Quoting Portions of a Signed Document

[Anonymous]

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

Ugly's the word for it, alright. ;-)

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

It would be nifty if there was a way to show that any continuous set
of bits were signed given only one signature on a whole document.
Intuitively, it seems to me that this might be provably inconsistent
with a secure hash.  Still, crypto results are full of surprises, so I
could imagine there is a way to do this.

>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.

Good point.  It is nice that people who quote out of context now in a
misleading way are convicted when you demand proof.

Still, my sense is that this would be a pretty useful thing to be able
to do and it would be technically interesting.

For example, you might want to reveal parts of a message signed by
somebody else without revealing the entire message.

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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