path: root/src/leap/mx/vendor/pgpy/packet/packets.py

""" packet.py
"""
import abc
import binascii
import calendar
import copy
import hashlib
import os
import re

from datetime import datetime

import six

from cryptography.hazmat.primitives import constant_time
from cryptography.hazmat.primitives.asymmetric import padding

from .fields import DSAPriv, DSAPub, DSASignature
from .fields import ECDSAPub, ECDSAPriv, ECDSASignature
from .fields import ECDHPub, ECDHPriv, ECDHCipherText
from .fields import ElGCipherText, ElGPriv, ElGPub
from .fields import OpaquePubKey
from .fields import OpaquePrivKey
from .fields import RSACipherText, RSAPriv, RSAPub, RSASignature
from .fields import String2Key
from .fields import SubPackets
from .fields import UserAttributeSubPackets

from .types import Packet
from .types import Primary
from .types import Private
from .types import Public
from .types import Sub
from .types import VersionedPacket

from ..constants import CompressionAlgorithm
from ..constants import HashAlgorithm
from ..constants import PubKeyAlgorithm
from ..constants import SignatureType
from ..constants import SymmetricKeyAlgorithm
from ..constants import TrustFlags
from ..constants import TrustLevel

from ..decorators import sdproperty

from ..errors import PGPDecryptionError

from ..symenc import _decrypt
from ..symenc import _encrypt

from ..types import Fingerprint

__all__ = ['PKESessionKey',
           'PKESessionKeyV3',
           'Signature',
           'SignatureV4',
           'SKESessionKey',
           'SKESessionKeyV4',
           'OnePassSignature',
           'OnePassSignatureV3',
           'PrivKey',
           'PubKey',
           'PubKeyV4',
           'PrivKeyV4',
           'PrivSubKey',
           'PrivSubKeyV4',
           'CompressedData',
           'SKEData',
           'Marker',
           'LiteralData',
           'Trust',
           'UserID',
           'PubSubKey',
           'PubSubKeyV4',
           'UserAttribute',
           'IntegrityProtectedSKEData',
           'IntegrityProtectedSKEDataV1',
           'MDC']


class PKESessionKey(VersionedPacket):
    __typeid__ = 0x01
    __ver__ = 0

    @abc.abstractmethod
    def decrypt_sk(self, pk):
        raise NotImplementedError()

    @abc.abstractmethod
    def encrypt_sk(self, pk, symalg, symkey):
        raise NotImplementedError()


class PKESessionKeyV3(PKESessionKey):
    """
    5.1.  Public-Key Encrypted Session Key Packets (Tag 1)

    A Public-Key Encrypted Session Key packet holds the session key used
    to encrypt a message.  Zero or more Public-Key Encrypted Session Key
    packets and/or Symmetric-Key Encrypted Session Key packets may
    precede a Symmetrically Encrypted Data Packet, which holds an
    encrypted message.  The message is encrypted with the session key,
    and the session key is itself encrypted and stored in the Encrypted
    Session Key packet(s).  The Symmetrically Encrypted Data Packet is
    preceded by one Public-Key Encrypted Session Key packet for each
    OpenPGP key to which the message is encrypted.  The recipient of the
    message finds a session key that is encrypted to their public key,
    decrypts the session key, and then uses the session key to decrypt
    the message.

    The body of this packet consists of:

     - A one-octet number giving the version number of the packet type.
       The currently defined value for packet version is 3.

     - An eight-octet number that gives the Key ID of the public key to
       which the session key is encrypted.  If the session key is
       encrypted to a subkey, then the Key ID of this subkey is used
       here instead of the Key ID of the primary key.

     - A one-octet number giving the public-key algorithm used.

     - A string of octets that is the encrypted session key.  This
       string takes up the remainder of the packet, and its contents are
       dependent on the public-key algorithm used.

    Algorithm Specific Fields for RSA encryption

     - multiprecision integer (MPI) of RSA encrypted value m**e mod n.

    Algorithm Specific Fields for Elgamal encryption:

     - MPI of Elgamal (Diffie-Hellman) value g**k mod p.

     - MPI of Elgamal (Diffie-Hellman) value m * y**k mod p.

    The value "m" in the above formulas is derived from the session key
    as follows.  First, the session key is prefixed with a one-octet
    algorithm identifier that specifies the symmetric encryption
    algorithm used to encrypt the following Symmetrically Encrypted Data
    Packet.  Then a two-octet checksum is appended, which is equal to the
    sum of the preceding session key octets, not including the algorithm
    identifier, modulo 65536.  This value is then encoded as described in
    PKCS#1 block encoding EME-PKCS1-v1_5 in Section 7.2.1 of [RFC3447] to
    form the "m" value used in the formulas above.  See Section 13.1 of
    this document for notes on OpenPGP's use of PKCS#1.

    Note that when an implementation forms several PKESKs with one
    session key, forming a message that can be decrypted by several keys,
    the implementation MUST make a new PKCS#1 encoding for each key.

    An implementation MAY accept or use a Key ID of zero as a "wild card"
    or "speculative" Key ID.  In this case, the receiving implementation
    would try all available private keys, checking for a valid decrypted
    session key.  This format helps reduce traffic analysis of messages.
    """
    __ver__ = 3

    @sdproperty
    def encrypter(self):
        return self._encrypter

    @encrypter.register(bytearray)
    def encrypter_bin(self, val):
        self._encrypter = binascii.hexlify(val).upper().decode('latin-1')

    @sdproperty
    def pkalg(self):
        return self._pkalg

    @pkalg.register(int)
    @pkalg.register(PubKeyAlgorithm)
    def pkalg_int(self, val):
        self._pkalg = PubKeyAlgorithm(val)

