--- Functions for the SSH-2 protocol.
-- @author Sven Klemm <sven@c3d2.de>
-- @copyright Same as Nmap--See http://nmap.org/book/man-legal.html

module(... or "ssh2",package.seeall)

require "bin"
require "base64"
require "openssl"
require "stdnse"

-- table holding transport layer functions
transport = {}

-- table of SSH-2 constants
local SSH2

--- Retrieve the size of the packet that is being received
--  and checks if it is fully received
-- 
--  This function is very similar to the function generated
--  with match.numbytes(num) function, except that this one
--  will check for the number of bytes on-the-fly, based on
--  the written on the SSH packet.
--
--  @param buffer The receive buffer
--  @return packet_length, packet_length or nil
--  the return is similar to the lua function string:find()
check_packet_length = function( buffer )
  local packet_length, offset
  offset, packet_length = bin.unpack( ">I", buffer )
  assert(packet_length)
  if packet_length + 4 > buffer:len() then return nil end
  return packet_length, packet_length
end

--- Receives a complete SSH packet, even if fragmented
--  this function is an abstraction layer to deal with
--  checking the packet size to know if there is any more
--  data to receive.
--
--  @param socket The socket used to receive the data
--  @return status True or false
--  @return packet The packet received
transport.receive_packet = function( socket )
  local status, packet = socket:receive_buf(check_packet_length)
  return status, packet
end

--- Pack a multiprecision integer for sending.
-- @param bn <code>openssl</code> bignum.
-- @return Packed multiprecision integer.
transport.pack_mpint = function( bn )
  local bytes, packed
  bytes = bn:num_bytes()
  packed = bn:tobin()
  if bytes % 8 == 0 then
    bytes = bytes + 1
    packed = string.char(0) .. packed
  end
  return bin.pack( ">IA", bytes, packed )
end

--- Build an SSH-2 packet.
-- @param payload Payload of the packet.
-- @return Packet to send on the wire.
transport.build = function( payload )
  local packet_length, padding_length
  padding_length = 8 - ( (payload:len() + 1 + 4 ) % 8 )
  packet_length = payload:len() + padding_length + 1
  return bin.pack( ">IcAA", packet_length, padding_length, payload, openssl.rand_pseudo_bytes( padding_length ) )
end

--- Extract the payload from a received SSH-2 packet.
-- @param packet Peceived SSH-2 packet.
-- @return Payload of the SSH-2 packet.
transport.payload = function( packet )
  local packet_length, padding_length, payload_length, payload, offset
  offset, packet_length, padding_length = bin.unpack( ">Ic", packet )
  assert(packet_length and padding_length)
  payload_length = packet_length - padding_length - 1
  if packet_length ~= packet:len() then
    stdnse.print_debug("SSH-2 packet doesn't match length: payload_length is %d but total length is only %d.", packet_length, packet:len())
    return nil
  end
  offset, payload = bin.unpack( ">A" .. payload_length, packet, offset )
  return payload
end

--- Build a <code>kexdh_init</code> packet.
transport.kexdh_init = function( e )
  return bin.pack( ">cA", SSH2.SSH_MSG_KEXDH_INIT, transport.pack_mpint( e ) )
end

--- Build a <code>kex_init</code> packet.
transport.kex_init = function( cookie, options )
  options = options or {}
  local kex_algorithms = "diffie-hellman-group1-sha1"
  local host_key_algorithms = options['host_key_algorithms'] or "ssh-dss,ssh-rsa"
  local encryption_algorithms = "aes128-cbc,3des-cbc,blowfish-cbc,aes192-cbc,aes256-cbc,aes128-ctr,aes192-ctr,aes256-ctr"
  local mac_algorithms = "hmac-md5,hmac-sha1,hmac-ripemd160"
  local compression_algorithms = "none"
  local languages = ""

  local payload = bin.pack( ">cAaa", SSH2.SSH_MSG_KEXINIT, cookie, kex_algorithms, host_key_algorithms )
  payload = payload .. bin.pack( ">aa", encryption_algorithms, encryption_algorithms )
  payload = payload .. bin.pack( ">aa", mac_algorithms, mac_algorithms )
  payload = payload .. bin.pack( ">aa", compression_algorithms, compression_algorithms )
  payload = payload .. bin.pack( ">aa", languages, languages )
  payload = payload .. bin.pack( ">cI", 0, 0 )

  return payload
end

--- Parse a <code>kexinit</code> package.
--
-- Returns an empty table in case of an error
transport.parse_kex_init = function( payload ) 
  local _, offset, msg_code, parsed, fields, fieldname
  parsed = {}

