RE: OCSP Algorithm Agility

["Seth Hitchings" <[e-mail]>]

I'm generally in favor of simpler approaches, such as those suggested by
Santosh, as they favor interoperability. Updating deployed applications to
understand new elements of the protocol could take just as long to roll out
as the new algorithm support that's purportedly lacking.

I can see a situation in which the CA cannot assume that the entire user
population understands a new algorithm such as SHA-256, and thus has to
continue using SHA-1 with RSA when signing certificates. Individual clients
that do understand SHA-256 may wish to receive OCSP responses signed using
SHA-256 with RSA, and would need a way to signal their desires to the OCSP
responder.

The question is, then, is there any security benefit to having the signature
algorithm on the OCSP response stronger than the signature algorithm on the
certificate? If you can break the algorithm and cause me to accept a
contrived OCSP response, could you not do the same thing with a certificate?

Seth

-----Original Message-----
From: [e-mail] [mailto:[e-mail]] On
Behalf Of Santosh Chokhani
Sent: Wednesday, October 10, 2007 2:28 PM
To: Hallam-Baker, Phillip; [e-mail]
Subject: RE: OCSP Algorithm Agility


See responses in-line below.

-----Original Message-----
From: Hallam-Baker, Phillip [mailto:[e-mail]] 
Sent: Tuesday, October 09, 2007 2:07 PM
To: Santosh Chokhani; [e-mail]
Subject: RE: OCSP Algorithm Agility

Not if they don't know that the client can understand them they can't.
[Santosh Chokhani] The Responder has two ways to know what the client
understands.  From certID or from CA.  If the client does not understand
the algorithm used in certificate signing, revocation information is
useless.

And the issue is not just hashing algorithms, it is signature
algorithms. 
Like your previous posts you continue to assume that cipher strength is
a linear quantity with only one dimension and that any client which
supports strength 2X must automatically support strength X. 
[Santosh Chokhani] I am making no such assumptions.  I am saying that if
an algorithm is good enough for a certificate, it is good enough for
revocation notification for that certificate.

That is not the case if you consider the practicalities of deploying ECC
in an RSA world. 

To give a practical example, VeriSign issues a cert for an ECC key
signed with an ECC key. The email program on Alice's machine in the DHS
supports ECC but not the DHS SCVP server or OCSP relay.
[Santosh Chokhani] And what is the problem with that?  They have no
choice but to use RSA.

In the real world the OCSP service does not have a necessary connection
to the CA. There are plenty of commercial OCSP offerings that report on
certificates issued by other CAs. 
[Santosh Chokhani] And how does this relate to algorithm agility?  If
the OCSP does not do ECDSA it won't.  If OCSP supports both RSA and
ECDSA, it can take the cue from the CA CRL to determine what algorithm
to use for signing OCSP responses.  If the OCSP does not consume CRL,
the mechanism that tells it revocation information can also tell it the
signature algorithm to use.

