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The authors of a free/libre work can change the license of their work any time, and they can even make it non-free again.

This does not affect copies that were made before that change (and any further copies from that "branch"), so I may still use/modify/distribute it according the original license.

Let’s assume the authors remove the license as well as all mentions that their work once was published under this license (and they forbid the Wayback Machine to publish older versions of their website).

I’m distributing their work (according to the terms of the original license). How can I prove that the work really was published under this license?

(I hope it will never be necessary to prove this, but better be safe than sorry.)

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  • I'm not sure you can actually prove anything related to digital files, at least without using cryptography.
    – svick
    Commented Jun 26, 2015 at 15:23
  • Let’s assume the authors remove the license as well as all mentions that their work once was published under this license That's a very big assumption. We all know what happens to someone who tries to get information removed from the Internet. (Spoiler: it doesn't work and usually backfires big time.)
    – Mason Wheeler
    Commented Sep 22, 2015 at 17:38
  • @MasonWheeler: How great it would be if the Streisand effect would always work, but there are soooo many niches without a big, caring crowd. Most CC material I reuse is self-published by the authors and typically no one else writes (or even knows) about the material, so if the author’s document is gone, the reference is gone.
    – unor
    Commented Sep 22, 2015 at 19:33

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By the license declaration they originally had bundled with their work. In this answer I said that the license declaration at least has to define which work is licensed and with which license. As you copy the work, you copy the license declaration.

Now, anyone can claim that digital data is easily tampered with, so to be secure you should take measures to document that the work was licensed that way at the moment you copy this. This documentation can be used in the worst case in court, but probably helps already to prohibit it goes that far.

If you used that so far, didn't document and the source now does the silent change it is far more difficult. If you know about the change beforehand, you can use that for documentation.

How far you go with the documentation is up to you.

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  • You could use for example a timestamping service to prove that a given archive of the the source code with the license existed at some time. Of course that proves not that you didn't just add the "license" yourself, but it increases your chances that someone believes you. After all, how do you know that the code was legally released with that license and not just stolen by someone who added the license file?
    – Josef
    Commented Sep 22, 2015 at 14:08

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