RE: OCSP Algorithm Agility

["Hallam-Baker, Phillip" <[e-mail]>]

The problem with KISS is that the definition of simple can depend markedly on one's point of view. Simplifying assumptions can lead to great operational complexities, assuming the world to be simpler than it is tends to lead to complexity in my experience.
 
I don't follow the assumption that the OCSP responder is using the 'certificate signing key'. which certificate, the end entity cert under test or the OCSP server cert? The OCSP server might well have multiple certs. The request is asking for a response + cert chain that meets specific criteria. The OCSP responder may not have any relation to the certificate signer whatsoever. If it is a CRL driven responder it might easily be using Suite B rather than RSA or vice versa.
 
 
Putting a statement of the algorithms offered into the OCSP responder cert would certainly be one approach to closing the attack. I am not convinced that the attack is relevant but it is certainly more relevant than a great number of features we have already included in the PKIX stack.
 

 

________________________________

	From: [e-mail] [mailto:[e-mail]] On Behalf Of Santosh Chokhani
	Sent: Wednesday, September 19, 2007 1:58 PM
	To: Hallam-Baker, Phillip; pkix
	Subject: RE: OCSP Algorithm Agility
	
	

	Personally I believe in KISS principle, i.e., the Responder uses the key pair (hopefully it does not generate many different keys pairs for different algorithms and key sizes) and hash used to sign it certificate.  Again that is the practical side of how PKIs are deployed.

	 

	If you really wanted to protect against the attack and want in-band algorithm negotiation, the Responder certificate could also contain structure like you propose in a supportedAlgorithms extension.  The client could make a determination if the Responder made the proper selection and detect the MITM attack.

	 

	This still has residual risk of certificate minting due to weak algorithms, but that is a such as bigger problem that crypto suite selection dwarfs in comparison.

	 

	
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	From: Hallam-Baker, Phillip [mailto:[e-mail]] 
	Sent: Wednesday, September 19, 2007 1:05 PM
	To: Santosh Chokhani; pkix
	Subject: RE: OCSP Algorithm Agility

	 

	True, but the attacker can only force the selection of a particular algorithm that is acceptable to both the requestor and responder as acceptably secure.

	 

	A negotiation has to start somewhere and there is always a situation where a downgrade attack is going to be possible. 

	 

	 

	What we could do is to allow the responder to echo the algorithm request in the response so that the responder can detect a downgrade attack. But that does not get us very much further. If a requestor offers a weak algorithm they are vulnerable to a downgrade attack.

	 

	We need a policy layer. One approach would be to infer the OCSP responder policy from the certificate. If the cert has an SHA-256 certificate the requestor might be able to presume that SHA-256 is supported by the OCSP responder. This is not necessarily the case though since the OCSP service might be entirely separate.

	 

	A better approach would be to use security policy statements distributed through the DNS, WS-Policy or whatever. 

	 

	 

	Downgrade attack is much less of a concern to me than the ability to effect a transition from SHA-1 to SHA-256 before the known vulnerabilities of SHA-1 allow an attacker to cause a compromise. If we switch to SHA-256 before the downgrade attack gives the attacker an advantage the problem is very small. The only circumstance where it is relevant is if the OCSP token is to be stored and used in a persistent document archive, and then only if it is not verified when it was obtained.

		 

		
________________________________


		From: Santosh Chokhani [mailto:[e-mail]] 
		Sent: Wednesday, September 19, 2007 10:50 AM
		To: Hallam-Baker, Phillip; pkix
		Subject: RE: OCSP Algorithm Agility

		The approach provides an attacker (MITM) an opportunity to force the OCSP Responder to select weaker algorithms.

		 

		
________________________________


		From: [e-mail] [mailto:[e-mail]] On Behalf Of Hallam-Baker, Phillip
		Sent: Tuesday, September 18, 2007 2:53 PM
		To: pkix
		Subject: OCSP Algorithm Agility

		 

		Looking at places where we currently use SHA-1 it appears that there will soon be a need to support SHA-256 in OCSP.

		In particular we need a mechanism to allow the requestor to state which algorithms they can accept from the responder. I did a sketch of a scheme (see below).

		The question is how best to get this processed. Algorithm agility is one of the issues that is blocking advance of OCSP on the standards track so if this was the only issue that was blocking advancement it would perhaps make sense to rev the OCSP RFC.

		Alternatively it might be considered preferable to address algorithms agility 'across the board'. Although the only other protocol that is analagous is SCVP. I don't think the mechanism specified there translates to OCSP.

		So I was thinking that a good place to start was probably to submit some variation of the following as a personal ID:  

		The extension would need to be something like

		 

		id-pkix-ocsp-algorithmselectors   OBJECT IDENTIFIER ::= { id-pkix-ocsp 3 }

		 

		AlgorithmSelectors ::= SEQUENCE OF AlgorithmSelectorEntry 

		 

		AlgorithmSelectorEntry ::= SEQUENCE {

		    SignatureAlgorithm   [0] OBJECT IDENTIFIER OPTIONAL,

		    DigestAlgorithm   [1] OBJECT IDENTIFIER OPTIONAL,

		    }

		 

		The responder SHOULD sign the reponse with a signature algorithm and digest algorithm pair that match one ot the following criteria:

		 

		1) The Signature and Digest algorithm both match an algorithm selector entry exactly.

		 

		or

		 

		2) The Signature Algorithm used matches an algorithm selector entry where only the Signature Algorithm OID is specified and the digest algorithm matches an algorithm selector entry where only the Digest Algorithm OID is specified.

		 

		 

		So for example a requestor can stipulate that they will accept any combination of RSA-1024, RSA-2048, SHA-1 and SHA-256 or DSA2 with SHA-256 as follows:

		 

		{SignatureAlgorithm   RSA-1024}

		{SignatureAlgorithm   RSA-2048}

		{DigestAlgorithm      SHA-1 }

		{DigestAlgorithm      SHA-256  }

		{SignatureAlgorithm   DSA2, DigestAlgorithm      SHA-1 }

		 

		The rationale here is that in most cases the signature and digest algorithms are 'mix and match'. But sometimes they are not, in particular if specific signature hardware is to be used.

		 

		I note that SCVP does something rather different but I don't see how the scheme specified there would fit OCSP.