text: # This file is part of Scapy
  # Copyright (C) 2007, 2008, 2009 Arnaud Ebalard
  # 2015, 2016, 2017 Maxence Tury
  # This program is published under a GPLv2 license
  
  """
  Common TLS fields & bindings.
  
  This module covers the record layer, along with the ChangeCipherSpec, Alert and
  ApplicationData submessages. For the Handshake type, see tls_handshake.py.
  
  See the TLS class documentation for more information.
  """
  
  import struct
  
  from scapy.config import conf
  from scapy.error import log_runtime
  from scapy.fields import ByteEnumField, PacketListField, StrField
  from scapy.compat import raw, chb, orb
  from scapy.utils import randstring
  from scapy.packet import Raw, Padding, bind_layers
  from scapy.layers.inet import TCP
  from scapy.layers.tls.session import _GenericTLSSessionInheritance
  from scapy.layers.tls.handshake import (_tls_handshake_cls, _TLSHandshake,
                                          TLS13ServerHello)
  from scapy.layers.tls.basefields import (_TLSVersionField, _tls_version,
                                           _TLSIVField, _TLSMACField,
                                           _TLSPadField, _TLSPadLenField,
                                           _TLSLengthField, _tls_type)
  from scapy.layers.tls.crypto.pkcs1 import pkcs_i2osp
  from scapy.layers.tls.crypto.cipher_aead import AEADTagError
  from scapy.layers.tls.crypto.cipher_stream import Cipher_NULL
  from scapy.layers.tls.crypto.ciphers import CipherError
  from scapy.layers.tls.crypto.h_mac import HMACError
  import scapy.modules.six as six
  if conf.crypto_valid_advanced:
      from scapy.layers.tls.crypto.cipher_aead import Cipher_CHACHA20_POLY1305
  
  # Util
  
  
  def _tls_version_check(version, min):
      """Returns if version >= min, or False if version == None"""
      if version is None:
          return False
      return version >= min
  
  ###############################################################################
  #   TLS Record Protocol                                                       #
  ###############################################################################
  
  
  class _TLSEncryptedContent(Raw):
      """
      When the content of a TLS record (more precisely, a TLSCiphertext) could
      not be deciphered, we use this class to represent the encrypted data.
      The MAC will still be parsed from the whole message, even though it could
      not been verified. When present (depending on cipher type and protocol
      version), the nonce_explicit, IV and/or padding will also be parsed.
      """
      name = "Encrypted Content"
  
  
  class _TLSMsgListField(PacketListField):
      """
      This is the actual content of the TLS record. As a TLS record may pack
      multiple sublayer messages (notably, several handshake messages),
      we inherit from PacketListField.
      """
  
      def __init__(self, name, default, length_from=None):
          if not length_from:
              length_from = self._get_length
          super(_TLSMsgListField, self).__init__(name, default, cls=None,
                                                 length_from=length_from)
  
      def _get_length(self, pkt):
          if pkt.deciphered_len is None:
              return pkt.len
          return pkt.deciphered_len
  
      def m2i(self, pkt, m):
          """
          Try to parse one of the TLS subprotocols (ccs, alert, handshake or
          application_data). This is used inside a loop managed by .getfield().
          """
          cls = Raw
          if pkt.type == 22:
              if len(m) >= 1:
                  msgtype = orb(m[0])
                  cls = _tls_handshake_cls.get(msgtype, Raw)
          elif pkt.type == 20:
              cls = TLSChangeCipherSpec
          elif pkt.type == 21:
              cls = TLSAlert
          elif pkt.type == 23:
              cls = TLSApplicationData
  
          if cls is Raw:
              return Raw(m)
          else:
              try:
                  return cls(m, tls_session=pkt.tls_session)
              except Exception:
                  if conf.debug_dissector:
                      raise
                  return Raw(m)
  
      def getfield(self, pkt, s):
          """
          If the decryption of the content did not fail with a CipherError,
          we begin a loop on the clear content in order to get as much messages
          as possible, of the type advertised in the record header. This is
          notably important for several TLS handshake implementations, which
          may for instance pack a server_hello, a certificate, a
          server_key_exchange and a server_hello_done, all in one record.
          Each parsed message may update the TLS context through their method
          .post_dissection_tls_session_update().
  
