xref: /external/python/cryptography/src/cryptography/hazmat/backends/openssl/backend.py

# 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 base64
import collections
import contextlib
import itertools
from contextlib import contextmanager

import asn1crypto.core

import six
from six.moves import range

from cryptography import utils, x509
from cryptography.exceptions import UnsupportedAlgorithm, _Reasons
from cryptography.hazmat.backends.interfaces import (
    CMACBackend, CipherBackend, DERSerializationBackend, DHBackend, DSABackend,
    EllipticCurveBackend, HMACBackend, HashBackend, PBKDF2HMACBackend,
    PEMSerializationBackend, RSABackend, ScryptBackend, X509Backend
)
from cryptography.hazmat.backends.openssl import aead
from cryptography.hazmat.backends.openssl.ciphers import _CipherContext
from cryptography.hazmat.backends.openssl.cmac import _CMACContext
from cryptography.hazmat.backends.openssl.decode_asn1 import (
    _CRL_ENTRY_REASON_ENUM_TO_CODE, _Integers
)
from cryptography.hazmat.backends.openssl.dh import (
    _DHParameters, _DHPrivateKey, _DHPublicKey, _dh_params_dup
)
from cryptography.hazmat.backends.openssl.dsa import (
    _DSAParameters, _DSAPrivateKey, _DSAPublicKey
)
from cryptography.hazmat.backends.openssl.ec import (
    _EllipticCurvePrivateKey, _EllipticCurvePublicKey
)
from cryptography.hazmat.backends.openssl.encode_asn1 import (
    _CRL_ENTRY_EXTENSION_ENCODE_HANDLERS,
    _CRL_EXTENSION_ENCODE_HANDLERS, _EXTENSION_ENCODE_HANDLERS,
    _OCSP_BASICRESP_EXTENSION_ENCODE_HANDLERS,
    _OCSP_REQUEST_EXTENSION_ENCODE_HANDLERS,
    _encode_asn1_int_gc, _encode_asn1_str_gc, _encode_name_gc, _txt2obj_gc,
)
from cryptography.hazmat.backends.openssl.hashes import _HashContext
from cryptography.hazmat.backends.openssl.hmac import _HMACContext
from cryptography.hazmat.backends.openssl.ocsp import (
    _OCSPRequest, _OCSPResponse
)
from cryptography.hazmat.backends.openssl.rsa import (
    _RSAPrivateKey, _RSAPublicKey
)
from cryptography.hazmat.backends.openssl.x25519 import (
    _X25519PrivateKey, _X25519PublicKey
)
from cryptography.hazmat.backends.openssl.x448 import (
    _X448PrivateKey, _X448PublicKey
)
from cryptography.hazmat.backends.openssl.x509 import (
    _Certificate, _CertificateRevocationList,
    _CertificateSigningRequest, _RevokedCertificate
)
from cryptography.hazmat.bindings.openssl import binding
from cryptography.hazmat.primitives import hashes, serialization
from cryptography.hazmat.primitives.asymmetric import dsa, ec, rsa
from cryptography.hazmat.primitives.asymmetric.padding import (
    MGF1, OAEP, PKCS1v15, PSS
)
from cryptography.hazmat.primitives.ciphers.algorithms import (
    AES, ARC4, Blowfish, CAST5, Camellia, ChaCha20, IDEA, SEED, TripleDES
)
from cryptography.hazmat.primitives.ciphers.modes import (
    CBC, CFB, CFB8, CTR, ECB, GCM, OFB, XTS
)
from cryptography.hazmat.primitives.kdf import scrypt
from cryptography.hazmat.primitives.serialization import ssh
from cryptography.x509 import ocsp


_MemoryBIO = collections.namedtuple("_MemoryBIO", ["bio", "char_ptr"])


@utils.register_interface(CipherBackend)
@utils.register_interface(CMACBackend)
@utils.register_interface(DERSerializationBackend)
@utils.register_interface(DHBackend)
@utils.register_interface(DSABackend)
@utils.register_interface(EllipticCurveBackend)
@utils.register_interface(HashBackend)
@utils.register_interface(HMACBackend)
@utils.register_interface(PBKDF2HMACBackend)
@utils.register_interface(RSABackend)
@utils.register_interface(PEMSerializationBackend)
@utils.register_interface(X509Backend)
@utils.register_interface_if(
    binding.Binding().lib.Cryptography_HAS_SCRYPT, ScryptBackend
)
class Backend(object):
    """
    OpenSSL API binding interfaces.
    """
    name = "openssl"

    def __init__(self):
        self._binding = binding.Binding()
        self._ffi = self._binding.ffi
        self._lib = self._binding.lib

        self._cipher_registry = {}
        self._register_default_ciphers()
        self.activate_osrandom_engine()
        self._dh_types = [self._lib.EVP_PKEY_DH]
        if self._lib.Cryptography_HAS_EVP_PKEY_DHX:
            self._dh_types.append(self._lib.EVP_PKEY_DHX)

    def openssl_assert(self, ok):
        return binding._openssl_assert(self._lib, ok)

    def activate_builtin_random(self):
        # Obtain a new structural reference.
        e = self._lib.ENGINE_get_default_RAND()
        if e != self._ffi.NULL:
            self._lib.ENGINE_unregister_RAND(e)
            # Reset the RNG to use the new engine.
            self._lib.RAND_cleanup()
            # decrement the structural reference from get_default_RAND
            res = self._lib.ENGINE_finish(e)
            self.openssl_assert(res == 1)

    @contextlib.contextmanager
    def _get_osurandom_engine(self):
        # Fetches an engine by id and returns it. This creates a structural
        # reference.
        e = self._lib.ENGINE_by_id(self._binding._osrandom_engine_id)
        self.openssl_assert(e != self._ffi.NULL)
        # Initialize the engine for use. This adds a functional reference.
        res = self._lib.ENGINE_init(e)
        self.openssl_assert(res == 1)

        try:
            yield e
        finally:
            # Decrement the structural ref incremented by ENGINE_by_id.
            res = self._lib.ENGINE_free(e)
            self.openssl_assert(res == 1)
            # Decrement the functional ref incremented by ENGINE_init.
            res = self._lib.ENGINE_finish(e)
            self.openssl_assert(res == 1)

    def activate_osrandom_engine(self):
        # Unregister and free the current engine.
        self.activate_builtin_random()
        with self._get_osurandom_engine() as e:
            # Set the engine as the default RAND provider.
            res = self._lib.ENGINE_set_default_RAND(e)
            self.openssl_assert(res == 1)
        # Reset the RNG to use the new engine.
        self._lib.RAND_cleanup()

    def osrandom_engine_implementation(self):
        buf = self._ffi.new("char[]", 64)
        with self._get_osurandom_engine() as e:
            res = self._lib.ENGINE_ctrl_cmd(e, b"get_implementation",
                                            len(buf), buf,
                                            self._ffi.NULL, 0)
            self.openssl_assert(res > 0)
        return self._ffi.string(buf).decode('ascii')

    def openssl_version_text(self):
        """
        Friendly string name of the loaded OpenSSL library. This is not
        necessarily the same version as it was compiled against.

        Example: OpenSSL 1.0.1e 11 Feb 2013
        """
        return self._ffi.string(
            self._lib.OpenSSL_version(self._lib.OPENSSL_VERSION)
        ).decode("ascii")

    def openssl_version_number(self):
        return self._lib.OpenSSL_version_num()

    def create_hmac_ctx(self, key, algorithm):
        return _HMACContext(self, key, algorithm)

    def _evp_md_from_algorithm(self, algorithm):
        if algorithm.name == "blake2b" or algorithm.name == "blake2s":
            alg = "{0}{1}".format(
                algorithm.name, algorithm.digest_size * 8
            ).encode("ascii")
        else:
            alg = algorithm.name.encode("ascii")

        evp_md = self._lib.EVP_get_digestbyname(alg)
        return evp_md

    def _evp_md_non_null_from_algorithm(self, algorithm):
        evp_md = self._evp_md_from_algorithm(algorithm)
        self.openssl_assert(evp_md != self._ffi.NULL)
        return evp_md

    def hash_supported(self, algorithm):
        evp_md = self._evp_md_from_algorithm(algorithm)
        return evp_md != self._ffi.NULL

    def hmac_supported(self, algorithm):
        return self.hash_supported(algorithm)

    def create_hash_ctx(self, algorithm):
        return _HashContext(self, algorithm)

    def cipher_supported(self, cipher, mode):
        try:
            adapter = self._cipher_registry[type(cipher), type(mode)]
        except KeyError:
            return False
        evp_cipher = adapter(self, cipher, mode)
        return self._ffi.NULL != evp_cipher

    def register_cipher_adapter(self, cipher_cls, mode_cls, adapter):
        if (cipher_cls, mode_cls) in self._cipher_registry:
            raise ValueError("Duplicate registration for: {0} {1}.".format(
                cipher_cls, mode_cls)
            )
        self._cipher_registry[cipher_cls, mode_cls] = adapter

    def _register_default_ciphers(self):
        for mode_cls in [CBC, CTR, ECB, OFB, CFB, CFB8, GCM]:
            self.register_cipher_adapter(
                AES,
                mode_cls,
                GetCipherByName("{cipher.name}-{cipher.key_size}-{mode.name}")
            )
        for mode_cls in [CBC, CTR, ECB, OFB, CFB]:
            self.register_cipher_adapter(
                Camellia,
                mode_cls,
                GetCipherByName("{cipher.name}-{cipher.key_size}-{mode.name}")
            )
        for mode_cls in [CBC, CFB, CFB8, OFB]:
            self.register_cipher_adapter(
                TripleDES,
                mode_cls,
                GetCipherByName("des-ede3-{mode.name}")
            )
        self.register_cipher_adapter(
            TripleDES,
            ECB,
            GetCipherByName("des-ede3")
        )
        for mode_cls in [CBC, CFB, OFB, ECB]:
            self.register_cipher_adapter(
                Blowfish,
                mode_cls,
                GetCipherByName("bf-{mode.name}")
            )
        for mode_cls in [CBC, CFB, OFB, ECB]:
            self.register_cipher_adapter(
                SEED,
                mode_cls,
                GetCipherByName("seed-{mode.name}")
            )
        for cipher_cls, mode_cls in itertools.product(
            [CAST5, IDEA],
            [CBC, OFB, CFB, ECB],
        ):
            self.register_cipher_adapter(
                cipher_cls,
                mode_cls,
                GetCipherByName("{cipher.name}-{mode.name}")
            )
        self.register_cipher_adapter(
            ARC4,
            type(None),
            GetCipherByName("rc4")
        )
        self.register_cipher_adapter(
            ChaCha20,
            type(None),
            GetCipherByName("chacha20")
        )
        self.register_cipher_adapter(AES, XTS, _get_xts_cipher)

    def create_symmetric_encryption_ctx(self, cipher, mode):
        return _CipherContext(self, cipher, mode, _CipherContext._ENCRYPT)

    def create_symmetric_decryption_ctx(self, cipher, mode):
        return _CipherContext(self, cipher, mode, _CipherContext._DECRYPT)

    def pbkdf2_hmac_supported(self, algorithm):
        return self.hmac_supported(algorithm)

    def derive_pbkdf2_hmac(self, algorithm, length, salt, iterations,
                           key_material):
        buf = self._ffi.new("unsigned char[]", length)
        evp_md = self._evp_md_non_null_from_algorithm(algorithm)
        key_material_ptr = self._ffi.from_buffer(key_material)
        res = self._lib.PKCS5_PBKDF2_HMAC(
            key_material_ptr,
            len(key_material),
            salt,
            len(salt),
            iterations,
            evp_md,
            length,
            buf
        )
        self.openssl_assert(res == 1)
        return self._ffi.buffer(buf)[:]

    def _consume_errors(self):
        return binding._consume_errors(self._lib)

    def _bn_to_int(self, bn):
        assert bn != self._ffi.NULL

        if not six.PY2:
            # Python 3 has constant time from_bytes, so use that.
            bn_num_bytes = self._lib.BN_num_bytes(bn)
            bin_ptr = self._ffi.new("unsigned char[]", bn_num_bytes)
            bin_len = self._lib.BN_bn2bin(bn, bin_ptr)
            # A zero length means the BN has value 0
            self.openssl_assert(bin_len >= 0)
            return int.from_bytes(self._ffi.buffer(bin_ptr)[:bin_len], "big")
        else:
            # Under Python 2 the best we can do is hex()
            hex_cdata = self._lib.BN_bn2hex(bn)
            self.openssl_assert(hex_cdata != self._ffi.NULL)
            hex_str = self._ffi.string(hex_cdata)
            self._lib.OPENSSL_free(hex_cdata)
            return int(hex_str, 16)

    def _int_to_bn(self, num, bn=None):
        """
        Converts a python integer to a BIGNUM. The returned BIGNUM will not
        be garbage collected (to support adding them to structs that take
        ownership of the object). Be sure to register it for GC if it will
        be discarded after use.
        """
        assert bn is None or bn != self._ffi.NULL

        if bn is None:
            bn = self._ffi.NULL

        if not six.PY2:
            # Python 3 has constant time to_bytes, so use that.

