# This file is dual licensed under the terms of the Apache License, Version
# 2.0, and the BSD License. See the LICENSE file in the root of this repository
# for complete details.

from __future__ import absolute_import, division, print_function

import datetime
import ipaddress

from email.utils import parseaddr

import idna

import six

from six.moves import urllib_parse

from cryptography import x509
from cryptography.x509.oid import (
    CRLEntryExtensionOID, CertificatePoliciesOID, ExtensionOID
)


def _obj2txt(backend, obj):
    # Set to 80 on the recommendation of
    # https://www.openssl.org/docs/crypto/OBJ_nid2ln.html#return_values
    buf_len = 80
    buf = backend._ffi.new("char[]", buf_len)
    res = backend._lib.OBJ_obj2txt(buf, buf_len, obj, 1)
    backend.openssl_assert(res > 0)
    return backend._ffi.buffer(buf, res)[:].decode()


def _decode_x509_name_entry(backend, x509_name_entry):
    obj = backend._lib.X509_NAME_ENTRY_get_object(x509_name_entry)
    backend.openssl_assert(obj != backend._ffi.NULL)
    data = backend._lib.X509_NAME_ENTRY_get_data(x509_name_entry)
    backend.openssl_assert(data != backend._ffi.NULL)
    value = _asn1_string_to_utf8(backend, data)
    oid = _obj2txt(backend, obj)

    return x509.NameAttribute(x509.ObjectIdentifier(oid), value)


def _decode_x509_name(backend, x509_name):
    count = backend._lib.X509_NAME_entry_count(x509_name)
    attributes = []
    for x in range(count):
        entry = backend._lib.X509_NAME_get_entry(x509_name, x)
        attributes.append(_decode_x509_name_entry(backend, entry))

    return x509.Name(attributes)


def _decode_general_names(backend, gns):
    num = backend._lib.sk_GENERAL_NAME_num(gns)
    names = []
    for i in range(num):
        gn = backend._lib.sk_GENERAL_NAME_value(gns, i)
        backend.openssl_assert(gn != backend._ffi.NULL)
        names.append(_decode_general_name(backend, gn))

    return names


def _decode_general_name(backend, gn):
    if gn.type == backend._lib.GEN_DNS:
        data = _asn1_string_to_bytes(backend, gn.d.dNSName)
        if not data:
            decoded = u""
        elif data.startswith(b"*."):
            # This is a wildcard name. We need to remove the leading wildcard,
            # IDNA decode, then re-add the wildcard. Wildcard characters should
            # always be left-most (RFC 2595 section 2.4).
            decoded = u"*." + idna.decode(data[2:])
        else:
            # Not a wildcard, decode away. If the string has a * in it anywhere
            # invalid this will raise an InvalidCodePoint
            decoded = idna.decode(data)
            if data.startswith(b"."):
                # idna strips leading periods. Name constraints can have that
                # so we need to re-add it. Sigh.
                decoded = u"." + decoded

        return x509.DNSName(decoded)
    elif gn.type == backend._lib.GEN_URI:
        data = _asn1_string_to_ascii(backend, gn.d.uniformResourceIdentifier)
        parsed = urllib_parse.urlparse(data)
        if parsed.hostname:
            hostname = idna.decode(parsed.hostname)
        else:
            hostname = ""
        if parsed.port:
            netloc = hostname + u":" + six.text_type(parsed.port)
        else:
            netloc = hostname