> -----Original Message-----
> From: [e-mail] 
> [mailto:[e-mail]] On Behalf Of Santosh Chokhani
> Sent: Tuesday, October 09, 2007 11:16 AM
> To: Hallam-Baker, Phillip; [e-mail]
> Subject: RE: OCSP Algorithm Agility
> 
> 
> The OCSP and SCVP can transition to stronger algorithms any 
> time they want.
> 
> The OCSP can take their clue from the CA or the certID field 
> to decide the hashing algorithm.  When multiple certID is 
> present, OCSP can take low water mark of these.
> 
> SCVP is no different than a client in terms of validating a 
> path.  In terms of signing a response, it can take the low 
> water mark of the hashing algorithms in the certificate chain.
> 
> -----Original Message-----
> From: [e-mail] 
> [mailto:[e-mail]]
> On Behalf Of Hallam-Baker, Phillip
> Sent: Monday, October 08, 2007 7:27 PM
> To: Santosh Chokhani; Andrews, Rick; [e-mail]
> Subject: RE: OCSP Algorithm Agility
> 
> 
> The objective here is to overcome a real deployment obstacle 
> that creates a lockstep issue when use of stronger algorithms 
> is attempted.
> 
> The CA cannot issue a certificate that employs a new 
> algorithm until it is known that all clients that might rely 
> on the certificate are capable of processing the new 
> certificate. Requiring lockstep updates to the OCSP and SCVP 
> infrastructure to be made at the same time renders transition 
> to stronger algorithms a much lengthier and much less 
> practical proposition.
> 
> 
> > -----Original Message-----
> > From: [e-mail]
> > [mailto:[e-mail]] On Behalf Of Santosh Chokhani
> > Sent: Saturday, October 06, 2007 4:12 PM
> > To: Andrews, Rick; [e-mail]
> > Subject: RE: OCSP Algorithm Agility
> > 
> > 
> > Rick,
> > 
> > SCVP or other intermediaries are red herring.  Whoever 
> processes the 
> > certificate processes the revocation information for that 
> certificate.
> > It does not change the following:
> > 
> > 1.  There is no security benefit in using a stronger algorithm for 
> > OCSP
> > response than for the certificate in question.   Neither, 
> there is any
> > benefit from interoperability viewpoint for these two to be 
> different.
> > 
> > 2.  If the OCSP Responder does not get a CRL, it can use the same 
> > mechanism to get the hashing algorithm used as it uses to get the 
> > revocation information.
> > 
> > -----Original Message-----
> > From: Andrews, Rick [mailto:[e-mail]]
> > Sent: Wednesday, October 03, 2007 3:29 PM
> > To: Santosh Chokhani; [e-mail]
> > Subject: RE: OCSP Algorithm Agility
> > 
> > Santosh,
> > 
> > Sorry for the long delay in responding - travel and vacation.
> >  
> > Not all OCSP responders work from CRLs, so they won't take 
> their cue 
> > from the CRL. Nor should they take their cue from the 
> signature on the 
> > cert in question, I believe. Let me try to restate my argument in a 
> > different way.
> >  
> > With SCVP delegated path validation, the client requesting a cert's 
> > status from an OCSP responder will be different from the 
> client at the 
> > other end of the SSL connection. Those two clients may have very 
> > different capabilities in terms of supported signature and hash 
> > algorithms. It's not realistic to expect that all SSL clients, all 
> > SCVP servers, and all CAs will be able to upgrade in 
> lockstep to new 
> > algorithms as they are developed. Allowing the OCSP client 
> and server 
> > to negotiate a mutually-acceptable set of algorithms is 
> essential to 
> > the deployment of newer, stronger algorithms.
> >  
> > Likewise, companies that run large OCSP responders may wish to 
> > gradually move to ECC-based signatures for all their OCSP 
> responses, 
> > even those for certs with RSA or DSA keys, because ECC 
> signatures are 
> > cheaper to produce. If algorithm agility is added to OCSP, those 
> > companies can gradually achieve the move to ECC without 
> disrupting the 
> > installed base of OCSP clients that don't support ECC.
> >  
> > -Rick Andrews
> > 
> > > -----Original Message-----
> > > From: [e-mail]
> > > [mailto:[e-mail]] On Behalf Of 
> Santosh Chokhani
> > > Sent: Friday, September 21, 2007 2:45 PM
> > > To: Paul Hoffman; Stephen Kent; [e-mail]
> > > Subject: RE: OCSP Algorithm Agility
> > > 
> > > 
> > > Paul,
> > > 
> > > Here are my views on this.
> > > 
> > > The client should be first asking for the algorithm suite
> > that signed
> > > the certificate in question.  There is no need for the
> > client to ask
> > > for anything stronger.  The client can ask for stronger suites as 
> > > secondary, if client has them.
> > > 
> > > In the scenario you cite, the Responder certificate will
> > not include
> > > RSA with SHA 1 any longer.  So, client will know that
> > Responder only
> > > supported his second choice and he should be ok with it.
> > > 
> > > -----Original Message-----
> > > From: [e-mail]
> > > [mailto:[e-mail]]
> > > On Behalf Of Paul Hoffman
> > > Sent: Friday, September 21, 2007 4:39 PM
> > > To: Stephen Kent; [e-mail]
> > > Subject: RE: OCSP Algorithm Agility
> > > 
> > > 
> > > At 2:07 PM -0400 9/21/07, Stephen Kent wrote:
> > > >How about defining an extension to be included in the cert
> > > issued to an
> > > >OCSP responder by a CA.  The extension would have an
> > ordered list of
> > > >algorithms (hash and signature if we want to address more
> > > than the hash
> > > >agility issue) accepted by the OCSP responder.  An OCSP
> > > client can use
> > > >this info to determine what is the "best" algorithm (or alg
> > > pair) that
> > > >it and the responder share. The combination of this
> > extension and an
> > > >OCSP negotiation procedure will allow the client to detect MITM 
> > > >downgrade attacks. In fact, if the client acquires the
> > > responder's cert
> > > >prior to making a request, there would not even be a 
> need for real 
> > > >negotiation, since the client would know what alg to 
> request in a 
> > > >response.
> > > 
> > > Imagine the list of algorithms is RSA-with-SHA1 first and
> > > DSA-with-SHA1 second. How does your negotiation work? The
> > client asks
> > > for this message to be signed with RSA-with-SHA1.
> > > But the server knows that RSA-with-SHA1 has been
> > compromised since it
> > > got that certificate from the CA. What does the server say to the 
> > > client to indicate that it only wants to sign with
> > DSA-with-SHA1? What
> > > prevents Mallory from saying the same thing to the client?
> > > 
> > > --Paul Hoffman, Director
> > > --VPN Consortium
> > > 
> > > 
> > > 
> > 
> > 
> 
> 
>