Subject: Identifying my version of a GPL program
From: Bernard Lang <>
Date: Wed, 20 Dec 2000 18:44:06 +0100


This is probably not new, but I do not recall seeing it.
The problem is real, not invented.

An organization Yoyodyne is producing software. They probably would
not mind making it free, but they have a problem.
   For reasons their own, they want to make sure that whoever is using
that software to collaborate with them is using one of their versions,
and not a modified variant by someone else.
  This software is intended to produce very specialized documents to be
sent to Yoyodyne (the communication is not interactive)

  My question is : can it be achieved in compatibility with the GPL.

  A proper way of checking which version is being used is simply to
attach a MD5 checksum of the binary used, computed dynamically at
execution time, to the documents.

  However this will not prevent someone from replacing it by a fixed,
precomputed value, and thus cheat the version checker.

I came up with one idea, using trademark.

Yoyodyne publishes its version in such a way that it attaches also a
trademarked sentence to the documents it produces.  And a comment is
attached to the Checksum computing code specifying that it is illegal
to attach that sentence to documents produced with non original
versions of the code (i.e. versions not issued by Yoyodyne).

  Now, despite the GPL licence, any cheating on the requirement to use
Yoyodyne version is willfull trademark infringement (and can be
sued). No one infringes by accident or thoughtlessness and produces
acceptable documents.

  By this does not restrict in any way the use of the code in any
other context than communicating with Yoyodyne.

well ... sorry if I was complicated ... maybe there is a much simpler


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