pycert.py

#!/usr/bin/env python
#
# This Source Code Form is subject to the terms of the Mozilla Public
# License, v. 2.0. If a copy of the MPL was not distributed with this
# file, You can obtain one at http://mozilla.org/MPL/2.0/.

"""
Reads a certificate specification from stdin or a file and outputs a
signed x509 certificate with the desired properties.

The input format is as follows:

issuer:<issuer distinguished name specification>
subject:<subject distinguished name specification>
[version:{1,2,3,4}]
[validity:<YYYYMMDD-YYYYMMDD|duration in days>]
[issuerKey:<key specification>]
[subjectKey:<key specification>]
[signature:{sha256WithRSAEncryption,sha1WithRSAEncryption,
            md5WithRSAEncryption,ecdsaWithSHA256}]
[serialNumber:<integer in the interval [1, 127]>]
[extension:<extension name:<extension-specific data>>]
[...]

Known extensions are:
basicConstraints:[cA],[pathLenConstraint]
keyUsage:[digitalSignature,nonRepudiation,keyEncipherment,
          dataEncipherment,keyAgreement,keyCertSign,cRLSign]
extKeyUsage:[serverAuth,clientAuth,codeSigning,emailProtection
             nsSGC, # Netscape Server Gated Crypto
             OCSPSigning,timeStamping]
subjectAlternativeName:[<dNSName|directoryName>,...]
authorityInformationAccess:<OCSP URI>
certificatePolicies:[<policy OID>,...]
nameConstraints:{permitted,excluded}:[<dNSName|directoryName>,...]
nsCertType:sslServer
TLSFeature:[<TLSFeature>,...]
embeddedSCTList:[<key specification>:<YYYYMMDD>,...]

Where:
  [] indicates an optional field or component of a field
  <> indicates a required component of a field
  {} indicates a choice of exactly one value among a set of values
  [a,b,c] indicates a list of potential values, of which zero or more
          may be used

For instance, the version field is optional. However, if it is
specified, it must have exactly one value from the set {1,2,3,4}.

Most fields have reasonable default values. By default one shared RSA
key is used for all signatures and subject public key information
fields. Using "issuerKey:<key specification>" or
"subjectKey:<key specification>" causes a different key be used for
signing or as the subject public key information field, respectively.
See pykey.py for the list of available specifications.
The signature algorithm is sha256WithRSAEncryption by default.

The validity period may be specified as either concrete notBefore and
notAfter values or as a validity period centered around 'now'. For the
latter, this will result in a notBefore of 'now' - duration/2 and a
notAfter of 'now' + duration/2.

Issuer and subject distinguished name specifications are of the form
'[stringEncoding]/C=XX/O=Example/CN=example.com'. C (country name), ST
(state or province name), L (locality name), O (organization name), OU
(organizational unit name), CN (common name) and emailAddress (email
address) are currently supported. The optional stringEncoding field may
be 'utf8String' or 'printableString'. If the given string does not
contain a '/', it is assumed to represent a common name. If an empty
string is provided, then an empty distinguished name is returned.
DirectoryNames also use this format. When specifying a directoryName in
a nameConstraints extension, the implicit form may not be used.

If an extension name has '[critical]' after it, it will be marked as
critical. Otherwise (by default), it will not be marked as critical.

TLSFeature values can either consist of a named value (currently only
'OCSPMustStaple' which corresponds to status_request) or a numeric TLS
feature value (see rfc7633 for more information).

