More about e-ink and digital paper. This Metafilter item on printable batteries makes an aside that got me thinking:
look carefully at your next contract ? make sure they can't rearrange the letters after you sign it!
The only way to prevent repurposing of a signature would be to tell easily e-paper from regular dumb paper, which eventually won't be possible. When anyone can change what's on a legal instrument such as a contract, what do you do?
That's what cryptographic signatures are for.
Because of the malleability of a digital document, a good cryptographic signature has to be verifiable against the document--any given person with (assuming a PGP-style public key model) the signatory's public key and software to do the math should be able to check that, indeed, someone with the paired private key (presumably the key's owner) signed that exact document. How's that done? Digital signatures include a hash of the document itself (a mathematically probable miniature version of the document, since I'm being a little non-technical).
So, when contracts might be made on digital paper, you'll have to sign cryptographically to make sure enough it won't be changed.
