Recover the Secret Components in a ForkCipher

Recently, a new cryptographic primitive has been proposed called ForkCiphers. This paper aims at proposing new generic cryptanalysis against such constructions. We give a generic method to apply existing decompositions againt the underlying block cipher \calE^\boldsymbolr on the forking variant Fork \calE -(r−1)-r₀-(r+1−r₀). As application, we consider the security of ForkSPN and ForkFN with secret inner functions. We provide a generic attack against ForkSPN-2-r₀-(4−r₀) based on the decomposition of SASAS. And also we extend the decomposition of Biryukov et al. against Feistel networks in SAC 2015 to get all the unknown round functions in ForkFN-r-r₀-r₁ for r≤ 6 and r₀+r₁≤ 8. Therefore, compared with the original block cipher, the forking version requires more iteration rounds to resist the recovery attack.