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A Practical Fault Attack on ARX-Like Ciphers with a Case Study on ChaCha20

This paper presents the first practical fault attack on the ChaCha family of addition-rotation-XOR (ARX)-based stream ciphers. ChaCha has recently been deployed for speeding up and strengthening HTTPS connections for Google Chrome on Android devices. In this paper, we propose differential fault analysis attacks on ChaCha without resorting to nonce misuse. We use the instruction skip and instruction replacement fault models, which are popularly mounted on microcontroller-based cryptographic implementations. We corroborate the attack propositions via practical fault injection experiments using a laser-based setup targeting an Atmel AVR 8-bit microcontroller-based implementation of ChaCha. Each of the proposed attacks can be repeated with 100% accuracy in our fault injection setup, and can recover the entire 256 bit secret key using 5-8 fault injections on an average.