        _c = {PubKeyAlgorithm.RSAEncryptOrSign: RSACipherText,
              PubKeyAlgorithm.RSAEncrypt: RSACipherText,
              PubKeyAlgorithm.ElGamal: ElGCipherText,
              PubKeyAlgorithm.FormerlyElGamalEncryptOrSign: ElGCipherText,
              PubKeyAlgorithm.ECDH: ECDHCipherText}

        ct = _c.get(self._pkalg, None)
        self.ct = ct() if ct is not None else ct

    def __init__(self):
        super(PKESessionKeyV3, self).__init__()
        self.encrypter = bytearray(8)
        self.pkalg = 0
        self.ct = None

    def __bytearray__(self):
        _bytes = bytearray()
        _bytes += super(PKESessionKeyV3, self).__bytearray__()
        _bytes += binascii.unhexlify(self.encrypter.encode())
        _bytes += bytearray([self.pkalg])
        _bytes += self.ct.__bytearray__() if self.ct is not None else b'\x00' * (self.header.length - 10)
        return _bytes

    def __copy__(self):
        sk = self.__class__()
        sk.header = copy.copy(self.header)
        sk._encrypter = self._encrypter
        sk.pkalg = self.pkalg
        if self.ct is not None:
            sk.ct = copy.copy(self.ct)

        return sk

    def decrypt_sk(self, pk):
        if self.pkalg == PubKeyAlgorithm.RSAEncryptOrSign:
            # pad up ct with null bytes if necessary
            ct = self.ct.me_mod_n.to_mpibytes()[2:]
            ct = b'\x00' * ((pk.keymaterial.__privkey__().key_size // 8) - len(ct)) + ct

            decrypter = pk.keymaterial.__privkey__().decrypt
            decargs = (ct, padding.PKCS1v15(),)

        elif self.pkalg == PubKeyAlgorithm.ECDH:
            decrypter = pk
            decargs = ()

        else:
            raise NotImplementedError(self.pkalg)

        m = bytearray(self.ct.decrypt(decrypter, *decargs))

        """
        The value "m" in the above formulas is derived from the session key
        as follows.  First, the session key is prefixed with a one-octet
        algorithm identifier that specifies the symmetric encryption
        algorithm used to encrypt the following Symmetrically Encrypted Data
        Packet.  Then a two-octet checksum is appended, which is equal to the
        sum of the preceding session key octets, not including the algorithm
        identifier, modulo 65536.  This value is then encoded as described in
        PKCS#1 block encoding EME-PKCS1-v1_5 in Section 7.2.1 of [RFC3447] to
        form the "m" value used in the formulas above.  See Section 13.1 of
        this document for notes on OpenPGP's use of PKCS#1.
        """

        symalg = SymmetricKeyAlgorithm(m[0])
        del m[0]

        symkey = m[:symalg.key_size // 8]
        del m[:symalg.key_size // 8]

        checksum = self.bytes_to_int(m[:2])
        del m[:2]

        if not sum(symkey) % 65536 == checksum:  # pragma: no cover
            raise PGPDecryptionError("{:s} decryption failed".format(self.pkalg.name))

        return (symalg, symkey)

    def encrypt_sk(self, pk, symalg, symkey):
        m = bytearray(self.int_to_bytes(symalg) + symkey)
        m += self.int_to_bytes(sum(bytearray(symkey)) % 65536, 2)

        if self.pkalg == PubKeyAlgorithm.RSAEncryptOrSign:
            encrypter = pk.keymaterial.__pubkey__().encrypt
            encargs = (bytes(m), padding.PKCS1v15(),)

        elif self.pkalg == PubKeyAlgorithm.ECDH:
            encrypter = pk
            encargs = (bytes(m),)

        else:
            raise NotImplementedError(self.pkalg)

        self.ct = self.ct.encrypt(encrypter, *encargs)
        self.update_hlen()

    def parse(self, packet):
        super(PKESessionKeyV3, self).parse(packet)
        self.encrypter = packet[:8]
        del packet[:8]

        self.pkalg = packet[0]
        del packet[0]

        if self.ct is not None:
            self.ct.parse(packet)

        else:  # pragma: no cover
            del packet[:(self.header.length - 18)]


class Signature(VersionedPacket):
    __typeid__ = 0x02
    __ver__ = 0


class SignatureV4(Signature):
    """
    5.2.3.  Version 4 Signature Packet Format

    The body of a version 4 Signature packet contains:

     - One-octet version number (4).

     - One-octet signature type.

     - One-octet public-key algorithm.

     - One-octet hash algorithm.

     - Two-octet scalar octet count for following hashed subpacket data.
       Note that this is the length in octets of all of the hashed
       subpackets; a pointer incremented by this number will skip over
       the hashed subpackets.

     - Hashed subpacket data set (zero or more subpackets).

     - Two-octet scalar octet count for the following unhashed subpacket
       data.  Note that this is the length in octets of all of the
       unhashed subpackets; a pointer incremented by this number will
       skip over the unhashed subpackets.

     - Unhashed subpacket data set (zero or more subpackets).

     - Two-octet field holding the left 16 bits of the signed hash
       value.

     - One or more multiprecision integers comprising the signature.
       This portion is algorithm specific, as described above.

    The concatenation of the data being signed and the signature data
    from the version number through the hashed subpacket data (inclusive)
    is hashed.  The resulting hash value is what is signed.  The left 16
    bits of the hash are included in the Signature packet to provide a
    quick test to reject some invalid signatures.

    There are two fields consisting of Signature subpackets.  The first
    field is hashed with the rest of the signature data, while the second
    is unhashed.  The second set of subpackets is not cryptographically
    protected by the signature and should include only advisory
    information.

    The algorithms for converting the hash function result to a signature
    are described in a section below.
    """
    __ver__ = 4

    @sdproperty
    def sigtype(self):
        return self._sigtype

    @sigtype.register(int)
    @sigtype.register(SignatureType)
    def sigtype_int(self, val):
        self._sigtype = SignatureType(val)

    @sdproperty
    def pubalg(self):
        return self._pubalg

    @pubalg.register(int)
    @pubalg.register(PubKeyAlgorithm)
    def pubalg_int(self, val):
        self._pubalg = PubKeyAlgorithm(val)

        sigs = {PubKeyAlgorithm.RSAEncryptOrSign: RSASignature,
                PubKeyAlgorithm.RSAEncrypt: RSASignature,
                PubKeyAlgorithm.RSASign: RSASignature,
                PubKeyAlgorithm.DSA: DSASignature,
                PubKeyAlgorithm.ECDSA: ECDSASignature, }

        if self.pubalg in sigs:
            self.signature = sigs[self.pubalg]()

