You can encrypt file with a crypto hash of its contents (e.g. `key=sha1(file)`).
That's a nice catch-22, as you need contents of the file to obtain the key to decrypt it.
Deduplication could be even more effective if the file was first split into variable-size content-dependent blocks using rolling hash (like rsync does) and then each block was encrypted this way separately (this way same MP3s with different ID Tags would still be mostly deduplicated).
Of course the more you make deduplication easier the more you indirectly disclose about contents of the file, so this is a security/privacy trade-off.
A system like that prevents browsing contents if you don't know what you're looking for, but it doesn't prevent asking questions like "Do you have a copy of this file?", or enforcing a blacklist based on file contents.
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You can encrypt file with a crypto hash of its contents (e.g. `key=sha1(file)`).
That's a nice catch-22, as you need contents of the file to obtain the key to decrypt it.
Deduplication could be even more effective if the file was first split into variable-size content-dependent blocks using rolling hash (like rsync does) and then each block was encrypted this way separately (this way same MP3s with different ID Tags would still be mostly deduplicated).
Of course the more you make deduplication easier the more you indirectly disclose about contents of the file, so this is a security/privacy trade-off.
A system like that prevents browsing contents if you don't know what you're looking for, but it doesn't prevent asking questions like "Do you have a copy of this file?", or enforcing a blacklist based on file contents.