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

From: John Boyer <jboyer@uwi.com>
Date: Tue, 20 Apr 1999 17:16:02 -0700
Message-ID: <017301be8b8c$24b904a0$9ccbf4cc@kuratowski.uwi.bc.ca>
To: <rdbrown@GlobeSet.com>
Cc: "Dsig group" <w3c-xml-sig-ws@w3.org>
I believe I answered this before.  When we say secure, we usually mean the
security offered by cryptography.  Although noone has ever proved it, we
believe that factoring 2048 bit keys, for example, is intractable.

I personally do not believe that the handwritten signature schemes are
intractably secure.  So the question is how much security do you get from a
tractable but difficult problem like reverse engineering the computation of
a very complex data structure?  That is a position that I am glad I don't
have to defend.  To me, though, it's beside the point.  Numerous
organizations have decided that, for a particular type of deployment, PKI is
too costly or impractical.  All they really want is a way to sign XML such
that the assertion that they forged an e-signature is at least as absurd as
asserting that they forged a paper signature.

This is not the binary on/off idealistic proof idea that we know and love.
It is the fuzzy, threshold of pain and liability idea that can be found in
the real world.  For walk-in business, for example, if the walk-in doesn't
have a certificate, how are you going to do business with the person?  You
can't issue him a cert. because you are the receiving party.

The pen people claim that their technology binds the document to the signer
in a way that digital certs. don't.  Users don't lose, misplace or leave
behind their hands.  Further arguments to the contrary should go to those
who manufacture this technology.  I only integrate with it, and I'm only
trying to say that these people have the core requirement of non-readability
of the signature blob.

John Boyer
Software Development Manager
UWI.Com -- The Internet Forms Company
jboyer@uwi.com
-----Original Message-----
From: Richard D. Brown <rdbrown@GlobeSet.com>
To: 'John Boyer' <jboyer@uwi.com>; 'Bede McCall' <bede@mitre.org>
Cc: 'Dsig group' <w3c-xml-sig-ws@w3.org>
Date: Tuesday, April 20, 1999 4:46 PM
Subject: RE: Fw: XML versus ASN.1/DER blob


>John,
>
>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 20:11:57 EDT

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