    @sdproperty
    def halg(self):
        return self._halg

    @halg.register(int)
    @halg.register(HashAlgorithm)
    def halg_int(self, val):
        try:
            self._halg = HashAlgorithm(val)

        except ValueError:  # pragma: no cover
            self._halg = val

    @property
    def signature(self):
        return self._signature

    @signature.setter
    def signature(self, val):
        self._signature = val

    @property
    def signer(self):
        return self.subpackets['Issuer'][-1].issuer

    def __init__(self):
        super(Signature, self).__init__()
        self._sigtype = None
        self._pubalg = None
        self._halg = None
        self.subpackets = SubPackets()
        self.hash2 = bytearray(2)
        self.signature = None

    def __bytearray__(self):
        _bytes = bytearray()
        _bytes += super(Signature, self).__bytearray__()
        _bytes += self.int_to_bytes(self.sigtype)
        _bytes += self.int_to_bytes(self.pubalg)
        _bytes += self.int_to_bytes(self.halg)
        _bytes += self.subpackets.__bytearray__()
        _bytes += self.hash2
        _bytes += self.signature.__bytearray__()

        return _bytes

    def __copy__(self):
        spkt = SignatureV4()
        spkt.header = copy.copy(self.header)
        spkt._sigtype = self._sigtype
        spkt._pubalg = self._pubalg
        spkt._halg = self._halg

        spkt.subpackets = copy.copy(self.subpackets)
        spkt.hash2 = copy.copy(self.hash2)
        spkt.signature = copy.copy(self.signature)

        return spkt

    def update_hlen(self):
        self.subpackets.update_hlen()
        super(SignatureV4, self).update_hlen()

    def parse(self, packet):
        super(Signature, self).parse(packet)
        self.sigtype = packet[0]
        del packet[0]

        self.pubalg = packet[0]
        del packet[0]

        self.halg = packet[0]
        del packet[0]

        self.subpackets.parse(packet)

        self.hash2 = packet[:2]
        del packet[:2]

        self.signature.parse(packet)


class SKESessionKey(VersionedPacket):
    __typeid__ = 0x03
    __ver__ = 0

    @abc.abstractmethod
    def decrypt_sk(self, passphrase):
        raise NotImplementedError()

    @abc.abstractmethod
    def encrypt_sk(self, passphrase, sk):
        raise NotImplementedError()


class SKESessionKeyV4(SKESessionKey):
    """
    5.3.  Symmetric-Key Encrypted Session Key Packets (Tag 3)

    The Symmetric-Key Encrypted Session Key packet holds the
    symmetric-key encryption of a session key used to encrypt a message.
    Zero or more Public-Key Encrypted Session Key packets and/or
    Symmetric-Key Encrypted Session Key packets may precede a
    Symmetrically Encrypted Data packet that holds an encrypted message.
    The message is encrypted with a session key, and the session key is
    itself encrypted and stored in the Encrypted Session Key packet or
    the Symmetric-Key Encrypted Session Key packet.

    If the Symmetrically Encrypted Data packet is preceded by one or
    more Symmetric-Key Encrypted Session Key packets, each specifies a
    passphrase that may be used to decrypt the message.  This allows a
    message to be encrypted to a number of public keys, and also to one
    or more passphrases.  This packet type is new and is not generated
    by PGP 2.x or PGP 5.0.

    The body of this packet consists of:

     - A one-octet version number.  The only currently defined version
       is 4.

     - A one-octet number describing the symmetric algorithm used.

     - A string-to-key (S2K) specifier, length as defined above.

     - Optionally, the encrypted session key itself, which is decrypted
       with the string-to-key object.

    If the encrypted session key is not present (which can be detected
    on the basis of packet length and S2K specifier size), then the S2K
    algorithm applied to the passphrase produces the session key for
    decrypting the file, using the symmetric cipher algorithm from the
    Symmetric-Key Encrypted Session Key packet.

    If the encrypted session key is present, the result of applying the
    S2K algorithm to the passphrase is used to decrypt just that
    encrypted session key field, using CFB mode with an IV of all zeros.
    The decryption result consists of a one-octet algorithm identifier
    that specifies the symmetric-key encryption algorithm used to
    encrypt the following Symmetrically Encrypted Data packet, followed
    by the session key octets themselves.

    Note: because an all-zero IV is used for this decryption, the S2K
    specifier MUST use a salt value, either a Salted S2K or an
    Iterated-Salted S2K.  The salt value will ensure that the decryption
    key is not repeated even if the passphrase is reused.
    """
    __ver__ = 4

    @property
    def symalg(self):
        return self.s2k.encalg

    def __init__(self):
        super(SKESessionKeyV4, self).__init__()
        self.s2k = String2Key()
        self.ct = bytearray()

    def __bytearray__(self):
        _bytes = bytearray()
        _bytes += super(SKESessionKeyV4, self).__bytearray__()
        _bytes += self.s2k.__bytearray__()[1:]
        _bytes += self.ct
        return _bytes

    def __copy__(self):
        sk = self.__class__()
        sk.header = copy.copy(self.header)
        sk.s2k = copy.copy(self.s2k)
        sk.ct = self.ct[:]

        return sk

    def parse(self, packet):
        super(SKESessionKeyV4, self).parse(packet)
        # prepend a valid usage identifier so this parses correctly
        packet.insert(0, 255)
        self.s2k.parse(packet, iv=False)

        ctend = self.header.length - len(self.s2k)
        self.ct = packet[:ctend]
        del packet[:ctend]

    def decrypt_sk(self, passphrase):
        # derive the first session key from our passphrase
        sk = self.s2k.derive_key(passphrase)
        del passphrase

        # if there is no ciphertext, then the first session key is the session key being used
        if len(self.ct) == 0:
            return self.symalg, sk

        # otherwise, we now need to decrypt the encrypted session key
        m = bytearray(_decrypt(bytes(self.ct), sk, self.symalg))
        del sk

        symalg = SymmetricKeyAlgorithm(m[0])
        del m[0]

        return symalg, bytes(m)

    def encrypt_sk(self, passphrase, sk):
        # generate the salt and derive the key to encrypt sk with from it
        self.s2k.salt = bytearray(os.urandom(8))
        esk = self.s2k.derive_key(passphrase)
        del passphrase

        self.ct = _encrypt(self.int_to_bytes(self.symalg) + sk, esk, self.symalg)

        # update header length and return sk
        self.update_hlen()


class OnePassSignature(VersionedPacket):
    __typeid__ = 0x04
    __ver__ = 0


class OnePassSignatureV3(OnePassSignature):
    """
    5.4.  One-Pass Signature Packets (Tag 4)

    The One-Pass Signature packet precedes the signed data and contains
    enough information to allow the receiver to begin calculating any
    hashes needed to verify the signature.  It allows the Signature
    packet to be placed at the end of the message, so that the signer
    can compute the entire signed message in one pass.

    A One-Pass Signature does not interoperate with PGP 2.6.x or
    earlier.

    The body of this packet consists of:

     - A one-octet version number.  The current version is 3.

     - A one-octet signature type.  Signature types are described in
       Section 5.2.1.

     - A one-octet number describing the hash algorithm used.

     - A one-octet number describing the public-key algorithm used.

     - An eight-octet number holding the Key ID of the signing key.

