OSEC

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Crypto Archives: Re: Electronic envelopes

Re: Electronic envelopes

jeromepsti.com
Sun, 26 Sep 1999 19:09:41 -0400

i though about your problem and this is my answer.
i hope you will like it :)

PROTOCOL:
1. you, Mister X, send your encrypted message to several notaries Alice, Carl
2. send the key to Bob and ask him to send it on a public network
   at 00:00:00 2020
3. at 00:00:00 2020, Alice and Carl listen to this public network and
   can decrypt the message at the same time.

REQUIREMENT:
- Bob, Alice, Carl don't know each other. so Alice or Carl can't corrupt
  Bob to have the key before the date D.
- the 'public network' has to be broadcast to allow Alice and Carl to listen
  at the same time.
- the public network must be fast enougth to ensure a good synchonisation.
  for example a IP multicast group (delay <2 sec depending on the distance
  between the sender and the receivers).

POSSIBLE ATTACKS:
1. break the encryption without the key. (out of scope of this protocol)

   Solution: choose a stronger cryptosystem :)

2. Bob decide to send the key on the public network before the date D.

   Solution: the key and the delivery time is signed by a public key.
   so if bob send it on a public network, every listener of this network
   will know that bob is corrupted. if the public network allow bob
   to send it anonymously, Mister X itself has to monitor the public
   network.
   - it is assumed that this attack is unlikely because Bob can't take
   profits from this action because he doesn't know Alice or Carl.

3. collaboration between Bob and Alice(or Carl): the only real threat.

   Kind of Solution: because Bob and Alice don't know each other, it
   will required for both to advertise to the other notaries their
   files and so advertized they are corrupted. if the number of
   notaries is large enougth, it is unlikely to have such advertizement
   which require a generalised corrumption.

This archive was generated by hypermail 2.0b3 on Sun Sep 26 1999 - 20:45:21 CDT