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# Copyright (C) 2013 Dr. Ralf Schlatterbeck Open Source Consulting.
# Reichergasse 131, A-3411 Weidling.
# Web: Email:
# All rights reserved
# ****************************************************************************
# This library is free software; you can redistribute it and/or modify
# it under the terms of the GNU Library General Public License as
# published by the Free Software Foundation; either version 2 of the
# License, or (at your option) any later version.
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# GNU Library General Public License for more details.
# You should have received a copy of the GNU Library General Public
# License along with this program; if not, write to the Free Software
# Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
# ****************************************************************************
import time
from M2Crypto import ASN1, X509, EVP, BIO, RSA, m2
from pyasn1.codec.der.decoder import decode as der_decode
from pyasn1.codec.der.encoder import encode as der_encode
from pyasn1.codec.ber.encoder import BitStringEncoder
from pyasn1.type.base import Asn1Item
from pyasn1.error import PyAsn1Error
from pyasn1.type.univ import OctetString, BitString
from base64 import b64decode
from base64 import encodestring as b64encode
from bitstring import Bitstring
class SPKAC_Decode_Error (ValueError) :
class SPKAC (object) :
""" Takes a Netscape SPKI/SPKAC certificate signing request which is
generated by Mozilla-based browsers when filling in a form with
a <keygen> element.
The constructor takes the base64-encoded SPKAC, and the optional
challenge from the <keygen> form element. The challenge is
returned by the browser inside the SPKAC together with the
public key. Both are signed by the public key and this class
automagically verfies they're correct. If something goes wrong,
SPKAC_Decode_Error is raised.
Subject attributes can either be set as keyword arguments in the
constructor or by later setting spkac.subject.<attr>.
Extensions can be specified as unnamed args or later with
>>> import test
>>> from M2Crypto import EVP, X509
>>> pkey = EVP.load_key_string (test.ca_key)
>>> cert = X509.load_cert_string (test.ca_crt)
>>> email = ''
Set the email *and* CN (common name) on spkac.subject.
Usually client certificates are used for authentication
purposes only. If the user loses the certificate we want as
little information as possible on the cert (as much as is
needed for authentication). So we set CN and emailAddress on
the certificate (and O and C are set from the issuer if not
explicitly set).
>>> ne = X509.new_extension
>>> spkac = SPKAC \\
... ( test.spkac_encoded
... , 'WtQWMrqzBqTvIZTm-g44Hsr1-qczPFiQlw5Wsw'
... , ne ('basicConstraints', 'CA:FALSE', critical = True)
... , CN = email
... , Email = email
... , O = cert.get_subject ().O
... )
>>> pe = spkac.push_extension
>>> ku = 'digitalSignature, keyEncipherment, keyAgreement'
>>> pe (ne ('keyUsage', ku, critical = True))
>>> pe (ne ('extendedKeyUsage', 'clientAuth, emailProtection, nsSGC'))
>>> spkac.subject.C = cert.get_subject ().C
>>> spkac.subject.Email
>>> spkac.subject.CN
>>> spkac.subject.C
>>> spkac.subject.O
>>> for e in spkac.extensions :
... print e.get_name ()
>>> crt = spkac.gen_crt (pkey, cert, 42).as_pem ()
>>> lines = crt.strip ().split ("\\n")
>>> print "%s\\n%s\\n%s" % (lines [0], "...", lines [-1])
>>> spkac.cert.check_purpose (m2.X509_PURPOSE_SSL_SERVER, 0)
>>> spkac.cert.check_purpose (m2.X509_PURPOSE_NS_SSL_SERVER, 0)
>>> spkac.cert.check_purpose (m2.X509_PURPOSE_ANY, 0)
>>> spkac.cert.check_purpose (m2.X509_PURPOSE_SSL_CLIENT, 0)
>>> spkac = SPKAC \\
... ( test.spkac_encoded
... , 'non-matching-challenge'
... )
Traceback (most recent call last):
SPKAC_Decode_Error: Challenge doesn't match: got "non-matching-challenge" expect "WtQWMrqzBqTvIZTm-g44Hsr1-qczPFiQlw5Wsw"
# Table of Object Identifiers for public key crypto
# We directly store the module from which the load_pub_key_bio
# routine can be used and the assignment method name.
# Seems the message digest algo is also specified here (md5)
signature_algorithms = \
{ (1, 2, 840, 113549, 1, 1, 4) : (RSA, 'assign_rsa', 'md5')
# May come in handy in the future.
hash_algorithms = \
{ (1, 3, 14, 3, 2, 26) : 'sha1'
def __init__ (self, b64val, challenge = None, *extensions, ** kw) :
""" This gets a base64 encoded value as returned by a browser
when filling in a form with a keygen element.
The optional challenge is the challenge string issued by the
web server and is checked against the signed challenge value
returned by the browser (inside the b64val).
We generally return a SPKAC_Decode_Error in case something
cannot be parsed.