     - A one-octet number holding a flag showing whether the signature
       is nested.  A zero value indicates that the next packet is
       another One-Pass Signature packet that describes another
       signature to be applied to the same message data.

    Note that if a message contains more than one one-pass signature,
    then the Signature packets bracket the message; that is, the first
    Signature packet after the message corresponds to the last one-pass
    packet and the final Signature packet corresponds to the first
    one-pass packet.
    """
    __ver__ = 3

    @sdproperty
    def sigtype(self):
        return self._sigtype

    @sigtype.register(int)
    @sigtype.register(SignatureType)
    def sigtype_int(self, val):
        self._sigtype = SignatureType(val)

    @sdproperty
    def pubalg(self):
        return self._pubalg

    @pubalg.register(int)
    @pubalg.register(PubKeyAlgorithm)
    def pubalg_int(self, val):
        self._pubalg = PubKeyAlgorithm(val)
        if self._pubalg in [PubKeyAlgorithm.RSAEncryptOrSign, PubKeyAlgorithm.RSAEncrypt, PubKeyAlgorithm.RSASign]:
            self.signature = RSASignature()

        elif self._pubalg == PubKeyAlgorithm.DSA:
            self.signature = DSASignature()

    @sdproperty
    def halg(self):
        return self._halg

    @halg.register(int)
    @halg.register(HashAlgorithm)
    def halg_int(self, val):
        try:
            self._halg = HashAlgorithm(val)

        except ValueError:  # pragma: no cover
            self._halg = val

    @sdproperty
    def signer(self):
        return self._signer

    @signer.register(str)
    @signer.register(six.text_type)
    def signer_str(self, val):
        self._signer = val

    @signer.register(bytearray)
    def signer_bin(self, val):
        self._signer = binascii.hexlify(val).upper().decode('latin-1')

    def __init__(self):
        super(OnePassSignatureV3, self).__init__()
        self._sigtype = None
        self._halg = None
        self._pubalg = None
        self._signer = b'\x00' * 8
        self.nested = False

    def __bytearray__(self):
        _bytes = bytearray()
        _bytes += super(OnePassSignatureV3, self).__bytearray__()
        _bytes += bytearray([self.sigtype])
        _bytes += bytearray([self.halg])
        _bytes += bytearray([self.pubalg])
        _bytes += binascii.unhexlify(six.b(self.signer))
        _bytes += bytearray([int(self.nested)])
        return _bytes

    def parse(self, packet):
        super(OnePassSignatureV3, self).parse(packet)
        self.sigtype = packet[0]
        del packet[0]

        self.halg = packet[0]
        del packet[0]

        self.pubalg = packet[0]
        del packet[0]

        self.signer = packet[:8]
        del packet[:8]

        self.nested = (packet[0] == 1)
        del packet[0]


class PrivKey(VersionedPacket, Primary, Private):
    __typeid__ = 0x05
    __ver__ = 0


class PubKey(VersionedPacket, Primary, Public):
    __typeid__ = 0x06
    __ver__ = 0

    @abc.abstractproperty
    def fingerprint(self):
        """compute and return the fingerprint of the key"""


class PubKeyV4(PubKey):
    __ver__ = 4

    @sdproperty
    def created(self):
        return self._created

    @created.register(datetime)
    def created_datetime(self, val):
        self._created = val

    @created.register(int)
    def created_int(self, val):
        self.created = datetime.utcfromtimestamp(val)

    @created.register(bytes)
    @created.register(bytearray)
    def created_bin(self, val):
        self.created = self.bytes_to_int(val)

    @sdproperty
    def pkalg(self):
        return self._pkalg

    @pkalg.register(int)
    @pkalg.register(PubKeyAlgorithm)
    def pkalg_int(self, val):
        self._pkalg = PubKeyAlgorithm(val)

        _c = {
            # True means public
            (True, PubKeyAlgorithm.RSAEncryptOrSign): RSAPub,
            (True, PubKeyAlgorithm.RSAEncrypt): RSAPub,
            (True, PubKeyAlgorithm.RSASign): RSAPub,
            (True, PubKeyAlgorithm.DSA): DSAPub,
            (True, PubKeyAlgorithm.ElGamal): ElGPub,
            (True, PubKeyAlgorithm.FormerlyElGamalEncryptOrSign): ElGPub,
            (True, PubKeyAlgorithm.ECDSA): ECDSAPub,
            (True, PubKeyAlgorithm.ECDH): ECDHPub,
            # False means private
            (False, PubKeyAlgorithm.RSAEncryptOrSign): RSAPriv,
            (False, PubKeyAlgorithm.RSAEncrypt): RSAPriv,
            (False, PubKeyAlgorithm.RSASign): RSAPriv,
            (False, PubKeyAlgorithm.DSA): DSAPriv,
            (False, PubKeyAlgorithm.ElGamal): ElGPriv,
            (False, PubKeyAlgorithm.FormerlyElGamalEncryptOrSign): ElGPriv,
            (False, PubKeyAlgorithm.ECDSA): ECDSAPriv,
            (False, PubKeyAlgorithm.ECDH): ECDHPriv,
        }

        k = (self.public, self.pkalg)
        km = _c.get(k, None)

        self.keymaterial = (km or (OpaquePubKey if self.public else OpaquePrivKey))()

        # km = _c.get(k, None)
        # self.keymaterial = km() if km is not None else km

    @property
    def public(self):
        return isinstance(self, PubKey) and not isinstance(self, PrivKey)

    @property
    def fingerprint(self):
        # A V4 fingerprint is the 160-bit SHA-1 hash of the octet 0x99, followed by the two-octet packet length,
        # followed by the entire Public-Key packet starting with the version field.  The Key ID is the
        # low-order 64 bits of the fingerprint.
        fp = hashlib.new('sha1')

        plen = self.keymaterial.publen()
        bcde_len = self.int_to_bytes(6 + plen, 2)

        # a.1) 0x99 (1 octet)
        # a.2) high-order length octet
        # a.3) low-order length octet
        fp.update(b'\x99' + bcde_len[:1] + bcde_len[-1:])
        # b) version number = 4 (1 octet);
        fp.update(b'\x04')
        # c) timestamp of key creation (4 octets);
        fp.update(self.int_to_bytes(calendar.timegm(self.created.timetuple()), 4))
        # d) algorithm (1 octet): 17 = DSA (example);
        fp.update(self.int_to_bytes(self.pkalg))
        # e) Algorithm-specific fields.
        fp.update(self.keymaterial.__bytearray__()[:plen])

