Half_and_Half

Half and Half

Crypto

Challenge

Mm, coffee. Best served with half and half!

Author: defund

half_and_half.py

Solution

As seen in the code

half = len(flag)//2
milk = flag[:half]
cream = flag[half:]

assert xor(milk, cream) == '\x15\x02\x07\x12\x1e\x100\x01\t\n\x01"'

Working operation:

• The flag is split into 2 halves.
• After which, the halves are XOR-ed together
• Resultant string is 12 chars long.

We know from the flag format that:

• milk starts with actf{
• cream ends with }

We can recover 6 of the 12 chars.

Python shell

def xor(x, y):
	o = ''
	for i in range(len(x)):
		o += chr(ord(x[i])^ord(y[i]))
	return o

encoded = '\x15\x02\x07\x12\x1e\x100\x01\t\n\x01"'

>>> print(xor('actf{', encoded))
taste

>>> print(xor('\x00' * 11 + '}', encoded)[-1])
_

Fill in the progress

actf{      _taste      }
123456789ABC123456789ABC

---

From the challenge, we have the hint of coffee

>>> print(xor('actf{coffee', encoded))
tastes_good

Fill in the progress again

actf{coffee_tastes_good}
123456789ABC123456789ABC

Flag

actf{coffee_tastes_good}