        # Note that building a URL in this fashion means it should be
        # semantically indistinguishable from the original but is not
        # guaranteed to be exactly the same.
        uri = urllib_parse.urlunparse((
            parsed.scheme,
            netloc,
            parsed.path,
            parsed.params,
            parsed.query,
            parsed.fragment
        ))
        return x509.UniformResourceIdentifier(uri)
    elif gn.type == backend._lib.GEN_RID:
        oid = _obj2txt(backend, gn.d.registeredID)
        return x509.RegisteredID(x509.ObjectIdentifier(oid))
    elif gn.type == backend._lib.GEN_IPADD:
        data = _asn1_string_to_bytes(backend, gn.d.iPAddress)
        data_len = len(data)
        if data_len == 8 or data_len == 32:
            # This is an IPv4 or IPv6 Network and not a single IP. This
            # type of data appears in Name Constraints. Unfortunately,
            # ipaddress doesn't support packed bytes + netmask. Additionally,
            # IPv6Network can only handle CIDR rather than the full 16 byte
            # netmask. To handle this we convert the netmask to integer, then
            # find the first 0 bit, which will be the prefix. If another 1
            # bit is present after that the netmask is invalid.
            base = ipaddress.ip_address(data[:data_len // 2])
            netmask = ipaddress.ip_address(data[data_len // 2:])
            bits = bin(int(netmask))[2:]
            prefix = bits.find('0')
            # If no 0 bits are found it is a /32 or /128
            if prefix == -1:
                prefix = len(bits)

            if "1" in bits[prefix:]:
                raise ValueError("Invalid netmask")

            ip = ipaddress.ip_network(base.exploded + u"/{0}".format(prefix))
        else:
            ip = ipaddress.ip_address(data)

        return x509.IPAddress(ip)
    elif gn.type == backend._lib.GEN_DIRNAME:
        return x509.DirectoryName(
            _decode_x509_name(backend, gn.d.directoryName)
        )
    elif gn.type == backend._lib.GEN_EMAIL:
        data = _asn1_string_to_ascii(backend, gn.d.rfc822Name)
        name, address = parseaddr(data)
        parts = address.split(u"@")
        if name or not address:
            # parseaddr has found a name (e.g. Name <email>) or the entire
            # value is an empty string.
            raise ValueError("Invalid rfc822name value")
        elif len(parts) == 1:
            # Single label email name. This is valid for local delivery. No
            # IDNA decoding can be done since there is no domain component.
            return x509.RFC822Name(address)
        else:
            # A normal email of the form user@domain.com. Let's attempt to
            # decode the domain component and return the entire address.
            return x509.RFC822Name(
                parts[0] + u"@" + idna.decode(parts[1])
            )
    elif gn.type == backend._lib.GEN_OTHERNAME:
        type_id = _obj2txt(backend, gn.d.otherName.type_id)
        value = _asn1_to_der(backend, gn.d.otherName.value)
        return x509.OtherName(x509.ObjectIdentifier(type_id), value)
    else:
        # x400Address or ediPartyName
        raise x509.UnsupportedGeneralNameType(
            "{0} is not a supported type".format(
                x509._GENERAL_NAMES.get(gn.type, gn.type)
            ),
            gn.type
        )


def _decode_ocsp_no_check(backend, ext):
    return x509.OCSPNoCheck()


def _decode_crl_number(backend, ext):
    asn1_int = backend._ffi.cast("ASN1_INTEGER *", ext)
    asn1_int = backend._ffi.gc(asn1_int, backend._lib.ASN1_INTEGER_free)
    return x509.CRLNumber(_asn1_integer_to_int(backend, asn1_int))


class _X509ExtensionParser(object):
    def __init__(self, ext_count, get_ext, handlers, unsupported_exts=None):
        self.ext_count = ext_count
        self.get_ext = get_ext
        self.handlers = handlers
        self.unsupported_exts = unsupported_exts

