SU CTF 2014 – Rolling Hash Write up

def RabinKarpRollingHash( str, a, n ):
        result = 0
        l = len(str)
        for i in range(0, l):
                result += ord(str[i]) * a ** (l - i - 1) % n
        print "result = ", result

RabinKarpRollingHash(flag, 256, 10**30)

output is 1317748575983887541099 What is the flag?

Well, the point is 10**30 is a very large number and calculating the modulus has no effect on output. also 256 is a power of 2.

Given code calculates ASCII code for each character of input and multiplies it in 256 ** (l-i-1) that makes their bits shift (8*k) then adds all results together. So representation of 1317748575983887541099 in binary contains concatenated binary codes of flag in reverse order. I used  this site to convert this number to binary.

chars = [
flag = ''
for ch in chars:
  flag = flag + chr(ch)

print flag

Well, that site failed in last bits and this code outputs ‘Good Lt’ but it’s very easy to guess that flag was Good Luck

EDIT: a better solution for the last step mentioned in comments:

flag = ''
number = 1317748575983887541099 
p = number
while p > 0:
  flag = chr(p%256) + flag
  p = p / 256

print flag
Mohammad Jafar Mashhadi

Mohammad Jafar Mashhadi

Your average genius.

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