (for instance lighttpd does not). Any recommendation from Let's Encrypt on this issue?




This is exactly the problem with OCSP. There's no way to tell if the remote server is down, or if a malicious actor sitting in your path is blocking it. So your browser can either a) make it super easy for all your OCSP-using sites to appear down, which will encourage users to use other, non-OCSP, sites, or b) silently fail, which makes the entire exercise pointless.

Stapling only partially mitigates this, as it doesn't currently work with intermediate certs, and at this point most sites have at least one intermediate cert.


Could you elaborate on why they don't work with intermediate certs?


RFC 6066 specifies that you can only have one certificate in an OCSP response - as with intermediate certs you need to be able to respond with a chain, this does not work. RFC 6961 defines a multiple response capability, but my understanding is that currently this is not sufficiently widely implemented to be useful yet.


Thanks! I thought it's enough if the stapled response contains information only about the intermediate cert, and the browser would accept that as good enough, if the chain it got in the handshake is valid.

https://bugzilla.mozilla.org/show_bug.cgi?id=611836 - this looks pretty abandoned (last comment 3 years ago) :/

and I found no bug for Chrome.


Chrome, as far as I know, does not do OCSP - https://www.imperialviolet.org/2012/02/05/crlsets.html


I'm pretty sure revocation checks like CRL and OCSP all fail-open (they still allow the connection if contacting the revocation server fails).


Some have argued that this is why CRL and (especially) OCSP are useless pieces of security theater: they don't actually protect against a crafted attack because they fail-open in the very situations that a determined adversary can trigger, so they only "protect" in situations where no real threat exists. It's simply feel-good bookkeeping.

Adam Langley, working on Google Chrome [1][2][3], has been very vocal about OCSP's faults, and Chrome began using its own auto-update to ship an aggregate of revocations of high-value certs directly to browsers out-of-band. Despite this being another famous instance of Chrome going against the grain of other browser vendors, I believe this was the correct solution: offering better protection for a curated subset of sites vs. pretending to -- but not actually -- protecting all sites.

[1] https://www.imperialviolet.org/2012/02/05/crlsets.html [2] https://www.imperialviolet.org/2014/04/19/revchecking.html [3] http://www.zdnet.com/article/chrome-does-certificate-revocat...


> I believe this was the correct solution: offering better protection for a curated subset of sites vs. pretending to -- but not actually -- protecting all sites.

I concur, but note that it is possible to do better and offer better revocation protection for all sites, with low bandwidth/storage costs: http://www.ccs.neu.edu/home/cbw/static/pdf/larisch-oakland17...


This paper -- the CRLite proposal -- is wonderfully well thought-out, experimentally tested, and meets the design goals much better and more elegantly than any other attempt to solve the certificate revocation problem.

Looks like it was posted here and got very little traction [1]; a shame. But it will be presented in a few days at the IEEE Symposium on Security and Privacy [2]. I hope it will get the coverage and examination it deserves.

[1] https://news.ycombinator.com/item?id=13982861 [2] https://www.ieee-security.org/TC/SP2017/program-papers.html


The one app chain (complex code signing) I worked on with OSCP, we defaulted to failsafe, but it could be overridden in the 'main' (enterprise CMS) app. The installer required OSCP or wouldn't install.

Basically the first and last mile were hard fails but everything in between was advisory if the signature checked out.




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