I was wondering if there is a file format that can be used as a defense against bulk scanning, but isn't necessarily an encrypted file where you would need a key to decode it. What I was thinking is encrypt the file, but store the key in the file in such a way that it requires an expensive computational operation to decode the key. When reading a single file it may take an extra couple seconds to open it, which could be ok for an end user, but it would make mass scanning impractical. One way of doing this I can think of is starting off with a random string that is recorded in the file header, then running it through bcrypt with a suitable cost factor, in order to derive the actual encryption key for the rest of the file.