Linear Consistency Test (LCT) in cryptanalysis of irregularly clocked LFSRs in the presence of noise
Guro Bu · NORA - Norwegian Open Research Archives · 2011
ENGELSK: Keystream generators using irregular clocking are frequently used to generate the keystream in a stream cipher. These generators are typically composed of two Linear Feedback Shift Registers (LFSRs), where the clocking of the second LFSR is controlled by the output sequence from the first LFSR. Various attacks exist to cryptanalyse such generators. Among the more popular are correlation attacks and algebraic attacks. When only the ciphertext is known, the usual approach is to use correlation methods. These attacks are accomplished in two stages. First, the initial state of the clocked LFSR is found, then the clock control sequence is reconstructed. Algebraic attacks, on the other hand, have only been applied in the known-plaintext scenario to cryptanalyse irregularly clocked generators. The Linear Consistency Test (LCT) is an algebraic key recovery attack which uses some guessed bits from the internal state of the generator to set up a set of equations used to determine whether the initial guess was correct or not. In this thesis, an extension of the LCT attack is implemented that successfully reconstructs the clock control sequence in the ciphertext only scenario. Experiments show that this attack can handle low to moderate levels of noise, and it is estimated to perform better than correlation attacks on irregularly clocked generators.