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.
From: Russ Housley [mailto:housley(_at_)vigilsec(_dot_)com]
Sent: Tuesday, May 02, 2006 1:23 PM
To: Jim Schaad; ietf-smime(_at_)imc(_dot_)org
Subject: RE: Protect Algorithm identifiers?
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
the parameters of the new round of hash algorithms that are
being looked at.
There is no protection for these parameters either in the
in the default settings of the validation code.
[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?
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
ones are not. The default setting will be vital.
At 11:38 PM 4/17/2006, Jim Schaad wrote:
In the process of reviewing documents dealing with multiple
processing, I suddenly realized that we currently do not
attribute which lets us verify that the correct digest and
signature algorithms have been used in verifying a
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
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,
could change the digest algorithms being used in both the
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
structure with the new digest algorithm data and the
body with the
updated body. This would currently be undetectable by a