[jdcwptsj] Distributed computation with untrusted clients

Assume a central server and many clients, some malicious.

1. Have each client establish a public key identity with a difficult-to-compute but easy-to-check challenge-response function, perhaps like Hashcash but better.  It is not cheap for a malicious adversary to manufacture many identities.
2. Clients submit completed work signed with their key.
3. Duplicate work units, so trustable keys can be discovered.

Let the central server be a Tor hidden service to avoid attack.