From: Ben.Goren@asu.edu
Date: Thu, 12 Jan 95 17:17:50 PST
To: cypherpunks@toad.com
Subject: Re: How do I know if its encrypted?
Message-ID: <v02110100ab3b6fb73f29@[129.219.97.131]>
MIME-Version: 1.0
Content-Type: text/plain


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.