Ufdjrw

There are 3 codes that need to do the following actions:

• A sends a message to B along with the CRC32 code.


• B receives this message and CRC32 code.


• B follows a 40% probability to change the message.


• B sends the message along with the original CRC32 code to C.


• C receives the message and CRC32 code and check whether it is correct or not.

For some reason, in part C when I compare the CRC's they are never equal, what am I missing?

Part A:

import socket
import struct
import sys
import binascii

def crc32(v):
 r = binascii.crc32(v.encode())
 return r


if len(sys.argv) != 3:
 print("Useage: python " + sys.argv[0] + " <ip> <liseten port>")
 sys.exit(-1)
s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
while True:
 print("Input text:")
 text = sys.stdin.readline().strip()
 ss = struct.pack("!50sL",text.encode(),crc32(text))
 s.sendto(ss,(sys.argv[1],int(sys.argv[2])))
 if text == "bye":
 break

Part B:

import socket
import operator
import sys
import binascii
import struct
import random

def crc32(v):
 return binascii.crc32(v.encode())

if len(sys.argv) != 3:
 print("Useage: python " + sys.argv[0] + " <liseten port>")
 sys.exit(-1)

s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
s.bind(("0.0.0.0", int(sys.argv[1])))
print("Waiting...")
while True:
 data, addr = s.recvfrom(1024)
 str,crc = struct.unpack("!50sL",data)
 str = str.decode("utf-8").replace("","")
 if random.randint(0,100) < 40:
 str = str + "x"
 print("str:%sncrc:%X" % (str,crc & 0xffffffff))
 str2 = str.encode("utf-8")
 tpack = struct.pack("!50sL", str2, crc)
 s.sendto(tpack,("127.0.0.1",int(sys.argv[2])))

 if str == "bye":
 break

Part C:

import socket
import operator
import sys
import binascii
import struct

def crc32(v):
 return binascii.crc32(v.encode())

if len(sys.argv) != 2:
 print("Useage: python " + sys.argv[0] + " <liseten port>")
 sys.exit(-1)

s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
s.bind(("0.0.0.0", int(sys.argv[1])))
print("Waiting...")
while True:
 data, addr = s.recvfrom(1024)
 str,crc = struct.unpack("!50sL",data)
 str = str.decode("utf-8")
 print("str:%sncrc:%X" % (str,crc & 0xffffffff))

 ncrc = crc32(str)
 if ncrc == crc:
 print("both messages are the same")
 if str == "bye":
 break

You forgot to replace the null bytes in Part C. You calculated the CRC in Part A before packing to 50 bytes, and removed them in Part B when displaying the received value.

str = str.decode("utf-8")

should b:

str = str.decode("utf-8").replace('','')

Note: str is a builtin function that you lose access to by using it as a variable name.