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I was looking at foaf+ssl http://www.w3.org/wiki/Foaf+ssl and wondering if its possible to spoof the modulus of the browser cert so that the foaf and browser cert modulus match

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I had never heard of FOAF+SSL, so you'll have to forgive me. After reading your link, I'm confused. Could you give a better explanation? Maybe an example scenario (e.g., trudy hacks bob's public FOAF document, etc)? – mikeazo♦ Sep 28 '11 at 12:21
What would "spoof the browser cert" mean? Create a new private key matching an existing public one? – Paŭlo Ebermann♦ Sep 28 '11 at 13:39
aren't identity certs just a public key? – Tyler Gillies Sep 29 '11 at 2:28

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I'm not sure exactly what you are asking. In the scenario, there are two certificates: Romeo's FOAF certificate and Juliet's SSL certificate. However Romeo's certificate is used in two different ways: once to authenticate (over mutually authenticated SSL) and once as posted on a domain he controls.

I believe your question may be, if an active adversary saw Juliet request the certificate information from Romeo's domain (as specified in the cert's alt name), could the adversary modify the response to match a bogus FOAF certificate it provided? The answer is yes, since this connection is not over SSL!

The real question is what happens in step 6? Does Juliet look in her friends databases for the URL or for the public key? If it is the former, there seems to be a security vulnerability. If it is the latter, it is ok, since an adversary has to substitute their own public key to form the HTTPS connection (which requires them to know the corresponding private key).

I once experimented with FOAF but don't know anything about FOAF+SSL beyond this article.

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Upon further consideration, it seems more troubling. Even if the query were made over SSL, there is nothing to prevent the adversary from substituting their own SSL certificate. Traditionally these attacks are prevented because the certificate is signed by a trusted CA but given the whole point of FOAF+SSL is to eliminate CAs and build a "web of trust," I'm becoming convinced there is no solution to this attack. – PulpSpy Oct 12 '11 at 20:44

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