        # and return the digest
        return Fingerprint(fp.hexdigest().upper())

    def __init__(self):
        super(PubKeyV4, self).__init__()
        self.created = datetime.utcnow()
        self.pkalg = 0
        self.keymaterial = None

    def __bytearray__(self):
        _bytes = bytearray()
        _bytes += super(PubKeyV4, self).__bytearray__()
        _bytes += self.int_to_bytes(calendar.timegm(self.created.timetuple()), 4)
        _bytes += self.int_to_bytes(self.pkalg)
        _bytes += self.keymaterial.__bytearray__()
        return _bytes

    def __copy__(self):
        pk = self.__class__()
        pk.header = copy.copy(self.header)
        pk.created = self.created
        pk.pkalg = self.pkalg
        pk.keymaterial = copy.copy(self.keymaterial)

        return pk

    def verify(self, subj, sigbytes, hash_alg):
        return self.keymaterial.verify(subj, sigbytes, hash_alg)

    def parse(self, packet):
        super(PubKeyV4, self).parse(packet)

        self.created = packet[:4]
        del packet[:4]

        self.pkalg = packet[0]
        del packet[0]

        # bound keymaterial to the remaining length of the packet
        pend = self.header.length - 6
        self.keymaterial.parse(packet[:pend])
        del packet[:pend]


class PrivKeyV4(PrivKey, PubKeyV4):
    __ver__ = 4

    @classmethod
    def new(cls, key_algorithm, key_size):
        # build a key packet
        pk = PrivKeyV4()
        pk.pkalg = key_algorithm
        if pk.keymaterial is None:
            raise NotImplementedError(key_algorithm)
        pk.keymaterial._generate(key_size)
        pk.update_hlen()
        return pk

    def pubkey(self):
        # return a copy of ourselves, but just the public half
        pk = PubKeyV4() if not isinstance(self, PrivSubKeyV4) else PubSubKeyV4()
        pk.created = self.created
        pk.pkalg = self.pkalg

        # copy over MPIs
        for pm in self.keymaterial.__pubfields__:
            setattr(pk.keymaterial, pm, copy.copy(getattr(self.keymaterial, pm)))

        if self.pkalg == PubKeyAlgorithm.ECDSA:
            pk.keymaterial.oid = self.keymaterial.oid

        if self.pkalg == PubKeyAlgorithm.ECDH:
            pk.keymaterial.oid = self.keymaterial.oid
            pk.keymaterial.kdf = copy.copy(self.keymaterial.kdf)

        pk.update_hlen()
        return pk

    @property
    def protected(self):
        return bool(self.keymaterial.s2k)

    @property
    def unlocked(self):
        if self.protected:
            return 0 not in list(self.keymaterial)
        return True  # pragma: no cover

    def protect(self, passphrase, enc_alg, hash_alg):
        self.keymaterial.encrypt_keyblob(passphrase, enc_alg, hash_alg)
        del passphrase
        self.update_hlen()

    def unprotect(self, passphrase):
        self.keymaterial.decrypt_keyblob(passphrase)
        del passphrase

    def sign(self, sigdata, hash_alg):
        return self.keymaterial.sign(sigdata, hash_alg)


class PrivSubKey(VersionedPacket, Sub, Private):
    __typeid__ = 0x07
    __ver__ = 0


class PrivSubKeyV4(PrivSubKey, PrivKeyV4):
    __ver__ = 4


class CompressedData(Packet):
    """
    5.6.  Compressed Data Packet (Tag 8)

    The Compressed Data packet contains compressed data.  Typically, this
    packet is found as the contents of an encrypted packet, or following
    a Signature or One-Pass Signature packet, and contains a literal data
    packet.

    The body of this packet consists of:

     - One octet that gives the algorithm used to compress the packet.

     - Compressed data, which makes up the remainder of the packet.

    A Compressed Data Packet's body contains an block that compresses
    some set of packets.  See section "Packet Composition" for details on
    how messages are formed.

    ZIP-compressed packets are compressed with raw RFC 1951 [RFC1951]
    DEFLATE blocks.  Note that PGP V2.6 uses 13 bits of compression.  If
    an implementation uses more bits of compression, PGP V2.6 cannot
    decompress it.

    ZLIB-compressed packets are compressed with RFC 1950 [RFC1950] ZLIB-
    style blocks.

    BZip2-compressed packets are compressed using the BZip2 [BZ2]
    algorithm.
    """
    __typeid__ = 0x08

    @sdproperty
    def calg(self):
        return self._calg

    @calg.register(int)
    @calg.register(CompressionAlgorithm)
    def calg_int(self, val):
        self._calg = CompressionAlgorithm(val)

    def __init__(self):
        super(CompressedData, self).__init__()
        self._calg = None
        self.packets = []

    def __bytearray__(self):
        _bytes = bytearray()
        _bytes += super(CompressedData, self).__bytearray__()
        _bytes += bytearray([self.calg])

        _pb = bytearray()
        for pkt in self.packets:
            _pb += pkt.__bytearray__()
        _bytes += self.calg.compress(bytes(_pb))

        return _bytes

    def parse(self, packet):
        super(CompressedData, self).parse(packet)
        self.calg = packet[0]
        del packet[0]

        cdata = bytearray(self.calg.decompress(packet[:self.header.length - 1]))
        del packet[:self.header.length - 1]

        while len(cdata) > 0:
            self.packets.append(Packet(cdata))


class SKEData(Packet):
    """
    5.7.  Symmetrically Encrypted Data Packet (Tag 9)

    The Symmetrically Encrypted Data packet contains data encrypted with
    a symmetric-key algorithm.  When it has been decrypted, it contains
    other packets (usually a literal data packet or compressed data
    packet, but in theory other Symmetrically Encrypted Data packets or
    sequences of packets that form whole OpenPGP messages).

    The body of this packet consists of:

     - Encrypted data, the output of the selected symmetric-key cipher
       operating in OpenPGP's variant of Cipher Feedback (CFB) mode.

    The symmetric cipher used may be specified in a Public-Key or
    Symmetric-Key Encrypted Session Key packet that precedes the
    Symmetrically Encrypted Data packet.  In that case, the cipher
    algorithm octet is prefixed to the session key before it is
    encrypted.  If no packets of these types precede the encrypted data,
    the IDEA algorithm is used with the session key calculated as the MD5
    hash of the passphrase, though this use is deprecated.

