client/server: registration process complete.

Server:
 REG: Decrypts, verifies & decapsulates client payload.
 REGOFFER: Generates new RSA & DH keypairs (registers for connecting client),
           generates a new UUID (also registers per client). Encrypts, digests
           and encapsulates payload sent to client.
 REGACK: Decrypts, verifies RSA signature, decapsulates client payload, uses
         supplied DH public key to generate shared secret. Strengthens secret
         using key derivation & registers per connecting client.
 REGFIN: Generates verification hash from shared secret, encrypts, digests &
         encapsulates payload for client as verification of registration process.

Client:
 REG: Generates RSA & DH keypair, encrypts, digests & encapsulates RSA public key
      payload for server.
 REGOFFER: Decrypts, verifies digest, decapsualtes server payload.
 REGACK: Encrypts, digially signs & encapsualtes DH public key. Registers
         extracted DH & RSA public keys and UUID from server.
 REGFIN: Decrypts, verifies RSA signature, decapsualtes server payload,
         generates shared secret from DH public key, derives key from
         shared secret, hashes and verifies payload contents from server.