Cryptanalysis of MORUS

Tomer Ashur (1) , Maria Eichlseder (2) , Martin M Lauridsen (3) , Gaëtan Leurent (4) , Brice Minaud (5) , Yann Rotella (4) , Yu Sasaki (6) , Benoît Viguier (7)

1 ESAT-COSIC - Computer Security and Industrial Cryptography [KU Leuven]
2 TU Graz - Graz University of Technology [Graz]
3 Datatilsynet - Danish Data Protection Agency
4 SECRET - Security, Cryptology and Transmissions
5 RHUL - Royal Holloway [University of London]
6 NTT Basic Research Laboratories [Tokio]
7 Radboud University [Nijmegen]

MORUS is a high-performance authenticated encryption algorithm submitted to the CAESAR competition, and recently selected as a finalist. There are three versions of MORUS: MORUS-640 with a 128-bit key, and MORUS-1280 with 128-bit or 256-bit keys. For all versions the security claim for confidentiality matches the key size. In this paper, we analyze the components of this algorithm (initialization, state update and tag generation), and report several results. As our main result, we present a linear correlation in the keystream of full MORUS, which can be used to distinguish its output from random and to recover some plaintext bits in the broadcast setting. For MORUS-1280, the correlation is 2 −76 , which can be exploited after around 2 152 encryptions, less than what would be expected for a 256-bit secure cipher. For MORUS-640, the same attack results in a correlation of 2 −73 , which does not violate the security claims of the cipher. To identify this correlation, we make use of rotational invariants in MORUS using linear masks that are invariant by word-rotations of the state. This motivates us to introduce single-word versions of MORUS called MiniMORUS, which simplifies the analysis. The attack has been implemented and verified on MiniMORUS, where it yields a correlation of 2 −16. We also study reduced versions of the initialization and finalization of MORUS, aiming to evaluate the security margin of these components. We show a forgery attack when finalization is reduced from 10 steps to 3, and a key-recovery attack in the nonce-misuse setting when initialization is reduced from 16 steps to 10. These additional results do not threaten the full MORUS, but studying all aspects of the design is useful to understand its strengths and weaknesses.

Mots clés: CAESAR; Authenticated Encryption; Confidentiality; MORUS; Linear Cryptanalysis; Nonce Respecting

Domaines: Informatique [cs]; Cryptographie et sécurité [cs.CR]

Tomer Ashur, Maria Eichlseder, Martin M Lauridsen, Gaëtan Leurent, Brice Minaud, et al.. Cryptanalysis of MORUS. ASIACRYPT 2018 - 24th Annual International Conference on the Theory and Application of Cryptology and Information Security, Dec 2018, Brisbane, Australia. pp.35-64, ⟨10.1007/978-3-030-03329-3_2⟩. ⟨hal-01944776⟩