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RE: Omitting Location and Transforms from SignedInfo

From: Greg Whitehead <gwhitehead@signio.com>
Date: Wed, 17 Nov 1999 16:09:20 -0800
Message-ID: <6B962A1EE646D31193270008C7A4BAB5381EB7@mail.paymentnet.com>
To: "'John Boyer'" <jboyer@uwi.com>, DSig Group <w3c-ietf-xmldsig@w3.org>
Hi John,

The question isn't so much whether transforms are signed or not, but what
they're allowed to do. I have to admit a certain ignorance of the
capabilities of the transforms proposed for inclusion in the initial version
of the spec, but I'm assuming that once Pandora's box is opened anything is
possible.

The bottom line is that any transform over SignedInfo with the ability to
modify itself (along with some other critical piece of information like
SignatureMethod or the DigestValue or Transforms in an ObjectReference) is
open to abuse. Given all of the restrictions that must be placed on
transforms over SignedInfo in the name of security, it seems like a clumsy
solution to the original problem.

I'm not discounting the original problem -- I just prefer a solution that
passes the object location problem off to the application. Even in the case
of URLs pointing to resources that haven't moved, an application verifying
the signature on an object probably has other uses for the object itself
(and should probably verify the signature against the version of the object
that it fetches, rather than a version that the signature verification core
fetches independently).

-Greg


-----Original Message-----
From: John Boyer [mailto:jboyer@uwi.com]
Sent: Wednesday, November 17, 1999 2:30 PM
To: Greg Whitehead; DSig Group
Subject: RE: Omitting Location and Transforms from SignedInfo


Hi Greg,

I apologize for the delay; I've had a lot of work to overcome recently.

I almost agree with your entire analysis.

Basically, in order to hijack the signature, it seems that you would have to
create a SignedInfo transform that omits itself as well as does the
interesting things you described.  By contrast, I rather assumed that we
would create a bottom turtle (to borrow one of Rohit's favorite phrases, and
also to follow the irony that the problem comes from self-reference, a
favorite of Russell's).

By bottom turtle I mean that SignedInfo should include its Transforms in its
text to be hashed even if the Transforms say to omit themselves.  I faced
this problem two years ago in XFDL, and the solution was to disregard the
filters if they tried to filter themselves out of the message to be
digested.  Actually, we do have a larger problem now because of that awful
XSLT transform.  I only want the transforms for the xpath transform because
I only want keep/omit abilities on the given markup, not the ability to turn
it into Mohawk.

Anyway, my point is that I agree with your analysis as given, but the
problem is the result of differing assumptions.  I assume that the
SignedInfo transforms are automatically covered by the SignatureValue and
hence are not modifiable.  Under that assumption, the security problem
vanishes before it gets started.

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



<Greg/>

Forget that, however, and consider this!

You sign object x without specifying any transform over SignedInfo (the
signature includes all SignedInfo elements):

	<Signature>
	<SignedInfo>
	...
	<ObjectReference location="x">
	...
	<DigestValue>xxx</DigestValue>
	</ObjectReference>
	</SignedInfo>
	...
	</Signature>

I modify your signature to refer to object y and I introduce a transform
that undoes this change (restores the original contents of the SignedInfo):

	<Signature>
	<SignedInfo>
	<Transforms>
	<Transform>
	replace SignedInfo with original contents
	</Transform>
	</Transforms>
	...
	<ObjectReference location="y">
	...
	<DigestValue>yyy</DigestValue>
	</ObjectReference>
	</SignedInfo>

As I understand the proposed verification logic, I have now hijacked your
signature. Specifically, we will first go find object y and verify that its
digest is yyy. We will then apply the transformation to SignedInfo, which
will restore it to what you originally signed. We will then verify your
signature of the transformed SignedInfo, which will succeed (implying that
you signed y).

You can't solve this problem by suggesting that verification of digest
values should take place after the transformations are applied, since you
are proposing to drop information like location.

Getting back to the need to drop location at all, your objection to
application specific object location logic is that it can't be cleanly
integrated with the core signature verification mechanism.  I don't buy
that.  It's a lot cleaner to use a callback to locate objects than to rely
on re-writing the ObjectReferences within the signature.

-Greg

-----Original Message-----
From: John Boyer [mailto:jboyer@uwi.com]
Sent: Friday, November 12, 1999 10:05 AM
To: Greg Whitehead; DSig Group
Subject: RE: Omitting Location and Transforms from SignedInfo


Hi Greg,

This is not the only concrete example.  If you go back to the letter I sent
(or indeed my IETF presentation conclusion) you will find that it is not
only possible for transforms to reduce security but also to obliterate it
entirely.  Rather than omitting the SignatureMethod, simply omit the
DigestValue from ObjectReferences, which disconnects the signer from the
signed.

Still, no application is going to create such signatures precisely because
they will make such newsworthy fodder for the Bruce Schneier types of the
world.

Essentially, signing SignedInfo is an implied ObjectReference.  Whatever
fears we have about omission from SignedInfo are precisely the same fears
for omission from referenced objects.  It is a dangerous but necessary
feature (and yes I can explain why (again) in painstaking detail for those
who require it, or you could just come to heckle my presentation of the
material at RSA2000).

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

I just thought of a concrete example:

SignatureMethod is included in SignedInfo to protect against a downgrade
attack, should one of the currently approved signature methods be broken.

Allowing arbitrary transformation of SignedInfo could potentially defeat
this protection by allowing an attacker to introduce a transformation that
substitutes in a broken SignatureMethod along with a reference to a modified
object and other changes (exploiting the broken signature method to produce
the original SignatureValue over the modified SignedInfo).

-Greg
Received on Wednesday, 17 November 1999 19:09:26 GMT

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