I've had a crazy idea for a way of protecting data using an MP3 collection. It's completely ridiculous, inefficient, and it can probably be shot to pieces. But it's fun.
MP3 streams consist of a bundle of frames. Frames begin with a 12-bit string of zeros, then there is a brief header which gives the bitrate and length of the frame, then the frame data. MP3 players should wait for the 12-bit sync, then read the frame, then wait for another sync (in most MP3 data this follows immediately).
It should be possible to bung in random bytes in there between frames and have players ignore it completely.
What if the bytes you stick in there comprise a filesystem? Say you use 10 6MB MP3s and pad them by say 10% with the filesystem data. 6MB filesystem! Enough for your most important secrets, and nobody is going to look for it there. Security through obscurity. Still, that's only version 1 of the protocol (ie. it occurred to me first).
Version 2: use a block cipher to encrypt the filesystem. Obviously, this is important as otherwise plaintext bytes are readily visible.
Version 3: hide the MP3s that you've used to create the filesystem in a collection of MP3s - a large but random number of MP3s that have been similarly padded, but with junk. Now you need the right MP3s in the right order. Choosing r MP3s in the right order from a set of n gives nPr combinations, which, for r n is approximately nr. For example, choosing 10 (in order) from a relatively modest collection of 500 (~ 29) is roughly equivalent to a 90-bit passphrase or a 15-character random password consisting of A-Z,a-z,0-9. But the nice thing about this is that humans should be good at reconstructing playlists from memory, even with thousands of MP3s to choose from.
Version 4: Stripe the bytes between different MP3s. Ensure that cipher blocks are split between MP3s. This ensures that you can't run a brute force crack attempt against part of the encrypted data because you can't dig a whole block out of any one file.
Version 5: (optionally) use some acoustic element of the assembled MP3 playlist as part of the passphrase for the block cipher. Entering one 'digit' of passphrase might be the equivalent of selecting a riff in the right song, or one particular lyric. Say you have to choose a 5-second segment from your 10 3-minute MP3s - that's about 8.5 bits of passphrase.
Version 6: swap some, but not all of your MP3s with P2Ps. There is an element of deniability - the random data may or may not be yours. Most MP3 collections I've seen have been collected from hundreds of different sources - and anyone using the system will have lots of MP3s with a mixture of junk and real padding, so to find many MP3s containing junk on any one system does not mean they have encrypted data. It just means they are guilty of piracy.
Told you it's completely ridiculous. But isn't it fun? :)