    The data is encrypted in CFB mode, with a CFB shift size equal to the
    cipher's block size.  The Initial Vector (IV) is specified as all
    zeros.  Instead of using an IV, OpenPGP prefixes a string of length
    equal to the block size of the cipher plus two to the data before it
    is encrypted.  The first block-size octets (for example, 8 octets for
    a 64-bit block length) are random, and the following two octets are
    copies of the last two octets of the IV.  For example, in an 8-octet
    block, octet 9 is a repeat of octet 7, and octet 10 is a repeat of
    octet 8.  In a cipher of length 16, octet 17 is a repeat of octet 15
    and octet 18 is a repeat of octet 16.  As a pedantic clarification,
    in both these examples, we consider the first octet to be numbered 1.

    After encrypting the first block-size-plus-two octets, the CFB state
    is resynchronized.  The last block-size octets of ciphertext are
    passed through the cipher and the block boundary is reset.

    The repetition of 16 bits in the random data prefixed to the message
    allows the receiver to immediately check whether the session key is
    incorrect.  See the "Security Considerations" section for hints on
    the proper use of this "quick check".
    """
    __typeid__ = 0x09

    def __init__(self):
        super(SKEData, self).__init__()
        self.ct = bytearray()

    def __bytearray__(self):
        _bytes = bytearray()
        _bytes += super(SKEData, self).__bytearray__()
        _bytes += self.ct
        return _bytes

    def __copy__(self):
        skd = self.__class__()
        skd.ct = self.ct[:]
        return skd

    def parse(self, packet):
        super(SKEData, self).parse(packet)
        self.ct = packet[:self.header.length]
        del packet[:self.header.length]

    def decrypt(self, key, alg):  # pragma: no cover
        pt = _decrypt(bytes(self.ct), bytes(key), alg)

        iv = bytes(pt[:alg.block_size // 8])
        del pt[:alg.block_size // 8]

        ivl2 = bytes(pt[:2])
        del pt[:2]

        if not constant_time.bytes_eq(iv[-2:], ivl2):
            raise PGPDecryptionError("Decryption failed")

        return pt


class Marker(Packet):
    __typeid__ = 0x0a

    def __init__(self):
        super(Marker, self).__init__()
        self.data = b'PGP'

    def __bytearray__(self):
        _bytes = bytearray()
        _bytes += super(Marker, self).__bytearray__()
        _bytes += self.data
        return _bytes

    def parse(self, packet):
        super(Marker, self).parse(packet)
        self.data = packet[:self.header.length]
        del packet[:self.header.length]


class LiteralData(Packet):
    """
    5.9.  Literal Data Packet (Tag 11)

    A Literal Data packet contains the body of a message; data that is
    not to be further interpreted.

    The body of this packet consists of:

     - A one-octet field that describes how the data is formatted.

    If it is a 'b' (0x62), then the Literal packet contains binary data.
    If it is a 't' (0x74), then it contains text data, and thus may need
    line ends converted to local form, or other text-mode changes.  The
    tag 'u' (0x75) means the same as 't', but also indicates that
    implementation believes that the literal data contains UTF-8 text.

    Early versions of PGP also defined a value of 'l' as a 'local' mode
    for machine-local conversions.  RFC 1991 [RFC1991] incorrectly stated
    this local mode flag as '1' (ASCII numeral one).  Both of these local
    modes are deprecated.

     - File name as a string (one-octet length, followed by a file
       name).  This may be a zero-length string.  Commonly, if the
       source of the encrypted data is a file, this will be the name of
       the encrypted file.  An implementation MAY consider the file name
       in the Literal packet to be a more authoritative name than the
       actual file name.

    If the special name "_CONSOLE" is used, the message is considered to
    be "for your eyes only".  This advises that the message data is
    unusually sensitive, and the receiving program should process it more
    carefully, perhaps avoiding storing the received data to disk, for
    example.

     - A four-octet number that indicates a date associated with the
       literal data.  Commonly, the date might be the modification date
       of a file, or the time the packet was created, or a zero that
       indicates no specific time.

     - The remainder of the packet is literal data.

       Text data is stored with <CR><LF> text endings (i.e., network-
       normal line endings).  These should be converted to native line
       endings by the receiving software.
    """
    __typeid__ = 0x0B

    @sdproperty
    def mtime(self):
        return self._mtime

    @mtime.register(datetime)
    def mtime_datetime(self, val):
        self._mtime = val

    @mtime.register(int)
    def mtime_int(self, val):
        self.mtime = datetime.utcfromtimestamp(val)

    @mtime.register(bytes)
    @mtime.register(bytearray)
    def mtime_bin(self, val):
        self.mtime = self.bytes_to_int(val)

    @property
    def contents(self):
        if self.format == 't':
            return self._contents.decode('latin-1')

        if self.format == 'u':
            return self._contents.decode('utf-8')

        return self._contents

    def __init__(self):
        super(LiteralData, self).__init__()
        self.format = 'b'
        self.filename = ''
        self.mtime = datetime.utcnow()
        self._contents = bytearray()

    def __bytearray__(self):
        _bytes = bytearray()
        _bytes += super(LiteralData, self).__bytearray__()
        _bytes += self.format.encode('latin-1')
        _bytes += bytearray([len(self.filename)])
        _bytes += self.filename.encode('latin-1')
        _bytes += self.int_to_bytes(calendar.timegm(self.mtime.timetuple()), 4)
        _bytes += self._contents
        return _bytes

    def __copy__(self):
        pkt = LiteralData()
        pkt.header = copy.copy(self.header)
        pkt.format = self.format
        pkt.filename = self.filename
        pkt.mtime = self.mtime
        pkt._contents = self._contents[:]

        return pkt

    def parse(self, packet):
        super(LiteralData, self).parse(packet)
        self.format = chr(packet[0])
        del packet[0]

        fnl = packet[0]
        del packet[0]

        self.filename = packet[:fnl].decode()
        del packet[:fnl]

        self.mtime = packet[:4]
        del packet[:4]

        self._contents = packet[:self.header.length - (6 + fnl)]
        del packet[:self.header.length - (6 + fnl)]


class Trust(Packet):
    """
    5.10.  Trust Packet (Tag 12)

    The Trust packet is used only within keyrings and is not normally
    exported.  Trust packets contain data that record the user's
    specifications of which key holders are trustworthy introducers,
    along with other information that implementing software uses for
    trust information.  The format of Trust packets is defined by a given
    implementation.