          If the decryption failed with a CipherError, presumably because we
          missed the session keys, we signal it by returning a
          _TLSEncryptedContent packet which simply contains the ciphered data.
          """
          tmp_len = self.length_from(pkt)
          lst = []
          ret = b""
          remain = s
          if tmp_len is not None:
              remain, ret = s[:tmp_len], s[tmp_len:]
  
          if remain == b"":
              if (((pkt.tls_session.tls_version or 0x0303) > 0x0200) and
                      hasattr(pkt, "type") and pkt.type == 23):
                  return ret, [TLSApplicationData(data=b"")]
              else:
                  return ret, [Raw(load=b"")]
  
          if False in six.itervalues(pkt.tls_session.rcs.cipher.ready):
              return ret, _TLSEncryptedContent(remain)
          else:
              while remain:
                  raw_msg = remain
                  p = self.m2i(pkt, remain)
                  if Padding in p:
                      pad = p[Padding]
                      remain = pad.load
                      del(pad.underlayer.payload)
                      if len(remain) != 0:
                          raw_msg = raw_msg[:-len(remain)]
                  else:
                      remain = b""
  
                  if isinstance(p, _GenericTLSSessionInheritance):
                      if not p.tls_session.frozen:
                          p.post_dissection_tls_session_update(raw_msg)
  
                  lst.append(p)
              return remain + ret, lst
  
      def i2m(self, pkt, p):
          """
          Update the context with information from the built packet.
          If no type was given at the record layer, we try to infer it.
          """
          cur = b""
          if isinstance(p, _GenericTLSSessionInheritance):
              if pkt.type is None:
                  if isinstance(p, TLSChangeCipherSpec):
                      pkt.type = 20
                  elif isinstance(p, TLSAlert):
                      pkt.type = 21
                  elif isinstance(p, _TLSHandshake):
                      pkt.type = 22
                  elif isinstance(p, TLSApplicationData):
                      pkt.type = 23
              p.tls_session = pkt.tls_session
              if not pkt.tls_session.frozen:
                  cur = p.raw_stateful()
                  p.post_build_tls_session_update(cur)
              else:
                  cur = raw(p)
          else:
              pkt.type = 23
              cur = raw(p)
          return cur
  
      def addfield(self, pkt, s, val):
          """
          Reconstruct the header because the TLS type may have been updated.
          Then, append the content.
          """
          res = b""
          for p in val:
              res += self.i2m(pkt, p)
          if (isinstance(pkt, _GenericTLSSessionInheritance) and
              _tls_version_check(pkt.tls_session.tls_version, 0x0304) and
                  not isinstance(pkt, TLS13ServerHello)):
              return s + res
          if not pkt.type:
              pkt.type = 0
          hdr = struct.pack("!B", pkt.type) + s[1:5]
          return hdr + res
  
  
  class TLS(_GenericTLSSessionInheritance):
      """
      The generic TLS Record message, based on section 6.2 of RFC 5246.
  
      When reading a TLS message, we try to parse as much as we can.
      In .pre_dissect(), according to the type of the current cipher algorithm
      (self.tls_session.rcs.cipher.type), we extract the 'iv', 'mac', 'pad' and
      'padlen'. Some of these fields may remain blank: for instance, when using
      a stream cipher, there is no IV nor any padding. The 'len' should always
      hold the length of the ciphered message; for the plaintext version, you
      should rely on the additional 'deciphered_len' attribute.
  
      XXX Fix 'deciphered_len' which should not be defined when failing with
      AEAD decryption. This is related to the 'decryption_success' below.
      Also, follow this behaviour in record_sslv2.py and record_tls13.py
  
      Once we have isolated the ciphered message aggregate (which should be one
      or several TLS messages of the same type), we try to decipher it. Either we
      succeed and store the clear data in 'msg', or we graciously fail with a
      CipherError and store the ciphered data in 'msg'.
  
      Unless the user manually provides the session secrets through the passing
      of a 'tls_session', obviously the ciphered messages will not be deciphered.
      Indeed, the need for a proper context may also present itself when trying
      to parse clear handshake messages.
  