    def parse(self, backend, x509_obj):
        extensions = []
        seen_oids = set()
        for i in range(self.ext_count(backend, x509_obj)):
            ext = self.get_ext(backend, x509_obj, i)
            backend.openssl_assert(ext != backend._ffi.NULL)
            crit = backend._lib.X509_EXTENSION_get_critical(ext)
            critical = crit == 1
            oid = x509.ObjectIdentifier(
                _obj2txt(backend, backend._lib.X509_EXTENSION_get_object(ext))
            )
            if oid in seen_oids:
                raise x509.DuplicateExtension(
                    "Duplicate {0} extension found".format(oid), oid
                )
            try:
                handler = self.handlers[oid]
            except KeyError:
                if critical:
                    raise x509.UnsupportedExtension(
                        "Critical extension {0} is not currently supported"
                        .format(oid), oid
                    )
                else:
                    # Dump the DER payload into an UnrecognizedExtension object
                    data = backend._lib.X509_EXTENSION_get_data(ext)
                    backend.openssl_assert(data != backend._ffi.NULL)
                    der = backend._ffi.buffer(data.data, data.length)[:]
                    unrecognized = x509.UnrecognizedExtension(oid, der)
                    extensions.append(
                        x509.Extension(oid, critical, unrecognized)
                    )
            else:
                # For extensions which are not supported by OpenSSL we pass the
                # extension object directly to the parsing routine so it can
                # be decoded manually.
                if self.unsupported_exts and oid in self.unsupported_exts:
                    ext_data = ext
                else:
                    ext_data = backend._lib.X509V3_EXT_d2i(ext)
                    if ext_data == backend._ffi.NULL:
                        backend._consume_errors()
                        raise ValueError(
                            "The {0} extension is invalid and can't be "
                            "parsed".format(oid)
                        )

                value = handler(backend, ext_data)
                extensions.append(x509.Extension(oid, critical, value))

            seen_oids.add(oid)

        return x509.Extensions(extensions)


def _decode_certificate_policies(backend, cp):
    cp = backend._ffi.cast("Cryptography_STACK_OF_POLICYINFO *", cp)
    cp = backend._ffi.gc(cp, backend._lib.sk_POLICYINFO_free)
    num = backend._lib.sk_POLICYINFO_num(cp)
    certificate_policies = []
    for i in range(num):
        qualifiers = None
        pi = backend._lib.sk_POLICYINFO_value(cp, i)
        oid = x509.ObjectIdentifier(_obj2txt(backend, pi.policyid))
        if pi.qualifiers != backend._ffi.NULL:
            qnum = backend._lib.sk_POLICYQUALINFO_num(pi.qualifiers)
            qualifiers = []
            for j in range(qnum):
                pqi = backend._lib.sk_POLICYQUALINFO_value(
                    pi.qualifiers, j
                )
                pqualid = x509.ObjectIdentifier(
                    _obj2txt(backend, pqi.pqualid)
                )
                if pqualid == CertificatePoliciesOID.CPS_QUALIFIER:
                    cpsuri = backend._ffi.buffer(
                        pqi.d.cpsuri.data, pqi.d.cpsuri.length
                    )[:].decode('ascii')
                    qualifiers.append(cpsuri)
                else:
                    assert pqualid == CertificatePoliciesOID.CPS_USER_NOTICE
                    user_notice = _decode_user_notice(
                        backend, pqi.d.usernotice
                    )
                    qualifiers.append(user_notice)

        certificate_policies.append(
            x509.PolicyInformation(oid, qualifiers)
        )

    return x509.CertificatePolicies(certificate_policies)


def _decode_user_notice(backend, un):
    explicit_text = None
    notice_reference = None

    if un.exptext != backend._ffi.NULL:
        explicit_text = _asn1_string_to_utf8(backend, un.exptext)

    if un.noticeref != backend._ffi.NULL:
        organization = _asn1_string_to_utf8(
            backend, un.noticeref.organization
        )

        num = backend._lib.sk_ASN1_INTEGER_num(
            un.noticeref.noticenos
        )
        notice_numbers = []
        for i in range(num):
            asn1_int = backend._lib.sk_ASN1_INTEGER_value(
                un.noticeref.noticenos, i
            )
            notice_num = _asn1_integer_to_int(backend, asn1_int)
            notice_numbers.append(notice_num)

        notice_reference = x509.NoticeReference(
            organization, notice_numbers
        )

    return x509.UserNotice(notice_reference, explicit_text)


def _decode_basic_constraints(backend, bc_st):
    basic_constraints = backend._ffi.cast("BASIC_CONSTRAINTS *", bc_st)
    basic_constraints = backend._ffi.gc(
        basic_constraints, backend._lib.BASIC_CONSTRAINTS_free
    )
    # The byte representation of an ASN.1 boolean true is \xff. OpenSSL
    # chooses to just map this to its ordinal value, so true is 255 and
    # false is 0.
    ca = basic_constraints.ca == 255
    path_length = _asn1_integer_to_int_or_none(
        backend, basic_constraints.pathlen
    )

