Encrypted tickets

[wavexx]

It would be very nice to use symmetric encryption for password-protected tickets. When the file is received, we could use a symmetric block cypher mode such as aes-xts to encrypt the file backed-up by the ticket. We could use the known password hash for validation instead of validating the content, so there's nothing "special" required to support it (the cypher is totally invisible to the user).

I was looking at PHP libraries, but I couldn't find any library that offers a convenient (seekable!) API to perform aes-xts on a file. I'm reluctant to use external tools, as this would complicate furthermore the setup and/or reduce the chance that this would be a standard feature in a DL installation, and not something that would be easily skipped.

[wavexx]

After some discussion, there's some extra consideration to be done:

• We should use an AEAD scheme for the ticket content (AES-GCM)
• We should validate at least the filename and file size in the scheme
• Should the ticket ID guarantee to always point to the same content?

The last is a tough question:

• Doing so prevents administrative maintenance of most kinds (Feature Request: replace the current file a ticket with a new version #38)
• Wouldn't prevent password/content leakage in case of remote compromise with an active attacker.