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Filename : factory.py
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# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.

"""
A Factory for SSH servers.

See also L{twisted.conch.openssh_compat.factory} for OpenSSH compatibility.

Maintainer: Paul Swartz
"""

from __future__ import division, absolute_import

from twisted.internet import protocol
from twisted.python import log

from twisted.conch import error
from twisted.conch.ssh import (_kex, transport, userauth, connection)

import random


class SSHFactory(protocol.Factory):
    """
    A Factory for SSH servers.
    """
    protocol = transport.SSHServerTransport

    services = {
        b'ssh-userauth':userauth.SSHUserAuthServer,
        b'ssh-connection':connection.SSHConnection
    }
    def startFactory(self):
        """
        Check for public and private keys.
        """
        if not hasattr(self,'publicKeys'):
            self.publicKeys = self.getPublicKeys()
        if not hasattr(self,'privateKeys'):
            self.privateKeys = self.getPrivateKeys()
        if not self.publicKeys or not self.privateKeys:
            raise error.ConchError('no host keys, failing')
        if not hasattr(self,'primes'):
            self.primes = self.getPrimes()


    def buildProtocol(self, addr):
        """
        Create an instance of the server side of the SSH protocol.

        @type addr: L{twisted.internet.interfaces.IAddress} provider
        @param addr: The address at which the server will listen.

        @rtype: L{twisted.conch.ssh.transport.SSHServerTransport}
        @return: The built transport.
        """
        t = protocol.Factory.buildProtocol(self, addr)
        t.supportedPublicKeys = self.privateKeys.keys()
        if not self.primes:
            log.msg('disabling non-fixed-group key exchange algorithms '
                    'because we cannot find moduli file')
            t.supportedKeyExchanges = [
                kexAlgorithm for kexAlgorithm in t.supportedKeyExchanges
                if _kex.isFixedGroup(kexAlgorithm) or
                     _kex.isEllipticCurve(kexAlgorithm)]
        return t


    def getPublicKeys(self):
        """
        Called when the factory is started to get the public portions of the
        servers host keys.  Returns a dictionary mapping SSH key types to
        public key strings.

        @rtype: L{dict}
        """
        raise NotImplementedError('getPublicKeys unimplemented')


    def getPrivateKeys(self):
        """
        Called when the factory is started to get the  private portions of the
        servers host keys.  Returns a dictionary mapping SSH key types to
        L{twisted.conch.ssh.keys.Key} objects.

        @rtype: L{dict}
        """
        raise NotImplementedError('getPrivateKeys unimplemented')


    def getPrimes(self):
        """
        Called when the factory is started to get Diffie-Hellman generators and
        primes to use.  Returns a dictionary mapping number of bits to lists
        of tuple of (generator, prime).

        @rtype: L{dict}
        """


    def getDHPrime(self, bits):
        """
        Return a tuple of (g, p) for a Diffe-Hellman process, with p being as
        close to bits bits as possible.

        @type bits: L{int}
        @rtype:     L{tuple}
        """
        primesKeys = sorted(self.primes.keys(), key=lambda i: abs(i - bits))
        realBits = primesKeys[0]
        return random.choice(self.primes[realBits])


    def getService(self, transport, service):
        """
        Return a class to use as a service for the given transport.

        @type transport:    L{transport.SSHServerTransport}
        @type service:      L{bytes}
        @rtype:             subclass of L{service.SSHService}
        """
        if service == b'ssh-userauth' or hasattr(transport, 'avatar'):
            return self.services[service]