    return x509.BasicConstraints(ca, path_length)


def _decode_subject_key_identifier(backend, asn1_string):
    asn1_string = backend._ffi.cast("ASN1_OCTET_STRING *", asn1_string)
    asn1_string = backend._ffi.gc(
        asn1_string, backend._lib.ASN1_OCTET_STRING_free
    )
    return x509.SubjectKeyIdentifier(
        backend._ffi.buffer(asn1_string.data, asn1_string.length)[:]
    )


def _decode_authority_key_identifier(backend, akid):
    akid = backend._ffi.cast("AUTHORITY_KEYID *", akid)
    akid = backend._ffi.gc(akid, backend._lib.AUTHORITY_KEYID_free)
    key_identifier = None
    authority_cert_issuer = None

    if akid.keyid != backend._ffi.NULL:
        key_identifier = backend._ffi.buffer(
            akid.keyid.data, akid.keyid.length
        )[:]

    if akid.issuer != backend._ffi.NULL:
        authority_cert_issuer = _decode_general_names(
            backend, akid.issuer
        )

    authority_cert_serial_number = _asn1_integer_to_int_or_none(
        backend, akid.serial
    )

    return x509.AuthorityKeyIdentifier(
        key_identifier, authority_cert_issuer, authority_cert_serial_number
    )


def _decode_authority_information_access(backend, aia):
    aia = backend._ffi.cast("Cryptography_STACK_OF_ACCESS_DESCRIPTION *", aia)
    aia = backend._ffi.gc(aia, backend._lib.sk_ACCESS_DESCRIPTION_free)
    num = backend._lib.sk_ACCESS_DESCRIPTION_num(aia)
    access_descriptions = []
    for i in range(num):
        ad = backend._lib.sk_ACCESS_DESCRIPTION_value(aia, i)
        backend.openssl_assert(ad.method != backend._ffi.NULL)
        oid = x509.ObjectIdentifier(_obj2txt(backend, ad.method))
        backend.openssl_assert(ad.location != backend._ffi.NULL)
        gn = _decode_general_name(backend, ad.location)
        access_descriptions.append(x509.AccessDescription(oid, gn))

    return x509.AuthorityInformationAccess(access_descriptions)


def _decode_key_usage(backend, bit_string):
    bit_string = backend._ffi.cast("ASN1_BIT_STRING *", bit_string)
    bit_string = backend._ffi.gc(bit_string, backend._lib.ASN1_BIT_STRING_free)
    get_bit = backend._lib.ASN1_BIT_STRING_get_bit
    digital_signature = get_bit(bit_string, 0) == 1
    content_commitment = get_bit(bit_string, 1) == 1
    key_encipherment = get_bit(bit_string, 2) == 1
    data_encipherment = get_bit(bit_string, 3) == 1
    key_agreement = get_bit(bit_string, 4) == 1
    key_cert_sign = get_bit(bit_string, 5) == 1
    crl_sign = get_bit(bit_string, 6) == 1
    encipher_only = get_bit(bit_string, 7) == 1
    decipher_only = get_bit(bit_string, 8) == 1
    return x509.KeyUsage(
        digital_signature,
        content_commitment,
        key_encipherment,
        data_encipherment,
        key_agreement,
        key_cert_sign,
        crl_sign,
        encipher_only,
        decipher_only
    )


def _decode_general_names_extension(backend, gns):
    gns = backend._ffi.cast("GENERAL_NAMES *", gns)
    gns = backend._ffi.gc(gns, backend._lib.GENERAL_NAMES_free)
    general_names = _decode_general_names(backend, gns)
    return general_names


def _decode_subject_alt_name(backend, ext):
    return x509.SubjectAlternativeName(
        _decode_general_names_extension(backend, ext)
    )


def _decode_issuer_alt_name(backend, ext):
    return x509.IssuerAlternativeName(
        _decode_general_names_extension(backend, ext)
    )


