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Paper 2024/858

Ascon-Keccak AEAD Algorithm

Abstract

The Ascon specification defines among others an encryption scheme offering authenticated encryption with associated data (AEAD) which is based on a duplex mode of a sponge. With that it is the first of such algorithm selected and about to be standardized by NIST. The sponge size is comparatively small, 320 bits, as expected for lightweight cryptography. With that, the strength of the defined AEAD algorithm is limited to 128 bits. Albeit, the definition of the Ascon AEAD algorithm integrates with the associated sponge, it is mathematically not bound to exactly this sponge function. Thus, the Ascon AEAD specification can be used with a different sponge and still operate as defined by the Ascon authors. This specification defines the Ascon-Keccak AEAD algorithm which replaces the Ascon sponge with the Keccak sponge, leaving the Ascon AEAD algorithm unchanged. The selected parameters for Ascon-Keccak AEAD offer two algorithm strengths: Ascon-Keccak 256 with a classic security strength of 256 bits and a quantum security strength of 128 bits. In addition, Ascon-Keccak 512 provides an algorithm with 512 bit classic security strength and 256 bit quantum security strength. The selected parameters for Ascon-Keccak 256 offer a significant higher performance on 64-bit architectures than Ascon-128 and Ascon-128a. The performance of Ascon-Keccak 512 is in league with Ascon-128. Yet, with the Keccak sponge size of 1600 bits, Ascon-Keccak cannot be considered a lightweight cryptographic algorithm any more. A reference implementation of the algorithm is provided as referenced in the document.