Guards in action: First-order SCA secure implementations of KETJE without additional randomness

Abstract

Recently the CAESAR competition has announced several finalists among the submitted authenticated encryption algorithms, after an open selection process during the last five years. Applications using these algorithms are rapidly increasing today. Devices implementing these applications are enormously susceptible to physical attacks, which are able to retrieve secret data through side-channel information such as power consumption or the electromagnetic radiations. In this work, we present a Side-Channel Analysis resistant hardware implementation of the whole family of authenticated encryption schemes Ketje. By changing just one parameter, any of the Ketje designs can be obtained, and tailored for different applications, either lightweight or high throughput.We introduce a new protected Keccak implementation, as well as unprotected and protected Ketje implementations, which allow both encryption and decryption modes in the same module. In order to secure these implementations we make use of the masking scheme known as Threshold Implementations and complement it with the technique of “Changing of the Guards”, achieving a first-order Side-Channel Analysis protected implementation with zero extra randomness needed. This way, no dedicated PRNG needs to be additionally implemented, avoiding issues such as the security of the PRNG itself or the quality of the randomness. We elaborate on the importance of the input dependencies and how the addition of linear blocks could thwart the security provided by a correctly protected module. This is of special importance when extending Keccakto implement Ketje. Furthermore, we provide an empirical evaluation of both Keccakand Ketjeimplementations, demonstrating how with our methodology we can construct a secure Ketjebased on a previously secured Keccak.