def _decode_name_constraints(backend, nc):
    nc = backend._ffi.cast("NAME_CONSTRAINTS *", nc)
    nc = backend._ffi.gc(nc, backend._lib.NAME_CONSTRAINTS_free)
    permitted = _decode_general_subtrees(backend, nc.permittedSubtrees)
    excluded = _decode_general_subtrees(backend, nc.excludedSubtrees)
    return x509.NameConstraints(
        permitted_subtrees=permitted, excluded_subtrees=excluded
    )


def _decode_general_subtrees(backend, stack_subtrees):
    if stack_subtrees == backend._ffi.NULL:
        return None

    num = backend._lib.sk_GENERAL_SUBTREE_num(stack_subtrees)
    subtrees = []

    for i in range(num):
        obj = backend._lib.sk_GENERAL_SUBTREE_value(stack_subtrees, i)
        backend.openssl_assert(obj != backend._ffi.NULL)
        name = _decode_general_name(backend, obj.base)
        subtrees.append(name)

    return subtrees


def _decode_policy_constraints(backend, pc):
    pc = backend._ffi.cast("POLICY_CONSTRAINTS *", pc)
    pc = backend._ffi.gc(pc, backend._lib.POLICY_CONSTRAINTS_free)

    require_explicit_policy = _asn1_integer_to_int_or_none(
        backend, pc.requireExplicitPolicy
    )
    inhibit_policy_mapping = _asn1_integer_to_int_or_none(
        backend, pc.inhibitPolicyMapping
    )

    return x509.PolicyConstraints(
        require_explicit_policy, inhibit_policy_mapping
    )


def _decode_extended_key_usage(backend, sk):
    sk = backend._ffi.cast("Cryptography_STACK_OF_ASN1_OBJECT *", sk)
    sk = backend._ffi.gc(sk, backend._lib.sk_ASN1_OBJECT_free)
    num = backend._lib.sk_ASN1_OBJECT_num(sk)
    ekus = []

    for i in range(num):
        obj = backend._lib.sk_ASN1_OBJECT_value(sk, i)
        backend.openssl_assert(obj != backend._ffi.NULL)
        oid = x509.ObjectIdentifier(_obj2txt(backend, obj))
        ekus.append(oid)

    return x509.ExtendedKeyUsage(ekus)


_DISTPOINT_TYPE_FULLNAME = 0
_DISTPOINT_TYPE_RELATIVENAME = 1


def _decode_crl_distribution_points(backend, cdps):
    cdps = backend._ffi.cast("Cryptography_STACK_OF_DIST_POINT *", cdps)
    cdps = backend._ffi.gc(cdps, backend._lib.sk_DIST_POINT_free)
    num = backend._lib.sk_DIST_POINT_num(cdps)

    dist_points = []
    for i in range(num):
        full_name = None
        relative_name = None
        crl_issuer = None
        reasons = None
        cdp = backend._lib.sk_DIST_POINT_value(cdps, i)
        if cdp.reasons != backend._ffi.NULL:
            # We will check each bit from RFC 5280
            # ReasonFlags ::= BIT STRING {
            #      unused                  (0),
            #      keyCompromise           (1),
            #      cACompromise            (2),
            #      affiliationChanged      (3),
            #      superseded              (4),
            #      cessationOfOperation    (5),
            #      certificateHold         (6),
            #      privilegeWithdrawn      (7),
            #      aACompromise            (8) }
            reasons = []
            get_bit = backend._lib.ASN1_BIT_STRING_get_bit
            if get_bit(cdp.reasons, 1):
                reasons.append(x509.ReasonFlags.key_compromise)

            if get_bit(cdp.reasons, 2):
                reasons.append(x509.ReasonFlags.ca_compromise)

            if get_bit(cdp.reasons, 3):
                reasons.append(x509.ReasonFlags.affiliation_changed)

            if get_bit(cdp.reasons, 4):
                reasons.append(x509.ReasonFlags.superseded)

            if get_bit(cdp.reasons, 5):
                reasons.append(x509.ReasonFlags.cessation_of_operation)

            if get_bit(cdp.reasons, 6):
                reasons.append(x509.ReasonFlags.certificate_hold)

            if get_bit(cdp.reasons, 7):
                reasons.append(x509.ReasonFlags.privilege_withdrawn)

            if get_bit(cdp.reasons, 8):
                reasons.append(x509.ReasonFlags.aa_compromise)

            reasons = frozenset(reasons)

        if cdp.CRLissuer != backend._ffi.NULL:
            crl_issuer = _decode_general_names(backend, cdp.CRLissuer)

