#!/usr/bin/python3
#
# Copyright 2017 The Android Open Source Project
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

from socket import *  # pylint: disable=wildcard-import
from scapy import all as scapy
import binascii
import struct

import csocket
import cstruct
import multinetwork_base
import net_test
import util
import xfrm

_ENCRYPTION_KEY_256 = binascii.unhexlify("308146eb3bd84b044573d60f5a5fd159"
                                         "57c7d4fe567a2120f35bae0f9869ec22")
_AUTHENTICATION_KEY_128 = binascii.unhexlify("af442892cdcd0ef650e9c299f9a8436a")

_ALGO_AUTH_NULL = (xfrm.XfrmAlgoAuth((b"digest_null", 0, 0)), b"")
_ALGO_HMAC_SHA1 = (xfrm.XfrmAlgoAuth((xfrm.XFRM_AALG_HMAC_SHA1, 128, 96)),
                   _AUTHENTICATION_KEY_128)

_ALGO_CRYPT_NULL = (xfrm.XfrmAlgo((b"ecb(cipher_null)", 0)), b"")
_ALGO_CBC_AES_256 = (xfrm.XfrmAlgo((xfrm.XFRM_EALG_CBC_AES, 256)),
                     _ENCRYPTION_KEY_256)

# Match all bits of the mark
MARK_MASK_ALL = 0xffffffff


def SetPolicySockopt(sock, family, opt_data):
  optlen = len(opt_data) if opt_data is not None else 0
  if family == AF_INET:
    csocket.Setsockopt(sock, IPPROTO_IP, xfrm.IP_XFRM_POLICY, opt_data, optlen)
  else:
    csocket.Setsockopt(sock, IPPROTO_IPV6, xfrm.IPV6_XFRM_POLICY, opt_data,
                       optlen)


def ApplySocketPolicy(sock, family, direction, spi, reqid, tun_addrs):
  """Create and apply an ESP policy to a socket.

  A socket may have only one policy per direction, so applying a policy will
  remove any policy that was previously applied in that direction.

  Args:
    sock: The socket that needs a policy
    family: AF_INET or AF_INET6
    direction: XFRM_POLICY_IN or XFRM_POLICY_OUT
    spi: 32-bit SPI in host byte order
    reqid: 32-bit ID matched against SAs
    tun_addrs: A tuple of (local, remote) addresses for tunnel mode, or None
      to request a transport mode SA.
  """
  # Create a selector that matches all packets of the specified address family.
  selector = xfrm.EmptySelector(family)

  # Create an XFRM policy and template.
  policy = xfrm.UserPolicy(direction, selector)
  template = xfrm.UserTemplate(family, spi, reqid, tun_addrs)

  # Set the policy and template on our socket.
  opt_data = policy.Pack() + template.Pack()

  # The policy family might not match the socket family. For example, we might
  # have an IPv4 policy on a dual-stack socket.
  sockfamily = sock.getsockopt(SOL_SOCKET, net_test.SO_DOMAIN)
  SetPolicySockopt(sock, sockfamily, opt_data)

def _GetCryptParameters(crypt_alg):
  """Looks up encryption algorithm's block and IV lengths.

  Args:
    crypt_alg: the encryption algorithm constant
  Returns:
    A tuple of the block size, and IV length
  """
  if crypt_alg == _ALGO_CRYPT_NULL:
    return (4, 0)
  if crypt_alg == _ALGO_CBC_AES_256:
    return (16, 16)
  return (0, 0)


def GetEspPacketLength(mode, version, udp_encap, payload,
                       auth_alg, crypt_alg):
  """Calculates encrypted length of a UDP packet with the given payload.

  Args:
    mode: XFRM_MODE_TRANSPORT or XFRM_MODE_TUNNEL.
    version: IPPROTO_IP for IPv4, IPPROTO_IPV6 for IPv6. The inner header.
    udp_encap: whether UDP encap overhead should be accounted for. Since the
               outermost IP header is ignored (payload only), only add for udp
               encap'd packets.
    payload: UDP payload bytes.
    auth_alg: The xfrm_base authentication algorithm used in the SA.
    crypt_alg: The xfrm_base encryption algorithm used in the SA.