  -- check for proper msg code
  offset, msg_code = bin.unpack( ">c", payload )
  if msg_code ~= SSH2.SSH_MSG_KEXINIT then return {} end

  offset, parsed.cookie = bin.unpack( ">A16", payload, offset )

  fields = {'kex_algorithms','server_host_key_algorithms',
    'encryption_algorithms_client_to_server','encryption_algorithms_server_to_client',
    'mac_algorithms_client_to_server','mac_algorithms_server_to_client',
    'compression_algorithms_client_to_server','compression_algorithms_server_to_client',
    'languages_client_to_server','languages_server_to_client'}
  for _, fieldname in pairs( fields ) do
    offset, parsed[fieldname] = bin.unpack( ">a", payload, offset )
  end

  return parsed
end


--- Fetch an SSH-2 host key.
-- @param host Nmap host table.
-- @param port Nmap port table.
-- @param key_type key type to fetch.
-- @return A table with the following fields: <code>key</code>,
-- <code>key_type</code>, <code>fp_input</code>, <code>bits</code>,
-- <code>full_key</code>, <code>algorithm</code>, and <code>fingerprint</code>.
fetch_host_key = function( host, port, key_type )
  local socket = nmap.new_socket()
  local status

  -- oakley group 2 prime taken from rfc 2409
  local prime = "FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E088A67CC74020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B302B0A6DF25F14374FE1356D6D51C245E485B576625E7EC6F44C42E9A637ED6B0BFF5CB6F406B7EDEE386BFB5A899FA5AE9F24117C4B1FE649286651ECE65381FFFFFFFFFFFFFFFF"

  status = socket:connect(host.ip, port.number)
  if not status then return end
  -- fetch banner
  status = socket:receive_lines(1)
  if not status then socket:close(); return end
  -- send our banner
  status = socket:send("SSH-2.0-Nmap-SSH2-Hostkey\r\n")
  if not status then socket:close(); return end

  local cookie = openssl.rand_bytes( 16 )
  local packet = transport.build( transport.kex_init( cookie, {host_key_algorithms=key_type} ) )
  status = socket:send( packet )
  if not status then socket:close(); return end

  local kex_init
  status, kex_init = transport.receive_packet( socket )
  if not status then socket:close(); return end
  kex_init = transport.parse_kex_init( transport.payload( kex_init ) )

  if not tostring(kex_init.server_host_key_algorithms):find( key_type, 1, true ) then
    -- server does not support host key type
    stdnse.print_debug( 2, "Hostkey type '%s' not supported by server.", key_type )
    return
  end

  local e, g, x, p
  -- e = g^x mod p
  g = openssl.bignum_dec2bn( "2" )
  p = openssl.bignum_hex2bn( prime )
  x = openssl.bignum_pseudo_rand( 1024 )
  e = openssl.bignum_mod_exp( g, x, p )

  packet = transport.build( transport.kexdh_init( e ) )
  status = socket:send( packet )
  if not status then socket:close(); return end

  local kexdh_reply
  status, kexdh_reply = transport.receive_packet( socket )
  kexdh_reply = transport.payload( kexdh_reply )
  -- check for proper msg code
  if kexdh_reply:byte(1) ~= SSH2.SSH_MSG_KEXDH_REPLY then
    return
  end

  local _,public_host_key,bits,algorithm
  _, _, public_host_key = bin.unpack( ">ca", kexdh_reply )

  if key_type == 'ssh-dss' then
    algorithm = "DSA"
    local p
    _, _, p = bin.unpack( ">aa", public_host_key )
    bits = openssl.bignum_bin2bn( p ):num_bits()
  elseif key_type == 'ssh-rsa' then
    algorithm = "RSA"
    local n
    _, _, _, n = bin.unpack( ">aaa", public_host_key )
    bits = openssl.bignum_bin2bn( n ):num_bits()
  else
    stdnse.print_debug( "Unsupported key type: %s", key_type )
  end

  return { key=public_host_key, key_type=key_type, fp_input=public_host_key, bits=bits,
           full_key=('%s %s'):format(key_type,base64.enc(public_host_key)),
           algorithm=algorithm, fingerprint=openssl.md5(public_host_key) }
end

-- constants

SSH2 = {
  SSH_MSG_DISCONNECT = 1,
  SSH_MSG_IGNORE = 2,
  SSH_MSG_UNIMPLEMENTED = 3,
  SSH_MSG_DEBUG = 4,
  SSH_MSG_SERVICE_REQUEST = 5,
  SSH_MSG_SERVICE_ACCEPT = 6,
  SSH_MSG_KEXINIT = 20,
  SSH_MSG_NEWKEYS = 21,
  SSH_MSG_KEXDH_INIT = 30,
  SSH_MSG_KEXDH_REPLY = 31,
}

