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# -*- test-case-name: twisted.conch.test.test_transport -*-
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
"""
SSH key exchange handling.
"""
from __future__ import absolute_import, division
from hashlib import sha1, sha256, sha384, sha512
from zope.interface import Attribute, implementer, Interface
from twisted.conch import error
from twisted.python.compat import long
class _IKexAlgorithm(Interface):
"""
An L{_IKexAlgorithm} describes a key exchange algorithm.
"""
preference = Attribute(
"An L{int} giving the preference of the algorithm when negotiating "
"key exchange. Algorithms with lower precedence values are more "
"preferred.")
hashProcessor = Attribute(
"A callable hash algorithm constructor (e.g. C{hashlib.sha256}) "
"suitable for use with this key exchange algorithm.")
class _IFixedGroupKexAlgorithm(_IKexAlgorithm):
"""
An L{_IFixedGroupKexAlgorithm} describes a key exchange algorithm with a
fixed prime / generator group.
"""
prime = Attribute(
"A L{long} giving the prime number used in Diffie-Hellman key "
"exchange, or L{None} if not applicable.")
generator = Attribute(
"A L{long} giving the generator number used in Diffie-Hellman key "
"exchange, or L{None} if not applicable. (This is not related to "
"Python generator functions.)")
class _IEllipticCurveExchangeKexAlgorithm(_IKexAlgorithm):
"""
An L{_IEllipticCurveExchangeKexAlgorithm} describes a key exchange algorithm
that uses an elliptic curve exchange between the client and server.
"""
class _IGroupExchangeKexAlgorithm(_IKexAlgorithm):
"""
An L{_IGroupExchangeKexAlgorithm} describes a key exchange algorithm
that uses group exchange between the client and server.
A prime / generator group should be chosen at run time based on the
requested size. See RFC 4419.
"""
@implementer(_IEllipticCurveExchangeKexAlgorithm)
class _ECDH256(object):
"""
Elliptic Curve Key Exchange with SHA-256 as HASH. Defined in
RFC 5656.
"""
preference = 1
hashProcessor = sha256
@implementer(_IEllipticCurveExchangeKexAlgorithm)
class _ECDH384(object):
"""
Elliptic Curve Key Exchange with SHA-384 as HASH. Defined in
RFC 5656.
"""
preference = 2
hashProcessor = sha384
@implementer(_IEllipticCurveExchangeKexAlgorithm)
class _ECDH512(object):
"""
Elliptic Curve Key Exchange with SHA-512 as HASH. Defined in
RFC 5656.
"""
preference = 3
hashProcessor = sha512
@implementer(_IGroupExchangeKexAlgorithm)
class _DHGroupExchangeSHA256(object):
"""
Diffie-Hellman Group and Key Exchange with SHA-256 as HASH. Defined in
RFC 4419, 4.2.
"""
preference = 4
hashProcessor = sha256
@implementer(_IGroupExchangeKexAlgorithm)
class _DHGroupExchangeSHA1(object):
"""
Diffie-Hellman Group and Key Exchange with SHA-1 as HASH. Defined in
RFC 4419, 4.1.
"""
preference = 5
hashProcessor = sha1
@implementer(_IFixedGroupKexAlgorithm)
class _DHGroup14SHA1(object):
"""
Diffie-Hellman key exchange with SHA-1 as HASH and Oakley Group 14
(2048-bit MODP Group). Defined in RFC 4253, 8.2.
"""
preference = 7
hashProcessor = sha1
# Diffie-Hellman primes from Oakley Group 14 (RFC 3526, 3).
prime = long('32317006071311007300338913926423828248817941241140239112842'
'00975140074170663435422261968941736356934711790173790970419175460587'
'32091950288537589861856221532121754125149017745202702357960782362488'
'84246189477587641105928646099411723245426622522193230540919037680524'
'23551912567971587011700105805587765103886184728025797605490356973256'
'15261670813393617995413364765591603683178967290731783845896806396719'
'00977202194168647225871031411336429319536193471636533209717077448227'
'98858856536920864529663607725026895550592836275112117409697299806841'
'05543595848665832916421362182310789909994486524682624169720359118525'
'07045361090559')
