5.3 BRUTE FORCE AGAIN

EXERCISE 5.3: BRUTE FORCE AGAIN

You should already have done some brute-force attacks in previous chapters, but it’s important to repeat the exercise until you develop intuition for the concept. Using our preceding fake HMAC, have the computer generate a random password of specific sizes and use brute-force methods to find out what it is. To be more specific, assume that you already know what the message is (e.g. “hello world”, “happy birthday”, or a message of your choosing). Write a program to create a random password of characters, prepend the password to the message, and then print out the MAC (hash). Take the output and iterate through all possible passwords until you find the right one. Start with a simple test of a single-letter character, then try two characters, and so forth. Mix things up by using different sets of characters such as all lowercase, lowercase and uppercase, either case plus numbers, and so forth.

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# ex5_3.py 

import hashlib 
from string import ascii_lowercase

class PasswordNotFoundException(Exception):
    pass 

def generate(len_of_password: int, alphabet: str = ascii_lowercase):
    if len_of_password <= 0: return

    elif len_of_password == 1: 
        for c in alphabet:
            yield c

    else:
        for c in alphabet:
            for next in generate(len_of_password-1, alphabet):
                yield c + next

def h(x: bytes) -> str:
    '''returns the sha256 digest of the string x.'''
    return hashlib.sha256(x).hexdigest()


def bruteforce(digest: str, plaintext: bytes, len_of_password: int = 5) -> str:
    '''
    Searches and returns the password p, satisfying: 
        sha256(p + plaintext) = digest.
    
    The space this bruteforce search is in, all possible
    lowercase words that are of length len_of_password.

    Raises the exception PasswordNotFoundException, if password
    was not found.
    '''

    counter = 0
    for p in generate(len_of_password=len_of_password):
        counter += 1 
        if h(p.encode() + plaintext) == digest:
            print(f"[+] Found after {counter} attempts")
            return p 
    else:
        raise PasswordNotFoundException()
    
if __name__ == '__main__':
    print(
        bruteforce(
            digest='bddd033f5cbc2e88194a941abc314d7f5259764453f99b367d762ad49d3ed47f',
            plaintext=b"hello world",
            len_of_password=5,
        )
    )

running ex5_3.py