            binary = num.to_bytes(int(num.bit_length() / 8.0 + 1), "big")
            bn_ptr = self._lib.BN_bin2bn(binary, len(binary), bn)
            self.openssl_assert(bn_ptr != self._ffi.NULL)
            return bn_ptr

        else:
            # Under Python 2 the best we can do is hex(), [2:] removes the 0x
            # prefix.
            hex_num = hex(num).rstrip("L")[2:].encode("ascii")
            bn_ptr = self._ffi.new("BIGNUM **")
            bn_ptr[0] = bn
            res = self._lib.BN_hex2bn(bn_ptr, hex_num)
            self.openssl_assert(res != 0)
            self.openssl_assert(bn_ptr[0] != self._ffi.NULL)
            return bn_ptr[0]

    def generate_rsa_private_key(self, public_exponent, key_size):
        rsa._verify_rsa_parameters(public_exponent, key_size)

        rsa_cdata = self._lib.RSA_new()
        self.openssl_assert(rsa_cdata != self._ffi.NULL)
        rsa_cdata = self._ffi.gc(rsa_cdata, self._lib.RSA_free)

        bn = self._int_to_bn(public_exponent)
        bn = self._ffi.gc(bn, self._lib.BN_free)

        res = self._lib.RSA_generate_key_ex(
            rsa_cdata, key_size, bn, self._ffi.NULL
        )
        self.openssl_assert(res == 1)
        evp_pkey = self._rsa_cdata_to_evp_pkey(rsa_cdata)

        return _RSAPrivateKey(self, rsa_cdata, evp_pkey)

    def generate_rsa_parameters_supported(self, public_exponent, key_size):
        return (public_exponent >= 3 and public_exponent & 1 != 0 and
                key_size >= 512)

    def load_rsa_private_numbers(self, numbers):
        rsa._check_private_key_components(
            numbers.p,
            numbers.q,
            numbers.d,
            numbers.dmp1,
            numbers.dmq1,
            numbers.iqmp,
            numbers.public_numbers.e,
            numbers.public_numbers.n
        )
        rsa_cdata = self._lib.RSA_new()
        self.openssl_assert(rsa_cdata != self._ffi.NULL)
        rsa_cdata = self._ffi.gc(rsa_cdata, self._lib.RSA_free)
        p = self._int_to_bn(numbers.p)
        q = self._int_to_bn(numbers.q)
        d = self._int_to_bn(numbers.d)
        dmp1 = self._int_to_bn(numbers.dmp1)
        dmq1 = self._int_to_bn(numbers.dmq1)
        iqmp = self._int_to_bn(numbers.iqmp)
        e = self._int_to_bn(numbers.public_numbers.e)
        n = self._int_to_bn(numbers.public_numbers.n)
        res = self._lib.RSA_set0_factors(rsa_cdata, p, q)
        self.openssl_assert(res == 1)
        res = self._lib.RSA_set0_key(rsa_cdata, n, e, d)
        self.openssl_assert(res == 1)
        res = self._lib.RSA_set0_crt_params(rsa_cdata, dmp1, dmq1, iqmp)
        self.openssl_assert(res == 1)
        res = self._lib.RSA_blinding_on(rsa_cdata, self._ffi.NULL)
        self.openssl_assert(res == 1)
        evp_pkey = self._rsa_cdata_to_evp_pkey(rsa_cdata)

        return _RSAPrivateKey(self, rsa_cdata, evp_pkey)

    def load_rsa_public_numbers(self, numbers):
        rsa._check_public_key_components(numbers.e, numbers.n)
        rsa_cdata = self._lib.RSA_new()
        self.openssl_assert(rsa_cdata != self._ffi.NULL)
        rsa_cdata = self._ffi.gc(rsa_cdata, self._lib.RSA_free)
        e = self._int_to_bn(numbers.e)
        n = self._int_to_bn(numbers.n)
        res = self._lib.RSA_set0_key(rsa_cdata, n, e, self._ffi.NULL)
        self.openssl_assert(res == 1)
        evp_pkey = self._rsa_cdata_to_evp_pkey(rsa_cdata)

        return _RSAPublicKey(self, rsa_cdata, evp_pkey)

    def _create_evp_pkey_gc(self):
        evp_pkey = self._lib.EVP_PKEY_new()
        self.openssl_assert(evp_pkey != self._ffi.NULL)
        evp_pkey = self._ffi.gc(evp_pkey, self._lib.EVP_PKEY_free)
        return evp_pkey

    def _rsa_cdata_to_evp_pkey(self, rsa_cdata):
        evp_pkey = self._create_evp_pkey_gc()
        res = self._lib.EVP_PKEY_set1_RSA(evp_pkey, rsa_cdata)
        self.openssl_assert(res == 1)
        return evp_pkey

    def _bytes_to_bio(self, data):
        """
        Return a _MemoryBIO namedtuple of (BIO, char*).

        The char* is the storage for the BIO and it must stay alive until the
        BIO is finished with.
        """
        data_ptr = self._ffi.from_buffer(data)
        bio = self._lib.BIO_new_mem_buf(
            data_ptr, len(data)
        )
        self.openssl_assert(bio != self._ffi.NULL)

        return _MemoryBIO(self._ffi.gc(bio, self._lib.BIO_free), data_ptr)

    def _create_mem_bio_gc(self):
        """
        Creates an empty memory BIO.
        """
        bio_method = self._lib.BIO_s_mem()
        self.openssl_assert(bio_method != self._ffi.NULL)
        bio = self._lib.BIO_new(bio_method)
        self.openssl_assert(bio != self._ffi.NULL)
        bio = self._ffi.gc(bio, self._lib.BIO_free)
        return bio

    def _read_mem_bio(self, bio):
        """
        Reads a memory BIO. This only works on memory BIOs.
        """
        buf = self._ffi.new("char **")
        buf_len = self._lib.BIO_get_mem_data(bio, buf)
        self.openssl_assert(buf_len > 0)
        self.openssl_assert(buf[0] != self._ffi.NULL)
        bio_data = self._ffi.buffer(buf[0], buf_len)[:]
        return bio_data

    def _evp_pkey_to_private_key(self, evp_pkey):
        """
        Return the appropriate type of PrivateKey given an evp_pkey cdata
        pointer.
        """

        key_type = self._lib.EVP_PKEY_id(evp_pkey)

        if key_type == self._lib.EVP_PKEY_RSA:
            rsa_cdata = self._lib.EVP_PKEY_get1_RSA(evp_pkey)
            self.openssl_assert(rsa_cdata != self._ffi.NULL)
            rsa_cdata = self._ffi.gc(rsa_cdata, self._lib.RSA_free)
            return _RSAPrivateKey(self, rsa_cdata, evp_pkey)
        elif key_type == self._lib.EVP_PKEY_DSA:
            dsa_cdata = self._lib.EVP_PKEY_get1_DSA(evp_pkey)
            self.openssl_assert(dsa_cdata != self._ffi.NULL)
            dsa_cdata = self._ffi.gc(dsa_cdata, self._lib.DSA_free)
            return _DSAPrivateKey(self, dsa_cdata, evp_pkey)
        elif key_type == self._lib.EVP_PKEY_EC:
            ec_cdata = self._lib.EVP_PKEY_get1_EC_KEY(evp_pkey)
            self.openssl_assert(ec_cdata != self._ffi.NULL)
            ec_cdata = self._ffi.gc(ec_cdata, self._lib.EC_KEY_free)
            return _EllipticCurvePrivateKey(self, ec_cdata, evp_pkey)
        elif key_type in self._dh_types:
            dh_cdata = self._lib.EVP_PKEY_get1_DH(evp_pkey)
            self.openssl_assert(dh_cdata != self._ffi.NULL)
            dh_cdata = self._ffi.gc(dh_cdata, self._lib.DH_free)
            return _DHPrivateKey(self, dh_cdata, evp_pkey)
        elif key_type == getattr(self._lib, "EVP_PKEY_X448", None):
            # EVP_PKEY_X448 is not present in OpenSSL < 1.1.1
            return _X448PrivateKey(self, evp_pkey)
        elif key_type == getattr(self._lib, "EVP_PKEY_X25519", None):
            # EVP_PKEY_X25519 is not present in OpenSSL < 1.1.0
            return _X25519PrivateKey(self, evp_pkey)
        else:
            raise UnsupportedAlgorithm("Unsupported key type.")

    def _evp_pkey_to_public_key(self, evp_pkey):
        """
        Return the appropriate type of PublicKey given an evp_pkey cdata
        pointer.
        """

        key_type = self._lib.EVP_PKEY_id(evp_pkey)

        if key_type == self._lib.EVP_PKEY_RSA:
            rsa_cdata = self._lib.EVP_PKEY_get1_RSA(evp_pkey)
            self.openssl_assert(rsa_cdata != self._ffi.NULL)
            rsa_cdata = self._ffi.gc(rsa_cdata, self._lib.RSA_free)
            return _RSAPublicKey(self, rsa_cdata, evp_pkey)
        elif key_type == self._lib.EVP_PKEY_DSA:
            dsa_cdata = self._lib.EVP_PKEY_get1_DSA(evp_pkey)
            self.openssl_assert(dsa_cdata != self._ffi.NULL)
            dsa_cdata = self._ffi.gc(dsa_cdata, self._lib.DSA_free)
            return _DSAPublicKey(self, dsa_cdata, evp_pkey)
        elif key_type == self._lib.EVP_PKEY_EC:
            ec_cdata = self._lib.EVP_PKEY_get1_EC_KEY(evp_pkey)
            self.openssl_assert(ec_cdata != self._ffi.NULL)
            ec_cdata = self._ffi.gc(ec_cdata, self._lib.EC_KEY_free)
            return _EllipticCurvePublicKey(self, ec_cdata, evp_pkey)
        elif key_type in self._dh_types:
            dh_cdata = self._lib.EVP_PKEY_get1_DH(evp_pkey)
            self.openssl_assert(dh_cdata != self._ffi.NULL)
            dh_cdata = self._ffi.gc(dh_cdata, self._lib.DH_free)
            return _DHPublicKey(self, dh_cdata, evp_pkey)
        elif key_type == getattr(self._lib, "EVP_PKEY_X448", None):
            # EVP_PKEY_X448 is not present in OpenSSL < 1.1.1
            return _X448PublicKey(self, evp_pkey)
        elif key_type == getattr(self._lib, "EVP_PKEY_X25519", None):
            # EVP_PKEY_X25519 is not present in OpenSSL < 1.1.0
            return _X25519PublicKey(self, evp_pkey)
        else:
            raise UnsupportedAlgorithm("Unsupported key type.")