If a serial number is not explicitly specified, it is automatically
generated based on the contents of the certificate.
"""

from pyasn1.codec.der import decoder
from pyasn1.codec.der import encoder
from pyasn1.type import constraint, namedtype, tag, univ, useful
from pyasn1_modules import rfc2459
from struct import pack
import base64
import datetime
import hashlib
import re
import sys

import pyct
import pykey

# The GeneralSubtree definition in pyasn1_modules.rfc2459 is incorrect.
# Where this definition uses a DefaultedNamedType, pyasn1_modules uses
# a NamedType, which results in the default value being explicitly
# encoded, which is incorrect for DER.
class GeneralSubtree(univ.Sequence):
    componentType = namedtype.NamedTypes(
        namedtype.NamedType('base', rfc2459.GeneralName()),
        namedtype.DefaultedNamedType('minimum', rfc2459.BaseDistance(0).subtype(
            implicitTag=tag.Tag(tag.tagClassContext, tag.tagFormatConstructed, 0))),
        namedtype.OptionalNamedType('maximum', rfc2459.BaseDistance().subtype(
            implicitTag=tag.Tag(tag.tagClassContext, tag.tagFormatConstructed, 1)))
    )


# The NameConstraints definition in pyasn1_modules.rfc2459 is incorrect.
# excludedSubtrees has a tag value of 1, not 0.
class NameConstraints(univ.Sequence):
    componentType = namedtype.NamedTypes(
        namedtype.OptionalNamedType('permittedSubtrees', rfc2459.GeneralSubtrees().subtype(
            implicitTag=tag.Tag(tag.tagClassContext, tag.tagFormatConstructed, 0))),
        namedtype.OptionalNamedType('excludedSubtrees', rfc2459.GeneralSubtrees().subtype(
            implicitTag=tag.Tag(tag.tagClassContext, tag.tagFormatConstructed, 1)))
    )


class Error(Exception):
    """Base class for exceptions in this module."""
    pass


class UnknownBaseError(Error):
    """Base class for handling unexpected input in this module."""
    def __init__(self, value):
        super(UnknownBaseError, self).__init__()
        self.value = value
        self.category = 'input'

    def __str__(self):
        return 'Unknown %s type "%s"' % (self.category, repr(self.value))


class UnknownAlgorithmTypeError(UnknownBaseError):
    """Helper exception type to handle unknown algorithm types."""

    def __init__(self, value):
        UnknownBaseError.__init__(self, value)
        self.category = 'algorithm'


class UnknownParameterTypeError(UnknownBaseError):
    """Helper exception type to handle unknown input parameters."""

    def __init__(self, value):
        UnknownBaseError.__init__(self, value)
        self.category = 'parameter'


class UnknownExtensionTypeError(UnknownBaseError):
    """Helper exception type to handle unknown input extensions."""

    def __init__(self, value):
        UnknownBaseError.__init__(self, value)
        self.category = 'extension'


class UnknownKeyPurposeTypeError(UnknownBaseError):
    """Helper exception type to handle unknown key purposes."""

    def __init__(self, value):
        UnknownBaseError.__init__(self, value)
        self.category = 'keyPurpose'


class UnknownKeyTargetError(UnknownBaseError):
    """Helper exception type to handle unknown key targets."""

    def __init__(self, value):
        UnknownBaseError.__init__(self, value)
        self.category = 'key target'


class UnknownVersionError(UnknownBaseError):
    """Helper exception type to handle unknown specified versions."""

    def __init__(self, value):
        UnknownBaseError.__init__(self, value)
        self.category = 'version'


class UnknownNameConstraintsSpecificationError(UnknownBaseError):
    """Helper exception type to handle unknown specified
    nameConstraints."""

    def __init__(self, value):
        UnknownBaseError.__init__(self, value)
        self.category = 'nameConstraints specification'


class UnknownDNTypeError(UnknownBaseError):
    """Helper exception type to handle unknown DN types."""

    def __init__(self, value):
        UnknownBaseError.__init__(self, value)
        self.category = 'DN'


class UnknownNSCertTypeError(UnknownBaseError):
    """Helper exception type to handle unknown nsCertType types."""

    def __init__(self, value):
        UnknownBaseError.__init__(self, value)
        self.category = 'nsCertType'


class UnknownTLSFeature(UnknownBaseError):
    """Helper exception type to handle unknown TLS Features."""

