Security/Key-Exchange/Diffie-Hellman is a challenge-response protocol that can do spontaneous session key exchange
(per Bruce's Applied Crytography:) Alice sends Y, g, n; Bob responds, same key now exists on both sides. Syntax can elide g,n in favor of a pointer to another context/server that vends those large numbers.
Perhaps allow PKCS #3 encoding of this request information. Then again...
Diffie-Hellman adds specs for
Diffie-Hellman: <N> <G>