        # Certificates may have a crl_issuer/reasons and no distribution
        # point so make sure it's not null.
        if cdp.distpoint != backend._ffi.NULL:
            # Type 0 is fullName, there is no #define for it in the code.
            if cdp.distpoint.type == _DISTPOINT_TYPE_FULLNAME:
                full_name = _decode_general_names(
                    backend, cdp.distpoint.name.fullname
                )
            # OpenSSL code doesn't test for a specific type for
            # relativename, everything that isn't fullname is considered
            # relativename.
            else:
                rns = cdp.distpoint.name.relativename
                rnum = backend._lib.sk_X509_NAME_ENTRY_num(rns)
                attributes = []
                for i in range(rnum):
                    rn = backend._lib.sk_X509_NAME_ENTRY_value(
                        rns, i
                    )
                    backend.openssl_assert(rn != backend._ffi.NULL)
                    attributes.append(
                        _decode_x509_name_entry(backend, rn)
                    )

                relative_name = x509.Name(attributes)

        dist_points.append(
            x509.DistributionPoint(
                full_name, relative_name, reasons, crl_issuer
            )
        )

    return x509.CRLDistributionPoints(dist_points)


def _decode_inhibit_any_policy(backend, asn1_int):
    asn1_int = backend._ffi.cast("ASN1_INTEGER *", asn1_int)
    asn1_int = backend._ffi.gc(asn1_int, backend._lib.ASN1_INTEGER_free)
    skip_certs = _asn1_integer_to_int(backend, asn1_int)
    return x509.InhibitAnyPolicy(skip_certs)


#    CRLReason ::= ENUMERATED {
#        unspecified             (0),
#        keyCompromise           (1),
#        cACompromise            (2),
#        affiliationChanged      (3),
#        superseded              (4),
#        cessationOfOperation    (5),
#        certificateHold         (6),
#             -- value 7 is not used
#        removeFromCRL           (8),
#        privilegeWithdrawn      (9),
#        aACompromise           (10) }
_CRL_ENTRY_REASON_CODE_TO_ENUM = {
    0: x509.ReasonFlags.unspecified,
    1: x509.ReasonFlags.key_compromise,
    2: x509.ReasonFlags.ca_compromise,
    3: x509.ReasonFlags.affiliation_changed,
    4: x509.ReasonFlags.superseded,
    5: x509.ReasonFlags.cessation_of_operation,
    6: x509.ReasonFlags.certificate_hold,
    8: x509.ReasonFlags.remove_from_crl,
    9: x509.ReasonFlags.privilege_withdrawn,
    10: x509.ReasonFlags.aa_compromise,
}


_CRL_ENTRY_REASON_ENUM_TO_CODE = {
    x509.ReasonFlags.unspecified: 0,
    x509.ReasonFlags.key_compromise: 1,
    x509.ReasonFlags.ca_compromise: 2,
    x509.ReasonFlags.affiliation_changed: 3,
    x509.ReasonFlags.superseded: 4,
    x509.ReasonFlags.cessation_of_operation: 5,
    x509.ReasonFlags.certificate_hold: 6,
    x509.ReasonFlags.remove_from_crl: 8,
    x509.ReasonFlags.privilege_withdrawn: 9,
    x509.ReasonFlags.aa_compromise: 10
}


def _decode_crl_reason(backend, enum):
    enum = backend._ffi.cast("ASN1_ENUMERATED *", enum)
    enum = backend._ffi.gc(enum, backend._lib.ASN1_ENUMERATED_free)
    code = backend._lib.ASN1_ENUMERATED_get(enum)

    try:
        return x509.CRLReason(_CRL_ENTRY_REASON_CODE_TO_ENUM[code])
    except KeyError:
        raise ValueError("Unsupported reason code: {0}".format(code))


