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RE: Who cares what MUST be signed? (was Locations...)

From: John Boyer <jboyer@uwi.com>
Date: Wed, 1 Dec 1999 09:12:26 -0800
To: "Daniel LaLiberte" <liberte@w3.org>
Cc: <w3c-ietf-xmldsig@w3.org>
Hi Daniel,

Taken out of the context in which I made the statements, yes my statement
may seem poorly worded.

True, the signer and verifier to mutually agree on the exact nature of an
agreement.  Indeed, it is really often the verifier that sets up the bulk of
what it said in the document because they want the signer to sign it and
send it back.

What I meant to say is that *at the moment of signing*, the signer of the
document must have a clear  definition of what he/she is signing as it is
the signer that is using his/her private key to authorize some transaction.
Once the signer's signature is affixed, if changes to the transforms are
allowed, then the document surrounding the bits covered by the signer's hash
can be changed.

If the signer intends to sign the document but must omit certain very
specific pieces so that others may complete the document (e.g. multiple
signatures, office use only sections, etc.), then changes to the document
other than those stated in the transforms should break the signature since
the changes cannot be guaranteed not to impact the bits signed by the
signer.  Thus, the signer must sign the transforms to prevent them from
being changed.

So, in the context of signature generation and verification mechanics (as
opposed to the context of transaction negotiations), it should be easy to
see that the signer does determine what must be signed since the definition
of what must be signed versus what must be omitted cannot change once the
signer has signed.

John Boyer
Software Development Manager
UWI.Com -- The Internet Forms Company

-----Original Message-----
From: w3c-ietf-xmldsig-request@w3.org
[mailto:w3c-ietf-xmldsig-request@w3.org]On Behalf Of Daniel LaLiberte
Sent: Wednesday, December 01, 1999 7:55 AM
To: Mark Bartel
Cc: w3c-ietf-xmldsig@w3.org
Subject: Who cares what MUST be signed? (was Locations...)

Mark Bartel writes:
 > 2. The signer of the document, not the verifier, determines what is
 > important to be signed.
 > I disagree with this.  I think both the signer and the verifier do this
 > determination.  It depends on the nature of the interaction.  I submit
 > following points:

I agree with Mark, for the reasons stated.  This issue seems
fundamentally important to the goals and requirements of signatures and
until this is worked out, there are likely to be misunderstandings and
disagreements elsewhere, as is evident from the discussions of the past
several weeks.

To reiterate, it seems that it is ONLY the verifier that cares what is
signed.  When does the signer really WANT to sign something except to
satisfy the needs of verifiers.  What advanatage does the signer have?
A signature seems to only obligate the signer.

I'll leave open the possibility that there is some advantage for the
signer, regardless of the needs of verifiers, but I can't think of any
right now.  Please provide some examples to inform this discussion.

 > a) The signer cares that *sufficient* information is signed.

...sufficient for the purposes of satisfying the needs of the verifier.

 > b) The verifier only cares that the information relevant to it is signed.
 > c) If the verifier chooses to ignore signed information, that doesn't
 > the assertion that the signer made.


 > d) If the signer is making an assertion intended to have legal force
 > will frequently not be the case), they will be concerned that information
 > that they are *not* asserting is *not* signed.


 > If location is signed, the document is moved, and the verifier doesn't
 > that doesn't change the assertion that the signer made.  This is points
 > b), and c).
 > And since it doesn't particularly matter where the bits come from, I
 > say that we should specify the signed location to mean that "this is
 > the resource was when I signed it" and nothing more.  Therefore d) would
 > be an issue.  I don't think it is reasonable for the assertion to be
 > is where the resource will be for all time".  If the verifying
 > requires that the resource be at the original location, it can do that
 > itself.  It seems to me that this is a lot like requiring an original of
 > contract rather than a photocopy; it is the recipient for which this
 > matters.  Sometime an organization will accept a copy, sometimes it

Daniel LaLiberte
Received on Wednesday, 1 December 1999 12:14:13 UTC

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