How to use the cryptography.utils function in cryptography

ctx = self._backend._lib.CMAC_CTX_new()

            self._backend.openssl_assert(ctx != self._backend._ffi.NULL)
            ctx = self._backend._ffi.gc(ctx, self._backend._lib.CMAC_CTX_free)

            key_ptr = self._backend._ffi.from_buffer(self._key)
            res = self._backend._lib.CMAC_Init(
                ctx, key_ptr, len(self._key),
                evp_cipher, self._backend._ffi.NULL
            )
            self._backend.openssl_assert(res == 1)

        self._ctx = ctx

    algorithm = utils.read_only_property("_algorithm")

    def update(self, data):
        res = self._backend._lib.CMAC_Update(self._ctx, data, len(data))
        self._backend.openssl_assert(res == 1)

    def finalize(self):
        buf = self._backend._ffi.new("unsigned char[]", self._output_length)
        length = self._backend._ffi.new("size_t *", self._output_length)
        res = self._backend._lib.CMAC_Final(
            self._ctx, buf, length
        )
        self._backend.openssl_assert(res == 1)

        self._ctx = None

        return self._backend._ffi.buffer(buf)[:]

# 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

from cryptography import utils
from cryptography.exceptions import (
    InvalidSignature, UnsupportedAlgorithm, _Reasons
)
from cryptography.hazmat.primitives import constant_time, hashes, interfaces


@utils.register_interface(interfaces.MACContext)
@utils.register_interface(hashes.HashContext)
class _HMACContext(object):
    def __init__(self, backend, key, algorithm, ctx=None):
        self._algorithm = algorithm
        self._backend = backend
        if ctx is None:
            ctx = self._backend._ffi.new("CCHmacContext *")
            try:
                alg = self._backend._supported_hmac_algorithms[algorithm.name]
            except KeyError:
                raise UnsupportedAlgorithm(
                    "{0} is not a supported HMAC hash on this backend.".format(
                        algorithm.name),
                    _Reasons.UNSUPPORTED_HASH
                )

            self._backend._lib.CCHmacInit(ctx, alg, key, len(key))

def from_issuer_subject_key_identifier(cls, ski):
        if isinstance(ski, SubjectKeyIdentifier):
            digest = ski.digest
        else:
            digest = ski.value.digest
            warnings.warn(
                "Extension objects are deprecated as arguments to "
                "from_issuer_subject_key_identifier and support will be "
                "removed soon. Please migrate to passing a "
                "SubjectKeyIdentifier directly.",
                utils.DeprecatedIn27,
                stacklevel=2,
            )

        return cls(
            key_identifier=digest,
            authority_cert_issuer=None,
            authority_cert_serial_number=None
        )

def derive(self, key_material):
        utils._check_byteslike("key_material", key_material)
        if self._used:
            raise AlreadyFinalized

        self._used = True
        return self._expand(key_material)

)
            name = self._backend._build_openssl_digest_name(algorithm)
            evp_md = self._backend._lib.EVP_get_digestbyname(name)
            if evp_md == self._backend._ffi.NULL:
                raise UnsupportedAlgorithm(
                    "{0} is not a supported hash on this backend.".format(
                        name),
                    _Reasons.UNSUPPORTED_HASH
                )
            res = self._backend._lib.EVP_DigestInit_ex(ctx, evp_md,
                                                       self._backend._ffi.NULL)
            self._backend.openssl_assert(res != 0)

        self._ctx = ctx

    algorithm = utils.read_only_property("_algorithm")

    def copy(self):
        copied_ctx = self._backend._lib.Cryptography_EVP_MD_CTX_new()
        copied_ctx = self._backend._ffi.gc(
            copied_ctx, self._backend._lib.Cryptography_EVP_MD_CTX_free
        )
        res = self._backend._lib.EVP_MD_CTX_copy_ex(copied_ctx, self._ctx)
        self._backend.openssl_assert(res != 0)
        return _HashContext(self._backend, self.algorithm, ctx=copied_ctx)

    def update(self, data):
        res = self._backend._lib.EVP_DigestUpdate(self._ctx, data, len(data))
        self._backend.openssl_assert(res != 0)

    def finalize(self):
        buf = self._backend._ffi.new("unsigned char[]",

return "".format(self._usages)

    def __eq__(self, other):
        if not isinstance(other, ExtendedKeyUsage):
            return NotImplemented

        return self._usages == other._usages

    def __ne__(self, other):
        return not self == other

    def __hash__(self):
        return hash(tuple(self._usages))