    def _oaep_hash_supported(self, algorithm):
        if self._lib.Cryptography_HAS_RSA_OAEP_MD:
            return isinstance(
                algorithm, (
                    hashes.SHA1,
                    hashes.SHA224,
                    hashes.SHA256,
                    hashes.SHA384,
                    hashes.SHA512,
                )
            )
        else:
            return isinstance(algorithm, hashes.SHA1)

    def rsa_padding_supported(self, padding):
        if isinstance(padding, PKCS1v15):
            return True
        elif isinstance(padding, PSS) and isinstance(padding._mgf, MGF1):
            return self.hash_supported(padding._mgf._algorithm)
        elif isinstance(padding, OAEP) and isinstance(padding._mgf, MGF1):
            return (
                self._oaep_hash_supported(padding._mgf._algorithm) and
                self._oaep_hash_supported(padding._algorithm) and
                (
                    (padding._label is None or len(padding._label) == 0) or
                    self._lib.Cryptography_HAS_RSA_OAEP_LABEL == 1
                )
            )
        else:
            return False

    def generate_dsa_parameters(self, key_size):
        if key_size not in (1024, 2048, 3072):
            raise ValueError("Key size must be 1024 or 2048 or 3072 bits.")

        ctx = self._lib.DSA_new()
        self.openssl_assert(ctx != self._ffi.NULL)
        ctx = self._ffi.gc(ctx, self._lib.DSA_free)

        res = self._lib.DSA_generate_parameters_ex(
            ctx, key_size, self._ffi.NULL, 0,
            self._ffi.NULL, self._ffi.NULL, self._ffi.NULL
        )

        self.openssl_assert(res == 1)

        return _DSAParameters(self, ctx)

    def generate_dsa_private_key(self, parameters):
        ctx = self._lib.DSAparams_dup(parameters._dsa_cdata)
        self.openssl_assert(ctx != self._ffi.NULL)
        ctx = self._ffi.gc(ctx, self._lib.DSA_free)
        self._lib.DSA_generate_key(ctx)
        evp_pkey = self._dsa_cdata_to_evp_pkey(ctx)

        return _DSAPrivateKey(self, ctx, evp_pkey)

    def generate_dsa_private_key_and_parameters(self, key_size):
        parameters = self.generate_dsa_parameters(key_size)
        return self.generate_dsa_private_key(parameters)

    def _dsa_cdata_set_values(self, dsa_cdata, p, q, g, pub_key, priv_key):
        res = self._lib.DSA_set0_pqg(dsa_cdata, p, q, g)
        self.openssl_assert(res == 1)
        res = self._lib.DSA_set0_key(dsa_cdata, pub_key, priv_key)
        self.openssl_assert(res == 1)

    def load_dsa_private_numbers(self, numbers):
        dsa._check_dsa_private_numbers(numbers)
        parameter_numbers = numbers.public_numbers.parameter_numbers

        dsa_cdata = self._lib.DSA_new()
        self.openssl_assert(dsa_cdata != self._ffi.NULL)
        dsa_cdata = self._ffi.gc(dsa_cdata, self._lib.DSA_free)

        p = self._int_to_bn(parameter_numbers.p)
        q = self._int_to_bn(parameter_numbers.q)
        g = self._int_to_bn(parameter_numbers.g)
        pub_key = self._int_to_bn(numbers.public_numbers.y)
        priv_key = self._int_to_bn(numbers.x)
        self._dsa_cdata_set_values(dsa_cdata, p, q, g, pub_key, priv_key)

        evp_pkey = self._dsa_cdata_to_evp_pkey(dsa_cdata)

        return _DSAPrivateKey(self, dsa_cdata, evp_pkey)

    def load_dsa_public_numbers(self, numbers):
        dsa._check_dsa_parameters(numbers.parameter_numbers)
        dsa_cdata = self._lib.DSA_new()
        self.openssl_assert(dsa_cdata != self._ffi.NULL)
        dsa_cdata = self._ffi.gc(dsa_cdata, self._lib.DSA_free)

        p = self._int_to_bn(numbers.parameter_numbers.p)
        q = self._int_to_bn(numbers.parameter_numbers.q)
        g = self._int_to_bn(numbers.parameter_numbers.g)
        pub_key = self._int_to_bn(numbers.y)
        priv_key = self._ffi.NULL
        self._dsa_cdata_set_values(dsa_cdata, p, q, g, pub_key, priv_key)

        evp_pkey = self._dsa_cdata_to_evp_pkey(dsa_cdata)

        return _DSAPublicKey(self, dsa_cdata, evp_pkey)

    def load_dsa_parameter_numbers(self, numbers):
        dsa._check_dsa_parameters(numbers)
        dsa_cdata = self._lib.DSA_new()
        self.openssl_assert(dsa_cdata != self._ffi.NULL)
        dsa_cdata = self._ffi.gc(dsa_cdata, self._lib.DSA_free)

        p = self._int_to_bn(numbers.p)
        q = self._int_to_bn(numbers.q)
        g = self._int_to_bn(numbers.g)
        res = self._lib.DSA_set0_pqg(dsa_cdata, p, q, g)
        self.openssl_assert(res == 1)

        return _DSAParameters(self, dsa_cdata)

    def _dsa_cdata_to_evp_pkey(self, dsa_cdata):
        evp_pkey = self._create_evp_pkey_gc()
        res = self._lib.EVP_PKEY_set1_DSA(evp_pkey, dsa_cdata)
        self.openssl_assert(res == 1)
        return evp_pkey

    def dsa_hash_supported(self, algorithm):
        return self.hash_supported(algorithm)

    def dsa_parameters_supported(self, p, q, g):
        return True

    def cmac_algorithm_supported(self, algorithm):
        return self.cipher_supported(
            algorithm, CBC(b"\x00" * algorithm.block_size)
        )

    def create_cmac_ctx(self, algorithm):
        return _CMACContext(self, algorithm)

    def create_x509_csr(self, builder, private_key, algorithm):
        if not isinstance(algorithm, hashes.HashAlgorithm):
            raise TypeError('Algorithm must be a registered hash algorithm.')

        if (
            isinstance(algorithm, hashes.MD5) and not
            isinstance(private_key, rsa.RSAPrivateKey)
        ):
            raise ValueError(
                "MD5 is not a supported hash algorithm for EC/DSA CSRs"
            )

        # Resolve the signature algorithm.
        evp_md = self._evp_md_non_null_from_algorithm(algorithm)

        # Create an empty request.
        x509_req = self._lib.X509_REQ_new()
        self.openssl_assert(x509_req != self._ffi.NULL)
        x509_req = self._ffi.gc(x509_req, self._lib.X509_REQ_free)

        # Set x509 version.
        res = self._lib.X509_REQ_set_version(x509_req, x509.Version.v1.value)
        self.openssl_assert(res == 1)

        # Set subject name.
        res = self._lib.X509_REQ_set_subject_name(
            x509_req, _encode_name_gc(self, builder._subject_name)
        )
        self.openssl_assert(res == 1)

        # Set subject public key.
        public_key = private_key.public_key()
        res = self._lib.X509_REQ_set_pubkey(
            x509_req, public_key._evp_pkey
        )
        self.openssl_assert(res == 1)

        # Add extensions.
        sk_extension = self._lib.sk_X509_EXTENSION_new_null()
        self.openssl_assert(sk_extension != self._ffi.NULL)
        sk_extension = self._ffi.gc(
            sk_extension,
            lambda x: self._lib.sk_X509_EXTENSION_pop_free(
                x, self._ffi.addressof(
                    self._lib._original_lib, "X509_EXTENSION_free"
                )
            )
        )
        # Don't GC individual extensions because the memory is owned by
        # sk_extensions and will be freed along with it.
        self._create_x509_extensions(
            extensions=builder._extensions,
            handlers=_EXTENSION_ENCODE_HANDLERS,
            x509_obj=sk_extension,
            add_func=self._lib.sk_X509_EXTENSION_insert,
            gc=False
        )
        res = self._lib.X509_REQ_add_extensions(x509_req, sk_extension)
        self.openssl_assert(res == 1)

        # Sign the request using the requester's private key.
        res = self._lib.X509_REQ_sign(
            x509_req, private_key._evp_pkey, evp_md
        )
        if res == 0:
            errors = self._consume_errors()
            self.openssl_assert(
                errors[0]._lib_reason_match(
                    self._lib.ERR_LIB_RSA,
                    self._lib.RSA_R_DIGEST_TOO_BIG_FOR_RSA_KEY
                )
            )

            raise ValueError("Digest too big for RSA key")

        return _CertificateSigningRequest(self, x509_req)

    def create_x509_certificate(self, builder, private_key, algorithm):
        if not isinstance(builder, x509.CertificateBuilder):
            raise TypeError('Builder type mismatch.')
        if not isinstance(algorithm, hashes.HashAlgorithm):
            raise TypeError('Algorithm must be a registered hash algorithm.')

        if (
            isinstance(algorithm, hashes.MD5) and not
            isinstance(private_key, rsa.RSAPrivateKey)
        ):
            raise ValueError(
                "MD5 is not a supported hash algorithm for EC/DSA certificates"
            )

        # Resolve the signature algorithm.
        evp_md = self._evp_md_non_null_from_algorithm(algorithm)

        # Create an empty certificate.
        x509_cert = self._lib.X509_new()
        x509_cert = self._ffi.gc(x509_cert, backend._lib.X509_free)

        # Set the x509 version.
        res = self._lib.X509_set_version(x509_cert, builder._version.value)
        self.openssl_assert(res == 1)

        # Set the subject's name.
        res = self._lib.X509_set_subject_name(
            x509_cert, _encode_name_gc(self, builder._subject_name)
        )
        self.openssl_assert(res == 1)

        # Set the subject's public key.
        res = self._lib.X509_set_pubkey(
            x509_cert, builder._public_key._evp_pkey
        )
        self.openssl_assert(res == 1)

        # Set the certificate serial number.
        serial_number = _encode_asn1_int_gc(self, builder._serial_number)
        res = self._lib.X509_set_serialNumber(x509_cert, serial_number)
        self.openssl_assert(res == 1)

        # Set the "not before" time.
        self._set_asn1_time(
            self._lib.X509_get_notBefore(x509_cert), builder._not_valid_before
        )

        # Set the "not after" time.
        self._set_asn1_time(
            self._lib.X509_get_notAfter(x509_cert), builder._not_valid_after
        )

        # Add extensions.
        self._create_x509_extensions(
            extensions=builder._extensions,
            handlers=_EXTENSION_ENCODE_HANDLERS,
            x509_obj=x509_cert,
            add_func=self._lib.X509_add_ext,
            gc=True
        )

        # Set the issuer name.
        res = self._lib.X509_set_issuer_name(
            x509_cert, _encode_name_gc(self, builder._issuer_name)
        )
        self.openssl_assert(res == 1)

        # Sign the certificate with the issuer's private key.
        res = self._lib.X509_sign(
            x509_cert, private_key._evp_pkey, evp_md
        )
        if res == 0:
            errors = self._consume_errors()
            self.openssl_assert(
                errors[0]._lib_reason_match(
                    self._lib.ERR_LIB_RSA,
                    self._lib.RSA_R_DIGEST_TOO_BIG_FOR_RSA_KEY
                )
            )
            raise ValueError("Digest too big for RSA key")

        return _Certificate(self, x509_cert)

    def _set_asn1_time(self, asn1_time, time):
        if time.year >= 2050:
            asn1_str = time.strftime('%Y%m%d%H%M%SZ').encode('ascii')
        else:
            asn1_str = time.strftime('%y%m%d%H%M%SZ').encode('ascii')
        res = self._lib.ASN1_TIME_set_string(asn1_time, asn1_str)
        self.openssl_assert(res == 1)

    def _create_asn1_time(self, time):
        asn1_time = self._lib.ASN1_TIME_new()
        self.openssl_assert(asn1_time != self._ffi.NULL)
        asn1_time = self._ffi.gc(asn1_time, self._lib.ASN1_TIME_free)
        self._set_asn1_time(asn1_time, time)
        return asn1_time

    def create_x509_crl(self, builder, private_key, algorithm):
        if not isinstance(builder, x509.CertificateRevocationListBuilder):
            raise TypeError('Builder type mismatch.')
        if not isinstance(algorithm, hashes.HashAlgorithm):
            raise TypeError('Algorithm must be a registered hash algorithm.')

