spkac.py

#!/usr/bin/python
# Copyright (C) 2013 Dr. Ralf Schlatterbeck Open Source Consulting.
# Reichergasse 131, A-3411 Weidling.
# Web: http://www.runtux.com Email: office@runtux.com
# 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
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# 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.error             import PyAsn1Error
from base64                   import b64decode
from bitstring                import Bitstring
from pem_object               import PEM_Object

class SPKAC_Decode_Error (ValueError) :
    pass

class SPKAC (PEM_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
        push_extension.
        >>> import test
        >>> from M2Crypto import EVP, X509
        >>> pkey  = EVP.load_key_string   (test.ca_key)
        >>> cert  = X509.load_cert_string (test.ca_crt)
        >>> email = 'testuser@example.com'

        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
        'testuser@example.com'
        >>> spkac.subject.CN
        'testuser@example.com'
        >>> spkac.subject.C
        'AT'
        >>> spkac.subject.O
        'runtux.com'
        >>> for e in spkac.extensions :
        ...     print e.get_name ()
        basicConstraints
        keyUsage
        extendedKeyUsage

        >>> crt   = spkac.gen_crt (pkey, cert, 42).as_pem ()
        >>> lines = crt.strip ().split ("\\n")
        >>> print "%s\\n%s\\n%s" % (lines [0], "...", lines [-1])
        -----BEGIN CERTIFICATE-----
        ...
        -----END CERTIFICATE-----

        >>> spkac.cert.check_purpose (m2.X509_PURPOSE_SSL_SERVER, 0)
        1
        >>> spkac.cert.check_purpose (m2.X509_PURPOSE_NS_SSL_SERVER, 0)
        1
        >>> spkac.cert.check_purpose (m2.X509_PURPOSE_ANY, 0)   
        1
        >>> spkac.cert.check_purpose (m2.X509_PURPOSE_SSL_CLIENT, 0)
        1

        >>> 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
            now.
        """
        if not not_before :
            not_before = long (time.time ())
        if not not_after :
            not_after  = not_before + 60 * 60 * 24 * 365
        nb        = ASN1.ASN1_UTCTIME ()
        nb.set_time                   (not_before)
        na        = ASN1.ASN1_UTCTIME ()
        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', 'DNS:foo.example.com')
        """
        self.extensions.push (ext)
    # end def push_extension

    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