    def __init__(self, value):
        UnknownBaseError.__init__(self, value)
        self.category = 'TLSFeature'


class InvalidSCTSpecification(Error):
    """Helper exception type to handle invalid SCT specifications."""

    def __init__(self, value):
        super(InvalidSCTSpecification, self).__init__()
        self.value = value

    def __str__(self):
        return repr('invalid SCT specification "{}"' % self.value)


class InvalidSerialNumber(Error):
    """Exception type to handle invalid serial numbers."""

    def __init__(self, value):
        super(InvalidSerialNumber, self).__init__()
        self.value = value

    def __str__(self):
        return repr(self.value)


def getASN1Tag(asn1Type):
    """Helper function for returning the base tag value of a given
    type from the pyasn1 package"""
    return asn1Type.baseTagSet.getBaseTag().asTuple()[2]

def stringToAccessDescription(string):
    """Helper function that takes a string representing a URI
    presumably identifying an OCSP authority information access
    location. Returns an AccessDescription usable by pyasn1."""
    accessMethod = rfc2459.id_ad_ocsp
    accessLocation = rfc2459.GeneralName()
    accessLocation.setComponentByName('uniformResourceIdentifier', string)
    sequence = univ.Sequence()
    sequence.setComponentByPosition(0, accessMethod)
    sequence.setComponentByPosition(1, accessLocation)
    return sequence

def stringToDN(string, tag=None):
    """Takes a string representing a distinguished name or directory
    name and returns a Name for use by pyasn1. See the documentation
    for the issuer and subject fields for more details. Takes an
    optional implicit tag in cases where the Name needs to be tagged
    differently."""
    if string and '/' not in string:
        string = '/CN=%s' % string
    rdns = rfc2459.RDNSequence()
    pattern = '/(C|ST|L|O|OU|CN|emailAddress)='
    split = re.split(pattern, string)
    # split should now be [[encoding], <type>, <value>, <type>, <value>, ...]
    if split[0]:
        encoding = split[0]
    else:
        encoding = 'utf8String'
    for pos, (nameType, value) in enumerate(zip(split[1::2], split[2::2])):
        ava = rfc2459.AttributeTypeAndValue()
        if nameType == 'C':
            ava.setComponentByName('type', rfc2459.id_at_countryName)
            nameComponent = rfc2459.X520countryName(value)
        elif nameType == 'ST':
            ava.setComponentByName('type', rfc2459.id_at_stateOrProvinceName)
            nameComponent = rfc2459.X520StateOrProvinceName()
        elif nameType == 'L':
            ava.setComponentByName('type', rfc2459.id_at_localityName)
            nameComponent = rfc2459.X520LocalityName()
        elif nameType == 'O':
            ava.setComponentByName('type', rfc2459.id_at_organizationName)
            nameComponent = rfc2459.X520OrganizationName()
        elif nameType == 'OU':
            ava.setComponentByName('type', rfc2459.id_at_organizationalUnitName)
            nameComponent = rfc2459.X520OrganizationalUnitName()
        elif nameType == 'CN':
            ava.setComponentByName('type', rfc2459.id_at_commonName)
            nameComponent = rfc2459.X520CommonName()
        elif nameType == 'emailAddress':
            ava.setComponentByName('type', rfc2459.emailAddress)
            nameComponent = rfc2459.Pkcs9email(value)
        else:
            raise UnknownDNTypeError(nameType)
        if not nameType == 'C' and not nameType == 'emailAddress':
            # The value may have things like '\0' (i.e. a slash followed by
            # the number zero) that have to be decoded into the resulting
            # '\x00' (i.e. a byte with value zero).
            nameComponent.setComponentByName(encoding, value.decode(encoding='string_escape'))
        ava.setComponentByName('value', nameComponent)
        rdn = rfc2459.RelativeDistinguishedName()
        rdn.setComponentByPosition(0, ava)
        rdns.setComponentByPosition(pos, rdn)
    if tag:
        name = rfc2459.Name().subtype(implicitTag=tag)
    else:
        name = rfc2459.Name()
    name.setComponentByPosition(0, rdns)
    return name

