I have a service that serves mp3 files to a small circle of people. In the case of a revelation, how could I know who had done that?
What technology could I use to sign every copy of the mp3 files that I share to a specific user?
How do moviemakers protect digital copies from being published to the public by cinema owners?
MP3s have a special place for ID3 tags (so the artist, song, etc info are embedded). You could easily set a field to have different value per user, but these fields are editable and could be stripped. You used to be able to do this in WinAmp back in the day; any MP3 making/editing software should easily be able to do this.
You could also add a one second block at the end of the song that contains "silence" at different frequencies where each person is assigned a different frequency. I have no idea how to actually do this as I don't engineer audio, but I can't imagine it would be too difficult.
If every user gets a slightly different copy of the file then it is easy to determine who leaked the file (assuming they don't modify the unique part of the file).
Think about a trivial example of sharing a document. If you make a code for each person you share the document to and one of the documents end up being leaked, you can check the document for that unique code. Whoever had that code assigned to them is the one that leaked it.
As far as specific technologies to do this with audio files I don't know about, much less make sure it's unnoticeable.