        if (
            isinstance(algorithm, hashes.MD5) and not
            isinstance(private_key, rsa.RSAPrivateKey)
        ):
            raise ValueError(
                "MD5 is not a supported hash algorithm for EC/DSA CRLs"
            )

        evp_md = self._evp_md_non_null_from_algorithm(algorithm)

        # Create an empty CRL.
        x509_crl = self._lib.X509_CRL_new()
        x509_crl = self._ffi.gc(x509_crl, backend._lib.X509_CRL_free)

        # Set the x509 CRL version. We only support v2 (integer value 1).
        res = self._lib.X509_CRL_set_version(x509_crl, 1)
        self.openssl_assert(res == 1)

        # Set the issuer name.
        res = self._lib.X509_CRL_set_issuer_name(
            x509_crl, _encode_name_gc(self, builder._issuer_name)
        )
        self.openssl_assert(res == 1)

        # Set the last update time.
        last_update = self._create_asn1_time(builder._last_update)
        res = self._lib.X509_CRL_set_lastUpdate(x509_crl, last_update)
        self.openssl_assert(res == 1)

        # Set the next update time.
        next_update = self._create_asn1_time(builder._next_update)
        res = self._lib.X509_CRL_set_nextUpdate(x509_crl, next_update)
        self.openssl_assert(res == 1)

        # Add extensions.
        self._create_x509_extensions(
            extensions=builder._extensions,
            handlers=_CRL_EXTENSION_ENCODE_HANDLERS,
            x509_obj=x509_crl,
            add_func=self._lib.X509_CRL_add_ext,
            gc=True
        )

        # add revoked certificates
        for revoked_cert in builder._revoked_certificates:
            # Duplicating because the X509_CRL takes ownership and will free
            # this memory when X509_CRL_free is called.
            revoked = self._lib.Cryptography_X509_REVOKED_dup(
                revoked_cert._x509_revoked
            )
            self.openssl_assert(revoked != self._ffi.NULL)
            res = self._lib.X509_CRL_add0_revoked(x509_crl, revoked)
            self.openssl_assert(res == 1)

        res = self._lib.X509_CRL_sign(
            x509_crl, private_key._evp_pkey, evp_md
        )
        if res == 0:
            errors = self._consume_errors()
            self.openssl_assert(
                errors[0]._lib_reason_match(
                    self._lib.ERR_LIB_RSA,
                    self._lib.RSA_R_DIGEST_TOO_BIG_FOR_RSA_KEY
                )
            )
            raise ValueError("Digest too big for RSA key")

        return _CertificateRevocationList(self, x509_crl)

    def _create_x509_extensions(self, extensions, handlers, x509_obj,
                                add_func, gc):
        for i, extension in enumerate(extensions):
            x509_extension = self._create_x509_extension(
                handlers, extension
            )
            self.openssl_assert(x509_extension != self._ffi.NULL)

            if gc:
                x509_extension = self._ffi.gc(
                    x509_extension, self._lib.X509_EXTENSION_free
                )
            res = add_func(x509_obj, x509_extension, i)
            self.openssl_assert(res >= 1)

    def _create_raw_x509_extension(self, extension, value):
        obj = _txt2obj_gc(self, extension.oid.dotted_string)
        return self._lib.X509_EXTENSION_create_by_OBJ(
            self._ffi.NULL, obj, 1 if extension.critical else 0, value
        )

    def _create_x509_extension(self, handlers, extension):
        if isinstance(extension.value, x509.UnrecognizedExtension):
            value = _encode_asn1_str_gc(self, extension.value.value)
            return self._create_raw_x509_extension(extension, value)
        elif isinstance(extension.value, x509.TLSFeature):
            asn1 = _Integers([x.value for x in extension.value]).dump()
            value = _encode_asn1_str_gc(self, asn1)
            return self._create_raw_x509_extension(extension, value)
        elif isinstance(extension.value, x509.PrecertPoison):
            asn1 = asn1crypto.core.Null().dump()
            value = _encode_asn1_str_gc(self, asn1)
            return self._create_raw_x509_extension(extension, value)
        else:
            try:
                encode = handlers[extension.oid]
            except KeyError:
                raise NotImplementedError(
                    'Extension not supported: {0}'.format(extension.oid)
                )

            ext_struct = encode(self, extension.value)
            nid = self._lib.OBJ_txt2nid(
                extension.oid.dotted_string.encode("ascii")
            )
            backend.openssl_assert(nid != self._lib.NID_undef)
            return self._lib.X509V3_EXT_i2d(
                nid, 1 if extension.critical else 0, ext_struct
            )

    def create_x509_revoked_certificate(self, builder):
        if not isinstance(builder, x509.RevokedCertificateBuilder):
            raise TypeError('Builder type mismatch.')

        x509_revoked = self._lib.X509_REVOKED_new()
        self.openssl_assert(x509_revoked != self._ffi.NULL)
        x509_revoked = self._ffi.gc(x509_revoked, self._lib.X509_REVOKED_free)
        serial_number = _encode_asn1_int_gc(self, builder._serial_number)
        res = self._lib.X509_REVOKED_set_serialNumber(
            x509_revoked, serial_number
        )
        self.openssl_assert(res == 1)
        rev_date = self._create_asn1_time(builder._revocation_date)
        res = self._lib.X509_REVOKED_set_revocationDate(x509_revoked, rev_date)
        self.openssl_assert(res == 1)
        # add CRL entry extensions
        self._create_x509_extensions(
            extensions=builder._extensions,
            handlers=_CRL_ENTRY_EXTENSION_ENCODE_HANDLERS,
            x509_obj=x509_revoked,
            add_func=self._lib.X509_REVOKED_add_ext,
            gc=True
        )
        return _RevokedCertificate(self, None, x509_revoked)

    def load_pem_private_key(self, data, password):
        return self._load_key(
            self._lib.PEM_read_bio_PrivateKey,
            self._evp_pkey_to_private_key,
            data,
            password,
        )

    def load_pem_public_key(self, data):
        mem_bio = self._bytes_to_bio(data)
        evp_pkey = self._lib.PEM_read_bio_PUBKEY(
            mem_bio.bio, self._ffi.NULL, self._ffi.NULL, self._ffi.NULL
        )
        if evp_pkey != self._ffi.NULL:
            evp_pkey = self._ffi.gc(evp_pkey, self._lib.EVP_PKEY_free)
            return self._evp_pkey_to_public_key(evp_pkey)
        else:
            # It's not a (RSA/DSA/ECDSA) subjectPublicKeyInfo, but we still
            # need to check to see if it is a pure PKCS1 RSA public key (not
            # embedded in a subjectPublicKeyInfo)
            self._consume_errors()
            res = self._lib.BIO_reset(mem_bio.bio)
            self.openssl_assert(res == 1)
            rsa_cdata = self._lib.PEM_read_bio_RSAPublicKey(
                mem_bio.bio, self._ffi.NULL, self._ffi.NULL, self._ffi.NULL
            )
            if rsa_cdata != self._ffi.NULL:
                rsa_cdata = self._ffi.gc(rsa_cdata, self._lib.RSA_free)
                evp_pkey = self._rsa_cdata_to_evp_pkey(rsa_cdata)
                return _RSAPublicKey(self, rsa_cdata, evp_pkey)
            else:
                self._handle_key_loading_error()

    def load_pem_parameters(self, data):
        mem_bio = self._bytes_to_bio(data)
        # only DH is supported currently
        dh_cdata = self._lib.PEM_read_bio_DHparams(
            mem_bio.bio, self._ffi.NULL, self._ffi.NULL, self._ffi.NULL)
        if dh_cdata != self._ffi.NULL:
            dh_cdata = self._ffi.gc(dh_cdata, self._lib.DH_free)
            return _DHParameters(self, dh_cdata)
        else:
            self._handle_key_loading_error()

    def load_der_private_key(self, data, password):
        # OpenSSL has a function called d2i_AutoPrivateKey that in theory
        # handles this automatically, however it doesn't handle encrypted
        # private keys. Instead we try to load the key two different ways.
        # First we'll try to load it as a traditional key.
        bio_data = self._bytes_to_bio(data)
        key = self._evp_pkey_from_der_traditional_key(bio_data, password)
        if key:
            return self._evp_pkey_to_private_key(key)
        else:
            # Finally we try to load it with the method that handles encrypted
            # PKCS8 properly.
            return self._load_key(
                self._lib.d2i_PKCS8PrivateKey_bio,
                self._evp_pkey_to_private_key,
                data,
                password,
            )

    def _evp_pkey_from_der_traditional_key(self, bio_data, password):
        key = self._lib.d2i_PrivateKey_bio(bio_data.bio, self._ffi.NULL)
        if key != self._ffi.NULL:
            key = self._ffi.gc(key, self._lib.EVP_PKEY_free)
            if password is not None:
                raise TypeError(
                    "Password was given but private key is not encrypted."
                )

            return key
        else:
            self._consume_errors()
            return None

    def load_der_public_key(self, data):
        mem_bio = self._bytes_to_bio(data)
        evp_pkey = self._lib.d2i_PUBKEY_bio(mem_bio.bio, self._ffi.NULL)
        if evp_pkey != self._ffi.NULL:
            evp_pkey = self._ffi.gc(evp_pkey, self._lib.EVP_PKEY_free)
            return self._evp_pkey_to_public_key(evp_pkey)
        else:
            # It's not a (RSA/DSA/ECDSA) subjectPublicKeyInfo, but we still
            # need to check to see if it is a pure PKCS1 RSA public key (not
            # embedded in a subjectPublicKeyInfo)
            self._consume_errors()
            res = self._lib.BIO_reset(mem_bio.bio)
            self.openssl_assert(res == 1)
            rsa_cdata = self._lib.d2i_RSAPublicKey_bio(
                mem_bio.bio, self._ffi.NULL
            )
            if rsa_cdata != self._ffi.NULL:
                rsa_cdata = self._ffi.gc(rsa_cdata, self._lib.RSA_free)
                evp_pkey = self._rsa_cdata_to_evp_pkey(rsa_cdata)
                return _RSAPublicKey(self, rsa_cdata, evp_pkey)
            else:
                self._handle_key_loading_error()

    def load_der_parameters(self, data):
        mem_bio = self._bytes_to_bio(data)
        dh_cdata = self._lib.d2i_DHparams_bio(
            mem_bio.bio, self._ffi.NULL
        )
        if dh_cdata != self._ffi.NULL:
            dh_cdata = self._ffi.gc(dh_cdata, self._lib.DH_free)
            return _DHParameters(self, dh_cdata)
        elif self._lib.Cryptography_HAS_EVP_PKEY_DHX:
            # We check to see if the is dhx.
            self._consume_errors()
            res = self._lib.BIO_reset(mem_bio.bio)
            self.openssl_assert(res == 1)
            dh_cdata = self._lib.Cryptography_d2i_DHxparams_bio(
                mem_bio.bio, self._ffi.NULL
            )
            if dh_cdata != self._ffi.NULL:
                dh_cdata = self._ffi.gc(dh_cdata, self._lib.DH_free)
                return _DHParameters(self, dh_cdata)

