Python cryptography.hazmat.primitives.padding.PKCS7 Examples

def aes_encrypt_b64(key, data):
    """
    This function encrypts the data using AES-128-CBC. It generates
    and adds an IV.
    This is used for PSKC.

    :param key: Encryption key (binary format)
    :type key: bytes
    :param data: Data to encrypt
    :type data: bytes
    :return: base64 encrypted output, containing IV and encrypted data
    :rtype: str
    """
    # pad data
    padder = padding.PKCS7(algorithms.AES.block_size).padder()
    padded_data = padder.update(data) + padder.finalize()
    iv = geturandom(16)
    encdata = aes_cbc_encrypt(key, iv, padded_data)
    return b64encode_and_unicode(iv + encdata) 

def aes_decrypt_b64(key, enc_data_b64):
    """
    This function decrypts base64 encoded data (containing the IV)
    using AES-128-CBC. Used for PSKC

    :param key: binary key
    :param enc_data_b64: base64 encoded data (IV + encdata)
    :type enc_data_b64: str
    :return: encrypted data
    """
    data_bin = base64.b64decode(enc_data_b64)
    iv = data_bin[:16]
    encdata = data_bin[16:]
    padded_data = aes_cbc_decrypt(key, iv, encdata)

    # remove padding
    unpadder = padding.PKCS7(algorithms.AES.block_size).unpadder()
    output = unpadder.update(padded_data) + unpadder.finalize()

    return output


# @log_with(log) 

def decrypt(self, k, a, iv, e, t):
        """ Decrypt according to the selected encryption and hashing
        functions.
        :param k: Encryption key (optional)
        :param a: Additional Authenticated Data
        :param iv: Initialization Vector
        :param e: Ciphertext
        :param t: Authentication Tag

        Returns plaintext or raises an error
        """
        hkey = k[:_inbytes(self.keysize)]
        dkey = k[_inbytes(self.keysize):]

        # verify mac
        if not constant_time.bytes_eq(t, self._mac(hkey, a, iv, e)):
            raise InvalidSignature('Failed to verify MAC')

        # decrypt
        cipher = Cipher(algorithms.AES(dkey), modes.CBC(iv),
                        backend=self.backend)
        decryptor = cipher.decryptor()
        d = decryptor.update(e) + decryptor.finalize()
        unpadder = PKCS7(self.blocksize).unpadder()
        return unpadder.update(d) + unpadder.finalize() 

def _serialize_secure_string(self, value):
        if self.cipher is None:
            raise SerializationError("Cannot generate secure string as cipher "
                                     "is not initialised")

        # convert the string to a UTF-16 byte string as that is what is
        # expected in Windows. If a byte string (native string in Python 2) was
        # passed in, the sender must make sure it is a valid UTF-16
        # representation and not UTF-8 or else the server will fail to decrypt
        # the secure string in most cases
        string_bytes = to_bytes(value, encoding='utf-16-le')

        padder = PKCS7(self.cipher.algorithm.block_size).padder()
        padded_data = padder.update(string_bytes) + padder.finalize()

        encryptor = self.cipher.encryptor()
        ss_value = encryptor.update(padded_data) + encryptor.finalize()
        ss_string = to_string(base64.b64encode(ss_value))

        return ss_string 

def test_validate_encryption(self):
        # Arrange
        self.bbs.require_encryption = True
        kek = KeyWrapper('key1')
        self.bbs.key_encryption_key = kek
        blob_name = self._create_small_blob('block_blob')

        # Act
        self.bbs.require_encryption = False
        self.bbs.key_encryption_key = None
        blob = self.bbs.get_blob_to_bytes(self.container_name, blob_name)

        encryption_data = _dict_to_encryption_data(loads(blob.metadata['encryptiondata']))
        iv = encryption_data.content_encryption_IV
        content_encryption_key = _validate_and_unwrap_cek(encryption_data, kek, None)
        cipher = _generate_AES_CBC_cipher(content_encryption_key, iv)
        decryptor = cipher.decryptor()
        unpadder = PKCS7(128).unpadder()

        content = decryptor.update(blob.content) + decryptor.finalize()
        content = unpadder.update(content) + unpadder.finalize()
        
        self.assertEqual(self.bytes, content) 

def _encrypt_from_parts(self, data, current_time, iv):
        if not isinstance(data, bytes):
            raise TypeError("data must be bytes.")