The base64 encoded value is ASN.1 and stores the following
Netscape SPKI/SPKAC data structure:
ASN.1 notation of this (see HTML-5 keygen docs)
PublicKeyAndChallenge ::= SEQUENCE {
spki SubjectPublicKeyInfo,
challenge IA5STRING
SignedPublicKeyAndChallenge ::= SEQUENCE {
publicKeyAndChallenge PublicKeyAndChallenge,
signatureAlgorithm AlgorithmIdentifier,
signature BIT STRING
try :
seq, rest = der_decode (b64decode (b64val))
except PyAsn1Error, e :
raise SPKAC_Decode_Error (e)
if rest :
raise SPKAC_Decode_Error ("ASN.1 decode: data after SPKAC value")
if len (seq) != 3 or len (seq [0]) != 2 or len (seq [1]) != 2 :
raise SPKAC_Decode_Error ("Unknown SPKAC data format")
if seq [1][1] :
raise SPKAC_Decode_Error ("Invalid Public Key Info")
self.signed = der_encode (seq [0])
self.spki = seq [0][0]
self.challenge = seq [0][1]
self.sig_algo = tuple (seq [1][0])
self.signature = Bitstring (seq [2]).as_string ()
if challenge and challenge != self.challenge :
msg = 'Challenge doesn\'t match: got "%s" expect "%s"' \
% (challenge, self.challenge)
raise SPKAC_Decode_Error (msg)
self.pkey, self.hash = self._compute_public_key_ ()
self.pkey.reset_context (md = self.hash)
self.pkey.verify_init ()
self.pkey.verify_update (self.signed)
r = self.pkey.verify_final (self.signature)
if r < 0 :
raise SPKAC_Decode_Error ("Error during signature verification")
elif r == 0 :
raise SPKAC_Decode_Error ("Invalid signature")
elif r != 1 :
raise RuntimeError ("Unexpected verify_final return code: %s" % r)
self.extensions = X509.X509_Extension_Stack ()
self.subject = X509.X509_Name ()
# unnamed args from constructor are interpreted as extensions
for e in extensions :
self.push_extension (e)
# additional keyword arguments are subject attributes
for k, v in kw.iteritems () :
setattr (self.subject, k, v)
# end def __init__
def gen_crt \
(self, ca_pkey, ca_crt, serial, not_before = None, not_after = None) :
""" Return as an X509 certificate.
We do this because M2Crypto doesn't wrap the SPKAC data
structures. So to generate a certificate we get the key
of the CA and sign the certificate with it.
Note that the caller will have to make sure to not generate
duplicate sequence numbers.
Note this will also set self.cert for later retrieval. In
addition self.cert is returned.
Default not_before is now.
Default validity time of not_after isn't set is 1 year from
if not not_before :
not_before = long (time.time ()) + time.timezone
if not not_after :
not_after = not_before + 60 * 60 * 24 * 365
nb.set_time (not_before)
na.set_time (not_after)
self.cert = cert = X509.X509 ()
cert.set_version (2)
cert.set_serial_number (serial)
cert.set_not_before (nb)
cert.set_not_after (na)
cert.set_pubkey (self.pkey)
cert.set_subject (self.subject)
cert.set_issuer (ca_crt.get_subject ())
for ext in self.extensions :
cert.add_ext (ext)
cert.sign (pkey = ca_pkey, md = 'sha1')
assert cert.verify (ca_pkey)
assert not cert.check_ca ()
return cert
# end def gen_crt
def push_extension (self, ext) :
""" Specify an X509 extension for export as CRT.
X509 extensions are specified with X509.new_extension, e.g.
>>> x = X509.new_extension ('subjectAltName', '')
self.extensions.push (ext)
# end def push_extension
def _as_pem (self, asn1val, header = None) :
""" Create base64 encoded version of asn1val and wrap in in
appropriate BEGIN/END lines for pem format. The BEGIN/END text
differs for different asn1 values, so we specify this as
"header". This is necessary since most constructors in
M2Crypto don't seem to have a version that directly accepts
ASN.1, so we generate the pem version here.
# Future: we might want to have default header
if header is None :
header = self.header
if isinstance (asn1val, Asn1Item) :
asn1val = der_encode (asn1val)
v = b64encode (asn1val)
return '-----BEGIN %s-----\n%s-----END %s-----\n' \
% (header, v, header)
# end def _as_pem
def _compute_public_key_ (self) :
""" Compute the public key as a EVP.PKey object from the
information in the SPKAC.
Note that there doesn't seem to be a direct way (at least in
M2Crypto) to directly obtain an EVP from just the public key
ASN.1 data. So we have to find out the signature algorithm
and create the appropriate public key object. Then this is
put into the PKey.
This currently only supports RSA keys -- seems only RSA is
wrapped in M2Crypto *and* we've never seen something other
than RSA returned from a keygen tag by a browser.
if self.sig_algo not in self.signature_algorithms :
raise SPKAC_Decode_Error ("Unknown signature algorithm")
buf = BIO.MemoryBuffer (self._as_pem (self.spki, 'PUBLIC KEY'))
mod, asg, hash = self.signature_algorithms [self.sig_algo]
# the following effectively does
# rsa = RSA.load_pub_key_bio (buf)
# pkey.assign_rsa (rsa)
alg = mod.load_pub_key_bio (buf)
pkey = EVP.PKey ()
method = getattr (pkey, asg)
method (alg)
return pkey, hash
# end def _compute_public_key_
# end class SPKAC