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RE: Protect Algorithm identifiers?


Jim:
I now understand your concerns. Hugo Krawczyk is advocating the use of a random number, but he is not advocating the use of the parameters field to carry them.
In the case of RSA, Hugo is suggesting the encryption of the random number and the hash value output with the private key. Verification is performed by decrypting the signature value, recovering the random number and hash value, and then computing the hash with the random number, and then comparing the output to the hash value from the signature.
In the case of DSA, Hugo has suggested using the r value as the random number. Recall that a DSA signature is two values: r and s. The r value is independent of the message content, and it can be computed before the s value, which depends on the hash value.
As you say, the proposal made by Hugo may not become widely accepted. But if it is, it does not depend on integrity protection of the parameters. 

Russ

At 04:37 PM 5/2/2006, Jim Schaad wrote:

Russ,

Wrong -- If one changes the parameters that are being used, one can change
the source document as well and still get a match of the hash values.

There have been proposals (which I am sure will not survive) to use a random
value and xor to strengthen the hash function.  Change the both the document
and the random in a well known manner would still result in a match on the
have value, but the document would no longer be the same.

Jim


> -----Original Message-----
> From: Russ Housley [mailto:housley@xxxxxxxxxxxx]
> Sent: Tuesday, May 02, 2006 1:23 PM
> To: Jim Schaad; ietf-smime@xxxxxxx
> Subject: RE: Protect Algorithm identifiers?
>
> Jim:
>
> 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?
>
> Russ
>
> At 04:24 PM 5/2/2006, Jim Schaad wrote:
> >Russ,
> >
> >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.
> >
> >Jim
> >
> >
> > > -----Original Message-----
> > > From: owner-ietf-smime@xxxxxxxxxxxx
> > > [mailto:owner-ietf-smime@xxxxxxxxxxxx] On Behalf Of Russ Housley
> > > Sent: Tuesday, May 02, 2006 12:50 PM
> > > To: jimsch@xxxxxxxxxx; ietf-smime@xxxxxxx
> > > 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
> > >
> > >
> > >
>
>
>