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RE: Omitting Location and Transforms from SignedInfo

From: John Boyer <jboyer@uwi.com>
Date: Fri, 12 Nov 1999 10:04:32 -0800
To: "Greg Whitehead" <gwhitehead@signio.com>, "DSig Group" <w3c-ietf-xmldsig@w3.org>
Message-ID: <NDBBLAOMJKOFPMBCHJOIIECFCCAA.jboyer@uwi.com>
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 Friday, 12 November 1999 13:05:22 GMT

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