Rework samplekex.py to use the new -proxycmd.

It now expects its standard input to be connected to the same PuTTY
its standard output is talking to, i.e. expects to be invoked as a
proxy command. It conducts the same sample key exchange as it used to,
but now reads the SSH greeting and first couple of packets back from
PuTTY and minimally checks that they're something like what it was
expecting.

(In the process, I've also fixed a mistake in the Python message code
enumeration, which caused one of those expect() calls to fail.)

contrib/encodelib.py

@@ -65,10 +65,11 @@ SSH2_MSG_KEXINIT = 20
 SSH2_MSG_NEWKEYS = 21
 SSH2_MSG_KEXDH_INIT = 30
 SSH2_MSG_KEXDH_REPLY = 31
-SSH2_MSG_KEX_DH_GEX_REQUEST = 30
+SSH2_MSG_KEX_DH_GEX_REQUEST_OLD = 30
 SSH2_MSG_KEX_DH_GEX_GROUP = 31
 SSH2_MSG_KEX_DH_GEX_INIT = 32
 SSH2_MSG_KEX_DH_GEX_REPLY = 33
+SSH2_MSG_KEX_DH_GEX_REQUEST = 34
 SSH2_MSG_KEXRSA_PUBKEY = 30
 SSH2_MSG_KEXRSA_SECRET = 31
 SSH2_MSG_KEXRSA_DONE = 32
@@ -113,3 +114,16 @@ def clearpkt(msgtype, *stuff):
         s += byte(random.randint(0,255))
     s = byte(padlen) + s
     return string(s)
+
+def decode_uint32(s):
+    assert len(s) == 4
+    return struct.unpack(">I", s)[0]
+
+def read_clearpkt(fh):
+    length_field = fh.read(4)
+    s = fh.read(decode_uint32(length_field))
+    import sys
+    padlen = ord(s[0])
+    s = s[1:-padlen]
+    msgtype = ord(s[0])
+    return msgtype, s[1:]