        self._handle_key_loading_error()

    def load_pem_x509_certificate(self, data):
        mem_bio = self._bytes_to_bio(data)
        x509 = self._lib.PEM_read_bio_X509(
            mem_bio.bio, self._ffi.NULL, self._ffi.NULL, self._ffi.NULL
        )
        if x509 == self._ffi.NULL:
            self._consume_errors()
            raise ValueError(
                "Unable to load certificate. See https://cryptography.io/en/la"
                "test/faq/#why-can-t-i-import-my-pem-file for more details."
            )

        x509 = self._ffi.gc(x509, self._lib.X509_free)
        return _Certificate(self, x509)

    def load_der_x509_certificate(self, data):
        mem_bio = self._bytes_to_bio(data)
        x509 = self._lib.d2i_X509_bio(mem_bio.bio, self._ffi.NULL)
        if x509 == self._ffi.NULL:
            self._consume_errors()
            raise ValueError("Unable to load certificate")

        x509 = self._ffi.gc(x509, self._lib.X509_free)
        return _Certificate(self, x509)

    def load_pem_x509_crl(self, data):
        mem_bio = self._bytes_to_bio(data)
        x509_crl = self._lib.PEM_read_bio_X509_CRL(
            mem_bio.bio, self._ffi.NULL, self._ffi.NULL, self._ffi.NULL
        )
        if x509_crl == self._ffi.NULL:
            self._consume_errors()
            raise ValueError(
                "Unable to load CRL. See https://cryptography.io/en/la"
                "test/faq/#why-can-t-i-import-my-pem-file for more details."
            )

        x509_crl = self._ffi.gc(x509_crl, self._lib.X509_CRL_free)
        return _CertificateRevocationList(self, x509_crl)

    def load_der_x509_crl(self, data):
        mem_bio = self._bytes_to_bio(data)
        x509_crl = self._lib.d2i_X509_CRL_bio(mem_bio.bio, self._ffi.NULL)
        if x509_crl == self._ffi.NULL:
            self._consume_errors()
            raise ValueError("Unable to load CRL")

        x509_crl = self._ffi.gc(x509_crl, self._lib.X509_CRL_free)
        return _CertificateRevocationList(self, x509_crl)

    def load_pem_x509_csr(self, data):
        mem_bio = self._bytes_to_bio(data)
        x509_req = self._lib.PEM_read_bio_X509_REQ(
            mem_bio.bio, self._ffi.NULL, self._ffi.NULL, self._ffi.NULL
        )
        if x509_req == self._ffi.NULL:
            self._consume_errors()
            raise ValueError(
                "Unable to load request. See https://cryptography.io/en/la"
                "test/faq/#why-can-t-i-import-my-pem-file for more details."
            )

        x509_req = self._ffi.gc(x509_req, self._lib.X509_REQ_free)
        return _CertificateSigningRequest(self, x509_req)

    def load_der_x509_csr(self, data):
        mem_bio = self._bytes_to_bio(data)
        x509_req = self._lib.d2i_X509_REQ_bio(mem_bio.bio, self._ffi.NULL)
        if x509_req == self._ffi.NULL:
            self._consume_errors()
            raise ValueError("Unable to load request")

        x509_req = self._ffi.gc(x509_req, self._lib.X509_REQ_free)
        return _CertificateSigningRequest(self, x509_req)

    def _load_key(self, openssl_read_func, convert_func, data, password):
        mem_bio = self._bytes_to_bio(data)

        userdata = self._ffi.new("CRYPTOGRAPHY_PASSWORD_DATA *")
        if password is not None:
            utils._check_byteslike("password", password)
            password_ptr = self._ffi.from_buffer(password)
            userdata.password = password_ptr
            userdata.length = len(password)

        evp_pkey = openssl_read_func(
            mem_bio.bio,
            self._ffi.NULL,
            self._ffi.addressof(
                self._lib._original_lib, "Cryptography_pem_password_cb"
            ),
            userdata,
        )

        if evp_pkey == self._ffi.NULL:
            if userdata.error != 0:
                errors = self._consume_errors()
                self.openssl_assert(errors)
                if userdata.error == -1:
                    raise TypeError(
                        "Password was not given but private key is encrypted"
                    )
                else:
                    assert userdata.error == -2
                    raise ValueError(
                        "Passwords longer than {0} bytes are not supported "
                        "by this backend.".format(userdata.maxsize - 1)
                    )
            else:
                self._handle_key_loading_error()

        evp_pkey = self._ffi.gc(evp_pkey, self._lib.EVP_PKEY_free)

        if password is not None and userdata.called == 0:
            raise TypeError(
                "Password was given but private key is not encrypted.")

        assert (
            (password is not None and userdata.called == 1) or
            password is None
        )

        return convert_func(evp_pkey)

    def _handle_key_loading_error(self):
        errors = self._consume_errors()

        if not errors:
            raise ValueError("Could not deserialize key data.")

        elif (
            errors[0]._lib_reason_match(
                self._lib.ERR_LIB_EVP, self._lib.EVP_R_BAD_DECRYPT
            ) or errors[0]._lib_reason_match(
                self._lib.ERR_LIB_PKCS12,
                self._lib.PKCS12_R_PKCS12_CIPHERFINAL_ERROR
            )
        ):
            raise ValueError("Bad decrypt. Incorrect password?")

        elif (
            errors[0]._lib_reason_match(
                self._lib.ERR_LIB_EVP, self._lib.EVP_R_UNKNOWN_PBE_ALGORITHM
            ) or errors[0]._lib_reason_match(
                self._lib.ERR_LIB_PEM, self._lib.PEM_R_UNSUPPORTED_ENCRYPTION
            )
        ):
            raise UnsupportedAlgorithm(
                "PEM data is encrypted with an unsupported cipher",
                _Reasons.UNSUPPORTED_CIPHER
            )

        elif any(
            error._lib_reason_match(
                self._lib.ERR_LIB_EVP,
                self._lib.EVP_R_UNSUPPORTED_PRIVATE_KEY_ALGORITHM
            )
            for error in errors
        ):
            raise ValueError("Unsupported public key algorithm.")

        else:
            assert errors[0].lib in (
                self._lib.ERR_LIB_EVP,
                self._lib.ERR_LIB_PEM,
                self._lib.ERR_LIB_ASN1,
            )
            raise ValueError("Could not deserialize key data.")

    def elliptic_curve_supported(self, curve):
        try:
            curve_nid = self._elliptic_curve_to_nid(curve)
        except UnsupportedAlgorithm:
            curve_nid = self._lib.NID_undef

        group = self._lib.EC_GROUP_new_by_curve_name(curve_nid)

        if group == self._ffi.NULL:
            errors = self._consume_errors()
            self.openssl_assert(
                curve_nid == self._lib.NID_undef or
                errors[0]._lib_reason_match(
                    self._lib.ERR_LIB_EC,
                    self._lib.EC_R_UNKNOWN_GROUP
                )
            )
            return False
        else:
            self.openssl_assert(curve_nid != self._lib.NID_undef)
            self._lib.EC_GROUP_free(group)
            return True

    def elliptic_curve_signature_algorithm_supported(
        self, signature_algorithm, curve
    ):
        # We only support ECDSA right now.
        if not isinstance(signature_algorithm, ec.ECDSA):
            return False

        return self.elliptic_curve_supported(curve)

    def generate_elliptic_curve_private_key(self, curve):
        """
        Generate a new private key on the named curve.
        """

        if self.elliptic_curve_supported(curve):
            ec_cdata = self._ec_key_new_by_curve(curve)

            res = self._lib.EC_KEY_generate_key(ec_cdata)
            self.openssl_assert(res == 1)

            evp_pkey = self._ec_cdata_to_evp_pkey(ec_cdata)

            return _EllipticCurvePrivateKey(self, ec_cdata, evp_pkey)
        else:
            raise UnsupportedAlgorithm(
                "Backend object does not support {0}.".format(curve.name),
                _Reasons.UNSUPPORTED_ELLIPTIC_CURVE
            )

    def load_elliptic_curve_private_numbers(self, numbers):
        public = numbers.public_numbers

        ec_cdata = self._ec_key_new_by_curve(public.curve)

        private_value = self._ffi.gc(
            self._int_to_bn(numbers.private_value), self._lib.BN_clear_free
        )
        res = self._lib.EC_KEY_set_private_key(ec_cdata, private_value)
        self.openssl_assert(res == 1)

        ec_cdata = self._ec_key_set_public_key_affine_coordinates(
            ec_cdata, public.x, public.y)

        evp_pkey = self._ec_cdata_to_evp_pkey(ec_cdata)

        return _EllipticCurvePrivateKey(self, ec_cdata, evp_pkey)

    def load_elliptic_curve_public_numbers(self, numbers):
        ec_cdata = self._ec_key_new_by_curve(numbers.curve)
        ec_cdata = self._ec_key_set_public_key_affine_coordinates(
            ec_cdata, numbers.x, numbers.y)
        evp_pkey = self._ec_cdata_to_evp_pkey(ec_cdata)

        return _EllipticCurvePublicKey(self, ec_cdata, evp_pkey)

    def load_elliptic_curve_public_bytes(self, curve, point_bytes):
        ec_cdata = self._ec_key_new_by_curve(curve)
        group = self._lib.EC_KEY_get0_group(ec_cdata)
        self.openssl_assert(group != self._ffi.NULL)
        point = self._lib.EC_POINT_new(group)
        self.openssl_assert(point != self._ffi.NULL)
        point = self._ffi.gc(point, self._lib.EC_POINT_free)
        with self._tmp_bn_ctx() as bn_ctx:
            res = self._lib.EC_POINT_oct2point(
                group, point, point_bytes, len(point_bytes), bn_ctx
            )
            if res != 1:
                self._consume_errors()
                raise ValueError("Invalid public bytes for the given curve")

        res = self._lib.EC_KEY_set_public_key(ec_cdata, point)
        self.openssl_assert(res == 1)
        evp_pkey = self._ec_cdata_to_evp_pkey(ec_cdata)
        return _EllipticCurvePublicKey(self, ec_cdata, evp_pkey)

    def derive_elliptic_curve_private_key(self, private_value, curve):
        ec_cdata = self._ec_key_new_by_curve(curve)

        get_func, group = self._ec_key_determine_group_get_func(ec_cdata)

        point = self._lib.EC_POINT_new(group)
        self.openssl_assert(point != self._ffi.NULL)
        point = self._ffi.gc(point, self._lib.EC_POINT_free)

        value = self._int_to_bn(private_value)
        value = self._ffi.gc(value, self._lib.BN_clear_free)

        with self._tmp_bn_ctx() as bn_ctx:
            res = self._lib.EC_POINT_mul(group, point, value, self._ffi.NULL,
                                         self._ffi.NULL, bn_ctx)
            self.openssl_assert(res == 1)

            bn_x = self._lib.BN_CTX_get(bn_ctx)
            bn_y = self._lib.BN_CTX_get(bn_ctx)

            res = get_func(group, point, bn_x, bn_y, bn_ctx)
            self.openssl_assert(res == 1)

        res = self._lib.EC_KEY_set_public_key(ec_cdata, point)
        self.openssl_assert(res == 1)
        private = self._int_to_bn(private_value)
        private = self._ffi.gc(private, self._lib.BN_clear_free)
        res = self._lib.EC_KEY_set_private_key(ec_cdata, private)
        self.openssl_assert(res == 1)

        evp_pkey = self._ec_cdata_to_evp_pkey(ec_cdata)

        return _EllipticCurvePrivateKey(self, ec_cdata, evp_pkey)