      For instance, suppose you sniffed the beginning of a DHE-RSA negotiation:
          t1 = TLS(<client_hello>)
          t2 = TLS(<server_hello | certificate | server_key_exchange>)
          t3 = TLS(<server_hello | certificate | server_key_exchange>,
                   tls_session=t1.tls_session)
      (Note that to do things properly, here 't1.tls_session' should actually be
      't1.tls_session.mirror()'. See session.py for explanations.)
  
      As no context was passed to t2, neither was any client_random. Hence Scapy
      will not be able to verify the signature of the server_key_exchange inside
      t2. However, it should be able to do so for t3, thanks to the tls_session.
      The consequence of not having a complete TLS context is even more obvious
      when trying to parse ciphered content, as we described before.
  
      Thus, in order to parse TLS-protected communications with Scapy:
      _either Scapy reads every message from one side of the TLS connection and
      builds every message from the other side (as such, it should know the
      secrets needed for the generation of the pre_master_secret), while passing
      the same tls_session context (this is how our automaton.py mostly works);
      _or, if Scapy did not build any TLS message, it has to create a TLS context
      and feed it with secrets retrieved by whatever technique. Note that the
      knowing the private key of the server certificate will not be sufficient
      if a PFS ciphersuite was used. However, if you got a master_secret somehow,
      use it with tls_session.(w|r)cs.derive_keys() and leave the rest to Scapy.
  
      When building a TLS message with raw_stateful, we expect the tls_session to
      have the right parameters for ciphering. Else, .post_build() might fail.
      """
      __slots__ = ["deciphered_len"]
      name = "TLS"
      fields_desc = [ByteEnumField("type", None, _tls_type),
                     _TLSVersionField("version", None, _tls_version),
                     _TLSLengthField("len", None),
                     _TLSIVField("iv", None),
                     _TLSMsgListField("msg", []),
                     _TLSMACField("mac", None),
                     _TLSPadField("pad", None),
                     _TLSPadLenField("padlen", None)]
  
      def __init__(self, *args, **kargs):
          self.deciphered_len = kargs.get("deciphered_len", None)
          super(TLS, self).__init__(*args, **kargs)
  
      @classmethod
      def dispatch_hook(cls, _pkt=None, *args, **kargs):
          """
          If the TLS class was called on raw SSLv2 data, we want to return an
          SSLv2 record instance. We acknowledge the risk of SSLv2 packets with a
          msglen of 0x1403, 0x1503, 0x1603 or 0x1703 which will never be casted
          as SSLv2 records but TLS ones instead, but hey, we can't be held
          responsible for low-minded extensibility choices.
          """
          if _pkt and len(_pkt) >= 2:
              byte0 = orb(_pkt[0])
              byte1 = orb(_pkt[1])
              if (byte0 not in _tls_type) or (byte1 != 3):
                  from scapy.layers.tls.record_sslv2 import SSLv2
                  return SSLv2
              else:
                  s = kargs.get("tls_session", None)
                  if s and _tls_version_check(s.tls_version, 0x0304):
                      if s.rcs and not isinstance(s.rcs.cipher, Cipher_NULL):
                          from scapy.layers.tls.record_tls13 import TLS13
                          return TLS13
          if _pkt and len(_pkt) < 5:
                  # Layer detected as TLS but too small to be a real packet (len<5).  # noqa: E501
                  # Those packets appear when sessions are interrupted or to flush buffers.  # noqa: E501
                  # Scapy should not try to decode them
              return conf.raw_layer
          return TLS
  
      # Parsing methods
  
      def _tls_auth_decrypt(self, hdr, s):
          """
          Provided with the record header and AEAD-ciphered data, return the
          sliced and clear tuple (nonce, TLSCompressed.fragment, mac). Note that
          we still return the slicing of the original input in case of decryption
          failure. Also, if the integrity check fails, a warning will be issued,
          but we still return the sliced (unauthenticated) plaintext.
          """
          try:
              read_seq_num = struct.pack("!Q", self.tls_session.rcs.seq_num)
              self.tls_session.rcs.seq_num += 1
              # self.type and self.version have not been parsed yet,
              # this is why we need to look into the provided hdr.
              add_data = read_seq_num + chb(hdr[0]) + hdr[1:3]
              # Last two bytes of add_data are appended by the return function
              return self.tls_session.rcs.cipher.auth_decrypt(add_data, s,
                                                              read_seq_num)
          except CipherError as e:
              return e.args
          except AEADTagError as e:
              pkt_info = self.firstlayer().summary()
              log_runtime.info("TLS: record integrity check failed [%s]", pkt_info)  # noqa: E501
              return e.args
  