  Return: the packet length.
  """

  crypt_iv_len, crypt_blk_size=_GetCryptParameters(crypt_alg)
  auth_trunc_len = auth_alg[0].trunc_len

  # Wrap in UDP payload
  payload_len = len(payload) + net_test.UDP_HDR_LEN

  # Size constants
  esp_hdr_len = len(xfrm.EspHdr) # SPI + Seq number
  icv_len = auth_trunc_len // 8

  # Add inner IP header if tunnel mode
  if mode == xfrm.XFRM_MODE_TUNNEL:
    payload_len += net_test.GetIpHdrLength(version)

  # Add ESP trailer
  payload_len += 2 # Pad Length + Next Header fields

  # Align to block size of encryption algorithm
  payload_len += util.GetPadLength(crypt_blk_size, payload_len)

  # Add initialization vector, header length and ICV length
  payload_len += esp_hdr_len + crypt_iv_len + icv_len

  # Add encap as needed
  if udp_encap:
    payload_len += net_test.UDP_HDR_LEN

  return payload_len


def GetEspTrailer(length, nexthdr):
  # ESP padding per RFC 4303 section 2.4.
  # For a null cipher with a block size of 1, padding is only necessary to
  # ensure that the 1-byte Pad Length and Next Header fields are right aligned
  # on a 4-byte boundary.
  esplen = length + 2  # Packet length plus Pad Length and Next Header.
  padlen = util.GetPadLength(4, esplen)
  # The pad bytes are consecutive integers starting from 0x01.
  padding = "".join((chr(i) for i in range(1, padlen + 1))).encode("utf-8")
  return padding + struct.pack("BB", padlen, nexthdr)


def EncryptPacketWithNull(packet, spi, seq, tun_addrs):
  """Apply null encryption to a packet.

  This performs ESP encapsulation on the given packet. The returned packet will
  be a tunnel mode packet if tun_addrs is provided.

  The input packet is assumed to be a UDP packet. The input packet *MUST* have
  its length and checksum fields in IP and UDP headers set appropriately. This
  can be done by "rebuilding" the scapy object. e.g.,
      ip6_packet = scapy.IPv6(bytes(ip6_packet))

  TODO: Support TCP

  Args:
    packet: a scapy.IPv6 or scapy.IP packet
    spi: security parameter index for ESP header in host byte order
    seq: sequence number for ESP header
    tun_addrs: A tuple of (local, remote) addresses for tunnel mode, or None
      to request a transport mode packet.

  Return:
    The encrypted packet (scapy.IPv6 or scapy.IP)
  """
  # The top-level packet changes in tunnel mode, which would invalidate
  # the passed-in packet pointer. For consistency, this function now returns
  # a new packet and does not modify the user's original packet.
  packet = packet.copy()
  udp_layer = packet.getlayer(scapy.UDP)
  if not udp_layer:
    raise ValueError("Expected a UDP packet")
  # Build an ESP header.
  esp_packet = scapy.Raw(xfrm.EspHdr((spi, seq)).Pack())

  if tun_addrs:
    tsrc_addr, tdst_addr = tun_addrs
    outer_version = net_test.GetAddressVersion(tsrc_addr)
    ip_type = {4: scapy.IP, 6: scapy.IPv6}[outer_version]
    new_ip_layer = ip_type(src=tsrc_addr, dst=tdst_addr)
    inner_layer = packet
    esp_nexthdr = {scapy.IPv6: IPPROTO_IPV6,
                   scapy.IP: IPPROTO_IPIP}[type(packet)]
  else:
    new_ip_layer = None
    inner_layer = udp_layer
    esp_nexthdr = IPPROTO_UDP

  trailer = GetEspTrailer(len(inner_layer), esp_nexthdr)

  # Assemble the packet.
  esp_packet.payload = scapy.Raw(inner_layer)
  packet = new_ip_layer if new_ip_layer else packet
  packet.payload = scapy.Raw(bytes(esp_packet) + trailer)

  # TODO: Can we simplify this and avoid the initial copy()?
  # Fix the IPv4/IPv6 headers.
  if type(packet) is scapy.IPv6:
    packet.nh = IPPROTO_ESP
    # Recompute plen.
    packet.plen = None
    packet = scapy.IPv6(bytes(packet))
  elif type(packet) is scapy.IP:
    packet.proto = IPPROTO_ESP
    # Recompute IPv4 len and checksum.
    packet.len = None
    packet.chksum = None
    packet = scapy.IP(bytes(packet))
  else:
    raise ValueError("First layer in packet should be IPv4 or IPv6: " + repr(packet))
  return packet


def DecryptPacketWithNull(packet):
  """Apply null decryption to a packet.

