Runs over a reliable ordered bidirectional point-to-point datagram stream.

Frame format:
   1  ui  Opcode
   ....   Opcode-specific data

Opcodes:
  ClientHello (0x00)
    4  ui  Magic (0xB673B08D)
    2  ui  Protocol Major Version (0)
    2  ui  Reserved (0/ignore)
   32      Client Transient Public Key (curveCt)
   24      Initial Client Nonce Nc
   16+     Box (contents ignored, empty) <take Nc> (Ct->S)
  ServerHello (0x01)
    4  ui  Magic (0x42D19719)
   24      Initial Server Nonce Ns
   48     Box [St] <take Ns> (S->Ct)
  ClientCommence (0x02)
  144     Box (C + vouch + metadata) <take Nc> (Ct->St)
            where vouch :=
              80  Box (Ct + S) <take Nc> (C->St)

          (Inner Nc is taken first)
  Message (0x03)
   17+     Box (payload) <take Nc or take Ns> (St->Ct) or (Ct->St)
             where payload :=
               1  ui  Reserved (0)
               ...    Data

"Take Nx" means take the initial nonce given and XOR it with the
associated 64-bit unsigned counter. If a counter wraps around to 0, the
connection is aborted.