Writeup from CSAW QUALS CTF’18

CSAW CTF QUALS 2018: Babycrypto

Category: Crypto
Challenge Points: 50
Solves: 295
Description:

yeeeeeeeeeeeeeeeeeeeeeeeeeeeeeet

single yeet yeeted with single yeet == 0

yeeet

what is yeet?

yeet is yeet

Attachments: ciphertext.txt
PRO TIP:Tell you what, the smallest crypto will always be XOR in CSAW! And a XOR a == 0 ==>

|single yeet yeeted with single yeet == 0

And that’s the hint! to suggest it is a XOR challenge. So, we start with the attachment file, which includes a base64 encoded ciphertext.
Perform a single byte xor, and you’ll find the flag in character 255.

Leon is a programmer who aspires to create programs that help people do less. He wants to put automation first, and scalability alongside. He dreams of a world where the endless and the infinite become realities to mankind, and where the true value of life is preserved.flag{diffie-hellman-g0ph3rzraOY1Jal4cHaFY9SWRyAQ6aH}

Complete Script

import string
def xor(a,b):
    return chr(ord(i)^ord(j) for i,j in zip(a,b))

ct = open('ciphertext.txt','rb').read().decode('base64')
for i in range(256):
    pt = xor(ct,chr(i)*len(ct))
    if all(c in string.printable for c in pt): print pt

Flag

flag{diffie-hellman-g0ph3rzraOY1Jal4cHaFY9SWRyAQ6aH}