    Trust packets SHOULD NOT be emitted to output streams that are
    transferred to other users, and they SHOULD be ignored on any input
    other than local keyring files.
    """
    __typeid__ = 0x0C

    @sdproperty
    def trustlevel(self):
        return self._trustlevel

    @trustlevel.register(int)
    @trustlevel.register(TrustLevel)
    def trustlevel_int(self, val):
        self._trustlevel = TrustLevel(val & 0x0F)

    @sdproperty
    def trustflags(self):
        return self._trustflags

    @trustflags.register(list)
    def trustflags_list(self, val):
        self._trustflags = val

    @trustflags.register(int)
    def trustflags_int(self, val):
        self._trustflags = TrustFlags & val

    def __init__(self):
        super(Trust, self).__init__()
        self.trustlevel = TrustLevel.Unknown
        self.trustflags = []

    def __bytearray__(self):
        _bytes = bytearray()
        _bytes += super(Trust, self).__bytearray__()
        _bytes += self.int_to_bytes(self.trustlevel + sum(self.trustflags), 2)
        return _bytes

    def parse(self, packet):
        super(Trust, self).parse(packet)
        # self.trustlevel = packet[0] & 0x1f
        t = self.bytes_to_int(packet[:2])
        del packet[:2]

        self.trustlevel = t
        self.trustflags = t


class UserID(Packet):
    """
    5.11.  User ID Packet (Tag 13)

    A User ID packet consists of UTF-8 text that is intended to represent
    the name and email address of the key holder.  By convention, it
    includes an RFC 2822 [RFC2822] mail name-addr, but there are no
    restrictions on its content.  The packet length in the header
    specifies the length of the User ID.
    """
    __typeid__ = 0x0D

    def __init__(self):
        super(UserID, self).__init__()
        self.name = ""
        self.comment = ""
        self.email = ""

    def __bytearray__(self):
        _bytes = bytearray()
        _bytes += super(UserID, self).__bytearray__()
        _bytes += self.text_to_bytes(self.name)
        if self.comment:
            _bytes += b' (' + self.text_to_bytes(self.comment) + b')'

        if self.email:
            _bytes += b' <' + self.text_to_bytes(self.email) + b'>'

        return _bytes

    def __copy__(self):
        uid = UserID()
        uid.header = copy.copy(self.header)
        uid.name = self.name
        uid.comment = self.comment
        uid.email = self.email
        return uid

    def parse(self, packet):
        super(UserID, self).parse(packet)

        uid_text = packet[:self.header.length].decode('latin-1')
        del packet[:self.header.length]

        # came across a UID packet with no payload. If that happens, don't bother trying to parse anything!
        if self.header.length > 0:
            uid = re.match(r"""^
                               # name should always match something
                               (?P<name>.+?)
                               # comment *optionally* matches text in parens following name
                               # this should never come after email and must be followed immediately by
                               # either the email field, or the end of the packet.
                               (\ \((?P<comment>.+?)\)(?=(\ <|$)))?
                               # email *optionally* matches text in angle brackets following name or comment
                               # this should never come before a comment, if comment exists,
                               # but can immediately follow name if comment does not exist
                               (\ <(?P<email>.+)>)?
                               $
                            """, uid_text, flags=re.VERBOSE).groupdict()

            self.name = uid['name']
            self.comment = uid['comment'] or ""
            self.email = uid['email'] or ""


class PubSubKey(VersionedPacket, Sub, Public):
    __typeid__ = 0x0E
    __ver__ = 0


class PubSubKeyV4(PubSubKey, PubKeyV4):
    __ver__ = 4


class UserAttribute(Packet):
    """
    5.12.  User Attribute Packet (Tag 17)

    The User Attribute packet is a variation of the User ID packet.  It
    is capable of storing more types of data than the User ID packet,
    which is limited to text.  Like the User ID packet, a User Attribute
    packet may be certified by the key owner ("self-signed") or any other
    key owner who cares to certify it.  Except as noted, a User Attribute
    packet may be used anywhere that a User ID packet may be used.

    While User Attribute packets are not a required part of the OpenPGP
    standard, implementations SHOULD provide at least enough
    compatibility to properly handle a certification signature on the
    User Attribute packet.  A simple way to do this is by treating the
    User Attribute packet as a User ID packet with opaque contents, but
    an implementation may use any method desired.

    The User Attribute packet is made up of one or more attribute
    subpackets.  Each subpacket consists of a subpacket header and a
    body.  The header consists of:

     - the subpacket length (1, 2, or 5 octets)

     - the subpacket type (1 octet)

    and is followed by the subpacket specific data.

    The only currently defined subpacket type is 1, signifying an image.
    An implementation SHOULD ignore any subpacket of a type that it does
    not recognize.  Subpacket types 100 through 110 are reserved for
    private or experimental use.
    """
    __typeid__ = 0x11

    @property
    def image(self):
        if 'Image' not in self.subpackets:
            self.subpackets.addnew('Image')
        return next(iter(self.subpackets['Image']))

    def __init__(self):
        super(UserAttribute, self).__init__()
        self.subpackets = UserAttributeSubPackets()

    def __bytearray__(self):
        _bytes = bytearray()
        _bytes += super(UserAttribute, self).__bytearray__()
        _bytes += self.subpackets.__bytearray__()
        return _bytes

    def parse(self, packet):
        super(UserAttribute, self).parse(packet)

        plen = len(packet)
        while self.header.length > (plen - len(packet)):
            self.subpackets.parse(packet)

    def update_hlen(self):
        self.subpackets.update_hlen()
        super(UserAttribute, self).update_hlen()


class IntegrityProtectedSKEData(VersionedPacket):
    __typeid__ = 0x12
    __ver__ = 0


class IntegrityProtectedSKEDataV1(IntegrityProtectedSKEData):
    """
    5.13.  Sym. Encrypted Integrity Protected Data Packet (Tag 18)

    The Symmetrically Encrypted Integrity Protected Data packet is a
    variant of the Symmetrically Encrypted Data packet.  It is a new
    feature created for OpenPGP that addresses the problem of detecting a
    modification to encrypted data.  It is used in combination with a
    Modification Detection Code packet.

    There is a corresponding feature in the features Signature subpacket
    that denotes that an implementation can properly use this packet
    type.  An implementation MUST support decrypting these packets and
    SHOULD prefer generating them to the older Symmetrically Encrypted
    Data packet when possible.  Since this data packet protects against
    modification attacks, this standard encourages its proliferation.
    While blanket adoption of this data packet would create
    interoperability problems, rapid adoption is nevertheless important.
    An implementation SHOULD specifically denote support for this packet,
    but it MAY infer it from other mechanisms.

    For example, an implementation might infer from the use of a cipher
    such as Advanced Encryption Standard (AES) or Twofish that a user
    supports this feature.  It might place in the unhashed portion of
    another user's key signature a Features subpacket.  It might also
    present a user with an opportunity to regenerate their own self-
    signature with a Features subpacket.