        padder = padding.PKCS7(algorithms.AES.block_size).padder()
        padded_data = padder.update(data) + padder.finalize()
        encryptor = Cipher(
            algorithms.AES(self._encryption_key), modes.CBC(iv), self._backend
        ).encryptor()
        ciphertext = encryptor.update(padded_data) + encryptor.finalize()

        basic_parts = (
            b"\x80" + struct.pack(">Q", current_time) + iv + ciphertext
        )

        h = HMAC(self._signing_key, hashes.SHA256(), backend=self._backend)
        h.update(basic_parts)
        hmac = h.finalize()
        return base64.urlsafe_b64encode(basic_parts + hmac) 

def _CKM_AES_CBC_encrypt(self, key, pt_data, session):
        iv = self._generate_random(_CBC_IV_SIZE, session)
        ck_mechanism = self._build_cbc_mechanism(iv)
        rv = self.lib.C_EncryptInit(session, ck_mechanism.mech, key)
        self._check_error(rv)

        padder = padding.PKCS7(_CBC_BLOCK_SIZE).padder()
        padded_pt_data = padder.update(pt_data)
        padded_pt_data += padder.finalize()

        pt_len = len(padded_pt_data)
        ct_len = self.ffi.new("CK_ULONG *", pt_len)

        ct = self.ffi.new("CK_BYTE[{}]".format(ct_len[0]))
        rv = self.lib.C_Encrypt(session, padded_pt_data, pt_len, ct, ct_len)
        self._check_error(rv)

        return {
            "iv": self.ffi.buffer(iv)[:],
            "ct": self.ffi.buffer(ct, ct_len[0])[:]
        } 

def _encrypt_from_parts(self, data, current_time, iv):
        utils._check_bytes("data", data)

        padder = padding.PKCS7(algorithms.AES.block_size).padder()
        padded_data = padder.update(data) + padder.finalize()
        encryptor = Cipher(
            algorithms.AES(self._encryption_key), modes.CBC(iv), self._backend
        ).encryptor()
        ciphertext = encryptor.update(padded_data) + encryptor.finalize()

        basic_parts = (
            b"\x80" + struct.pack(">Q", current_time) + iv + ciphertext
        )

        h = HMAC(self._signing_key, hashes.SHA256(), backend=self._backend)
        h.update(basic_parts)
        hmac = h.finalize()
        return base64.urlsafe_b64encode(basic_parts + hmac) 

def _CKM_AES_CBC_decrypt(self, key, iv, ct_data, session):
        iv = self.ffi.new("CK_BYTE[{}]".format(len(iv)), iv)
        ck_mechanism = self._build_cbc_mechanism(iv)
        rv = self.lib.C_DecryptInit(session, ck_mechanism.mech, key)
        self._check_error(rv)

        ct_len = len(ct_data)
        pt_len = self.ffi.new("CK_ULONG *", ct_len)
        pt = self.ffi.new("CK_BYTE[{0}]".format(pt_len[0]))
        rv = self.lib.C_Decrypt(session, ct_data, ct_len, pt, pt_len)
        self._check_error(rv)
        pt = self.ffi.buffer(pt, pt_len[0])[:]

        unpadder = padding.PKCS7(_CBC_BLOCK_SIZE).unpadder()
        unpadded_pt = unpadder.update(pt)
        unpadded_pt += unpadder.finalize()
        return unpadded_pt 

def _encrypt_from_parts(self, data, current_time, iv):
        utils._check_bytes("data", data)

        padder = padding.PKCS7(algorithms.AES.block_size).padder()
        padded_data = padder.update(data) + padder.finalize()
        encryptor = Cipher(
            algorithms.AES(self._encryption_key), modes.CBC(iv), self._backend
        ).encryptor()
        ciphertext = encryptor.update(padded_data) + encryptor.finalize()

        basic_parts = (
            b"\x80" + struct.pack(">Q", current_time) + iv + ciphertext
        )

        h = HMAC(self._signing_key, hashes.SHA256(), backend=self._backend)
        h.update(basic_parts)
        hmac = h.finalize()
        return base64.urlsafe_b64encode(basic_parts + hmac) 

def _encrypt_from_parts(self, data, current_time, iv):
        if not isinstance(data, bytes):
            raise TypeError("data must be bytes.")

