From: pstemari@erinet.com (Paul J. Ste. Marie)
Date: Fri, 13 Jan 95 22:08:55 PST
To: Ben Goren <cypherpunks@toad.com
Subject: Re: How do I know if its encrypted?
Message-ID: <9501140600.AA07244@eri.erinet.com>
MIME-Version: 1.0
Content-Type: text/plain


At 10:07 PM 1/13/95, Ben Goren wrote:
> ... Alice hashes her file and uses that hash as the key to encrypt the file.
>She sends the file to Dave, and sends the original hash when she wants it
>back. Dave decrypts, and confirms the hash. 
>
>Unfortunately, this still doesn't quite close the loop--Dave knows the
>contents of the file once Alice sends the key. It does, however, make it
>very difficult for Dave to know anything about Alice's file. ... 

This seems overly complicated.  If Dave has a known public key, then Alice 
should be able to hash her file, sign the hash, encrypt (the hash, her 
public key, and the file) with Dave's public key, and (anonymously) sends 
the result to Dave's (encrypted) address.  Dave then decrypts, verifies the 
sig, and stores the file, hash, and PK together, indexed by the hash.

When Alice wants the file back, she signs (the hash and her encrypted return 
address), encrypts the result with Dave's key, and sends it off.  Dave 
decrypts the request, fetchs the public key based on the decrypted hash, 
verifies the signature, encrypts the file with Alice's provided public key, 
and sends it back to the encrypted return address. 

To avoid Dave's knowing the file contents, Alice can encrypt it before the 
described protocol and decrypt it afterwards.  The protocol is subject to a 
replay attack, but the result of the replay would cause the file to be sent 
to the original sender and not to the replayer.

The signed hash in the first step prevents people from spamming Dave with 
files that have Alice's public key.  Alice only requires an encrypted 
address and public key for Dave, and Dave validates the retrieval request 
against the public sent in the first step.

    --Paul J. Ste. Marie
      pstemari@well.sf.ca.us, pstemari@erinet.com