A key-exchange service implementing the "diffie-hellman-group-exchange-sha1" key-exchange algorithm.

Constants
MINIMUM_BITS = 1024
MAXIMUM_BITS = 8192
KEXDH_GEX_GROUP = 31
KEXDH_GEX_INIT = 32
KEXDH_GEX_REPLY = 33
KEXDH_GEX_REQUEST = 34