        padder = padding.PKCS7(algorithms.AES.block_size).padder()
        padded_data = padder.update(data) + padder.finalize()
        encryptor = Cipher(
            algorithms.AES(self._encryption_key), modes.CBC(iv), self._backend
        ).encryptor()
        ciphertext = encryptor.update(padded_data) + encryptor.finalize()

        basic_parts = (
            b"\x80" + struct.pack(">Q", current_time) + iv + ciphertext
        )

        h = HMAC(self._signing_key, hashes.SHA256(), backend=self._backend)
        h.update(basic_parts)
        hmac = h.finalize()
        return base64.urlsafe_b64encode(basic_parts + hmac) 

def _encrypt_from_parts(self, data, current_time, iv):
        utils._check_bytes("data", data)

        padder = padding.PKCS7(algorithms.AES.block_size).padder()
        padded_data = padder.update(data) + padder.finalize()
        encryptor = Cipher(
            algorithms.AES(self._encryption_key), modes.CBC(iv), self._backend
        ).encryptor()
        ciphertext = encryptor.update(padded_data) + encryptor.finalize()

        basic_parts = (
            b"\x80" + struct.pack(">Q", current_time) + iv + ciphertext
        )

        h = HMAC(self._signing_key, hashes.SHA256(), backend=self._backend)
        h.update(basic_parts)
        hmac = h.finalize()
        return base64.urlsafe_b64encode(basic_parts + hmac) 

def _encrypt_from_parts(self, data, current_time, iv):
        if not isinstance(data, bytes):
            raise TypeError("data must be bytes.")

        padder = padding.PKCS7(algorithms.AES.block_size).padder()
        padded_data = padder.update(data) + padder.finalize()
        encryptor = Cipher(
            algorithms.AES(self._encryption_key), modes.CBC(iv), self._backend
        ).encryptor()
        ciphertext = encryptor.update(padded_data) + encryptor.finalize()

        basic_parts = (
            b"\x80" + struct.pack(">Q", current_time) + iv + ciphertext
        )

        h = HMAC(self._signing_key, hashes.SHA256(), backend=self._backend)
        h.update(basic_parts)
        hmac = h.finalize()
        return base64.urlsafe_b64encode(basic_parts + hmac) 

def get_decryption_aes_key(self, key: bytes, material_description: Dict[str, Any]) -> bytes:
        """
        Get decryption key for a given S3 object

        :param key: Base64 decoded version of x-amz-key
        :param material_description: JSON decoded x-amz-matdesc
        :return: Raw AES key bytes
        """

        # So it seems when java just calls Cipher.getInstance('AES') it'll default to AES/ECB/PKCS5Padding
        aesecb = self._cipher.decryptor()
        padded_result = await self._loop.run_in_executor(None, lambda: (aesecb.update(key) + aesecb.finalize()))

        unpadder = PKCS7(AES.block_size).unpadder()
        result = await self._loop.run_in_executor(None, lambda: (unpadder.update(padded_result) + unpadder.finalize()))

        return result 

def get_encryption_aes_key(self) -> Tuple[bytes, Dict[str, str], str]:
        """
        Get encryption key to encrypt an S3 object

        :return: Raw AES key bytes, Stringified JSON x-amz-matdesc, Base64 encoded x-amz-key
        """

        random_bytes = os.urandom(32)

        padder = PKCS7(AES.block_size).padder()
        padded_result = await self._loop.run_in_executor(
            None, lambda: (padder.update(random_bytes) + padder.finalize()))

        aesecb = self._cipher.encryptor()
        encrypted_result = await self._loop.run_in_executor(
            None, lambda: (aesecb.update(padded_result) + aesecb.finalize()))

        return random_bytes, {}, base64.b64encode(encrypted_result).decode() 

def _encrypt_from_parts(self, data, current_time, iv):
        utils._check_bytes("data", data)

        padder = padding.PKCS7(algorithms.AES.block_size).padder()
        padded_data = padder.update(data) + padder.finalize()
        encryptor = Cipher(
            algorithms.AES(self._encryption_key), modes.CBC(iv), self._backend
        ).encryptor()
        ciphertext = encryptor.update(padded_data) + encryptor.finalize()

        basic_parts = (
            b"\x80" + struct.pack(">Q", current_time) + iv + ciphertext
        )

        h = HMAC(self._signing_key, hashes.SHA256(), backend=self._backend)
        h.update(basic_parts)
        hmac = h.finalize()
        return base64.urlsafe_b64encode(basic_parts + hmac) 

def decrypt_mdmcert(response: bytes, decrypt_with: RSAPrivateKeyWithSerialization) -> bytes:
    """Decrypt a .plist.b64.p7 supplied by mdmcert.download.