  This performs ESP decapsulation on the given packet. The input packet is
  assumed to be a UDP packet. This function will remove the ESP header and
  trailer bytes from an ESP packet.

  TODO: Support TCP

  Args:
    packet: a scapy.IPv6 or scapy.IP packet

  Returns:
    A tuple of decrypted packet (scapy.IPv6 or scapy.IP) and EspHdr
  """
  esp_hdr, esp_data = cstruct.Read(bytes(packet.payload), xfrm.EspHdr)
  # Parse and strip ESP trailer.
  pad_len, esp_nexthdr = struct.unpack("BB", esp_data[-2:])
  trailer_len = pad_len + 2 # Add the size of the pad_len and next_hdr fields.
  LayerType = {
          IPPROTO_IPIP: scapy.IP,
          IPPROTO_IPV6: scapy.IPv6,
          IPPROTO_UDP: scapy.UDP}[esp_nexthdr]
  next_layer = LayerType(esp_data[:-trailer_len])
  if esp_nexthdr in [IPPROTO_IPIP, IPPROTO_IPV6]:
    # Tunnel mode decap is simple. Return the inner packet.
    return next_layer, esp_hdr

  # Cut out the ESP header.
  packet.payload = next_layer
  # Fix the IPv4/IPv6 headers.
  if type(packet) is scapy.IPv6:
    packet.nh = IPPROTO_UDP
    packet.plen = None # Recompute packet length.
    packet = scapy.IPv6(bytes(packet))
  elif type(packet) is scapy.IP:
    packet.proto = IPPROTO_UDP
    packet.len = None # Recompute packet length.
    packet.chksum = None # Recompute IPv4 checksum.
    packet = scapy.IP(bytes(packet))
  else:
    raise ValueError("First layer in packet should be IPv4 or IPv6: " + repr(packet))
  return packet, esp_hdr


class XfrmBaseTest(multinetwork_base.MultiNetworkBaseTest):
  """Base test class for all XFRM-related testing."""

  def _isIcmpv6(self, payload):
    if not isinstance(payload, scapy.IPv6):
      return False
    if payload.nh == IPPROTO_ICMPV6:
      return True
    return payload.nh == IPPROTO_HOPOPTS and payload.payload.nh == IPPROTO_ICMPV6

  def _ExpectEspPacketOn(self, netid, spi, seq, length, src_addr, dst_addr):
    """Read a packet from a netid and verify its properties.

    Args:
      netid: netid from which to read an ESP packet
      spi: SPI of the ESP packet in host byte order
      seq: sequence number of the ESP packet
      length: length of the packet's ESP payload or None to skip this check
      src_addr: source address of the packet or None to skip this check
      dst_addr: destination address of the packet or None to skip this check

    Returns:
      scapy.IP/IPv6: the read packet
    """
    packets = []
    for packet in self.ReadAllPacketsOn(netid):
      if not self._isIcmpv6(packet):
        packets.append(packet)

    self.assertEqual(1, len(packets))
    packet = packets[0]
    if length is not None:
      self.assertEqual(length, len(packet.payload))
    if dst_addr is not None:
      self.assertEqual(dst_addr, packet.dst)
    if src_addr is not None:
      self.assertEqual(src_addr, packet.src)
    # extract the ESP header
    esp_hdr, _ = cstruct.Read(bytes(packet.payload), xfrm.EspHdr)
    self.assertEqual(xfrm.EspHdr((spi, seq)), esp_hdr)
    return packet


# TODO: delete this when we're more diligent about deleting our SAs.
class XfrmLazyTest(XfrmBaseTest):
  """Base test class Xfrm tests that cleans XFRM state on teardown."""
  def setUp(self):
    super(XfrmBaseTest, self).setUp()
    self.xfrm = xfrm.Xfrm()
    self.xfrm.FlushSaInfo()
    self.xfrm.FlushPolicyInfo()

  def tearDown(self):
    super(XfrmBaseTest, self).tearDown()
    self.xfrm.FlushSaInfo()
    self.xfrm.FlushPolicyInfo()
    self.xfrm.close()