generator = 2
# Which ECDH hash function to use is dependent on the size.
_kexAlgorithms = {
b"diffie-hellman-group-exchange-sha256": _DHGroupExchangeSHA256(),
b"diffie-hellman-group-exchange-sha1": _DHGroupExchangeSHA1(),
b"diffie-hellman-group14-sha1": _DHGroup14SHA1(),
b"ecdh-sha2-nistp256": _ECDH256(),
b"ecdh-sha2-nistp384": _ECDH384(),
b"ecdh-sha2-nistp521": _ECDH512(),
}
def getKex(kexAlgorithm):
"""
Get a description of a named key exchange algorithm.
@param kexAlgorithm: The key exchange algorithm name.
@type kexAlgorithm: L{bytes}
@return: A description of the key exchange algorithm named by
C{kexAlgorithm}.
@rtype: L{_IKexAlgorithm}
@raises ConchError: if the key exchange algorithm is not found.
"""
if kexAlgorithm not in _kexAlgorithms:
raise error.ConchError(
"Unsupported key exchange algorithm: %s" % (kexAlgorithm,))
return _kexAlgorithms[kexAlgorithm]
def isEllipticCurve(kexAlgorithm):
"""
Returns C{True} if C{kexAlgorithm} is an elliptic curve.
@param kexAlgorithm: The key exchange algorithm name.
@type kexAlgorithm: C{str}
@return: C{True} if C{kexAlgorithm} is an elliptic curve,
otherwise C{False}.
@rtype: C{bool}
"""
return _IEllipticCurveExchangeKexAlgorithm.providedBy(getKex(kexAlgorithm))
def isFixedGroup(kexAlgorithm):
"""
Returns C{True} if C{kexAlgorithm} has a fixed prime / generator group.
@param kexAlgorithm: The key exchange algorithm name.
@type kexAlgorithm: L{bytes}
@return: C{True} if C{kexAlgorithm} has a fixed prime / generator group,
otherwise C{False}.
@rtype: L{bool}
"""
return _IFixedGroupKexAlgorithm.providedBy(getKex(kexAlgorithm))
def getHashProcessor(kexAlgorithm):
"""
Get the hash algorithm callable to use in key exchange.
@param kexAlgorithm: The key exchange algorithm name.
@type kexAlgorithm: L{bytes}
@return: A callable hash algorithm constructor (e.g. C{hashlib.sha256}).
@rtype: C{callable}
"""
kex = getKex(kexAlgorithm)
return kex.hashProcessor
def getDHGeneratorAndPrime(kexAlgorithm):
"""
Get the generator and the prime to use in key exchange.
@param kexAlgorithm: The key exchange algorithm name.
@type kexAlgorithm: L{bytes}
@return: A L{tuple} containing L{long} generator and L{long} prime.
@rtype: L{tuple}
"""
kex = getKex(kexAlgorithm)
return kex.generator, kex.prime
def getSupportedKeyExchanges():
"""
Get a list of supported key exchange algorithm names in order of
preference.
@return: A C{list} of supported key exchange algorithm names.
@rtype: C{list} of L{bytes}
"""
from cryptography.hazmat.backends import default_backend
from cryptography.hazmat.primitives.asymmetric import ec
from twisted.conch.ssh.keys import _curveTable
backend = default_backend()
kexAlgorithms = _kexAlgorithms.copy()
for keyAlgorithm in list(kexAlgorithms):
if keyAlgorithm.startswith(b"ecdh"):
keyAlgorithmDsa = keyAlgorithm.replace(b"ecdh", b"ecdsa")
supported = backend.elliptic_curve_exchange_algorithm_supported(
ec.ECDH(), _curveTable[keyAlgorithmDsa])
if not supported:
kexAlgorithms.pop(keyAlgorithm)
return sorted(
kexAlgorithms,
key = lambda kexAlgorithm: kexAlgorithms[kexAlgorithm].preference)