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The variant of Diffie-Hellman key exchange in which the server
sends a different prime modulus every time (known in SSH as 'group
exchange') was revised by RFC 4419 to change the format of the
SSH_MSG_KEX_DH_GEX_REQUEST message, and also its message
number. PuTTY didn't get round to supporting the revised message for
quite a long time, but as of 0.65 it will now do so.
(The OpenSSH server is removing support for these messages. It has backwards-compatibility code for old versions of PuTTY, but assumes that 0.65 onwards will support the revised message. If for some reason the backwards-compatibility arrangements don't work, the symptom is a message like error: Hm, kex protocol error: type 30 seq 1 [preauth] in the SSH server log.)