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kex_group1.py
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kex_group1.py
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# Copyright (C) 2011 Jeff Forcier <jeff@bitprophet.org>
#
# This file is part of ssh.
#
# 'ssh' is free software; you can redistribute it and/or modify it under the
# terms of the GNU Lesser General Public License as published by the Free
# Software Foundation; either version 2.1 of the License, or (at your option)
# any later version.
#
# 'ssh' is distrubuted in the hope that it will be useful, but WITHOUT ANY
# WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
# A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more
# details.
#
# You should have received a copy of the GNU Lesser General Public License
# along with 'ssh'; if not, write to the Free Software Foundation, Inc.,
# 51 Franklin Street, Suite 500, Boston, MA 02110-1335 USA.
"""
Standard SSH key exchange ("kex" if you wanna sound cool). Diffie-Hellman of
1024 bit key halves, using a known "p" prime and "g" generator.
"""
from Crypto.Hash import SHA
from ssh.common import *
from ssh import util
from ssh.message import Message
from ssh.ssh_exception import SSHException
_MSG_KEXDH_INIT, _MSG_KEXDH_REPLY = range(30, 32)
# draft-ietf-secsh-transport-09.txt, page 17
P = 0xFFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E088A67CC74020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B302B0A6DF25F14374FE1356D6D51C245E485B576625E7EC6F44C42E9A637ED6B0BFF5CB6F406B7EDEE386BFB5A899FA5AE9F24117C4B1FE649286651ECE65381FFFFFFFFFFFFFFFFL
G = 2
class KexGroup1(object):
name = 'diffie-hellman-group1-sha1'
def __init__(self, transport):
self.transport = transport
self.x = 0L
self.e = 0L
self.f = 0L
def start_kex(self):
self._generate_x()
if self.transport.server_mode:
# compute f = g^x mod p, but don't send it yet
self.f = pow(G, self.x, P)
self.transport._expect_packet(_MSG_KEXDH_INIT)
return
# compute e = g^x mod p (where g=2), and send it
self.e = pow(G, self.x, P)
m = Message()
m.add_byte(chr(_MSG_KEXDH_INIT))
m.add_mpint(self.e)
self.transport._send_message(m)
self.transport._expect_packet(_MSG_KEXDH_REPLY)
def parse_next(self, ptype, m):
if self.transport.server_mode and (ptype == _MSG_KEXDH_INIT):
return self._parse_kexdh_init(m)
elif not self.transport.server_mode and (ptype == _MSG_KEXDH_REPLY):
return self._parse_kexdh_reply(m)
raise SSHException('KexGroup1 asked to handle packet type %d' % ptype)
### internals...
def _generate_x(self):
# generate an "x" (1 < x < q), where q is (p-1)/2.
# p is a 128-byte (1024-bit) number, where the first 64 bits are 1.
# therefore q can be approximated as a 2^1023. we drop the subset of
# potential x where the first 63 bits are 1, because some of those will be
# larger than q (but this is a tiny tiny subset of potential x).
while 1:
x_bytes = self.transport.rng.read(128)
x_bytes = chr(ord(x_bytes[0]) & 0x7f) + x_bytes[1:]
if (x_bytes[:8] != '\x7F\xFF\xFF\xFF\xFF\xFF\xFF\xFF') and \
(x_bytes[:8] != '\x00\x00\x00\x00\x00\x00\x00\x00'):
break
self.x = util.inflate_long(x_bytes)
def _parse_kexdh_reply(self, m):
# client mode
host_key = m.get_string()
self.f = m.get_mpint()
if (self.f < 1) or (self.f > P - 1):
raise SSHException('Server kex "f" is out of range')
sig = m.get_string()
K = pow(self.f, self.x, P)
# okay, build up the hash H of (V_C || V_S || I_C || I_S || K_S || e || f || K)
hm = Message()
hm.add(self.transport.local_version, self.transport.remote_version,
self.transport.local_kex_init, self.transport.remote_kex_init)
hm.add_string(host_key)
hm.add_mpint(self.e)
hm.add_mpint(self.f)
hm.add_mpint(K)
self.transport._set_K_H(K, SHA.new(str(hm)).digest())
self.transport._verify_key(host_key, sig)
self.transport._activate_outbound()
def _parse_kexdh_init(self, m):
# server mode
self.e = m.get_mpint()
if (self.e < 1) or (self.e > P - 1):
raise SSHException('Client kex "e" is out of range')
K = pow(self.e, self.x, P)
key = str(self.transport.get_server_key())
# okay, build up the hash H of (V_C || V_S || I_C || I_S || K_S || e || f || K)
hm = Message()
hm.add(self.transport.remote_version, self.transport.local_version,
self.transport.remote_kex_init, self.transport.local_kex_init)
hm.add_string(key)
hm.add_mpint(self.e)
hm.add_mpint(self.f)
hm.add_mpint(K)
H = SHA.new(str(hm)).digest()
self.transport._set_K_H(K, H)
# sign it
sig = self.transport.get_server_key().sign_ssh_data(self.transport.rng, H)
# send reply
m = Message()
m.add_byte(chr(_MSG_KEXDH_REPLY))
m.add_string(key)
m.add_mpint(self.f)
m.add_string(str(sig))
self.transport._send_message(m)
self.transport._activate_outbound()