HMAC(secret, <time_t in hex>)
and the hash function is SHA256
+
Possible future nonce-based authentication:
server keeps big nonce counter for each client
also server keeps bitmap of the previous ?64 nonces,
whether client has sent them
-client picks.... xxx
+difficult because client-generated nonces would have to never go
+backwaards which basically means never-rewinding state on the client.