      def _tls_decrypt(self, s):
          """
          Provided with stream- or block-ciphered data, return the clear version.
          The cipher should have been updated with the right IV early on,
          which should not be at the beginning of the input.
          In case of decryption failure, a CipherError will be raised with
          the slicing of the original input as first argument.
          """
          return self.tls_session.rcs.cipher.decrypt(s)
  
      def _tls_hmac_verify(self, hdr, msg, mac):
          """
          Provided with the record header, the TLSCompressed.fragment and the
          HMAC, return True if the HMAC is correct. If we could not compute the
          HMAC because the key was missing, there is no sense in verifying
          anything, thus we also return True.
  
          Meant to be used with a block cipher or a stream cipher.
          It would fail with an AEAD cipher, because rcs.hmac would be None.
          See RFC 5246, section 6.2.3.
          """
          read_seq_num = struct.pack("!Q", self.tls_session.rcs.seq_num)
          self.tls_session.rcs.seq_num += 1
  
          mac_len = self.tls_session.rcs.mac_len
          if mac_len == 0:            # should be TLS_NULL_WITH_NULL_NULL
              return True
          if len(mac) != mac_len:
              return False
  
          alg = self.tls_session.rcs.hmac
          version = struct.unpack("!H", hdr[1:3])[0]
          try:
              if version > 0x300:
                  h = alg.digest(read_seq_num + hdr + msg)
              elif version == 0x300:
                  h = alg.digest_sslv3(read_seq_num + hdr[:1] + hdr[3:5] + msg)
              else:
                  raise Exception("Unrecognized version.")
          except HMACError:
              h = mac
          return h == mac
  
      def _tls_decompress(self, s):
          """
          Provided with the TLSCompressed.fragment,
          return the TLSPlaintext.fragment.
          """
          alg = self.tls_session.rcs.compression
          return alg.decompress(s)
  
      def pre_dissect(self, s):
          """
          Decrypt, verify and decompress the message,
          i.e. apply the previous methods according to the reading cipher type.
          If the decryption was successful, 'len' will be the length of the
          TLSPlaintext.fragment. Else, it should be the length of the
          _TLSEncryptedContent.
          """
          if len(s) < 5:
              raise Exception("Invalid record: header is too short.")
  
          msglen = struct.unpack('!H', s[3:5])[0]
          hdr, efrag, r = s[:5], s[5:5 + msglen], s[msglen + 5:]
  
          iv = mac = pad = b""
          self.padlen = None
          decryption_success = False
  
          cipher_type = self.tls_session.rcs.cipher.type
  
          if cipher_type == 'block':
              version = struct.unpack("!H", s[1:3])[0]
  
              # Decrypt
              try:
                  if version >= 0x0302:
                      # Explicit IV for TLS 1.1 and 1.2
                      block_size = self.tls_session.rcs.cipher.block_size
                      iv, efrag = efrag[:block_size], efrag[block_size:]
                      self.tls_session.rcs.cipher.iv = iv
                      pfrag = self._tls_decrypt(efrag)
                  else:
                      # Implicit IV for SSLv3 and TLS 1.0
                      pfrag = self._tls_decrypt(efrag)
              except CipherError as e:
                  # This will end up dissected as _TLSEncryptedContent.
                  cfrag = e.args[0]
              else:
                  decryption_success = True
                  # Excerpt below better corresponds to TLS 1.1 IV definition,
                  # but the result is the same as with TLS 1.2 anyway.
                  # This leading *IV* has been decrypted by _tls_decrypt with a
                  # random IV, hence it does not correspond to anything.
                  # What actually matters is that we got the first encrypted block  # noqa: E501
                  # in order to decrypt the second block (first data block).
                  # if version >= 0x0302:
                  #    block_size = self.tls_session.rcs.cipher.block_size
                  #    iv, pfrag = pfrag[:block_size], pfrag[block_size:]
                  #    l = struct.unpack('!H', hdr[3:5])[0]
                  #    hdr = hdr[:3] + struct.pack('!H', l-block_size)
  