def stringToAlgorithmIdentifiers(string):
    """Helper function that converts a description of an algorithm
    to a representation usable by the pyasn1 package and a hash
    algorithm constant for use by pykey."""
    algorithmIdentifier = rfc2459.AlgorithmIdentifier()
    algorithmType = None
    algorithm = None
    if string == 'sha1WithRSAEncryption':
        algorithmType = pykey.HASH_SHA1
        algorithm = rfc2459.sha1WithRSAEncryption
    elif string == 'sha256WithRSAEncryption':
        algorithmType = pykey.HASH_SHA256
        algorithm = univ.ObjectIdentifier('1.2.840.113549.1.1.11')
    elif string == 'md5WithRSAEncryption':
        algorithmType = pykey.HASH_MD5
        algorithm = rfc2459.md5WithRSAEncryption
    elif string == 'ecdsaWithSHA256':
        algorithmType = pykey.HASH_SHA256
        algorithm = univ.ObjectIdentifier('1.2.840.10045.4.3.2')
    else:
        raise UnknownAlgorithmTypeError(string)
    algorithmIdentifier.setComponentByName('algorithm', algorithm)
    return (algorithmIdentifier, algorithmType)

def datetimeToTime(dt):
    """Takes a datetime object and returns an rfc2459.Time object with
    that time as its value as a GeneralizedTime"""
    time = rfc2459.Time()
    time.setComponentByName('generalTime', useful.GeneralizedTime(dt.strftime('%Y%m%d%H%M%SZ')))
    return time

def serialBytesToString(serialBytes):
    """Takes a list of integers in the interval [0, 255] and returns
    the corresponding serial number string."""
    serialBytesLen = len(serialBytes)
    if serialBytesLen > 127:
        raise InvalidSerialNumber("{} bytes is too long".format(serialBytesLen))
    # Prepend the ASN.1 INTEGER tag and length bytes.
    stringBytes = [getASN1Tag(univ.Integer), serialBytesLen] + serialBytes
    return ''.join(chr(b) for b in stringBytes)

class Certificate(object):
    """Utility class for reading a certificate specification and
    generating a signed x509 certificate"""

    def __init__(self, paramStream):
        self.versionValue = 2 # a value of 2 is X509v3
        self.signature = 'sha256WithRSAEncryption'
        self.issuer = 'Default Issuer'
        actualNow = datetime.datetime.utcnow()
        self.now = datetime.datetime.strptime(str(actualNow.year), '%Y')
        aYearAndAWhile = datetime.timedelta(days=400)
        self.notBefore = self.now - aYearAndAWhile
        self.notAfter = self.now + aYearAndAWhile
        self.subject = 'Default Subject'
        self.extensions = None
        self.savedEmbeddedSCTListData = None
        self.subjectKey = pykey.keyFromSpecification('default')
        self.issuerKey = pykey.keyFromSpecification('default')
        self.serialNumber = None
        self.decodeParams(paramStream)
        # If a serial number wasn't specified, generate one based on
        # the certificate contents.
        if not self.serialNumber:
            self.serialNumber = self.generateSerialNumber()
        # This has to be last because the SCT signature depends on the
        # contents of the certificate.
        if self.savedEmbeddedSCTListData:
            self.addEmbeddedSCTListData()

