: Observe that the LSB of the keystream is directly tied to the LSB of the initial state. Bit-Stepping : Assume the first bits of the state are known. Simulate the T-function for the next bit (
: 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) ti_moe_more
-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
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 : Observe that the LSB of the keystream
-th bit of the state update does not depend on bits higher than
: 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]. : The state size or the complexity of
Compare the resulting bit with the observed ciphertext/keystream bit.