                  # Extract padding ('pad' actually includes the trailing padlen)
                  padlen = orb(pfrag[-1]) + 1
                  mfrag, pad = pfrag[:-padlen], pfrag[-padlen:]
                  self.padlen = padlen
  
                  # Extract MAC
                  tmp_len = self.tls_session.rcs.mac_len
                  if tmp_len != 0:
                      cfrag, mac = mfrag[:-tmp_len], mfrag[-tmp_len:]
                  else:
                      cfrag, mac = mfrag, b""
  
                  # Verify integrity
                  chdr = hdr[:3] + struct.pack('!H', len(cfrag))
                  is_mac_ok = self._tls_hmac_verify(chdr, cfrag, mac)
                  if not is_mac_ok:
                      pkt_info = self.firstlayer().summary()
                      log_runtime.info("TLS: record integrity check failed [%s]", pkt_info)  # noqa: E501
  
          elif cipher_type == 'stream':
              # Decrypt
              try:
                  pfrag = self._tls_decrypt(efrag)
              except CipherError as e:
                  # This will end up dissected as _TLSEncryptedContent.
                  cfrag = e.args[0]
              else:
                  decryption_success = True
                  mfrag = pfrag
  
                  # Extract MAC
                  tmp_len = self.tls_session.rcs.mac_len
                  if tmp_len != 0:
                      cfrag, mac = mfrag[:-tmp_len], mfrag[-tmp_len:]
                  else:
                      cfrag, mac = mfrag, b""
  
                  # Verify integrity
                  chdr = hdr[:3] + struct.pack('!H', len(cfrag))
                  is_mac_ok = self._tls_hmac_verify(chdr, cfrag, mac)
                  if not is_mac_ok:
                      pkt_info = self.firstlayer().summary()
                      log_runtime.info("TLS: record integrity check failed [%s]", pkt_info)  # noqa: E501
  
          elif cipher_type == 'aead':
              # Authenticated encryption
              # crypto/cipher_aead.py prints a warning for integrity failure
              if (conf.crypto_valid_advanced and
                      isinstance(self.tls_session.rcs.cipher, Cipher_CHACHA20_POLY1305)):  # noqa: E501
                  iv = b""
                  cfrag, mac = self._tls_auth_decrypt(hdr, efrag)
              else:
                  iv, cfrag, mac = self._tls_auth_decrypt(hdr, efrag)
              decryption_success = True       # see XXX above
  
          frag = self._tls_decompress(cfrag)
  
          if (decryption_success and
                  not isinstance(self.tls_session.rcs.cipher, Cipher_NULL)):
              self.deciphered_len = len(frag)
          else:
              self.deciphered_len = None
  
          reconstructed_body = iv + frag + mac + pad
  
          return hdr + reconstructed_body + r
  
      def post_dissect(self, s):
          """
          Commit the pending r/w state if it has been triggered (e.g. by an
          underlying TLSChangeCipherSpec or a SSLv2ClientMasterKey). We update
          nothing if the prcs was not set, as this probably means that we're
          working out-of-context (and we need to keep the default rcs).
          """
          if self.tls_session.triggered_prcs_commit:
              if self.tls_session.prcs is not None:
                  self.tls_session.rcs = self.tls_session.prcs
                  self.tls_session.prcs = None
              self.tls_session.triggered_prcs_commit = False
          if self.tls_session.triggered_pwcs_commit:
              if self.tls_session.pwcs is not None:
                  self.tls_session.wcs = self.tls_session.pwcs
                  self.tls_session.pwcs = None
              self.tls_session.triggered_pwcs_commit = False
          return s
  
      def do_dissect_payload(self, s):
          """
          Try to dissect the following data as a TLS message.
          Note that overloading .guess_payload_class() would not be enough,
          as the TLS session to be used would get lost.
          """
          if s:
              try:
                  p = TLS(s, _internal=1, _underlayer=self,
                          tls_session=self.tls_session)
              except KeyboardInterrupt:
                  raise
              except Exception:
                  p = conf.raw_layer(s, _internal=1, _underlayer=self)
              self.add_payload(p)
  
