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RE: Fw: XML versus ASN.1/DER blob

From: Richard D. Brown <rdbrown@GlobeSet.com>
Date: Tue, 20 Apr 1999 18:46:39 -0500
To: "'John Boyer'" <jboyer@uwi.com>, "'Bede McCall'" <bede@mitre.org>
Cc: "'Dsig group'" <w3c-xml-sig-ws@w3.org>
Message-ID: <003301be8b88$09dd6530$0bc0010a@artemis.globeset.com>

Though it is probably not the subject of this mailing to discuss about the
strength of a particular signature technology, I cannot resist asking what
makes the PenOP scheme secure. Unless, the encryption that you refer to in
step 3 makes use of a strong public-key algorithm, I do not get it.
Attaching a biometric token to a message is no more secure than attaching a
digital certificate to the message - in fact this does not prove anything...

Richard D. Brown

> -----Original Message-----
> From: w3c-xml-sig-ws-request@w3.org
> [mailto:w3c-xml-sig-ws-request@w3.org]On Behalf Of John Boyer
> Sent: Tuesday, April 20, 1999 4:34 PM
> To: Bede McCall
> Cc: Dsig group
> Subject: Re: Fw: XML versus ASN.1/DER blob
> Hi Bede,
> Actually, you start out incorrect, but then begin deriving
> some of the very
> ideas behind handwritten signature technologies.  First off,
> by handwritten
> signature technology, I am not referring to a simple signature bitmap
> (although one can be included for display purposes).
> PenOp is an example of a technology that binds the
> handwritten signature to
> the document.  Further, they would argue that they do a better job of
> binding the document to the signer than cryptography does.
> I'm not sure I
> agree that they do a better job than cryptography in any area
> of signature
> technology, but I'm not prepared to argue that the technology
> is useless, in
> part because several deployments of XFDL use PenOp signatures.
> They bind the document to the signature as follows:
> 1) 90 or so measures of a person's handwriting style are
> recorded into a
> "biometric token".
> 2) a secure hash of the document is computed (sha-1 or md5)
> 3) 1 and 2 are concatenated and the result is encrypted
> Now, as I just said in a very pleasant phone conversation
> with Don Eastlake,
> I'm quite sure that the use of encryption is simply to
> obfuscate matters and
> does not represent a cryptographic strength security
> solution.  Though a
> tractable problem, it is still quite difficult and expensive
> to reverse
> engineer this technology.  Therefore, it makes sense for many types of
> deployments where one expects high volume, relatively low
> value transactions
> where the organization wants to avoid the cost and difficulty
> of setting up
> a PKI for walk-in business.  It is probably not "reasonable
> doubt" to assert
> that the organization broke the signature technology since,
> based on the
> value of the transaction, it is probably too costly to do
> that or it is less
> expensive to forge the person's signature (the "old fashioned" way).
> So you see, the opaqueness of the blob (offered by the
> encryption) is how
> these handwritten signature technologies can claim to bind
> the signature
> (biometric data) to the document (secure hash).  Hence, if we
> only support
> signing technologies that allow a human readable format, then
> we are, by
> definition, excluding handwritten signature technologies.
> Maybe this is what the group wants to do, but I think it is at least
> important that we know we're making this choice rather than
> having it sail
> by unnoticed.  In UWI's case, I'm pretty sure our executive
> will not want to
> throw out a good revenue stream because of the standard, so
> XFDL will most
> likely to continue to support this form of 'signed' XML.  It
> will just be
> unfortunate that the signed XML standard cannot accommodate
> all kinds of
> signed XML.
> John Boyer
> Software Development Manager
> UWI.Com -- The Internet Forms Company
> jboyer@uwi.com
> >The basic problem with handwritten signatures is that they can't be
> >bound to a particular document without an extra step.  Specifically,
> >you can't simply store a signature bitmap with a digital document and
> >later claim that the mere fact of "close proximity" (largely
> a fiction
> >when you get down to the device level) between these objects in your
> >logical file system implies that the two things necessarily belong
> >together.  The situation is almost exactly the same as the one you
> >have with a photocopy of a "signed" document:  it's easy to "edit"
> >photocopies, so a photocopy of a signature is essentially worthless.
> >A collection of "handwritten signature bits" is equally worthless for
> >the same reason.
> >
> >When you sign a paper document, what binds your signature to the
> >document happens at the mechanical/chemical level.  Using existing
> >technology, the only way to bind signatures to digital documents is
> >through cryptographic means.  I'm not a cryptographer, but maybe a
> >digitized handwritten signature might be used as a source of keying
> >material.  In the end, though, you still need a way to bind the
> >handwritten signature bits (or any biometric "signature", like a
> >retina scan or a thumbprint) to the document bits.
> >
> >By itself, a digitized handwritten signature is basically a display
> >element or attribute you might want to cryptographically bind to a
> >document.  This doesn't mean that handwritten signatures aren't
> >important -- particularly for legal documents -- mainly because
> >humans couldn't possibly recognize or describe their own "digital
> >signatures".
> >
> >I think part of the problem here comes from the fact that we overload
> >the terms "signature" and "signing".  For example, cryptographically
> >"signing" something has side effects that make the act of signing
> >traceable to something only the signer possesses (specifically, a
> >key).  A "digital signature" only exists as some side effects of
> >signing.  These side effects are different each time you sign
> >something different, so you can't be expected to recognize
> or describe
> >your "digital signature" any more than you can be expected to
> >recognize or describe the unique mechanical/chemical binding that
> >exists between your handritten signature and a particular sheet of
> >paper.
> >
> >
> >--Bede
> >
Received on Tuesday, 20 April 1999 19:46:18 UTC

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