    def _ec_key_new_by_curve(self, curve):
        curve_nid = self._elliptic_curve_to_nid(curve)
        ec_cdata = self._lib.EC_KEY_new_by_curve_name(curve_nid)
        self.openssl_assert(ec_cdata != self._ffi.NULL)
        return self._ffi.gc(ec_cdata, self._lib.EC_KEY_free)

    def load_der_ocsp_request(self, data):
        mem_bio = self._bytes_to_bio(data)
        request = self._lib.d2i_OCSP_REQUEST_bio(mem_bio.bio, self._ffi.NULL)
        if request == self._ffi.NULL:
            self._consume_errors()
            raise ValueError("Unable to load OCSP request")

        request = self._ffi.gc(request, self._lib.OCSP_REQUEST_free)
        return _OCSPRequest(self, request)

    def load_der_ocsp_response(self, data):
        mem_bio = self._bytes_to_bio(data)
        response = self._lib.d2i_OCSP_RESPONSE_bio(mem_bio.bio, self._ffi.NULL)
        if response == self._ffi.NULL:
            self._consume_errors()
            raise ValueError("Unable to load OCSP response")

        response = self._ffi.gc(response, self._lib.OCSP_RESPONSE_free)
        return _OCSPResponse(self, response)

    def create_ocsp_request(self, builder):
        ocsp_req = self._lib.OCSP_REQUEST_new()
        self.openssl_assert(ocsp_req != self._ffi.NULL)
        ocsp_req = self._ffi.gc(ocsp_req, self._lib.OCSP_REQUEST_free)
        cert, issuer, algorithm = builder._request
        evp_md = self._evp_md_non_null_from_algorithm(algorithm)
        certid = self._lib.OCSP_cert_to_id(
            evp_md, cert._x509, issuer._x509
        )
        self.openssl_assert(certid != self._ffi.NULL)
        onereq = self._lib.OCSP_request_add0_id(ocsp_req, certid)
        self.openssl_assert(onereq != self._ffi.NULL)
        self._create_x509_extensions(
            extensions=builder._extensions,
            handlers=_OCSP_REQUEST_EXTENSION_ENCODE_HANDLERS,
            x509_obj=ocsp_req,
            add_func=self._lib.OCSP_REQUEST_add_ext,
            gc=True,
        )
        return _OCSPRequest(self, ocsp_req)

    def _create_ocsp_basic_response(self, builder, private_key, algorithm):
        basic = self._lib.OCSP_BASICRESP_new()
        self.openssl_assert(basic != self._ffi.NULL)
        basic = self._ffi.gc(basic, self._lib.OCSP_BASICRESP_free)
        evp_md = self._evp_md_non_null_from_algorithm(
            builder._response._algorithm
        )
        certid = self._lib.OCSP_cert_to_id(
            evp_md, builder._response._cert._x509,
            builder._response._issuer._x509
        )
        self.openssl_assert(certid != self._ffi.NULL)
        certid = self._ffi.gc(certid, self._lib.OCSP_CERTID_free)
        if builder._response._revocation_reason is None:
            reason = -1
        else:
            reason = _CRL_ENTRY_REASON_ENUM_TO_CODE[
                builder._response._revocation_reason
            ]
        if builder._response._revocation_time is None:
            rev_time = self._ffi.NULL
        else:
            rev_time = self._create_asn1_time(
                builder._response._revocation_time
            )

        next_update = self._ffi.NULL
        if builder._response._next_update is not None:
            next_update = self._create_asn1_time(
                builder._response._next_update
            )

        this_update = self._create_asn1_time(builder._response._this_update)

        res = self._lib.OCSP_basic_add1_status(
            basic,
            certid,
            builder._response._cert_status.value,
            reason,
            rev_time,
            this_update,
            next_update
        )
        self.openssl_assert(res != self._ffi.NULL)
        # okay, now sign the basic structure
        evp_md = self._evp_md_non_null_from_algorithm(algorithm)
        responder_cert, responder_encoding = builder._responder_id
        flags = self._lib.OCSP_NOCERTS
        if responder_encoding is ocsp.OCSPResponderEncoding.HASH:
            flags |= self._lib.OCSP_RESPID_KEY

        if builder._certs is not None:
            for cert in builder._certs:
                res = self._lib.OCSP_basic_add1_cert(basic, cert._x509)
                self.openssl_assert(res == 1)

        self._create_x509_extensions(
            extensions=builder._extensions,
            handlers=_OCSP_BASICRESP_EXTENSION_ENCODE_HANDLERS,
            x509_obj=basic,
            add_func=self._lib.OCSP_BASICRESP_add_ext,
            gc=True,
        )

        res = self._lib.OCSP_basic_sign(
            basic, responder_cert._x509, private_key._evp_pkey,
            evp_md, self._ffi.NULL, flags
        )
        if res != 1:
            errors = self._consume_errors()
            self.openssl_assert(
                errors[0]._lib_reason_match(
                    self._lib.ERR_LIB_X509,
                    self._lib.X509_R_KEY_VALUES_MISMATCH
                )
            )
            raise ValueError("responder_cert must be signed by private_key")

        return basic

    def create_ocsp_response(self, response_status, builder, private_key,
                             algorithm):
        if response_status is ocsp.OCSPResponseStatus.SUCCESSFUL:
            basic = self._create_ocsp_basic_response(
                builder, private_key, algorithm
            )
        else:
            basic = self._ffi.NULL

        ocsp_resp = self._lib.OCSP_response_create(
            response_status.value, basic
        )
        self.openssl_assert(ocsp_resp != self._ffi.NULL)
        ocsp_resp = self._ffi.gc(ocsp_resp, self._lib.OCSP_RESPONSE_free)
        return _OCSPResponse(self, ocsp_resp)

    def elliptic_curve_exchange_algorithm_supported(self, algorithm, curve):
        return (
            self.elliptic_curve_supported(curve) and
            isinstance(algorithm, ec.ECDH)
        )

    def _ec_cdata_to_evp_pkey(self, ec_cdata):
        evp_pkey = self._create_evp_pkey_gc()
        res = self._lib.EVP_PKEY_set1_EC_KEY(evp_pkey, ec_cdata)
        self.openssl_assert(res == 1)
        return evp_pkey

    def _elliptic_curve_to_nid(self, curve):
        """
        Get the NID for a curve name.
        """

        curve_aliases = {
            "secp192r1": "prime192v1",
            "secp256r1": "prime256v1"
        }

        curve_name = curve_aliases.get(curve.name, curve.name)

        curve_nid = self._lib.OBJ_sn2nid(curve_name.encode())
        if curve_nid == self._lib.NID_undef:
            raise UnsupportedAlgorithm(
                "{0} is not a supported elliptic curve".format(curve.name),
                _Reasons.UNSUPPORTED_ELLIPTIC_CURVE
            )
        return curve_nid

    @contextmanager
    def _tmp_bn_ctx(self):
        bn_ctx = self._lib.BN_CTX_new()
        self.openssl_assert(bn_ctx != self._ffi.NULL)
        bn_ctx = self._ffi.gc(bn_ctx, self._lib.BN_CTX_free)
        self._lib.BN_CTX_start(bn_ctx)
        try:
            yield bn_ctx
        finally:
            self._lib.BN_CTX_end(bn_ctx)

    def _ec_key_determine_group_get_func(self, ctx):
        """
        Given an EC_KEY determine the group and what function is required to
        get point coordinates.
        """
        self.openssl_assert(ctx != self._ffi.NULL)

        nid_two_field = self._lib.OBJ_sn2nid(b"characteristic-two-field")
        self.openssl_assert(nid_two_field != self._lib.NID_undef)

        group = self._lib.EC_KEY_get0_group(ctx)
        self.openssl_assert(group != self._ffi.NULL)

        method = self._lib.EC_GROUP_method_of(group)
        self.openssl_assert(method != self._ffi.NULL)

        nid = self._lib.EC_METHOD_get_field_type(method)
        self.openssl_assert(nid != self._lib.NID_undef)

        if nid == nid_two_field and self._lib.Cryptography_HAS_EC2M:
            get_func = self._lib.EC_POINT_get_affine_coordinates_GF2m
        else:
            get_func = self._lib.EC_POINT_get_affine_coordinates_GFp

        assert get_func

        return get_func, group

    def _ec_key_set_public_key_affine_coordinates(self, ctx, x, y):
        """
        Sets the public key point in the EC_KEY context to the affine x and y
        values.
        """

        if x < 0 or y < 0:
            raise ValueError(
                "Invalid EC key. Both x and y must be non-negative."
            )

        x = self._ffi.gc(self._int_to_bn(x), self._lib.BN_free)
        y = self._ffi.gc(self._int_to_bn(y), self._lib.BN_free)
        res = self._lib.EC_KEY_set_public_key_affine_coordinates(ctx, x, y)
        if res != 1:
            self._consume_errors()
            raise ValueError("Invalid EC key.")

        return ctx

    def _private_key_bytes(self, encoding, format, encryption_algorithm,
                           evp_pkey, cdata):
        if not isinstance(format, serialization.PrivateFormat):
            raise TypeError(
                "format must be an item from the PrivateFormat enum"
            )

        # X9.62 encoding is only valid for EC public keys
        if encoding is serialization.Encoding.X962:
            raise ValueError("X9.62 format is only valid for EC public keys")

        # Raw format and encoding are only valid for X25519, Ed25519, X448, and
        # Ed448 keys. We capture those cases before this method is called so if
        # we see those enum values here it means the caller has passed them to
        # a key that doesn't support raw type
        if format is serialization.PrivateFormat.Raw:
            raise ValueError("raw format is invalid with this key or encoding")

        if encoding is serialization.Encoding.Raw:
            raise ValueError("raw encoding is invalid with this key or format")

        if not isinstance(encryption_algorithm,
                          serialization.KeySerializationEncryption):
            raise TypeError(
                "Encryption algorithm must be a KeySerializationEncryption "
                "instance"
            )

        if isinstance(encryption_algorithm, serialization.NoEncryption):
            password = b""
            passlen = 0
            evp_cipher = self._ffi.NULL
        elif isinstance(encryption_algorithm,
                        serialization.BestAvailableEncryption):
            # This is a curated value that we will update over time.
            evp_cipher = self._lib.EVP_get_cipherbyname(
                b"aes-256-cbc"
            )
            password = encryption_algorithm.password
            passlen = len(password)
            if passlen > 1023:
                raise ValueError(
                    "Passwords longer than 1023 bytes are not supported by "
                    "this backend"
                )
        else:
            raise ValueError("Unsupported encryption type")

        key_type = self._lib.EVP_PKEY_id(evp_pkey)
        if encoding is serialization.Encoding.PEM:
            if format is serialization.PrivateFormat.PKCS8:
                write_bio = self._lib.PEM_write_bio_PKCS8PrivateKey
                key = evp_pkey
            else:
                assert format is serialization.PrivateFormat.TraditionalOpenSSL
                if key_type == self._lib.EVP_PKEY_RSA:
                    write_bio = self._lib.PEM_write_bio_RSAPrivateKey
                elif key_type == self._lib.EVP_PKEY_DSA:
                    write_bio = self._lib.PEM_write_bio_DSAPrivateKey
                else:
                    assert key_type == self._lib.EVP_PKEY_EC
                    write_bio = self._lib.PEM_write_bio_ECPrivateKey

                key = cdata
        elif encoding is serialization.Encoding.DER:
            if format is serialization.PrivateFormat.TraditionalOpenSSL:
                if not isinstance(
                    encryption_algorithm, serialization.NoEncryption
                ):
                    raise ValueError(
                        "Encryption is not supported for DER encoded "
                        "traditional OpenSSL keys"
                    )

                return self._private_key_bytes_traditional_der(key_type, cdata)
            else:
                assert format is serialization.PrivateFormat.PKCS8
                write_bio = self._lib.i2d_PKCS8PrivateKey_bio
                key = evp_pkey
        else:
            raise TypeError("encoding must be Encoding.PEM or Encoding.DER")

        bio = self._create_mem_bio_gc()
        res = write_bio(
            bio,
            key,
            evp_cipher,
            password,
            passlen,
            self._ffi.NULL,
            self._ffi.NULL
        )
        self.openssl_assert(res == 1)
        return self._read_mem_bio(bio)

    def _private_key_bytes_traditional_der(self, key_type, cdata):
        if key_type == self._lib.EVP_PKEY_RSA:
            write_bio = self._lib.i2d_RSAPrivateKey_bio
        elif key_type == self._lib.EVP_PKEY_EC:
            write_bio = self._lib.i2d_ECPrivateKey_bio
        else:
            self.openssl_assert(key_type == self._lib.EVP_PKEY_DSA)
            write_bio = self._lib.i2d_DSAPrivateKey_bio

        bio = self._create_mem_bio_gc()
        res = write_bio(bio, cdata)
        self.openssl_assert(res == 1)
        return self._read_mem_bio(bio)

    def _public_key_bytes(self, encoding, format, key, evp_pkey, cdata):
        if not isinstance(encoding, serialization.Encoding):
            raise TypeError("encoding must be an item from the Encoding enum")

