RE: OCSP Algorithm Agility

["Andrews, Rick" <[e-mail]>]

Santosh,
 
I don't think that's realistic in the mass market. Let's say a major
financial institution wants to buy a SHA-256 cert from us because
they're concerned about SHA-1 weaknesses. Their clients might be
ordinary users with ordinary browsers on ordinary OSes, who will then
need to rely on the SHA-256 cert (and therefore need SHA-256 to check
the signature on the FI's cert as well as the signature on the OCSP
responder's cert). The financial institution has no way to force these
clients to upgrade to SHA-256. Even if clients really want to rely on
the cert they have to wait until their browser vendor or their OS vendor
build in SHA-256 support.
 
Even if we created separate CAs to sign SHA-1 certs and SHA-256 certs,
the situation would not be improved for the end client.
 
The alternative is for the FI to set up two parallel web sites, once
with a SHA-1 cert and one with a SHA-256 cert, and somehow direct
clients to the appropriate site. I doubt anyone would want to do this.
 
-Rick Andrews


________________________________

	From: Santosh Chokhani [mailto:[e-mail]] 
	Sent: Thursday, September 20, 2007 5:40 PM
	To: Andrews, Rick; pkix
	Subject: RE: OCSP Algorithm Agility
	
	

	Rick,

	 

	Let us say that CA X starts issuing SHa-256 certificates.
Clients that rely on that CA, will need to upgrade to SHA-256.  The CA
can start signing the CRLs using SHA-256.  The OCSP Responder R can take
its cue from the CRL and sign those responses with SHA-256.

	 

	Now, if the same OCSP Responder R also serves CA Y who is still
using SHA-1.  Again, taking the cue from CRL, the same Responder R can
sign the responses using SHA-1.

	 

	Next, you might argue that VeriSign has a large set of disjoint
customers using the same CA.  I, do not think it is a big deal to have
two physically or logically distinct CAs one for SHA-256 population and
one for SHA-1 population.

	 

	
________________________________


	From: [e-mail]
[mailto:[e-mail]] On Behalf Of Andrews, Rick
	Sent: Thursday, September 20, 2007 5:49 PM
	To: Santosh Chokhani; Hallam-Baker, Phillip; pkix
	Subject: RE: OCSP Algorithm Agility

	 

	Santosh,

	 

	I think it's an installed-base problem. If we start issuing
certs with SHA-256, it will take a long time for all OCSP clients to
build in the support. Until SHA-2 is ubiqitous, we need the server to be
agile with respect to algorithms.

	 

	-Rick Andrews

		 

		
________________________________


		From: [e-mail]
[mailto:[e-mail]] On Behalf Of Santosh Chokhani
		Sent: Thursday, September 20, 2007 10:54 AM
		To: Hallam-Baker, Phillip; pkix
		Subject: RE: OCSP Algorithm Agility

		I am still have misgiving about the need for in-band
signaling for SHA-1 Vs SHA-256.

		 

		For practical PKI, if the certificates are still SHA-1,
why does the OCSP needs to go to SHA-256?  If the certificates have
started using SHA-256, client better process SHA-256.

		 

		I can envision the RFC to accommodate algorithms.  I am
not sure in-band signaling is that important.

		
________________________________


		From: [e-mail]
[mailto:[e-mail]] On Behalf Of Hallam-Baker, Phillip
		Sent: Wednesday, September 19, 2007 5:38 PM
		To: Santosh Chokhani; pkix
		Subject: RE: OCSP Algorithm Agility

		 

		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.

			 

			
________________________________


			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.