Quoting Portions of a Signed Document

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It would be neat if you could quote people and prove that they signed
the particular paragraph quoted without supplying the entire text.  Is
there a way to do this?  (It seems impossible, but so does mental
poker.)

A crude approach would be to sign every paragraph or line separately,
but that's obviously inelegant.

Monty Cantsin
Editor in Chief
Smile Magazine
http://www.neoism.org/squares/smile_index.html
http://www.neoism.org/squares/cantsin_10.htm

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On Tue, 25 Nov 1997, Anonymous wrote:

> A crude approach would be to sign every paragraph or line separately,
> but that's obviously inelegant.

Or use a MIME attachment to store secure hashes (heck, these could even be
CRC32's if you're not too paranoid which you could pack into 5 bytes of
ascii or something) of each of the lines in the body, then a valid
signature of all the hashes at the end. 

When you reply to such a message, you'll need to reattach the hash
attachment to the reply along with some sort of translation table that
says line 20 in this message is quoted from line 502 in the original.

You'll also need some way to do quotes that won't interfere with the hash
testing... maybe using block quotes such as

:Begin_Quote
text
:End_Quote

or delete the ">" chars.  But you'll get in trouble with auto text
justifications...

i.e. reply signature mime attachment:
*Quoted Lines used: 15=234, 16=235, 17=236, etc...
*From Original Mime attachment:
*Begin body hashes
line1hash line2hash line3hash line4hash line5hash.... lineNhash
*Begin body hash signature
slkdjflskdjfjsdlfjsdlf
*End signature

Or when you quote the message, you could have the begin_quote blocks have
the line translation in them.

If you don't mind shoving HTML in the text, the begin quote blocks in the
body of the reply could include the line number.  i.e.

(using square brackets so those using HTML readers will see this.)

[Strong][QuoteLine55-69]
text_of_lines55-69_from_original_message_here.
[/QUOTE][/Strong]



Problem with this scheme:

But, then what do we do about quoted quotes from the reply of the
reply????  We could recurse the stuff through, but then deep replies will
have a large number of attachments if we chose to retain them...  Possible
solution, don't keep quoted quote signatures, or just keep the message
id's of them so they can be verified from an archive server or
something...


While you will need special mailer clients to decode the attachments and
verify the quotes, you still want some compatibility with regular mail
clients...

=====================================Kaos=Keraunos=Kybernetos==============
.+.^.+.|  Ray Arachelian    |Prying open my 3rd eye.  So good to see |./|\.
..\|/..|sunder@sundernet.com|you once again. I thought you were      |/\|/\
<--*-->| ------------------ |hiding, and you thought that I had run  |\/|\/
../|\..| "A toast to Odin,  |away chasing the tail of dogma. I opened|.\|/.
.+.v.+.|God of screwdrivers"|my eye and there we were....            |.....
======================= http://www.sundernet.com ==========================

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In <199711252109.WAA05443@basement.replay.com>, on 11/25/97 
   at 10:09 PM, nobody@REPLAY.COM (Anonymous) said:


>-----BEGIN PGP SIGNED MESSAGE-----

>It would be neat if you could quote people and prove that they signed the
>particular paragraph quoted without supplying the entire text.  Is there
>a way to do this?  (It seems impossible, but so does mental poker.)

>A crude approach would be to sign every paragraph or line separately, but
>that's obviously inelegant.

Well this could be done by creating a document signature and then a
collection of sub signatures but it can get ugly real quick.

What level of granularity does one use for the sub signature?

Paragraph: Contentious block of text separated by a line of white space.
Line:      Block of text ending with a CRLF.
Word:      Block of text separated by a white space.

Then what does the sub signature really tell you? Yes you can verify that
the quote was written by someone but it may be taken completely out of
context. How about when several blocks of text from different messages are
combined. Each individual block checks out but by combining them the text
has a completely different meaning than the original document.