    def generateSerialNumber(self):
        """Generates a serial number for this certificate based on its
        contents. Intended to be reproducible for compatibility with
        the build system on OS X (see the comment above main, later in
        this file)."""
        hasher = hashlib.sha256()
        hasher.update(str(self.versionValue))
        hasher.update(self.signature)
        hasher.update(self.issuer)
        hasher.update(str(self.notBefore))
        hasher.update(str(self.notAfter))
        hasher.update(self.subject)
        if self.extensions:
            for extension in self.extensions:
                hasher.update(str(extension))
        if self.savedEmbeddedSCTListData:
            # savedEmbeddedSCTListData is
            # (embeddedSCTListSpecification, critical), where |critical|
            # may be None
            hasher.update(self.savedEmbeddedSCTListData[0])
            if (self.savedEmbeddedSCTListData[1]):
                hasher.update(self.savedEmbeddedSCTListData[1])
        serialBytes = [ord(c) for c in hasher.digest()[:20]]
        # Ensure that the most significant bit isn't set (which would
        # indicate a negative number, which isn't valid for serial
        # numbers).
        serialBytes[0] &= 0x7f
        # Also ensure that the least significant bit on the most
        # significant byte is set (to prevent a leading zero byte,
        # which also wouldn't be valid).
        serialBytes[0] |= 0x01
        return serialBytesToString(serialBytes)

    def decodeParams(self, paramStream):
        for line in paramStream.readlines():
            self.decodeParam(line.strip())

    def decodeParam(self, line):
        param = line.split(':')[0]
        value = ':'.join(line.split(':')[1:])
        if param == 'version':
            self.setVersion(value)
        elif param == 'subject':
            self.subject = value
        elif param == 'issuer':
            self.issuer = value
        elif param == 'validity':
            self.decodeValidity(value)
        elif param == 'extension':
            self.decodeExtension(value)
        elif param == 'issuerKey':
            self.setupKey('issuer', value)
        elif param == 'subjectKey':
            self.setupKey('subject', value)
        elif param == 'signature':
            self.signature = value
        elif param == 'serialNumber':
            serialNumber = int(value)
            # Ensure only serial numbers that conform to the rules listed in
            # generateSerialNumber() are permitted.
            if serialNumber < 1 or serialNumber > 127:
                raise InvalidSerialNumber(value)
            self.serialNumber = serialBytesToString([serialNumber])
        else:
            raise UnknownParameterTypeError(param)

    def setVersion(self, version):
        intVersion = int(version)
        if intVersion >= 1 and intVersion <= 4:
            self.versionValue = intVersion - 1
        else:
            raise UnknownVersionError(version)

    def decodeValidity(self, duration):
        match = re.search('([0-9]{8})-([0-9]{8})', duration)
        if match:
            self.notBefore = datetime.datetime.strptime(match.group(1), '%Y%m%d')
            self.notAfter = datetime.datetime.strptime(match.group(2), '%Y%m%d')
        else:
            delta = datetime.timedelta(days=(int(duration) / 2))
            self.notBefore = self.now - delta
            self.notAfter = self.now + delta

    def decodeExtension(self, extension):
        match = re.search(r'([a-zA-Z]+)(\[critical\])?:(.*)', extension)
        if not match:
            raise UnknownExtensionTypeError(extension)
        extensionType = match.group(1)
        critical = match.group(2)
        value = match.group(3)
        if extensionType == 'basicConstraints':
            self.addBasicConstraints(value, critical)
        elif extensionType == 'keyUsage':
            self.addKeyUsage(value, critical)
        elif extensionType == 'extKeyUsage':
            self.addExtKeyUsage(value, critical)
        elif extensionType == 'subjectAlternativeName':
            self.addSubjectAlternativeName(value, critical)
        elif extensionType == 'authorityInformationAccess':
            self.addAuthorityInformationAccess(value, critical)
        elif extensionType == 'certificatePolicies':
            self.addCertificatePolicies(value, critical)
        elif extensionType == 'nameConstraints':
            self.addNameConstraints(value, critical)
        elif extensionType == 'nsCertType':
            self.addNSCertType(value, critical)
        elif extensionType == 'TLSFeature':
            self.addTLSFeature(value, critical)
        elif extensionType == 'embeddedSCTList':
            self.savedEmbeddedSCTListData = (value, critical)
        else:
            raise UnknownExtensionTypeError(extensionType)