        # Compressed/UncompressedPoint are only valid for EC keys and those
        # cases are handled by the ECPublicKey public_bytes method before this
        # method is called
        if format in (serialization.PublicFormat.UncompressedPoint,
                      serialization.PublicFormat.CompressedPoint):
            raise ValueError("Point formats are not valid for this key type")

        # Raw format and encoding are only valid for X25519, Ed25519, X448, and
        # Ed448 keys. We capture those cases before this method is called so if
        # we see those enum values here it means the caller has passed them to
        # a key that doesn't support raw type
        if format is serialization.PublicFormat.Raw:
            raise ValueError("raw format is invalid with this key or encoding")

        if encoding is serialization.Encoding.Raw:
            raise ValueError("raw encoding is invalid with this key or format")

        if (
            format is serialization.PublicFormat.OpenSSH or
            encoding is serialization.Encoding.OpenSSH
        ):
            if (
                format is not serialization.PublicFormat.OpenSSH or
                encoding is not serialization.Encoding.OpenSSH
            ):
                raise ValueError(
                    "OpenSSH format must be used with OpenSSH encoding"
                )
            return self._openssh_public_key_bytes(key)
        elif format is serialization.PublicFormat.SubjectPublicKeyInfo:
            if encoding is serialization.Encoding.PEM:
                write_bio = self._lib.PEM_write_bio_PUBKEY
            else:
                assert encoding is serialization.Encoding.DER
                write_bio = self._lib.i2d_PUBKEY_bio

            key = evp_pkey
        elif format is serialization.PublicFormat.PKCS1:
            # Only RSA is supported here.
            assert self._lib.EVP_PKEY_id(evp_pkey) == self._lib.EVP_PKEY_RSA
            if encoding is serialization.Encoding.PEM:
                write_bio = self._lib.PEM_write_bio_RSAPublicKey
            else:
                assert encoding is serialization.Encoding.DER
                write_bio = self._lib.i2d_RSAPublicKey_bio

            key = cdata
        else:
            raise TypeError(
                "format must be an item from the PublicFormat enum"
            )

        bio = self._create_mem_bio_gc()
        res = write_bio(bio, key)
        self.openssl_assert(res == 1)
        return self._read_mem_bio(bio)

    def _openssh_public_key_bytes(self, key):
        if isinstance(key, rsa.RSAPublicKey):
            public_numbers = key.public_numbers()
            return b"ssh-rsa " + base64.b64encode(
                ssh._ssh_write_string(b"ssh-rsa") +
                ssh._ssh_write_mpint(public_numbers.e) +
                ssh._ssh_write_mpint(public_numbers.n)
            )
        elif isinstance(key, dsa.DSAPublicKey):
            public_numbers = key.public_numbers()
            parameter_numbers = public_numbers.parameter_numbers
            return b"ssh-dss " + base64.b64encode(
                ssh._ssh_write_string(b"ssh-dss") +
                ssh._ssh_write_mpint(parameter_numbers.p) +
                ssh._ssh_write_mpint(parameter_numbers.q) +
                ssh._ssh_write_mpint(parameter_numbers.g) +
                ssh._ssh_write_mpint(public_numbers.y)
            )
        else:
            assert isinstance(key, ec.EllipticCurvePublicKey)
            public_numbers = key.public_numbers()
            try:
                curve_name = {
                    ec.SECP256R1: b"nistp256",
                    ec.SECP384R1: b"nistp384",
                    ec.SECP521R1: b"nistp521",
                }[type(public_numbers.curve)]
            except KeyError:
                raise ValueError(
                    "Only SECP256R1, SECP384R1, and SECP521R1 curves are "
                    "supported by the SSH public key format"
                )

            point = key.public_bytes(
                serialization.Encoding.X962,
                serialization.PublicFormat.UncompressedPoint
            )
            return b"ecdsa-sha2-" + curve_name + b" " + base64.b64encode(
                ssh._ssh_write_string(b"ecdsa-sha2-" + curve_name) +
                ssh._ssh_write_string(curve_name) +
                ssh._ssh_write_string(point)
            )

    def _parameter_bytes(self, encoding, format, cdata):
        if encoding is serialization.Encoding.OpenSSH:
            raise TypeError(
                "OpenSSH encoding is not supported"
            )

        # Only DH is supported here currently.
        q = self._ffi.new("BIGNUM **")
        self._lib.DH_get0_pqg(cdata,
                              self._ffi.NULL,
                              q,
                              self._ffi.NULL)
        if encoding is serialization.Encoding.PEM:
            if q[0] != self._ffi.NULL:
                write_bio = self._lib.PEM_write_bio_DHxparams
            else:
                write_bio = self._lib.PEM_write_bio_DHparams
        elif encoding is serialization.Encoding.DER:
            if q[0] != self._ffi.NULL:
                write_bio = self._lib.Cryptography_i2d_DHxparams_bio
            else:
                write_bio = self._lib.i2d_DHparams_bio
        else:
            raise TypeError("encoding must be an item from the Encoding enum")

        bio = self._create_mem_bio_gc()
        res = write_bio(bio, cdata)
        self.openssl_assert(res == 1)
        return self._read_mem_bio(bio)

    def generate_dh_parameters(self, generator, key_size):
        if key_size < 512:
            raise ValueError("DH key_size must be at least 512 bits")

        if generator not in (2, 5):
            raise ValueError("DH generator must be 2 or 5")

        dh_param_cdata = self._lib.DH_new()
        self.openssl_assert(dh_param_cdata != self._ffi.NULL)
        dh_param_cdata = self._ffi.gc(dh_param_cdata, self._lib.DH_free)

        res = self._lib.DH_generate_parameters_ex(
            dh_param_cdata,
            key_size,
            generator,
            self._ffi.NULL
        )
        self.openssl_assert(res == 1)

        return _DHParameters(self, dh_param_cdata)

    def _dh_cdata_to_evp_pkey(self, dh_cdata):
        evp_pkey = self._create_evp_pkey_gc()
        res = self._lib.EVP_PKEY_set1_DH(evp_pkey, dh_cdata)
        self.openssl_assert(res == 1)
        return evp_pkey

    def generate_dh_private_key(self, parameters):
        dh_key_cdata = _dh_params_dup(parameters._dh_cdata, self)

        res = self._lib.DH_generate_key(dh_key_cdata)
        self.openssl_assert(res == 1)

        evp_pkey = self._dh_cdata_to_evp_pkey(dh_key_cdata)

        return _DHPrivateKey(self, dh_key_cdata, evp_pkey)

    def generate_dh_private_key_and_parameters(self, generator, key_size):
        return self.generate_dh_private_key(
            self.generate_dh_parameters(generator, key_size))

    def load_dh_private_numbers(self, numbers):
        parameter_numbers = numbers.public_numbers.parameter_numbers

        dh_cdata = self._lib.DH_new()
        self.openssl_assert(dh_cdata != self._ffi.NULL)
        dh_cdata = self._ffi.gc(dh_cdata, self._lib.DH_free)

        p = self._int_to_bn(parameter_numbers.p)
        g = self._int_to_bn(parameter_numbers.g)

        if parameter_numbers.q is not None:
            q = self._int_to_bn(parameter_numbers.q)
        else:
            q = self._ffi.NULL

        pub_key = self._int_to_bn(numbers.public_numbers.y)
        priv_key = self._int_to_bn(numbers.x)

        res = self._lib.DH_set0_pqg(dh_cdata, p, q, g)
        self.openssl_assert(res == 1)

        res = self._lib.DH_set0_key(dh_cdata, pub_key, priv_key)
        self.openssl_assert(res == 1)

        codes = self._ffi.new("int[]", 1)
        res = self._lib.Cryptography_DH_check(dh_cdata, codes)
        self.openssl_assert(res == 1)

        # DH_check will return DH_NOT_SUITABLE_GENERATOR if p % 24 does not
        # equal 11 when the generator is 2 (a quadratic nonresidue).
        # We want to ignore that error because p % 24 == 23 is also fine.
        # Specifically, g is then a quadratic residue. Within the context of
        # Diffie-Hellman this means it can only generate half the possible
        # values. That sounds bad, but quadratic nonresidues leak a bit of
        # the key to the attacker in exchange for having the full key space
        # available. See: https://crypto.stackexchange.com/questions/12961
        if codes[0] != 0 and not (
            parameter_numbers.g == 2 and
            codes[0] ^ self._lib.DH_NOT_SUITABLE_GENERATOR == 0
        ):
            raise ValueError(
                "DH private numbers did not pass safety checks."
            )

        evp_pkey = self._dh_cdata_to_evp_pkey(dh_cdata)

        return _DHPrivateKey(self, dh_cdata, evp_pkey)

    def load_dh_public_numbers(self, numbers):
        dh_cdata = self._lib.DH_new()
        self.openssl_assert(dh_cdata != self._ffi.NULL)
        dh_cdata = self._ffi.gc(dh_cdata, self._lib.DH_free)

        parameter_numbers = numbers.parameter_numbers

        p = self._int_to_bn(parameter_numbers.p)
        g = self._int_to_bn(parameter_numbers.g)

        if parameter_numbers.q is not None:
            q = self._int_to_bn(parameter_numbers.q)
        else:
            q = self._ffi.NULL

        pub_key = self._int_to_bn(numbers.y)

        res = self._lib.DH_set0_pqg(dh_cdata, p, q, g)
        self.openssl_assert(res == 1)

        res = self._lib.DH_set0_key(dh_cdata, pub_key, self._ffi.NULL)
        self.openssl_assert(res == 1)

        evp_pkey = self._dh_cdata_to_evp_pkey(dh_cdata)

        return _DHPublicKey(self, dh_cdata, evp_pkey)

    def load_dh_parameter_numbers(self, numbers):
        dh_cdata = self._lib.DH_new()
        self.openssl_assert(dh_cdata != self._ffi.NULL)
        dh_cdata = self._ffi.gc(dh_cdata, self._lib.DH_free)

        p = self._int_to_bn(numbers.p)
        g = self._int_to_bn(numbers.g)

        if numbers.q is not None:
            q = self._int_to_bn(numbers.q)
        else:
            q = self._ffi.NULL

        res = self._lib.DH_set0_pqg(dh_cdata, p, q, g)
        self.openssl_assert(res == 1)

        return _DHParameters(self, dh_cdata)

    def dh_parameters_supported(self, p, g, q=None):
        dh_cdata = self._lib.DH_new()
        self.openssl_assert(dh_cdata != self._ffi.NULL)
        dh_cdata = self._ffi.gc(dh_cdata, self._lib.DH_free)

        p = self._int_to_bn(p)
        g = self._int_to_bn(g)

