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WRITEUP.md

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Challenge name: Follow the Currents

Description:

go with the flow... Source

P.S: given files: enc, source.py

Solution:

A simple bruteforce challenge! all source code do is generating key with keystream function and XOR it with flag. Keystream has 2 bytes for initial. We bruteforce all 65536 of them, generate the key and compute the plaintext and check if it contains "actf{".

with open("enc","rb") as f:
    ciphertext = f.read()
    
plain = []
for j in range(65535):
    kb = j.to_bytes(2, 'big')
    k = keystream(kb)
    for i in ciphertext:
        plain.append(i ^ next(k))
        
    m = bytes(plain)
    if (b"actf{" in m):
        print (m)
    plain = []

final exploit.py

Output

b"x''R@\xc5\x05\xfb/9\xe7\x10\x13L\xc5\xd6\x19 minutes left before the ctf starts so i have no idea what to put here other than the flag which is actf{low_entropy_keystream}"
b'\x88\x86\x02\x15\x8b are like 30 minutes left before the ctf starts so i have no idea what to put here other than the flag which is actf{low_entropy_keystream}'
b'uX\x07\x80^\xec\xe0\x1e\x069\xe7\x10\x13L\xc5\xd6\x19 minutes left before the ctf starts so i have no idea what to put here other than the flag which is actf{low_entropy_keystream}'
b'\x85\xf9"\xc7\x95\t\x84\x97L like 30 minutes left before the ctf starts so i have no idea what to put here other than the flag which is actf{low_entropy_keystream}'
b'there are like 30 minutes left before the ctf starts so i have no idea what to put here other than the flag which is actf{low_entropy_keystream}'
b'\x84\xc9@5\xae\xc5\x05\xfb/9\xe7\x10\x13L\xc5\xd6\x19 minutes left before the ctf starts so i have no idea what to put here other than the flag which is actf{low_entropy_keystream}'
b'y\x17E\xa0{\t\x84\x97L like 30 minutes left before the ctf starts so i have no idea what to put here other than the flag which is actf{low_entropy_keystream}'
b'\x89\xb6`\xe7\xb0\xec\xe0\x1e\x069\xe7\x10\x13L\xc5\xd6\x19 minutes left before the ctf starts so i have no idea what to put here other than the flag which is actf{low_entropy_keystream}'

The Plaintext

there are like 30 minutes left before the ctf starts so i have no idea what to put here other than the flag which is actf{low_entropy_keystream}