cryptopals set 1

Occam's Razor

The most straightforward theory is preferable to all others of several sufficient possible explanations for the same state of facts. In other words: The simplest explanation is always the most probable.

Overview

For reference, here is where I got these challenges: https://cryptopals.com/

Problem 1

Convert hex to base64

The string: 49276d206b696c6c696e6720796f757220627261696e206c696b65206120706f69736f6e6f7573206d757368726f6f6d

Should produce:

SSdtIGtpbGxpbmcgeW91ciBicmFpbiBsaWtlIGEgcG9pc29ub3VzIG11c2hyb29t

Solution

import base64

def tobase64(hec):
    return base64.b64encode(bytes.fromhex(hec))

hec = '49276d206b696c6c696e6720796f757220627261696e206c696b65206120706f69736f6e6f7573206d757368726f6f6d'
print(tobase64(hec))

This one was pretty straightforward. Convert the hexadecimal to raw bytes with the bytes.fromhex() function, and then base64 encode that output with base64.b64encode().

Problem 2

Fixed XOR

Write a function that takes two equal-length buffers and produces their XOR combination.

If your function works properly, then when you feed it the string:

1c0111001f010100061a024b53535009181c

… after hex decoding, and when XOR‘d against:

686974207468652062756c6c277320657965

… should produce:

746865206b696420646f6e277420706c6179

Solution

import binascii

def fixed_xor(one,two):
    one_bytes = binascii.a2b_hex(one)
    two_bytes = binascii.a2b_hex(two)
	if len(one_bytes) != len(two_bytes):
		return -1
	xored = bytes(a^b for (a,b) in zip(one_bytes, two_bytes))
	return xored

one = "1c0111001f010100061a024b53535009181c"
two = "686974207468652062756c6c277320657965"

ret = fixed_xor(one,two)
print(f'hex:{binascii.hexlify(ret)\nascii: {ret.decode("ascii")}')

Problem 3

Single-byte XOR cipher

The hex encoded string:

1b37373331363f78151b7f2b783431333d78397828372d363c78373e783a393b3736

… has been XOR’d against a single character. Find the key, decrypt the message.

You can do this by hand. But don’t: write code to do it for you.

How? Devise some method for “scoring” a piece of English plaintext. Character frequency is a good metric. Evaluate each output and choose the one with the best score.

Achievement Unlocked

You now have our permission to make "ETAOIN SHRDLU" jokes on Twitter.

Solution

Things are quickly getting interesting.