    def setupKey(self, subjectOrIssuer, value):
        if subjectOrIssuer == 'subject':
            self.subjectKey = pykey.keyFromSpecification(value)
        elif subjectOrIssuer == 'issuer':
            self.issuerKey = pykey.keyFromSpecification(value)
        else:
            raise UnknownKeyTargetError(subjectOrIssuer)

    def addExtension(self, extensionType, extensionValue, critical):
        if not self.extensions:
            self.extensions = []
        encapsulated = univ.OctetString(encoder.encode(extensionValue))
        extension = rfc2459.Extension()
        extension.setComponentByName('extnID', extensionType)
        # critical is either the string '[critical]' or None.
        # We only care whether or not it is truthy.
        if critical:
            extension.setComponentByName('critical', True)
        extension.setComponentByName('extnValue', encapsulated)
        self.extensions.append(extension)

    def addBasicConstraints(self, basicConstraints, critical):
        cA = basicConstraints.split(',')[0]
        pathLenConstraint = basicConstraints.split(',')[1]
        basicConstraintsExtension = rfc2459.BasicConstraints()
        basicConstraintsExtension.setComponentByName('cA', cA == 'cA')
        if pathLenConstraint:
            pathLenConstraintValue = \
                univ.Integer(int(pathLenConstraint)).subtype(
                    subtypeSpec=constraint.ValueRangeConstraint(0, 64))
            basicConstraintsExtension.setComponentByName('pathLenConstraint',
                                                         pathLenConstraintValue)
        self.addExtension(rfc2459.id_ce_basicConstraints, basicConstraintsExtension, critical)

    def addKeyUsage(self, keyUsage, critical):
        keyUsageExtension = rfc2459.KeyUsage(keyUsage)
        self.addExtension(rfc2459.id_ce_keyUsage, keyUsageExtension, critical)

    def keyPurposeToOID(self, keyPurpose):
        if keyPurpose == 'serverAuth':
            # the OID for id_kp_serverAuth is incorrect in the
            # pyasn1-modules implementation
            return univ.ObjectIdentifier('1.3.6.1.5.5.7.3.1')
        if keyPurpose == 'clientAuth':
            return rfc2459.id_kp_clientAuth
        if keyPurpose == 'codeSigning':
            return rfc2459.id_kp_codeSigning
        if keyPurpose == 'emailProtection':
            return rfc2459.id_kp_emailProtection
        if keyPurpose == 'nsSGC':
            return univ.ObjectIdentifier('2.16.840.1.113730.4.1')
        if keyPurpose == 'OCSPSigning':
            return univ.ObjectIdentifier('1.3.6.1.5.5.7.3.9')
        if keyPurpose == 'timeStamping':
            return rfc2459.id_kp_timeStamping
        raise UnknownKeyPurposeTypeError(keyPurpose)

    def addExtKeyUsage(self, extKeyUsage, critical):
        extKeyUsageExtension = rfc2459.ExtKeyUsageSyntax()
        for count, keyPurpose in enumerate(extKeyUsage.split(',')):
            extKeyUsageExtension.setComponentByPosition(count, self.keyPurposeToOID(keyPurpose))
        self.addExtension(rfc2459.id_ce_extKeyUsage, extKeyUsageExtension, critical)

    def addSubjectAlternativeName(self, names, critical):
        subjectAlternativeName = rfc2459.SubjectAltName()
        for count, name in enumerate(names.split(',')):
            generalName = rfc2459.GeneralName()
            if '/' in name:
                directoryName = stringToDN(name,
                                           tag.Tag(tag.tagClassContext, tag.tagFormatSimple, 4))
                generalName.setComponentByName('directoryName', directoryName)
            else:
                # The string may have things like '\0' (i.e. a slash
                # followed by the number zero) that have to be decoded into
                # the resulting '\x00' (i.e. a byte with value zero).
                generalName.setComponentByName('dNSName', name.decode(encoding='string_escape'))
            subjectAlternativeName.setComponentByPosition(count, generalName)
        self.addExtension(rfc2459.id_ce_subjectAltName, subjectAlternativeName, critical)