    In order to decrypt this we need to:
    - decode the payload using unhexlify()
    - find the private key that corresponded to the request.

    Args:
        response (bytes): The still encryped and hex encoded payload
        decrypt_with (RSAPrivateKeyWithSerialization): The private key that should be used to decrypt the payload.

    Returns:
        bytes - the decrypted response
    """
    decoded_payload = unhexlify(response)

    # try:
    result = decrypt_smime_content(decoded_payload, decrypt_with)
    # except ValueError as e:
    #     return abort(400, e)
    # result = decrypt_with.decrypt(
    #     decoded_payload,
    #     padding.PKCS7(block_size=8)
    # )
    return result 

def _encrypt_from_parts(self, data, current_time, iv):
        if not isinstance(data, bytes):
            raise TypeError("data must be bytes.")

        padder = padding.PKCS7(algorithms.AES.block_size).padder()
        padded_data = padder.update(data) + padder.finalize()
        encryptor = Cipher(
            algorithms.AES(self._encryption_key), modes.CBC(iv), self._backend
        ).encryptor()
        ciphertext = encryptor.update(padded_data) + encryptor.finalize()

        basic_parts = (
            b"\x80" + struct.pack(">Q", current_time) + iv + ciphertext
        )

        h = HMAC(self._signing_key, hashes.SHA256(), backend=self._backend)
        h.update(basic_parts)
        hmac = h.finalize()
        return base64.urlsafe_b64encode(basic_parts + hmac) 

def get_encryptor(key, iv=None):
        algoer = algorithms.AES(key) #这里的AES算法（若要换成des算法，这里换成TripleDES，该加密库中，DES 事实上等于 TripleDES 的密钥长度为 64bit 时的加解密）
        cipher = Cipher(algoer, modes.CBC(iv), backend=default_backend()) #这里的CBC模式
        def enc(bitstring):
            padder    = padding.PKCS7(algoer.block_size).padder()
            bitstring = padder.update(bitstring) + padder.finalize()
            encryptor = cipher.encryptor()
            return encryptor.update(bitstring) + encryptor.finalize()
        def dec(bitstring):
            decryptor = cipher.decryptor()
            ddata     = decryptor.update(bitstring) + decryptor.finalize()
            unpadder  = padding.PKCS7(algoer.block_size).unpadder()
            return unpadder.update(ddata) + unpadder.finalize()
        class f:pass
        f.encrypt = enc
        f.decrypt = dec
        return f 

def decrypt(self, pk, *args):
        km = pk.keymaterial
        if km.oid == EllipticCurveOID.Curve25519:
            v = x25519.X25519PublicKey.from_public_bytes(self.p.x)
            s = km.__privkey__().exchange(v)
        else:
            # assemble the public component of ephemeral key v
            v = ec.EllipticCurvePublicNumbers(self.p.x, self.p.y, km.oid.curve()).public_key(default_backend())
            # compute s using the inverse of how it was derived during encryption
            s = km.__privkey__().exchange(ec.ECDH(), v)

        # derive the wrapping key
        z = km.kdf.derive_key(s, km.oid, PubKeyAlgorithm.ECDH, pk.fingerprint)

        # unwrap and unpad m
        _m = aes_key_unwrap(z, self.c, default_backend())

        padder = PKCS7(64).unpadder()
        return padder.update(_m) + padder.finalize() 

def _wrap_object_with_symmetric_key(plaintext):
    import json
    import base64
    from cryptography.hazmat.primitives import padding
    from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
    from cryptography.hazmat.backends import default_backend

