Bogus digital signatures, Re: OT: utility account transfer frauds

[George Ross]

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> > Is it not sufficient that each party generate their own random string,
> sign
> > the concatenation of that string with the document, and publish the string
> > along with the signature.  No-one is then signing anyone else's text, but
> > all can verify that everyone signed the same document.
> 
> I presume that both strings would be appended to the text (so both sides get
> to vary it).

Is that necessary?  Since the idea is to defend against someone else 
contriving the text to force a particular hash, just concatenating your own 
random string before signing should suffice.  Everyone signs their own 
concatenation, but all can verify that it's the same document that's been 
signed since the strings are all published.

(Aside: I think I would concatenate the random string on to the front of 
the document before signing; and the length of the string would also be 
random.  Actually, truely random isn't necessary, as long as it's not 
predictable in advance by the other parties.)

Whether the contents of the document itself can be interpreted in more than 
once way is a real, but separate, issue.
-- 
Dr George D M Ross, School of Informatics, University of Edinburgh
   Kings Buildings, Mayfield Road, Edinburgh, Scotland, EH9 3JZ
Mail: gdmr@inf.ed.ac.uk   Voice: +44 131 650 5147   Fax: +44 131 667 7209
PGP DSA: 1024/AD758CC5 B91E D430 1E0D 5883 EF6A  426C B676 5C2B AD75 8CC5



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