Quoting Portions of a Signed Document

[Bill Stewart]

>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 at ix.netcom.com
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