#!/usr/bin/python

import socket
import struct
import threading
import time
import sys
import random
import re
import logging
from optparse import OptionParser
from scapy.all import *

def calculate_tcp_length(pkt):
        iphdr_len = pkt[IP].ihl * 4
        total_len = pkt[IP].len
        tcp_hdrlen = pkt[TCP].dataofs * 4
        tcp_payload_len = total_len - iphdr_len - tcp_hdrlen
        return tcp_payload_len

def recalculate_tcp_md5_signature(pkt,password):
    
    destination_ip = pkt[IP].dst
    source_ip = pkt[IP].src
    reserved = 0
    protocol = socket.IPPROTO_TCP

    tcp_hdrlen = pkt[TCP].dataofs *4
    tcp_payload_len =calculate_tcp_length(pkt)
    total_length = tcp_hdrlen + tcp_payload_len
    
    #pseudo header
    psh = struct.pack("!4s4sBBH",
                      source_ip,
                      destination_ip,
                      reserved,
                      protocol,
                      total_length
                      )

    iphdr_len = pkt[IP].ihl * 4
    tcp_header = pkt[iphdr_len:iphdr_len+20]
    tcp_payload = pkt[iphdr_len+20:]
    
    tcp_sport = pkt[TCP].sport
    tcp_dport = pkt[TCP].dport
    tcp_seq = pkt[TCP].seq
    tcp_ack = pkt[TCP].ack
    tcp_dataofs = pkt[TCP].dataofs
    tcp_reserved = pkt[TCP].reserved
    tcp_flags = pkt[TCP].flags
    tcp_window = pkt[TCP].window
    tcp_chksum = pkt[TCP].chksum
    tcp_urgptr = pkt[TCP].urgptr