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Ascon (cipher)

Ascon is a family of lightweight authenticated ciphers that had been selected by US National Institute of Standards and Technology (NIST) for future standardization of the lightweight cryptography.^[2]

Ascon was developed in 2014 by a team of researchers from Graz University of Technology, Infineon Technologies, Lamarr Security Research, and Radboud University.^[3] The cipher family was chosen as a finalist of the CAESAR Competition^[3] in February 2019.

NIST had announced its decision on February 7, 2023^[3] with the following intermediate steps that would lead to the eventual standardization:^[2]

• Publication of NIST IR 8454 describing the process of evaluation and selection that was used;
• Preparation of a new draft for public comments;
• Public workshop to be held on June 21-22, 2023.

The design is based on a sponge construction along the lines of SpongeWrap and MonkeyDuplex. This design makes it easy to reuse Ascon in multiple ways (as a cipher, hash, or a MAC).^[4] As of February 2023, the Ascon suite contained seven ciphers,^[3] including:^[5]

• Ascon-128 and Ascon-128a authenticated ciphers;
• Ascon-Hash cryptographic hash;
• Ascon-Xof extendable-output function;
• Ascon-80pq cipher with an "increased" 160-bit key.

The main components have been borrowed from other designs:^[4]

• substitution layer utilizes a modified S-box from the χ function of Keccak;
• permutation layer functions are similar to the ${\displaystyle \Sigma }$ofSHA-2.

The ciphers are parameterizable by the key length k (up to 128 bits), "rate" (block size) r, and two numbers of rounds a, b. All algorithms support authenticated encryption with plaintext P and additional authenticated data A (that remains unencrypted). The encryption input also includes a public nonce N, the output - authentication tag T, size of the ciphertext C is the same as that of P. The decryption uses N, A, C, and T as inputs and produces either P or signals verification failure if the message has been altered. Nonce and tag have the same size as the key K (k bits).^[6]

In the CAESAR submission, two sets of parameters were recommended:^[6]

The data in both A and P is padded with a single bit with the value of 1 and a number of zeros to the nearest multiple of r bits. As an exception, if A is an empty string, there is no padding at all.^[7]

The state consists of 320 bits, so the capacity ${\displaystyle c=320-r}$.^[8] The state is initialized by an initialization vector IV (constant for each cipher type, e.g., hex 80400c0600000000 for Ascon-128) concatenated with K and N.^[9]

The initial state is transformed by applying a times the transformation function p (${\displaystyle p^{a}}$). On encryption, each word of A || P is XORed into the state and the p is applied b times (${\displaystyle p^{b}}$). The ciphertext C is contained in the first r bits of the result of the XOR. Decryption is near-identical to encryption.^[8] The final stage that produces the tag T consists of another application of ${\displaystyle p^{a}}$; the special values are XORed into the last c bits after the initialization, the end of A, and before the finalization.^[7]

Transformation p consists of three layers:

• ${\displaystyle p_{C}}$, XORing the round constants;
• ${\displaystyle p_{S}}$, application of 5-bit S-boxes;
• ${\displaystyle p_{L}}$, application of linear diffusion.

Hash values of an empty string (i.e., a zero-length input text) for both the XOF and non-XOF variants.^[10]

Ascon-Hash("")
0x 7346bc14f036e87ae03d0997913088f5f68411434b3cf8b54fa796a80d251f91
Ascon-HashA("")
0x aecd027026d0675f9de7a8ad8ccf512db64b1edcf0b20c388a0c7cc617aaa2c4
Ascon-Xof("", 32)
0x 5d4cbde6350ea4c174bd65b5b332f8408f99740b81aa02735eaefbcf0ba0339e
Ascon-XofA("", 32)
0x 7c10dffd6bb03be262d72fbe1b0f530013c6c4eadaabde278d6f29d579e3908d

Even a small change in the message will (with overwhelming probability) result in a different hash, due to the avalanche effect.

Ascon-Hash("The quick brown fox jumps over the lazy dog")
0x 3375fb43372c49cbd48ac5bb6774e7cf5702f537b2cf854628edae1bd280059e
Ascon-Hash("The quick brown fox jumps over the lazy dog.")
0x c9744340ed476ac235dd979d12f5010a7523146ee90b57ccc4faeb864efcd048

• Speck - An earlier lightweight cipher family released by the U.S. National Security Agency

• NIST (2023a). "Lightweight Cryptography Standardization Process: NIST Selects Ascon". nist.gov. National Institute of Standards and Technology.
• NIST (2023b). "NIST Selects 'Lightweight Cryptography' Algorithms to Protect Small Devices". nist.gov. National Institute of Standards and Technology.
• Dobraunig, Christoph; Eichlseder, Maria; Mendel, Florian; Schläffer, Martin (2016). "Ascon v1.2: Submission to the CAESAR Competition" (PDF). nist.gov. National Institute of Standards and Technology.
• Dobraunig, Christoph; Eichlseder, Maria; Mendel, Florian; Schläffer, Martin (22 June 2021). "Ascon v1.2: Lightweight Authenticated Encryption and Hashing". Journal of Cryptology. 34 (3). doi:10.1007/s00145-021-09398-9. eISSN 1432-1378. hdl:2066/235128. ISSN 0933-2790. S2CID 253633576.

• TU Graz. "Ascon: Publications". tugraz.at.

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