Writeup for InCTF 2014 - Crypto 100 - XOR decipher

xor_decipher.py

#!/usr/bin/env python
 
import hashlib
 
 
"""
one.txt
This sentence is encrypted using XOR cipher.
"""
 
plain_text = open('one.txt','r').read().strip()
 
 
"""
one.txt.enc
LAcbGEUKHQEGDgsaHU8bGEUcFgwAEhUNHQtSHhYQFghSMyorWAwbGw0cCkE=
"""
 
cipher_text = open('one.txt.enc','r').read().decode("base64")
 
print plain_text
print '---------------------------------------------------------------------'
 
print cipher_text
print '---------------------------------------------------------------------'
 
plain_text = [ord(i) for i in plain_text]
cipher_text = [ord(i) for i in cipher_text]
 
key = ''
for i in range(len(plain_text)):
    c = ((plain_text[i] ^ cipher_text[i]))
    key += chr(c)
 
print key
print '---------------------------------------------------------------------'
print 'VERFICIATION'
key='xorkey'
 
key_text = [ord(i) for i in key]
msg = ''
check = len(key_text)
j=0
for i in range(len(cipher_text)):
    if check > j:
        c = ((key_text[j] ^ cipher_text[i]))
        msg+= chr(c)
        if j+1 == check:
            j=0
        else:
            j+=1
 
print '--------------------------------------------------'
print msg
print '--------------------------------------------------'
 
print '---------------------------------------------------------------------'
print 'CAPTURE THE FLAG'
key='xorkey'
 
 
"""
Second.txt.enc
LAcXSz02Kk8RAhURHR1SAhZZGU8EDhcAWBgXCg5ZGwcdAgYcWAkdGUUYWAwbGw0cCk8TBQFZCwcdHgkdWAEdH0UQFk8CGQQaDAYRDktZLAcXSwMVGQhSDQoLWBsaAhZZFAoEDglZERxSHw0cWAsXCBcACBsbBAtZEwoLSwwNCwoeDUs= 
"""
 
key_text = [ord(i) for i in key]
cipher_text = open('two.txt.enc','r').read().decode("base64")
cipher_text = [ord(i) for i in cipher_text]
 
 
flag = ''
check = len(key_text)
j=0
for i in range(len(cipher_text)):
    if check > j:
        c = ((key_text[j] ^ cipher_text[i]))
        flag+= chr(c)
        if j+1 == check:
            j=0
        else:
            j+=1
 
print '--------------------------------------------------'
print flag
print '--------------------------------------------------'