    #Pad data
    padder = padding.PKCS7(128).padder()
    padded_plaintext = padder.update(plaintext)
    padded_plaintext += padder.finalize()
    #Generate random symmetric key and intialization vector
    key = os.urandom(32)
    iv = os.urandom(16)
    #Encrypt object
    backend = default_backend()
    cipher = Cipher(algorithms.AES(key), modes.CBC(iv), backend=backend)
    encryptor = cipher.encryptor()
    ciphertext = encryptor.update(padded_plaintext) + encryptor.finalize()
    wrapped_object= {'ciphertext': base64.b64encode(ciphertext), 'symmetric_key':base64.b64encode(key), 'iv':base64.b64encode(iv)}
    return wrapped_object 

def _encrypt_from_parts(self, data, current_time, iv):
        utils._check_bytes("data", data)

        padder = padding.PKCS7(algorithms.AES.block_size).padder()
        padded_data = padder.update(data) + padder.finalize()
        encryptor = Cipher(
            algorithms.AES(self._encryption_key), modes.CBC(iv), self._backend
        ).encryptor()
        ciphertext = encryptor.update(padded_data) + encryptor.finalize()

        basic_parts = (
            b"\x80" + struct.pack(">Q", current_time) + iv + ciphertext
        )

        h = HMAC(self._signing_key, hashes.SHA256(), backend=self._backend)
        h.update(basic_parts)
        hmac = h.finalize()
        return base64.urlsafe_b64encode(basic_parts + hmac) 

def _get_blob_encryptor_and_padder(cek, iv, should_pad):
    encryptor = None
    padder = None

    if cek is not None and iv is not None:
        cipher = _generate_AES_CBC_cipher(cek, iv)
        encryptor = cipher.encryptor()
        padder = PKCS7(128).padder() if should_pad else None

    return encryptor, padder 

def _encrypt_blob(blob, key_encryption_key):
    '''
    Encrypts the given blob using AES256 in CBC mode with 128 bit padding.
    Wraps the generated content-encryption-key using the user-provided key-encryption-key (kek). 
    Returns a json-formatted string containing the encryption metadata. This method should
    only be used when a blob is small enough for single shot upload. Encrypting larger blobs
    is done as a part of the _upload_blob_chunks method.

    :param bytes blob:
        The blob to be encrypted.
    :param object key_encryption_key:
        The user-provided key-encryption-key. Must implement the following methods:
        wrap_key(key)--wraps the specified key using an algorithm of the user's choice.
        get_key_wrap_algorithm()--returns the algorithm used to wrap the specified symmetric key.
        get_kid()--returns a string key id for this key-encryption-key.
    :return: A tuple of json-formatted string containing the encryption metadata and the encrypted blob data.
    :rtype: (str, bytes)
    '''

    _validate_not_none('blob', blob)
    _validate_not_none('key_encryption_key', key_encryption_key)
    _validate_key_encryption_key_wrap(key_encryption_key)

    # AES256 uses 256 bit (32 byte) keys and always with 16 byte blocks
    content_encryption_key = urandom(32)
    initialization_vector = urandom(16)

    cipher = _generate_AES_CBC_cipher(content_encryption_key, initialization_vector)

    # PKCS7 with 16 byte blocks ensures compatibility with AES.
    padder = PKCS7(128).padder()
    padded_data = padder.update(blob) + padder.finalize()

    # Encrypt the data.
    encryptor = cipher.encryptor()
    encrypted_data = encryptor.update(padded_data) + encryptor.finalize()
    encryption_data = _generate_encryption_data_dict(key_encryption_key, content_encryption_key,
                                                     initialization_vector)
    encryption_data['EncryptionMode'] = 'FullBlob'

    return dumps(encryption_data), encrypted_data 

def _decrypt(message, encryption_data, key_encryption_key=None, resolver=None):
    '''
    Decrypts the given ciphertext using AES256 in CBC mode with 128 bit padding.
    Unwraps the content-encryption-key using the user-provided or resolved key-encryption-key (kek). Returns the original plaintex.

    :param str message:
        The ciphertext to be decrypted.
    :param _EncryptionData encryption_data:
        The metadata associated with this ciphertext.
    :param object key_encryption_key:
        The user-provided key-encryption-key. Must implement the following methods:
        unwrap_key(key, algorithm)--returns the unwrapped form of the specified symmetric key using the string-specified algorithm.
        get_kid()--returns a string key id for this key-encryption-key.
    :param function resolver(kid):
        The user-provided key resolver. Uses the kid string to return a key-encryption-key implementing the interface defined above.
    :return: The decrypted plaintext.
    :rtype: str
    '''
    _validate_not_none('message', message)
    content_encryption_key = _validate_and_unwrap_cek(encryption_data, key_encryption_key, resolver)

    if not (_EncryptionAlgorithm.AES_CBC_256 == encryption_data.encryption_agent.encryption_algorithm):
        raise ValueError(_ERROR_UNSUPPORTED_ENCRYPTION_ALGORITHM)

    cipher = _generate_AES_CBC_cipher(content_encryption_key, encryption_data.content_encryption_IV)