    def addAuthorityInformationAccess(self, ocspURI, critical):
        sequence = univ.Sequence()
        accessDescription = stringToAccessDescription(ocspURI)
        sequence.setComponentByPosition(0, accessDescription)
        self.addExtension(rfc2459.id_pe_authorityInfoAccess, sequence, critical)

    def addCertificatePolicies(self, policyOIDs, critical):
        policies = rfc2459.CertificatePolicies()
        for pos, policyOID in enumerate(policyOIDs.split(',')):
            if policyOID == 'any':
                policyOID = '2.5.29.32.0'
            policy = rfc2459.PolicyInformation()
            policyIdentifier = rfc2459.CertPolicyId(policyOID)
            policy.setComponentByName('policyIdentifier', policyIdentifier)
            policies.setComponentByPosition(pos, policy)
        self.addExtension(rfc2459.id_ce_certificatePolicies, policies, critical)

    def addNameConstraints(self, constraints, critical):
        nameConstraints = NameConstraints()
        if constraints.startswith('permitted:'):
            (subtreesType, subtreesTag) = ('permittedSubtrees', 0)
        elif constraints.startswith('excluded:'):
            (subtreesType, subtreesTag) = ('excludedSubtrees', 1)
        else:
            raise UnknownNameConstraintsSpecificationError(constraints)
        generalSubtrees = rfc2459.GeneralSubtrees().subtype(
            implicitTag=tag.Tag(tag.tagClassContext, tag.tagFormatConstructed, subtreesTag))
        subtrees = constraints[(constraints.find(':') + 1):]
        for pos, name in enumerate(subtrees.split(',')):
            generalName = rfc2459.GeneralName()
            if '/' in name:
                directoryName = stringToDN(name,
                                           tag.Tag(tag.tagClassContext, tag.tagFormatSimple, 4))
                generalName.setComponentByName('directoryName', directoryName)
            else:
                generalName.setComponentByName('dNSName', name)
            generalSubtree = GeneralSubtree()
            generalSubtree.setComponentByName('base', generalName)
            generalSubtrees.setComponentByPosition(pos, generalSubtree)
        nameConstraints.setComponentByName(subtreesType, generalSubtrees)
        self.addExtension(rfc2459.id_ce_nameConstraints, nameConstraints, critical)

    def addNSCertType(self, certType, critical):
        if certType != 'sslServer':
            raise UnknownNSCertTypeError(certType)
        self.addExtension(univ.ObjectIdentifier('2.16.840.1.113730.1.1'), univ.BitString("'01'B"),
                          critical)

    def addTLSFeature(self, features, critical):
        namedFeatures = {'OCSPMustStaple': 5}
        featureList = [f.strip() for f in features.split(',')]
        sequence = univ.Sequence()
        for feature in featureList:
            featureValue = 0
            try:
                featureValue = int(feature)
            except ValueError:
                try:
                    featureValue = namedFeatures[feature]
                except:
                    raise UnknownTLSFeature(feature)
            sequence.setComponentByPosition(len(sequence),
                                            univ.Integer(featureValue))
        self.addExtension(univ.ObjectIdentifier('1.3.6.1.5.5.7.1.24'), sequence,
                          critical)

    def addEmbeddedSCTListData(self):
        (scts, critical) = self.savedEmbeddedSCTListData
        encodedSCTs = []
        for sctSpec in scts.split(','):
            match = re.search('(\w+):(\d{8})', sctSpec)
            if not match:
                raise InvalidSCTSpecification(sctSpec)
            keySpec = match.group(1)
            key = pykey.keyFromSpecification(keySpec)
            time = datetime.datetime.strptime(match.group(2), '%Y%m%d')
            tbsCertificate = self.getTBSCertificate()
            tbsDER = encoder.encode(tbsCertificate)
            sct = pyct.SCT(key, time, tbsDER, self.issuerKey)
            signed = sct.signAndEncode()
            lengthPrefix = pack('!H', len(signed))
            encodedSCTs.append(lengthPrefix + signed)
        encodedSCTBytes = "".join(encodedSCTs)
        lengthPrefix = pack('!H', len(encodedSCTBytes))
        extensionBytes = lengthPrefix + encodedSCTBytes
        self.addExtension(univ.ObjectIdentifier('1.3.6.1.4.1.11129.2.4.2'),
                          univ.OctetString(extensionBytes), critical)

