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#!/usr/bin/env python3
import cgi
import os
import json
from base64 import b64decode, b64encode
from Crypto.Cipher import AES
from Crypto.Util.strxor import strxor
from Crypto.Util.Padding import pad, unpad
from Crypto.Util.number import long_to_bytes

from secret import key, secret_message

block_length = 16
aes = AES.new(key, AES.MODE_ECB)

def chunk(data, length):
    return [data[i:i+length] for i in range(0, len(data), length)]

def sign(msg, iv=None):
    msg = pad(msg, block_length)
    if not iv:
        iv = os.urandom(block_length)
    blocks = chunk(msg, block_length)

    signature = iv
    for block in [long_to_bytes(len(blocks), block_length)] + blocks:
        signature = aes.encrypt(strxor(signature, block))
    return iv, signature

def verify(msg, iv, signature):
    _, computed_signature = sign(msg, iv)
    return computed_signature == signature

msg_ = b'{"authenticated":false,"text":"oto_przyklad_uzycia_podpisu"}'
iv_, sig_ = sign(msg_)

print("Content-type: text/html")
print()
print( """
<html>
<head><title>Zadanie 2: AES CBC MAC</title></head>
<meta charset="utf-8">
<body>
""")
print("""
<p> sign(%r) == b64decode(%r), b64decode(%r) </p>
""" % (msg_, b64encode(iv_), b64encode(sig_)))

form = cgi.FieldStorage()
message = form.getvalue('message', '')
iv = form.getvalue('iv', '')
signature = form.getvalue('signature', '')

print('<p>')
def x(message, iv, signature):
    try:
        message = b64decode(message)
        iv = b64decode(iv)
        signature = b64decode(signature)
    except Exception as e:
        print('niepoprawne kodowanie: ', e)
    if len(iv) != 16 or len(signature) != 16:
        print('Zła długość.')
        return

    try:
        if verify(message, iv, signature):
            d = json.loads(message)
            if d['authenticated']:
                print('Oto sekretna wiadomość: ', secret_message)
            else:
                print('Tym razem niestety nic')
        else:
            print('Niepoprawny podpis.')
    except Exception as e:
        print(e)
if message and iv and signature:
    x(message, iv, signature)

print('</p>')

print("""


  <p>Wszystkie rzeczy trzeba najpierw zakodować za pomocą base64!

  <form method="post">
    <p>message: <input type="text" name="message"/></p>
    <p>iv: <input type="text" name="iv"/></p>
    <p>signature: <input type="text" name="signature"/></p>
  <input type="submit">
  </form>

</body>

</html>
""")