Breaking Panther

Panther is a sponge-based lightweight authenticated encryption scheme published at Indocrypt 2021. Its round function is based on four Nonlinear Feedback Shift Registers (NFSRs). We show here that it is possible to fully recover the secret key of the construction by using a single known plaintext-ciphertext pair and with minimal computational resources. Furthermore, we show that in a known ciphertext setting an attacker is able with the knowledge of a single ciphertext to decrypt all plaintext blocks expect for the very first ones and can forge the tag with only one call and probability one. As we demonstrate, the problem of the design comes mainly from the low number of iterations of the round function during the absorption phase. All of our attacks have been implemented and validated.