        if q is not None:
            q = self._int_to_bn(q)
        else:
            q = self._ffi.NULL

        res = self._lib.DH_set0_pqg(dh_cdata, p, q, g)
        self.openssl_assert(res == 1)

        codes = self._ffi.new("int[]", 1)
        res = self._lib.Cryptography_DH_check(dh_cdata, codes)
        self.openssl_assert(res == 1)

        return codes[0] == 0

    def dh_x942_serialization_supported(self):
        return self._lib.Cryptography_HAS_EVP_PKEY_DHX == 1

    def x509_name_bytes(self, name):
        x509_name = _encode_name_gc(self, name)
        pp = self._ffi.new("unsigned char **")
        res = self._lib.i2d_X509_NAME(x509_name, pp)
        self.openssl_assert(pp[0] != self._ffi.NULL)
        pp = self._ffi.gc(
            pp, lambda pointer: self._lib.OPENSSL_free(pointer[0])
        )
        self.openssl_assert(res > 0)
        return self._ffi.buffer(pp[0], res)[:]

    def x25519_load_public_bytes(self, data):
        # When we drop support for CRYPTOGRAPHY_OPENSSL_LESS_THAN_111 we can
        # switch this to EVP_PKEY_new_raw_public_key
        if len(data) != 32:
            raise ValueError("An X25519 public key is 32 bytes long")

        evp_pkey = self._create_evp_pkey_gc()
        res = self._lib.EVP_PKEY_set_type(evp_pkey, self._lib.NID_X25519)
        backend.openssl_assert(res == 1)
        res = self._lib.EVP_PKEY_set1_tls_encodedpoint(
            evp_pkey, data, len(data)
        )
        backend.openssl_assert(res == 1)
        return _X25519PublicKey(self, evp_pkey)

    def x25519_load_private_bytes(self, data):
        # When we drop support for CRYPTOGRAPHY_OPENSSL_LESS_THAN_111 we can
        # switch this to EVP_PKEY_new_raw_private_key and drop the
        # zeroed_bytearray garbage.
        # OpenSSL only has facilities for loading PKCS8 formatted private
        # keys using the algorithm identifiers specified in
        # https://tools.ietf.org/html/draft-ietf-curdle-pkix-09.
        # This is the standard PKCS8 prefix for a 32 byte X25519 key.
        # The form is:
        #    0:d=0  hl=2 l=  46 cons: SEQUENCE
        #    2:d=1  hl=2 l=   1 prim: INTEGER           :00
        #    5:d=1  hl=2 l=   5 cons: SEQUENCE
        #    7:d=2  hl=2 l=   3 prim: OBJECT            :1.3.101.110
        #    12:d=1  hl=2 l=  34 prim: OCTET STRING      (the key)
        # Of course there's a bit more complexity. In reality OCTET STRING
        # contains an OCTET STRING of length 32! So the last two bytes here
        # are \x04\x20, which is an OCTET STRING of length 32.
        if len(data) != 32:
            raise ValueError("An X25519 private key is 32 bytes long")

        pkcs8_prefix = b'0.\x02\x01\x000\x05\x06\x03+en\x04"\x04 '
        with self._zeroed_bytearray(48) as ba:
            ba[0:16] = pkcs8_prefix
            ba[16:] = data
            bio = self._bytes_to_bio(ba)
            evp_pkey = backend._lib.d2i_PrivateKey_bio(bio.bio, self._ffi.NULL)

        self.openssl_assert(evp_pkey != self._ffi.NULL)
        evp_pkey = self._ffi.gc(evp_pkey, self._lib.EVP_PKEY_free)
        self.openssl_assert(
            self._lib.EVP_PKEY_id(evp_pkey) == self._lib.EVP_PKEY_X25519
        )
        return _X25519PrivateKey(self, evp_pkey)

    def _evp_pkey_keygen_gc(self, nid):
        evp_pkey_ctx = self._lib.EVP_PKEY_CTX_new_id(nid, self._ffi.NULL)
        self.openssl_assert(evp_pkey_ctx != self._ffi.NULL)
        evp_pkey_ctx = self._ffi.gc(evp_pkey_ctx, self._lib.EVP_PKEY_CTX_free)
        res = self._lib.EVP_PKEY_keygen_init(evp_pkey_ctx)
        self.openssl_assert(res == 1)
        evp_ppkey = self._ffi.new("EVP_PKEY **")
        res = self._lib.EVP_PKEY_keygen(evp_pkey_ctx, evp_ppkey)
        self.openssl_assert(res == 1)
        self.openssl_assert(evp_ppkey[0] != self._ffi.NULL)
        evp_pkey = self._ffi.gc(evp_ppkey[0], self._lib.EVP_PKEY_free)
        return evp_pkey

    def x25519_generate_key(self):
        evp_pkey = self._evp_pkey_keygen_gc(self._lib.NID_X25519)
        return _X25519PrivateKey(self, evp_pkey)

    def x25519_supported(self):
        return self._lib.CRYPTOGRAPHY_OPENSSL_110_OR_GREATER

    def x448_load_public_bytes(self, data):
        if len(data) != 56:
            raise ValueError("An X448 public key is 56 bytes long")

        evp_pkey = self._lib.EVP_PKEY_new_raw_public_key(
            self._lib.NID_X448, self._ffi.NULL, data, len(data)
        )
        self.openssl_assert(evp_pkey != self._ffi.NULL)
        evp_pkey = self._ffi.gc(evp_pkey, self._lib.EVP_PKEY_free)
        return _X448PublicKey(self, evp_pkey)

    def x448_load_private_bytes(self, data):
        if len(data) != 56:
            raise ValueError("An X448 private key is 56 bytes long")

        data_ptr = self._ffi.from_buffer(data)
        evp_pkey = self._lib.EVP_PKEY_new_raw_private_key(
            self._lib.NID_X448, self._ffi.NULL, data_ptr, len(data)
        )
        self.openssl_assert(evp_pkey != self._ffi.NULL)
        evp_pkey = self._ffi.gc(evp_pkey, self._lib.EVP_PKEY_free)
        return _X448PrivateKey(self, evp_pkey)

    def x448_generate_key(self):
        evp_pkey = self._evp_pkey_keygen_gc(self._lib.NID_X448)
        return _X448PrivateKey(self, evp_pkey)

    def x448_supported(self):
        return not self._lib.CRYPTOGRAPHY_OPENSSL_LESS_THAN_111

    def derive_scrypt(self, key_material, salt, length, n, r, p):
        buf = self._ffi.new("unsigned char[]", length)
        key_material_ptr = self._ffi.from_buffer(key_material)
        res = self._lib.EVP_PBE_scrypt(
            key_material_ptr, len(key_material), salt, len(salt), n, r, p,
            scrypt._MEM_LIMIT, buf, length
        )
        if res != 1:
            errors = self._consume_errors()
            if not self._lib.CRYPTOGRAPHY_OPENSSL_LESS_THAN_111:
                # This error is only added to the stack in 1.1.1+
                self.openssl_assert(
                    errors[0]._lib_reason_match(
                        self._lib.ERR_LIB_EVP,
                        self._lib.ERR_R_MALLOC_FAILURE
                    ) or
                    errors[0]._lib_reason_match(
                        self._lib.ERR_LIB_EVP,
                        self._lib.EVP_R_MEMORY_LIMIT_EXCEEDED
                    )
                )

            # memory required formula explained here:
            # https://blog.filippo.io/the-scrypt-parameters/
            min_memory = 128 * n * r // (1024**2)
            raise MemoryError(
                "Not enough memory to derive key. These parameters require"
                " {} MB of memory.".format(min_memory)
            )
        return self._ffi.buffer(buf)[:]

    def aead_cipher_supported(self, cipher):
        cipher_name = aead._aead_cipher_name(cipher)
        return (
            self._lib.EVP_get_cipherbyname(cipher_name) != self._ffi.NULL
        )

    @contextlib.contextmanager
    def _zeroed_bytearray(self, length):
        """
        This method creates a bytearray, which we copy data into (hopefully
        also from a mutable buffer that can be dynamically erased!), and then
        zero when we're done.
        """
        ba = bytearray(length)
        try:
            yield ba
        finally:
            self._zero_data(ba, length)

    def _zero_data(self, data, length):
        # We clear things this way because at the moment we're not
        # sure of a better way that can guarantee it overwrites the
        # memory of a bytearray and doesn't just replace the underlying char *.
        for i in range(length):
            data[i] = 0

    @contextlib.contextmanager
    def _zeroed_null_terminated_buf(self, data):
        """
        This method takes bytes, which can be a bytestring or a mutable
        buffer like a bytearray, and yields a null-terminated version of that
        data. This is required because PKCS12_parse doesn't take a length with
        its password char * and ffi.from_buffer doesn't provide null
        termination. So, to support zeroing the data via bytearray we
        need to build this ridiculous construct that copies the memory, but
        zeroes it after use.
        """
        if data is None:
            yield self._ffi.NULL
        else:
            data_len = len(data)
            buf = self._ffi.new("char[]", data_len + 1)
            self._ffi.memmove(buf, data, data_len)
            try:
                yield buf
            finally:
                # Cast to a uint8_t * so we can assign by integer
                self._zero_data(self._ffi.cast("uint8_t *", buf), data_len)

    def load_key_and_certificates_from_pkcs12(self, data, password):
        if password is not None:
            utils._check_byteslike("password", password)

        bio = self._bytes_to_bio(data)
        p12 = self._lib.d2i_PKCS12_bio(bio.bio, self._ffi.NULL)
        if p12 == self._ffi.NULL:
            self._consume_errors()
            raise ValueError("Could not deserialize PKCS12 data")

        p12 = self._ffi.gc(p12, self._lib.PKCS12_free)
        evp_pkey_ptr = self._ffi.new("EVP_PKEY **")
        x509_ptr = self._ffi.new("X509 **")
        sk_x509_ptr = self._ffi.new("Cryptography_STACK_OF_X509 **")
        with self._zeroed_null_terminated_buf(password) as password_buf:
            res = self._lib.PKCS12_parse(
                p12, password_buf, evp_pkey_ptr, x509_ptr, sk_x509_ptr
            )

        if res == 0:
            self._consume_errors()
            raise ValueError("Invalid password or PKCS12 data")

        cert = None
        key = None
        additional_certificates = []

        if evp_pkey_ptr[0] != self._ffi.NULL:
            evp_pkey = self._ffi.gc(evp_pkey_ptr[0], self._lib.EVP_PKEY_free)
            key = self._evp_pkey_to_private_key(evp_pkey)

        if x509_ptr[0] != self._ffi.NULL:
            x509 = self._ffi.gc(x509_ptr[0], self._lib.X509_free)
            cert = _Certificate(self, x509)

        if sk_x509_ptr[0] != self._ffi.NULL:
            sk_x509 = self._ffi.gc(sk_x509_ptr[0], self._lib.sk_X509_free)
            num = self._lib.sk_X509_num(sk_x509_ptr[0])
            for i in range(num):
                x509 = self._lib.sk_X509_value(sk_x509, i)
                x509 = self._ffi.gc(x509, self._lib.X509_free)
                self.openssl_assert(x509 != self._ffi.NULL)
                additional_certificates.append(_Certificate(self, x509))

        return (key, cert, additional_certificates)


class GetCipherByName(object):
    def __init__(self, fmt):
        self._fmt = fmt

    def __call__(self, backend, cipher, mode):
        cipher_name = self._fmt.format(cipher=cipher, mode=mode).lower()
        return backend._lib.EVP_get_cipherbyname(cipher_name.encode("ascii"))


def _get_xts_cipher(backend, cipher, mode):
    cipher_name = "aes-{0}-xts".format(cipher.key_size // 2)
    return backend._lib.EVP_get_cipherbyname(cipher_name.encode("ascii"))


backend = Backend()