      # Building methods
  
      def _tls_compress(self, s):
          """
          Provided with the TLSPlaintext.fragment,
          return the TLSCompressed.fragment.
          """
          alg = self.tls_session.wcs.compression
          return alg.compress(s)
  
      def _tls_auth_encrypt(self, s):
          """
          Return the TLSCiphertext.fragment for AEAD ciphers, i.e. the whole
          GenericAEADCipher. Also, the additional data is computed right here.
          """
          write_seq_num = struct.pack("!Q", self.tls_session.wcs.seq_num)
          self.tls_session.wcs.seq_num += 1
          add_data = (write_seq_num +
                      pkcs_i2osp(self.type, 1) +
                      pkcs_i2osp(self.version, 2) +
                      pkcs_i2osp(len(s), 2))
          return self.tls_session.wcs.cipher.auth_encrypt(s, add_data,
                                                          write_seq_num)
  
      def _tls_hmac_add(self, hdr, msg):
          """
          Provided with the record header (concatenation of the TLSCompressed
          type, version and length fields) and the TLSCompressed.fragment,
          return the concatenation of the TLSCompressed.fragment and the HMAC.
  
          Meant to be used with a block cipher or a stream cipher.
          It would fail with an AEAD cipher, because wcs.hmac would be None.
          See RFC 5246, section 6.2.3.
          """
          write_seq_num = struct.pack("!Q", self.tls_session.wcs.seq_num)
          self.tls_session.wcs.seq_num += 1
  
          alg = self.tls_session.wcs.hmac
          version = struct.unpack("!H", hdr[1:3])[0]
          if version > 0x300:
              h = alg.digest(write_seq_num + hdr + msg)
          elif version == 0x300:
              h = alg.digest_sslv3(write_seq_num + hdr[:1] + hdr[3:5] + msg)
          else:
              raise Exception("Unrecognized version.")
          return msg + h
  
      def _tls_pad(self, s):
          """
          Provided with the concatenation of the TLSCompressed.fragment and the
          HMAC, append the right padding and return it as a whole.
          This is the TLS-style padding: while SSL allowed for random padding,
          TLS (misguidedly) specifies the repetition of the same byte all over,
          and this byte must be equal to len(<entire padding>) - 1.
  
          Meant to be used with a block cipher only.
          """
          padding = b""
          block_size = self.tls_session.wcs.cipher.block_size
          padlen = block_size - ((len(s) + 1) % block_size)
          if padlen == block_size:
              padlen = 0
          pad_pattern = chb(padlen)
          padding = pad_pattern * (padlen + 1)
          return s + padding
  
      def _tls_encrypt(self, s):
          """
          Return the stream- or block-ciphered version of the concatenated input.
          In case of GenericBlockCipher, no IV has been specifically prepended to
          the output, so this might not be the whole TLSCiphertext.fragment yet.
          """
          return self.tls_session.wcs.cipher.encrypt(s)
  
      def post_build(self, pkt, pay):
          """
          Apply the previous methods according to the writing cipher type.
          """
          # Compute the length of TLSPlaintext fragment
          hdr, frag = pkt[:5], pkt[5:]
          tmp_len = len(frag)
          hdr = hdr[:3] + struct.pack("!H", tmp_len)
  
          # Compression
          cfrag = self._tls_compress(frag)
          tmp_len = len(cfrag)  # Update the length as a result of compression
          hdr = hdr[:3] + struct.pack("!H", tmp_len)
  
          cipher_type = self.tls_session.wcs.cipher.type
  
          if cipher_type == 'block':
              # Integrity
              mfrag = self._tls_hmac_add(hdr, cfrag)
  
              # Excerpt below better corresponds to TLS 1.1 IV definition,
              # but the result is the same as with TLS 1.2 anyway.
              # if self.version >= 0x0302:
              #    l = self.tls_session.wcs.cipher.block_size
              #    iv = randstring(l)
              #    mfrag = iv + mfrag
  