    # decrypt data
    decrypted_data = message
    decryptor = cipher.decryptor()
    decrypted_data = (decryptor.update(decrypted_data) + decryptor.finalize())

    # unpad data
    unpadder = PKCS7(128).unpadder()
    decrypted_data = (unpadder.update(decrypted_data) + unpadder.finalize())

    return decrypted_data 

def decrypt(crypted):
    """Decrypts with AES-128 and removes PKCS#7 padding."""
    iv = KEY
    backend = default_backend()
    cipher = Cipher(algorithms.AES(KEY), modes.CBC(iv), backend=backend)
    decryptor = cipher.decryptor()
    plain = decryptor.update(crypted)
    unpadder = padding.PKCS7(BLOCK_SIZE).unpadder()
    unpadded_data = unpadder.update(plain) + unpadder.finalize()
    return unpadded_data 

def _decrypt_data(self, data, timestamp, ttl):
        current_time = int(time.time())
        if ttl is not None:
            if timestamp + ttl < current_time:
                raise InvalidToken

            if current_time + _MAX_CLOCK_SKEW < timestamp:
                raise InvalidToken

        self._verify_signature(data)

        iv = data[9:25]
        ciphertext = data[25:-32]
        decryptor = Cipher(
            algorithms.AES(self._encryption_key), modes.CBC(iv), self._backend
        ).decryptor()
        plaintext_padded = decryptor.update(ciphertext)
        try:
            plaintext_padded += decryptor.finalize()
        except ValueError:
            raise InvalidToken
        unpadder = padding.PKCS7(algorithms.AES.block_size).unpadder()

        unpadded = unpadder.update(plaintext_padded)
        try:
            unpadded += unpadder.finalize()
        except ValueError:
            raise InvalidToken
        return unpadded 

def encrypt(plain):
    """Adds PKCS#7 padding and encrypts with AES-128."""
    iv = KEY
    backend = default_backend()
    padder = padding.PKCS7(BLOCK_SIZE).padder()
    padded_data = padder.update(bytes(plain)) + padder.finalize()
    cipher = Cipher(algorithms.AES(KEY), modes.CBC(iv), backend=backend)
    encryptor = cipher.encryptor()
    crypted = encryptor.update(padded_data) + encryptor.finalize()
    return crypted 

def _unwrap_object_with_symmetric_key(wrapped_object):
    import json
    import base64
    from cryptography.hazmat.primitives import padding
    from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
    from cryptography.hazmat.backends import default_backend

    backend = default_backend()
    #Decode base64
    try:
        key = base64.b64decode(wrapped_object['symmetric_key'])
        iv =  base64.b64decode(wrapped_object['iv'])
        ciphertext =  base64.b64decode(wrapped_object['ciphertext'])
    except Exception as err:
        _log.error("Failed to decode base64 encoding of key, IV or ciphertext, err={}".format(err))
        raise
    #Decrypt
    try:
        cipher = Cipher(algorithms.AES(key), modes.CBC(iv), backend=backend)
        decryptor = cipher.decryptor()
        padded_plaintext = decryptor.update(ciphertext) + decryptor.finalize()
    except Exception as err:
        _log.error("_unwrap_object_with_symmetric_key: Failed to decrypt, err={}".format(err))
        raise
    #Remove padding
    unpadder = padding.PKCS7(128).unpadder()
    plaintext = unpadder.update(padded_plaintext)
    return plaintext + unpadder.finalize()

#TODO: Add integrity protection of object, i.e.,
# implement proper RSA-KEM+DEM1 or RSA-REACT 

def AES_cbc_decrypt(token: bytes, ciphertext: bytes) -> bytes:
    """Decrypt cipher text with device token."""
    key, iv = key_iv(token)
    cipher = Cipher(algorithms.AES(key), modes.CBC(iv), backend=_backend)
    decryptor = cipher.decryptor()
    padded_plaintext = decryptor.update(bytes(ciphertext)) \
        + decryptor.finalize()
    unpadder = padding.PKCS7(128).unpadder()
    unpadded_plaintext = unpadder.update(padded_plaintext)
    unpadded_plaintext += unpadder.finalize()
    return unpadded_plaintext