ITS_HA2_2A.py

#28 b4 3e 63 17 c3 a1 5d f5 09 68 26 50 4f dc a9 f2 1c a9 c3 e5 5e 89 1c ae 72 aa 1e 75 a9 07 46 2f dc 24 d0 be 21 b4 71 a1 2f 29 c8 13 a5 7f 41 cc 94 6b e4 1b 66 ad 10 4b 16 0f 23 06 46 dd db

#4 blocks
#1: 28 b4 3e 63 17 c3 a1 5d f5 09 68 26 50 4f dc a9
#2: f2 1c a9 c3 e5 5e 89 1c ae 72 aa 1e 75 a9 07 46
#3: 2f dc 24 d0 be 21 b4 71 a1 2f 29 c8 13 a5 7f 41
#4: cc 94 6b e4 1b 66 ad 10 4b 16 0f 23 06 46 dd db

import base64
import os
import subprocess

def decrypt_block(block_no):
    for byte_no in range(16): #go through every byte
        c1fake = bytearray(os.urandom(16)) #gen random bytearray

        for j in range(byte_no): #calc c1fake for valid padding
            c1fake[-(j+1)] = i2[-(j+1)] ^ byte_no+1 #c'1[16] ^ i2 = 02


        for test_hex in range(256): #find byte for correct padding
            c2 = blocks[block_no] #block to be decrypted
            testbyte = test_hex #generated byte in hex 

            c1fake[-(byte_no+1)] = testbyte #insert test byte in fake c1 block
            c1fake_c2 = bytes(c1fake) + bytes(c2) #merge blocks

            c1fake_c2_b64 = base64.b64encode(c1fake_c2) #base64 encode bytestring
            c1fake_c2_b64_str = c1fake_c2_b64.decode('utf-8') #convert to string
            c1fake_c2_b64_str_urlsafe = c1fake_c2_b64_str.replace('=', '%3D') #make url safe
            c1fake_c2_b64_str_urlsafe = c1fake_c2_b64_str_urlsafe.replace('+', '%2B')
            c1fake_c2_b64_str_urlsafe = c1fake_c2_b64_str_urlsafe.replace('/', '%2F')
            
            url = 'http://gruenau5.informatik.hu-berlin.de:8888/store_secret/'+c1fake_c2_b64_str_urlsafe #build url
            command = subprocess.Popen(["curl","--silent",url], stdout = subprocess.PIPE)
            answer = command.communicate()

            if answer[0] == 'Secret succesfully received': #if padding is right
                break

        c1 = blocks[block_no-1] #get real c1 block
        i2[-(byte_no+1)] = testbyte ^ byte_no+1 #calc intermediate state i2
        p2[-(byte_no+1)] = c1[-(byte_no+1)] ^ i2[-(byte_no+1)] #calc plaintext p2
        
    return(p2) #return plaintext 

if __name__ == "__main__":
    cipher = "KLQ+YxfDoV31CWgmUE/cqfIcqcPlXokcrnKqHnWpB0Yv3CTQviG0caEvKcgTpX9BzJRr5BtmrRBLFg8jBkbd2w==" #cipher
    cipher_hex = base64.b64decode(cipher) #base64 decode cipher
    testbyte = 0
    size = 16

    block1 = bytearray(cipher_hex[:16]) #partition cipher in blocks
    block2 = bytearray(cipher_hex[16:32])
    block3 = bytearray(cipher_hex[32:48])
    block4 = bytearray(cipher_hex[48:])
    blocks = [block1,block2,block3,block4]

    message = ""

    for i in range(1,4):
        p2 = bytearray(size)
        i2 = bytearray(size)
        message = message+decrypt_block(i) #decrypt blocks 2 to 4 (block 1 is IV)
    
    print(message+"\n")