@utils.register_interface(ExtensionType)
class OCSPNoCheck(object):
    oid = ExtensionOID.OCSP_NO_CHECK


@utils.register_interface(ExtensionType)
class PrecertPoison(object):
    oid = ExtensionOID.PRECERT_POISON


@utils.register_interface(ExtensionType)
class TLSFeature(object):
    oid = ExtensionOID.TLS_FEATURE

    def __init__(self, features):
        features = list(features)
        if (

"Backend object does not implement HashBackend.",
                _Reasons.BACKEND_MISSING_INTERFACE
            )

        if not isinstance(algorithm, HashAlgorithm):
            raise TypeError("Expected instance of hashes.HashAlgorithm.")
        self._algorithm = algorithm

        self._backend = backend

        if ctx is None:
            self._ctx = self._backend.create_hash_ctx(self.algorithm)
        else:
            self._ctx = ctx

    algorithm = utils.read_only_property("_algorithm")

    def update(self, data):
        if self._ctx is None:
            raise AlreadyFinalized("Context was already finalized.")
        if not isinstance(data, bytes):
            raise TypeError("data must be bytes.")
        self._ctx.update(data)

    def copy(self):
        if self._ctx is None:
            raise AlreadyFinalized("Context was already finalized.")
        return Hash(
            self.algorithm, backend=self._backend, ctx=self._ctx.copy()
        )

    def finalize(self):

def __init__(self, initialization_vector):
        utils._check_byteslike("initialization_vector", initialization_vector)
        self._initialization_vector = initialization_vector

initialization_vector = utils.read_only_property("_initialization_vector")
    validate_for_algorithm = _check_iv_length


@utils.register_interface(Mode)
@utils.register_interface(ModeWithInitializationVector)
class CFB8(object):
    name = "CFB8"

    def __init__(self, initialization_vector):
        if not isinstance(initialization_vector, bytes):
            raise TypeError("initialization_vector must be bytes")

        self._initialization_vector = initialization_vector

    initialization_vector = utils.read_only_property("_initialization_vector")
    validate_for_algorithm = _check_iv_length


@utils.register_interface(Mode)
@utils.register_interface(ModeWithNonce)
class CTR(object):
    name = "CTR"

    def __init__(self, nonce):
        if not isinstance(nonce, bytes):
            raise TypeError("nonce must be bytes")

        self._nonce = nonce

    nonce = utils.read_only_property("_nonce")

"Backend object does not implement HashBackend.",
                _Reasons.BACKEND_MISSING_INTERFACE
            )

        if not isinstance(algorithm, HashAlgorithm):
            raise TypeError("Expected instance of hashes.HashAlgorithm.")
        self._algorithm = algorithm

        self._backend = backend

        if ctx is None:
            self._ctx = self._backend.create_hash_ctx(self.algorithm)
        else:
            self._ctx = ctx

    algorithm = utils.read_only_property("_algorithm")

    def update(self, data):
        if self._ctx is None:
            raise AlreadyFinalized("Context was already finalized.")
        if not isinstance(data, bytes):
            raise TypeError("data must be bytes.")
        self._ctx.update(data)

    def copy(self):
        if self._ctx is None:
            raise AlreadyFinalized("Context was already finalized.")
        return Hash(
            self.algorithm, backend=self._backend, ctx=self._ctx.copy()
        )

    def finalize(self):