def _decode_invalidity_date(backend, inv_date):
    generalized_time = backend._ffi.cast(
        "ASN1_GENERALIZEDTIME *", inv_date
    )
    generalized_time = backend._ffi.gc(
        generalized_time, backend._lib.ASN1_GENERALIZEDTIME_free
    )
    return x509.InvalidityDate(
        _parse_asn1_generalized_time(backend, generalized_time)
    )


def _decode_cert_issuer(backend, ext):
    """
    This handler decodes the CertificateIssuer entry extension directly
    from the X509_EXTENSION object. This is necessary because this entry
    extension is not directly supported by OpenSSL 0.9.8.
    """

    data_ptr_ptr = backend._ffi.new("const unsigned char **")
    value = backend._lib.X509_EXTENSION_get_data(ext)
    data_ptr_ptr[0] = value.data
    gns = backend._lib.d2i_GENERAL_NAMES(
        backend._ffi.NULL, data_ptr_ptr, value.length
    )

    # Check the result of d2i_GENERAL_NAMES() is valid. Usually this is covered
    # in _X509ExtensionParser but since we are responsible for decoding this
    # entry extension ourselves, we have to this here.
    if gns == backend._ffi.NULL:
        backend._consume_errors()
        raise ValueError(
            "The {0} extension is corrupted and can't be parsed".format(
                CRLEntryExtensionOID.CERTIFICATE_ISSUER))

    gns = backend._ffi.gc(gns, backend._lib.GENERAL_NAMES_free)
    return x509.CertificateIssuer(_decode_general_names(backend, gns))


def _asn1_to_der(backend, asn1_type):
    buf = backend._ffi.new("unsigned char **")
    res = backend._lib.i2d_ASN1_TYPE(asn1_type, buf)
    backend.openssl_assert(res >= 0)
    backend.openssl_assert(buf[0] != backend._ffi.NULL)
    buf = backend._ffi.gc(
        buf, lambda buffer: backend._lib.OPENSSL_free(buffer[0])
    )
    return backend._ffi.buffer(buf[0], res)[:]


def _asn1_integer_to_int(backend, asn1_int):
    bn = backend._lib.ASN1_INTEGER_to_BN(asn1_int, backend._ffi.NULL)
    backend.openssl_assert(bn != backend._ffi.NULL)
    bn = backend._ffi.gc(bn, backend._lib.BN_free)
    return backend._bn_to_int(bn)


def _asn1_integer_to_int_or_none(backend, asn1_int):
    if asn1_int == backend._ffi.NULL:
        return None
    else:
        return _asn1_integer_to_int(backend, asn1_int)


def _asn1_string_to_bytes(backend, asn1_string):
    return backend._ffi.buffer(asn1_string.data, asn1_string.length)[:]


def _asn1_string_to_ascii(backend, asn1_string):
    return _asn1_string_to_bytes(backend, asn1_string).decode("ascii")


def _asn1_string_to_utf8(backend, asn1_string):
    buf = backend._ffi.new("unsigned char **")
    res = backend._lib.ASN1_STRING_to_UTF8(buf, asn1_string)
    if res == -1:
        raise ValueError(
            "Unsupported ASN1 string type. Type: {0}".format(asn1_string.type)
        )

    backend.openssl_assert(buf[0] != backend._ffi.NULL)
    buf = backend._ffi.gc(
        buf, lambda buffer: backend._lib.OPENSSL_free(buffer[0])
    )
    return backend._ffi.buffer(buf[0], res)[:].decode('utf8')


def _parse_asn1_time(backend, asn1_time):
    backend.openssl_assert(asn1_time != backend._ffi.NULL)
    generalized_time = backend._lib.ASN1_TIME_to_generalizedtime(
        asn1_time, backend._ffi.NULL
    )
    backend.openssl_assert(generalized_time != backend._ffi.NULL)
    generalized_time = backend._ffi.gc(
        generalized_time, backend._lib.ASN1_GENERALIZEDTIME_free
    )
    return _parse_asn1_generalized_time(backend, generalized_time)


def _parse_asn1_generalized_time(backend, generalized_time):
    time = _asn1_string_to_ascii(
        backend, backend._ffi.cast("ASN1_STRING *", generalized_time)
    )
    return datetime.datetime.strptime(time, "%Y%m%d%H%M%SZ")


