Impossible and improbable differential cryptanalysis of Spook algorithm

Bolel, Onur
In recent years, the number of IoT devices increased considerably and the security of IoT devices became an important issue. Furthermore, most IoT devices have constrained resources in terms of memory, area and power. Therefore, cryptographic algorithms that provide their security should be suitable for the implementation on the constrained devices. In 2013, NIST initiated a lightweight cryptography project to define the standards of lightweight cryptography. In 2018, the lightweight cryptography project turned into a competition-like process to choose the most convenient algorithms for constrained devices as a NIST standard. 57 algorithms were applied to the project. NIST published all algorithms for public evaluation and encouraged third-party analyses to reveal the weaknesses of algorithms. 32 algorithms were chosen as round 2 candidates. In this thesis, we have investigated the Spook algorithm, which is one of the round 2 candidates of the NIST’s lightweight cryptography competition. Spook is an AEAD algorithm that uses duplex sponge construction and tweakable block cipher. Besides, Spook has an internal permutation which is Shadow-512. We have worked on Shadow-512 permutation to find a distinguisher. Shadow-512 permutation was designed as 6-Step. The outputs of Shadow-512 permutation should seem random after the 6-Step operation. However, we have found two different 6-Step impossible differential distinguishers that cover full Shadow-512. Besides, we have found 7-Step impossible distinguisher and 8-Step improbable distinguisher by adding one or more additional steps to Shadow-512. The 8-Step improbable differential covers the largest number of steps of Shadow-512 compared to previously found distinguishers in other published papers. To conclude, we can distinguish 6-, 7-, 8-Step of Shadow-512 from a random permutation by using our distinguishers.
Citation Formats
O. Bolel, “Impossible and improbable differential cryptanalysis of Spook algorithm,” M.S. - Master of Science, Middle East Technical University, 2021.