: 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].
is a cryptographic challenge from the 2024 BuckeyeCTF , focusing on exploiting a flawed implementation of a Custom T-Function based stream cipher [1, 2]. Challenge Overview
The graph above illustrates the of T-functions: bit only depends on bits , allowing for the bit-by-bit recovery used in the exploit.
: Once the initial state is recovered, re-run the generator to produce the full keystream and XOR it with the provided ciphertext to reveal the flag [2, 6].
: 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)
-th bits of the input [3]. While T-functions are often used to create long-period sequences, improper implementation can lead to significant linear vulnerabilities. Key Vulnerabilities
The vulnerability in stems from the predictable bit-propagation within the T-function: Bit-by-Bit Leakage : Because the