Re^4: Encrypt files on server and then decrypt when user downloads

[Anonymous Monk]

If the files are encrypted with a user-supplied password/passphrase/public key, and decrypted by the server on download, it would give a fairly good balance between security and convenience.

The attackers, if they get control of the server, could then only decrypt the files the users download by sniffing passphrases.

[BrowserUk]

The attackers, if they get control of the server, could then only decrypt the files the users download by sniffing passphrases.

If the bad guys get access to the server, tweaking the download script to forward the filenames and associated decryption key to some dead-drop would be trivial.

Done with a little care, the server/site owner need never know the script had be tweaked unless they happened across the two or three lines of code that would need to be added to the source. Done with a little more care -- secreted into the source of a standard module called by the decryption script -- even that risk of discovery can be mitigated.

Far better that the user's decryption key is never seen by the server. And actually relatively easy to arrange. If the site doesn't want to require the user to download the tool and do the decryption manually, then they only need write the decryption code in javascript. It then gets downloaded to the users machine automatically and it prompts for the user's key locally.

It still requires the user to supply an encryption key prior to the file's creation; and to have a separately encrypted copy of a given file for each user that will download it.