CRYPTOHACK

Bean Counter

Description

I've struggled to get PyCrypto's counter mode doing what I want, so I've turned ECB mode into CTR myself. My counter can go both upwards and downwards to throw off cryptanalysts! There's no chance they'll be able to read my picture.

Help

This page offers a convenient way for you to interact with the challenge functions. You can also use GET requests to send and receive data directly from the listed routes/endpoints if you wish. For more information see the FAQ.

Your aim is to recover the FLAG value. Once you have have it, submit it on the CryptoHack Symmetric Ciphers page.

Source

from Crypto.Cipher import AES


KEY = ?


class StepUpCounter(object):
    def __init__(self, value=os.urandom(16), step_up=False):
        self.value = value.hex()
        self.step = 1
        self.stup = step_up

    def increment(self):
        if self.stup:
            self.newIV = hex(int(self.value, 16) + self.step)
        else:
            self.newIV = hex(int(self.value, 16) - self.stup)
        self.value = self.newIV[2:len(self.newIV)]
        return bytes.fromhex(self.value.zfill(32))

    def __repr__(self):
        self.increment()
        return self.value



@chal.route('/bean_counter/encrypt/')
def encrypt():
    cipher = AES.new(KEY, AES.MODE_ECB)
    ctr = StepUpCounter()

    out = []
    with open("challenge_files/bean_flag.png", 'rb') as f:
        block = f.read(16)
        while block:
            keystream = cipher.encrypt(ctr.increment())
            xored = [a^b for a, b in zip(block, keystream)]
            out.append(bytes(xored).hex())
            block = f.read(16)

    return {"encrypted": ''.join(out)}


Interact

encrypt()
Output

XOR tool

Use this form to XOR two hex strings together.

Output
Hex Encoder/Decoder

This is a convenient encoder designed for ASCII <-> Hex translations. It won't work for decoding hex to byte streams and will just show [unprintable] in that case.

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