Cryptanalysis

I made one crypto challenge each for CODEGATE CTF 2022 quals and finals. In this article, I’m gonna explain how the interpolation attack works, and show the way to solve my challenge “Hidden Command Service” from the finals. Interpolation Attack Once upon a time, there was a dangerous attack technique called ‘differential cryptanalysis’… Then, two crypto-magicians made a scheme which is provably safe from the differential cryptanalysis, called KN-Cipher. However, it soon turned out that the structure of S-boxes used in those ciphers are vulnerable to a new attack called ‘interpolation attack’, and a variant of KN-Cipher called PURE is broken....

I made one crypto challenge each for CODEGATE CTF 2022 quals and finals. In this article, I’m gonna explain how the cube attack works, and show the way to solve my challenge “Happy S-box” from the quals. Cube Attack Consider a multivariate equation over $\text{GF}(2)$, $f(x_1, \dots, x_n)$. There will be multiple monomials in $f$, and what we want to do is to pick one monomial $g = x_{i_1}x_{i_2} \cdots x_{i_k}$ and divide $f$ by $g$....