Half a croissant, on a plate, with a sign in front of it saying '50c'
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digital signature on paper

Digital signature on paper
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Every time you print a letter the only way to authenticate yourself is to add a manual signature to the paper with your expensive bic pen. When the person who signs a lot of papers every day, usually came out with a printed signature scanned in advance or worst solution. A better solution will be producing a summary of the content (aka checksum) plus a timestamp and digitally sign with your private key (see Public-key cryptography). The result code will be printed in the paper document with QR code barcodes (from wikipedia: “A QR Code is a matrix code (or two-dimensional bar code) created by Japanese corporation Denso-Wave in 1994. The “QR” is derived from “Quick Response”, as the creator intended the code to allow its contents to be decoded at high speed.”). When receiver want to check the integrity of the content of the letter and the source, it will use his phone whit a typical QR code reader and check with the source public key if the checksum is correct.

All this technical details can be hidden from user view integrating qr code generator in a web editor like google docs and handle a server for private-public key.

derte84, Nov 12 2009

someone mentioned this on twitter this morning - is this what you want? http://www.mylivesignature.com/
[po, Nov 12 2009]

[link]






       mebbe embed the checksum in the wording, somehow.
FlyingToaster, Nov 12 2009
  

       Po, the service you link to typesets or scans words and sends them back to you as an image. It doesn't prove that you wrote something - in fact that makes it easier for someone else to give the impression that you "signed" something you really didn't.   

       So, in a way, that link is the opposite of the idea. Not to mention fugly.
jutta, Nov 12 2009
  

       Let me firmly re-iterate: The authenticity of a document is only as good as the ability to verify it's source. A fax is a higher quality of source because, generally speaking, it comes from an easily traced phone connection. An encryption signature/anti-tampering index works only if we have a pub/priv key system already set up, otherwise it is meaningless.
WcW, Nov 12 2009
  

       Yeah, you'll need an infrastructure of trust in which to trade keys. This has its own issues as a paper document may outlast its issuing key. That is, a document may fail authentication because it was printed 5 years before its owner lost his private key and was added to a revocation list. The document is still valid, but may not be provably so.   

       Plus, just because I believe you created a document doesn't mean I believe what you've written in it. More to the point, many documents I sign are not, in fact, created by me (driver's license, checks, receipts, contracts, etc).   

       Is there a scenario where there's a legitimate need for something like this? Maybe a will or living will. What happens when a document spans many pages? Is there a barcode on each page?
phoenix, Nov 13 2009
  

       I seen no application for this, but so i did with Twitter... [+]
loonquawl, Nov 13 2009
  
      
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