    def getVersion(self):
        return rfc2459.Version(self.versionValue).subtype(
            explicitTag=tag.Tag(tag.tagClassContext, tag.tagFormatSimple, 0))

    def getSerialNumber(self):
        return decoder.decode(self.serialNumber)[0]

    def getIssuer(self):
        return stringToDN(self.issuer)

    def getValidity(self):
        validity = rfc2459.Validity()
        validity.setComponentByName('notBefore', self.getNotBefore())
        validity.setComponentByName('notAfter', self.getNotAfter())
        return validity

    def getNotBefore(self):
        return datetimeToTime(self.notBefore)

    def getNotAfter(self):
        return datetimeToTime(self.notAfter)

    def getSubject(self):
        return stringToDN(self.subject)

    def getTBSCertificate(self):
        (signatureOID, _) = stringToAlgorithmIdentifiers(self.signature)
        tbsCertificate = rfc2459.TBSCertificate()
        tbsCertificate.setComponentByName('version', self.getVersion())
        tbsCertificate.setComponentByName('serialNumber', self.getSerialNumber())
        tbsCertificate.setComponentByName('signature', signatureOID)
        tbsCertificate.setComponentByName('issuer', self.getIssuer())
        tbsCertificate.setComponentByName('validity', self.getValidity())
        tbsCertificate.setComponentByName('subject', self.getSubject())
        tbsCertificate.setComponentByName('subjectPublicKeyInfo',
                                          self.subjectKey.asSubjectPublicKeyInfo())
        if self.extensions:
            extensions = rfc2459.Extensions().subtype(
                explicitTag=tag.Tag(tag.tagClassContext, tag.tagFormatSimple, 3))
            for count, extension in enumerate(self.extensions):
                extensions.setComponentByPosition(count, extension)
            tbsCertificate.setComponentByName('extensions', extensions)
        return tbsCertificate

    def toDER(self):
        (signatureOID, hashAlgorithm) = stringToAlgorithmIdentifiers(self.signature)
        certificate = rfc2459.Certificate()
        tbsCertificate = self.getTBSCertificate()
        certificate.setComponentByName('tbsCertificate', tbsCertificate)
        certificate.setComponentByName('signatureAlgorithm', signatureOID)
        tbsDER = encoder.encode(tbsCertificate)
        certificate.setComponentByName('signatureValue', self.issuerKey.sign(tbsDER, hashAlgorithm))
        return encoder.encode(certificate)

    def toPEM(self):
        output = '-----BEGIN CERTIFICATE-----'
        der = self.toDER()
        b64 = base64.b64encode(der)
        while b64:
            output += '\n' + b64[:64]
            b64 = b64[64:]
        output += '\n-----END CERTIFICATE-----'
        return output


# The build harness will call this function with an output
# file-like object and a path to a file containing a
# specification. This will read the specification and output
# the certificate as PEM.
# This utility tries as hard as possible to ensure that two
# runs with the same input will have the same output. This is
# particularly important when building on OS X, where we
# generate everything twice for unified builds. During the
# unification step, if any pair of input files differ, the build
# system throws an error.
# The one concrete failure mode is if one run happens before
# midnight on New Year's Eve and the next run happens after
# midnight.
def main(output, inputPath):
    with open(inputPath) as configStream:
        output.write(Certificate(configStream).toPEM())


# When run as a standalone program, this will read a specification from
# stdin and output the certificate as PEM to stdout.
if __name__ == '__main__':
    print Certificate(sys.stdin).toPEM()