, the state can be recovered one bit at a time starting from the Least Significant Bit (LSB) [4, 6].
The challenge provides a Python script that encrypts a flag using a custom-built keystream generator. The core of the generator relies on a , which is a type of mapping where the -th bit of the output only depends on the ti_moe_more
The graph above illustrates the of T-functions: bit only depends on bits , allowing for the bit-by-bit recovery used in the exploit. , the state can be recovered one bit
Compare the resulting bit with the observed ciphertext/keystream bit. Solution Strategy (Write-up)
: Since there may be multiple candidates for a bit that satisfy the equation temporarily, use a recursive search or a queue-based approach to find the state that consistently produces the correct keystream for the entire length of the flag [3, 4].
To solve the challenge, you must reverse the keystream generator to recover the initial state (the key):
: The state size or the complexity of the mixing function is insufficient to prevent a guess-and-determine attack or a simple breadth-first search on the bit transitions [3, 5]. Solution Strategy (Write-up)