Tamuro challenge - Plaintext Checksums

gistfile1.txt

We know nothing about the contents of these encrypted messages other than it's a stream cipher
using the same private encryption key with each message and foolishly, the crc is of the
plaintext, not the ciphertext. Extract the keystream and use it to decrypt the secret flag.

flag (hex string) = 22360906580dc6f4d26fc4b0d8327932d87cf1

Message Format: stream_cipher(k, msg) || crc32-big_endian(msg)
CRC Check = 0xfc891918 (crc-32-bzip2)

Encrypted messages:
  d6e06a975295f388
  c23c5d20bfe8712d73ce
  8ec42d8623d774d58be529
  8a74715ad68b1dd2583dea166bc660182b980294
  6440124b5f091178d03351b1a4c3677a
  89941795df97acb1e035348a6196be
  3e8054579212eb
  9c707e4b02ea
  f2915b8f677ed6b156dd16dcc9b1f3af94
  b1f23a80d7
  08cb3dfbc775bc54142ae71d
  ec343da5662aebf3a9
  ca78c3124c9d18557d3c1d403427cfed3d0f5f1d1d32d324
  42994ab15e58e6b84be7932ba0
  683013518ae498ab5c09d1a80fe9d101a2b5b77ec2c4
  a61c3d1944ceb50c03ce5d3255287fcd074c481b1fa6fd
  2e60cad541d270fb66bdc1c0c84b9d39e9f5
  d176a7e8b2425c963913dff7b9f67abd4be2d1
  aae2cc53aa0d67f72eaf4facbf001cc5054e5a0048542c7570
  a86eb94e6fa27910b241b67d355cceaa3fc2e3365a
  76c7e67ae482e28fcd825a9a5e86

Challenge generation code, with secrets removed, is in the next file,

gistfile2.txt

flag = "DELETED"

import binascii
import secrets
import random
import crcmod

def generate_encryption_key():
  return secrets.token_bytes(256 // 8)
  
def encrypt(k, m):
  raise Exception("DELETED CODE")

def crc32(m):
  crc_function = crcmod.mkCrcFun(0x104C11DB7, rev=False, initCrc=0, xorOut=0xFFFFFFFF)
  return int(crc_function(m)).to_bytes(4, byteorder='big')

def checksum_and_encrypt(k, m):
  if isinstance(m, str):
    m = bytes(m, 'latin1')
  return (encrypt(k, m) + crc32(m))

def generate_flag(secret_key):
  encrypted_flag = binascii.hexlify(encrypt(secret_key, bytes(flag, 'ascii'))).decode('ascii')
  print("flag = {}".format(encrypted_flag))
  return encrypted_flag
  
def generate_encrypted_messages(secret_key):
  c = []
  msg_lengths = list(range(1, len(flag)+3))
  random.shuffle(msg_lengths)
  
  print("Format: stream_cipher(k, msg) || crc32-big_endian(msg)")
  print("CRC Check = {} (crc-32-bzip2)".format(hex(int.from_bytes(crc32(bytes("123456789", 'ascii')), byteorder='big'))))
  print("Encrypted messages:")
  for msg_length in msg_lengths:
    p = secrets.token_bytes(msg_length)
    m = checksum_and_encrypt(secret_key, p)
    c.append(m)
    print("  " + binascii.hexlify(m).decode('ascii'))  
  print("\n\n\n")
  return c

if __name__ == "__main__":
  secret_key = generate_encryption_key()
  encrypted_flag = generate_flag(secret_key)
  msgs = generate_encrypted_messages(secret_key)