Block ciphering system, using permutations to hide the core ciphering function of each encryption round

Abstract

In a system, a server provides a digital signal processing function ƒ to an executing device in an obfuscated form. The function ƒ includes a function cascade of signal processing functions ƒ, 1≦i≦N (e.g., FC(x)≡ƒ∘ . . . ∘ƒ(x)). The server includes a processor for selecting a set of 2N invertible permutations p, 1≦i≦2N; calculating a set of N functions g, where gis functionally equivalent to p∘ƒ∘p, for 1≦i≦N; and calculating a set of N−1 functions h, where his functionally equivalent to p∘p, for 2≦i≦N. The server equips the executing device with an execution device function cascade that includes y∘h∘y∘h∘ . . . ∘y, where y, . . . , yare function parameters (e.g., ED(y, . . . , y)≡y∘h∘y∘h∘ . . . ∘y), and provides the functions g, . . . gto the executing device. The executing device obtains the functions g, . . . , gand a processor for loading the execution device function cascade and applying the loaded execution device function cascade to the functions g, . . . , g(e.g., ED(g, . . . , g)).