              # Add padding
              pfrag = self._tls_pad(mfrag)
  
              # Encryption
              if self.version >= 0x0302:
                  # Explicit IV for TLS 1.1 and 1.2
                  tmp_len = self.tls_session.wcs.cipher.block_size
                  iv = randstring(tmp_len)
                  self.tls_session.wcs.cipher.iv = iv
                  efrag = self._tls_encrypt(pfrag)
                  efrag = iv + efrag
              else:
                  # Implicit IV for SSLv3 and TLS 1.0
                  efrag = self._tls_encrypt(pfrag)
  
          elif cipher_type == "stream":
              # Integrity
              mfrag = self._tls_hmac_add(hdr, cfrag)
              # Encryption
              efrag = self._tls_encrypt(mfrag)
  
          elif cipher_type == "aead":
              # Authenticated encryption (with nonce_explicit as header)
              efrag = self._tls_auth_encrypt(cfrag)
  
          if self.len is not None:
              # The user gave us a 'len', let's respect this ultimately
              hdr = hdr[:3] + struct.pack("!H", self.len)
          else:
              # Update header with the length of TLSCiphertext.fragment
              hdr = hdr[:3] + struct.pack("!H", len(efrag))
  
          # Now we commit the pending write state if it has been triggered (e.g.
          # by an underlying TLSChangeCipherSpec or a SSLv2ClientMasterKey). We
          # update nothing if the pwcs was not set. This probably means that
          # we're working out-of-context (and we need to keep the default wcs).
          if self.tls_session.triggered_pwcs_commit:
              if self.tls_session.pwcs is not None:
                  self.tls_session.wcs = self.tls_session.pwcs
                  self.tls_session.pwcs = None
              self.tls_session.triggered_pwcs_commit = False
  
          return hdr + efrag + pay
  
  
  ###############################################################################
  #   TLS ChangeCipherSpec                                                      #
  ###############################################################################
  
  _tls_changecipherspec_type = {1: "change_cipher_spec"}
  
  
  class TLSChangeCipherSpec(_GenericTLSSessionInheritance):
      """
      Note that, as they are not handshake messages, the ccs messages do not get
      appended to the list of messages whose integrity gets verified through the
      Finished messages.
      """
      name = "TLS ChangeCipherSpec"
      fields_desc = [ByteEnumField("msgtype", 1, _tls_changecipherspec_type)]
  
      def post_dissection_tls_session_update(self, msg_str):
          self.tls_session.triggered_prcs_commit = True
  
      def post_build_tls_session_update(self, msg_str):
          # Unlike for dissection case, we cannot commit pending write
          # state as current write state. We need to delay this after
          # the ChangeCipherSpec message has indeed been sent
          self.tls_session.triggered_pwcs_commit = True
  
  
  ###############################################################################
  #   TLS Alert                                                                 #
  ###############################################################################
  
  _tls_alert_level = {1: "warning", 2: "fatal"}
  
  _tls_alert_description = {
      0: "close_notify", 10: "unexpected_message",
      20: "bad_record_mac", 21: "decryption_failed",
      22: "record_overflow", 30: "decompression_failure",
      40: "handshake_failure", 41: "no_certificate_RESERVED",
      42: "bad_certificate", 43: "unsupported_certificate",
      44: "certificate_revoked", 45: "certificate_expired",
      46: "certificate_unknown", 47: "illegal_parameter",
      48: "unknown_ca", 49: "access_denied",
      50: "decode_error", 51: "decrypt_error",
      60: "export_restriction_RESERVED", 70: "protocol_version",
      71: "insufficient_security", 80: "internal_error",
      90: "user_canceled", 100: "no_renegotiation",
      110: "unsupported_extension", 111: "certificate_unobtainable",
      112: "unrecognized_name", 113: "bad_certificate_status_response",
      114: "bad_certificate_hash_value", 115: "unknown_psk_identity"}
  
  
  class TLSAlert(_GenericTLSSessionInheritance):
      name = "TLS Alert"
      fields_desc = [ByteEnumField("level", None, _tls_alert_level),
                     ByteEnumField("descr", None, _tls_alert_description)]
  
      def post_dissection_tls_session_update(self, msg_str):
          pass
  
      def post_build_tls_session_update(self, msg_str):
          pass
  
  
  ###############################################################################
  #   TLS Application Data                                                      #
  ###############################################################################
  
  class TLSApplicationData(_GenericTLSSessionInheritance):
      name = "TLS Application Data"
      fields_desc = [StrField("data", "")]
  
      def post_dissection_tls_session_update(self, msg_str):
          pass
  
      def post_build_tls_session_update(self, msg_str):
          pass
  
  
  ###############################################################################
  #   Bindings                                                                  #
  ###############################################################################
  
  bind_layers(TCP, TLS, sport=443)
  bind_layers(TCP, TLS, dport=443)
  