Here's a solution: Alice sends a file to Dave's DataHaven. When Alice wants her file back, she sends to Dave a secure hash of the file, a key with which to decrypt it, and a handful of plaintext at the beginning of the file. Dave decrypts the file that matches the hash with the key Alice gave him; if the file begins as Alice says it should, Dave returns the file to Alice. This way, only those people who have an intimate knowledge of the files can recover them. The hash isn't vital; Dave could try to decrypt each file on the server with the key Alice gives him until he matches the plaintext--but that's neither elegant nor friendly to the CPU. People can send Dave whatever kind of file they like, but they'll only get it back if it's been encrypted properly. If Dave charges to store a file, he'll gladly welcome as much spammage as people want to send him. If he likewise charges to return a file, he'll just as glady send a file to whoever gives him the cash and can identify it. Dave can have a policy whereby he deletes a file after returning it, unless Alice pays more to keep it there. Thus, Bad Bobby can send his naughty pictures to Dave, tell the 'net how to get them--but the first person who neglects to include the fee to leave the pictures there winds up blocking out everybody else. Similarly, Samaritan Sam could get into a spending war with Bobby. Each time Bobby sends Dave his smut, Sam retrieves the file without paying for its continued storage--and takes a sneak peak at the pictures before deleting them himself. Dave has no way of knowing what people are storing on his machine until the owner asks for the file back, and so cannot be held liable for the storage of "undesirable" files. Mallet could seize the entire archive and have nothing useful. Eve could recover information leaving the haven, but presumably Alice will further encrypt her files however she chooses. Or, as a service, Dave could agree to encrypt the file with an assymetric cipher before returning it. b& -- Ben.Goren@asu.edu, Arizona State University School of Music Finger ben@tux.music.asu.edu for PGP public key ID 0xCFF23BD5.