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RE: Protect Algorithm identifiers?

2006-05-02 13:45:44


If one has confidence in the hash algorithm and the hash values match, then there is not a problem, regardless of the parameter values. Right?


At 04:24 PM 5/2/2006, Jim Schaad wrote:

As I have stated, what really worries me is if one starts to play with the
parameters of the new round of hash algorithms that are being looked at.
There is no protection for these parameters either in the signature or in
the default settings of the validation code.


> -----Original Message-----
> From: owner-ietf-smime(_at_)mail(_dot_)imc(_dot_)org
> [mailto:owner-ietf-smime(_at_)mail(_dot_)imc(_dot_)org] On Behalf Of Russ 
> Sent: Tuesday, May 02, 2006 12:50 PM
> To: jimsch(_at_)exmsft(_dot_)com; ietf-smime(_at_)imc(_dot_)org
> Subject: Re: Protect Algorithm identifiers?
> Jim:
> If the recipient has confidence in the hash algorithm, I do
> not see any problem with the current documents.  I think that
> implementations are going to need to be modified to provide
> an interface for users to indicate which ones are acceptable
> and which ones are not.  The default setting will be vital.
> Russ
> At 11:38 PM 4/17/2006, Jim Schaad wrote:
> >In the process of reviewing documents dealing with multiple
> signature
> >processing, I suddenly realized that we currently do not have any
> >attribute which lets us verify that the correct digest and signature
> >algorithms have been used in verifying a SignerInfo.  The
> question is do we need to do this?
> >
> >More details on what I mean:
> >
> >When you create a signer info you:
> >
> >1.  Hash the body of the message, place the digest value as a signed
> >attribute and the digest algorithm into the SignerInfo
> structure in an
> >unprotected location.
> >
> >2.  Create the sequence of signed attributes, hash the
> value, create a
> >signature value using your private key and place the signature
> >algorithm and the signature in unprotected locations.
> >
> >The signature does not need any additional protection, however one
> >could change the digest algorithms being used in both the
> signature and
> >body digest locations without a verifier being able to know
> that it has happened.
> >
> >
> >The attack I envision would be to find a body that has a
> digest of the
> >same length, but uses a different algorithm and update the
> SignerInfo
> >structure with the new digest algorithm data and the body with the
> >updated body.  This would currently be undetectable by a verifier.
> >
> >Jim

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