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Re: Digital signing of printed documents

On 7/4/05, Dave Howorth <dhoworth@mrc-lmb.cam.ac.uk> wrote:
> Alphonse Ogulla wrote:
> > Hi good people,
> > I wish to get your views on how I can implement a system that will
> > capture text of a financial document that is to be printed, run a hash
> > algorithm (SHA-1) over the document text, store an electronic copy of
> > the document and its digital signature to disk and lastly print the
> > document with the electronic signature at the last line of the
> > document. The financial document can be a ticket/receipt produced by a
> > POS application or an invoice/delivery note generated by an accounting
> > software.
> ...
> > Do you think this is a reasonable way of going about this project? Is
> > Samba-CUPS and bash/perl scripting the best tools to use or are there
> > others? What is your recommendation and advise? Your comments, remarks
> > or criticism are welcome.
> As others have mentioned, perhaps you need to tell us more about the
> project's purpose - what risks is it designed to protect against?
> For example, the existence of the _c.txt leads me to suspect that you're
> worried the _b.txt files may be changed. But it's not clear why you
> wouldn't just store everything on the CD if that's the case?
> And I'm not sure of the purpose of printing the hash on the paper
> ticket, since it may or may not correspond to the text that's actually
> printed on a specific piece of paper that's presented later. When does
> the printed hash get used?
The primary purpose of the project is to assure auditors and tax
authorities that printed documents are true if the electronic
signatures match. It also provides a means of the authorities to check
in a reliable way what taxes a certain business is to pay. The
accounting system will continue to produce reports based on document

Only signatures that are used to authenticate _b.txt files are stored
on CD-R so that once a record has been saved on CD-R, it can no longer
be altered. Its is important in order to keep permanent records of
electronic endorsments.

The digital signature at the last line of the invoice is basically a
unique identifier derived from the text of the invoice. This
requirement is again a local reglulation concerning electronic invoice
validatiion for operation under fical requirements.

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