    This packet contains data encrypted with a symmetric-key algorithm
    and protected against modification by the SHA-1 hash algorithm.  When
    it has been decrypted, it will typically contain other packets (often
    a Literal Data packet or Compressed Data packet).  The last decrypted
    packet in this packet's payload MUST be a Modification Detection Code
    packet.

    The body of this packet consists of:

     - A one-octet version number.  The only currently defined value is
       1.

     - Encrypted data, the output of the selected symmetric-key cipher
       operating in Cipher Feedback mode with shift amount equal to the
       block size of the cipher (CFB-n where n is the block size).

    The symmetric cipher used MUST be specified in a Public-Key or
    Symmetric-Key Encrypted Session Key packet that precedes the
    Symmetrically Encrypted Data packet.  In either case, the cipher
    algorithm octet is prefixed to the session key before it is
    encrypted.

    The data is encrypted in CFB mode, with a CFB shift size equal to the
    cipher's block size.  The Initial Vector (IV) is specified as all
    zeros.  Instead of using an IV, OpenPGP prefixes an octet string to
    the data before it is encrypted.  The length of the octet string
    equals the block size of the cipher in octets, plus two.  The first
    octets in the group, of length equal to the block size of the cipher,
    are random; the last two octets are each copies of their 2nd
    preceding octet.  For example, with a cipher whose block size is 128
    bits or 16 octets, the prefix data will contain 16 random octets,
    then two more octets, which are copies of the 15th and 16th octets,
    respectively.  Unlike the Symmetrically Encrypted Data Packet, no
    special CFB resynchronization is done after encrypting this prefix
    data.  See "OpenPGP CFB Mode" below for more details.

    The repetition of 16 bits in the random data prefixed to the message
    allows the receiver to immediately check whether the session key is
    incorrect.

    The plaintext of the data to be encrypted is passed through the SHA-1
    hash function, and the result of the hash is appended to the
    plaintext in a Modification Detection Code packet.  The input to the
    hash function includes the prefix data described above; it includes
    all of the plaintext, and then also includes two octets of values
    0xD3, 0x14.  These represent the encoding of a Modification Detection
    Code packet tag and length field of 20 octets.

    The resulting hash value is stored in a Modification Detection Code
    (MDC) packet, which MUST use the two octet encoding just given to
    represent its tag and length field.  The body of the MDC packet is
    the 20-octet output of the SHA-1 hash.

    The Modification Detection Code packet is appended to the plaintext
    and encrypted along with the plaintext using the same CFB context.

    During decryption, the plaintext data should be hashed with SHA-1,
    including the prefix data as well as the packet tag and length field
    of the Modification Detection Code packet.  The body of the MDC
    packet, upon decryption, is compared with the result of the SHA-1
    hash.

    Any failure of the MDC indicates that the message has been modified
    and MUST be treated as a security problem.  Failures include a
    difference in the hash values, but also the absence of an MDC packet,
    or an MDC packet in any position other than the end of the plaintext.
    Any failure SHOULD be reported to the user.

    Note: future designs of new versions of this packet should consider
    rollback attacks since it will be possible for an attacker to change
    the version back to 1.
    """
    __ver__ = 1

    def __init__(self):
        super(IntegrityProtectedSKEDataV1, self).__init__()
        self.ct = bytearray()

    def __bytearray__(self):
        _bytes = bytearray()
        _bytes += super(IntegrityProtectedSKEDataV1, self).__bytearray__()
        _bytes += self.ct
        return _bytes

    def __copy__(self):
        skd = self.__class__()
        skd.ct = self.ct[:]
        return skd

    def parse(self, packet):
        super(IntegrityProtectedSKEDataV1, self).parse(packet)
        self.ct = packet[:self.header.length - 1]
        del packet[:self.header.length - 1]

    def encrypt(self, key, alg, data):
        iv = alg.gen_iv()
        data = iv + iv[-2:] + data

        mdc = MDC()
        mdc.mdc = binascii.hexlify(hashlib.new('SHA1', data + b'\xd3\x14').digest())
        mdc.update_hlen()

        data += mdc.__bytes__()
        self.ct = _encrypt(data, key, alg)
        self.update_hlen()

    def decrypt(self, key, alg):
        # iv, ivl2, pt = super(IntegrityProtectedSKEDataV1, self).decrypt(key, alg)
        pt = _decrypt(bytes(self.ct), bytes(key), alg)

        # do the MDC checks
        _expected_mdcbytes = b'\xd3\x14' + hashlib.new('SHA1', pt[:-20]).digest()
        if not constant_time.bytes_eq(bytes(pt[-22:]), _expected_mdcbytes):
            raise PGPDecryptionError("Decryption failed")  # pragma: no cover

        iv = bytes(pt[:alg.block_size // 8])
        del pt[:alg.block_size // 8]

        ivl2 = bytes(pt[:2])
        del pt[:2]

        if not constant_time.bytes_eq(iv[-2:], ivl2):
            raise PGPDecryptionError("Decryption failed")  # pragma: no cover

        return pt


class MDC(Packet):
    """
    5.14.  Modification Detection Code Packet (Tag 19)

    The Modification Detection Code packet contains a SHA-1 hash of
    plaintext data, which is used to detect message modification.  It is
    only used with a Symmetrically Encrypted Integrity Protected Data
    packet.  The Modification Detection Code packet MUST be the last
    packet in the plaintext data that is encrypted in the Symmetrically
    Encrypted Integrity Protected Data packet, and MUST appear in no
    other place.

    A Modification Detection Code packet MUST have a length of 20 octets.
    The body of this packet consists of:

     - A 20-octet SHA-1 hash of the preceding plaintext data of the
       Symmetrically Encrypted Integrity Protected Data packet,
       including prefix data, the tag octet, and length octet of the
       Modification Detection Code packet.

    Note that the Modification Detection Code packet MUST always use a
    new format encoding of the packet tag, and a one-octet encoding of
    the packet length.  The reason for this is that the hashing rules for
    modification detection include a one-octet tag and one-octet length
    in the data hash.  While this is a bit restrictive, it reduces
    complexity.
    """
    __typeid__ = 0x13

    def __init__(self):
        super(MDC, self).__init__()
        self.mdc = ''

    def __bytearray__(self):
        return super(MDC, self).__bytearray__() + binascii.unhexlify(self.mdc)

    def parse(self, packet):
        super(MDC, self).parse(packet)
        self.mdc = binascii.hexlify(packet[:20])
        del packet[:20]