_EXTENSION_HANDLERS = {
    ExtensionOID.BASIC_CONSTRAINTS: _decode_basic_constraints,
    ExtensionOID.SUBJECT_KEY_IDENTIFIER: _decode_subject_key_identifier,
    ExtensionOID.KEY_USAGE: _decode_key_usage,
    ExtensionOID.SUBJECT_ALTERNATIVE_NAME: _decode_subject_alt_name,
    ExtensionOID.EXTENDED_KEY_USAGE: _decode_extended_key_usage,
    ExtensionOID.AUTHORITY_KEY_IDENTIFIER: _decode_authority_key_identifier,
    ExtensionOID.AUTHORITY_INFORMATION_ACCESS: (
        _decode_authority_information_access
    ),
    ExtensionOID.CERTIFICATE_POLICIES: _decode_certificate_policies,
    ExtensionOID.CRL_DISTRIBUTION_POINTS: _decode_crl_distribution_points,
    ExtensionOID.OCSP_NO_CHECK: _decode_ocsp_no_check,
    ExtensionOID.INHIBIT_ANY_POLICY: _decode_inhibit_any_policy,
    ExtensionOID.ISSUER_ALTERNATIVE_NAME: _decode_issuer_alt_name,
    ExtensionOID.NAME_CONSTRAINTS: _decode_name_constraints,
    ExtensionOID.POLICY_CONSTRAINTS: _decode_policy_constraints,
}

_REVOKED_EXTENSION_HANDLERS = {
    CRLEntryExtensionOID.CRL_REASON: _decode_crl_reason,
    CRLEntryExtensionOID.INVALIDITY_DATE: _decode_invalidity_date,
    CRLEntryExtensionOID.CERTIFICATE_ISSUER: _decode_cert_issuer,
}

_REVOKED_UNSUPPORTED_EXTENSIONS = set([
    CRLEntryExtensionOID.CERTIFICATE_ISSUER,
])

_CRL_EXTENSION_HANDLERS = {
    ExtensionOID.CRL_NUMBER: _decode_crl_number,
    ExtensionOID.AUTHORITY_KEY_IDENTIFIER: _decode_authority_key_identifier,
    ExtensionOID.ISSUER_ALTERNATIVE_NAME: _decode_issuer_alt_name,
    ExtensionOID.AUTHORITY_INFORMATION_ACCESS: (
        _decode_authority_information_access
    ),
}

_CERTIFICATE_EXTENSION_PARSER = _X509ExtensionParser(
    ext_count=lambda backend, x: backend._lib.X509_get_ext_count(x),
    get_ext=lambda backend, x, i: backend._lib.X509_get_ext(x, i),
    handlers=_EXTENSION_HANDLERS
)

_CSR_EXTENSION_PARSER = _X509ExtensionParser(
    ext_count=lambda backend, x: backend._lib.sk_X509_EXTENSION_num(x),
    get_ext=lambda backend, x, i: backend._lib.sk_X509_EXTENSION_value(x, i),
    handlers=_EXTENSION_HANDLERS
)

_REVOKED_CERTIFICATE_EXTENSION_PARSER = _X509ExtensionParser(
    ext_count=lambda backend, x: backend._lib.X509_REVOKED_get_ext_count(x),
    get_ext=lambda backend, x, i: backend._lib.X509_REVOKED_get_ext(x, i),
    handlers=_REVOKED_EXTENSION_HANDLERS,
    unsupported_exts=_REVOKED_UNSUPPORTED_EXTENSIONS
)

_CRL_EXTENSION_PARSER = _X509ExtensionParser(
    ext_count=lambda backend, x: backend._lib.X509_CRL_get_ext_count(x),
    get_ext=lambda backend, x, i: backend._lib.X509_CRL_get_ext(x, i),
    handlers=_CRL_EXTENSION_HANDLERS,
)