The best thing right now is for a user to lookup the referring document in
the archives and verify the signature. It also give him the advantage of
reading the quote in its full context.

In an environment where a public archive of messages are not available
then other means of obtaining the source document are available (contact
the original author, contact the person quoting the original document). In
some environments it may be beneficial to attach the original document to
the message when sending.

Considering that a signed quote would require the Author to format his
signatures in this way I for one would not do so nor could I see a reason
for doing so.

Side Note: The above is in reference to small documents and E-Mail. In
large documents it may be desirable to sign every page or sign each
chapter in addition to signing the entire document. A case may be made for
subsignatures in a E-Mail message in which there are separate signatures
for the text of a message and accompanying attachments. In both cases
there should be a meta-signature that covers the entire document.

- -- 
- ---------------------------------------------------------------
William H. Geiger III  http://users.invweb.net/~whgiii
Geiger Consulting    Cooking With Warp 4.0

Author of E-Secure - PGP Front End for MR/2 Ice
PGP & MR/2 the only way for secure e-mail.
OS/2 PGP 2.6.3a at: http://users.invweb.net/~whgiii/pgpmr2.html                        
- ---------------------------------------------------------------

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> It would be neat if you could quote people and prove that they signed
> the particular paragraph quoted without supplying the entire text.  Is
> there a way to do this? 

Yes. To do it using standard proggies, hash each of your paragraphs and put the
results in a PGP-signed message. Quoter provides signature and signed hashes. 
Reader hashes your quoted paragraph and checks the signed message to see if it
was one of the original paragraphs. So simple I can do it. :) 

[Disclaimer: I think.]

A more cool-sounding solution would be to concatenate those hashes and sign
them (not a hash of them) as one packet. Since RSA sigs are secret-key
decryptions (right?), the reader can re-encrypt it and check any individual
hash. Not more secure, though, and it takes a lot of processor time.

No idea how to do it with bit-level granularity.

> (It seems impossible, but so does mental poker.) 

Mental poker? Easy.

> 
> A crude approach would be to sign every paragraph or line separately,
> but that's obviously inelegant.

Allows you a tad too much freedom with their quotes, too...

> 
> Monty Cantsin
> Editor in Chief
> Smile Magazine
> http://www.neoism.org/squares/smile_index.html
> http://www.neoism.org/squares/cantsin_10.htm

---------------------------------------------------------------------------
Randall Farmer
    rfarmer@hiwaay.net
    http://hiwaay.net/~rfarmer

> It would be neat if you could quote people and prove that they signed
> the particular paragraph quoted without supplying the entire text.  Is
> there a way to do this?  (It seems impossible, but so does mental
> poker.)

	Assuming that they digitally signed the entire document, simply
put it on the web, quote the part you want and reference the original
signed peice in the message.

>Cantsin> A crude approach would be to sign every paragraph 
>Cantsin> or line separately, but that's obviously inelegant.
>
>Geiger> Well this could be done by creating a document signature 
>Geiger> and then a collection of sub signatures but it can get ugly real quick.

Creating chains of hashes lets you do this without having to
do signatures on each piece - you just sign the hash at the end.
So you'd create 
	hash_page_1 = hash( hash(page_1_para_1), hash(page_1_para_2)...)
	hash_final  = hash( hash_page_1, hash_page_2, ... )
	sign( hash_final, signaturekey )
or whatever hierarchy you like, and to demonstrate you've got page_2_para_2
correctly, you provide the hashes for all the page, and the hashes for
all the paragraphs on page 2.

But then Geiger brings out the other important point:
>Then what does the sub signature really tell you? Yes you can verify that
>the quote was written by someone but it may be taken completely out of
>context. How about when several blocks of text from different messages are
>combined. Each individual block checks out but by combining them the text
>has a completely different meaning than the original document.

				Thanks! 
					Bill
Bill Stewart, stewarts@ix.netcom.com
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