International Association
for Cryptologic Research

We present the Deoxys family of authenticated encryption schemes, which consists of Deoxys-I and Deoxys-II . Both are nonce-based authenticated encryption schemes with associated data and have either 128- or 256-bit keys. Deoxys-I is similar to OCB : It is single-pass but insecure when nonces are repeated; in contrast, Deoxys-II is nonce-misuse resistant. Deoxys-II was selected as first choice in the final portfolio of the CAESAR competition for the defense-in-depth category. Deoxys uses a new family of tweakable block ciphers as internal primitive, Deoxys-TBC , which follows the TWEAKEY framework (Jean, Nikolić, and Peyrin, ASIACRYPT 2014) and relies on the AES round function. Our benchmarks indicate that Deoxys does not sacrifice efficiency for security and performs very well both in software (e.g., Deoxys-I efficiency is similar to AES-GCM ) and hardware.

We present an invariant subspace attack on the block cipher Midori64, proposed at Asiacrypt 2015. Our analysis shows that Midori64 has a class of 232 weak keys. Under any such key, the cipher can be distinguished with only a single chosen query, and the key can be recovered in 216 time with two chosen queries. As both the distinguisher and the key recovery have very low complexities, we confirm our analysis by implementing the attacks. Some tweaks of round constants make Midori64 more resistant to the attacks, but some lead to even larger weak-key classes. To eliminate the dependency on the round constants, we investigate alternative S-boxes for Midori64 that provide certain level of security against the found invariant subspace attacks, regardless of the choice of the round constants. Our search for S-boxes is enhanced with a dedicated tool which evaluates the depth of any given 4-bit S-box that satisfies certain design criteria. The tool may be of independent interest to future S-box designs.

We show generic attacks on unbalanced Feistel ciphers based on the meet-in-the-middle technique. We analyze two general classes of unbalanced Feistel structures, namely contracting Feistels and expanding Feistels. In both of the cases, we consider the practical scenario where the round functions are keyless and known to the adversary. In the case of contracting Feistels with 4 branches, we show attacks on 16 rounds when the key length k (in bits) is as large as the block length n (in bits), and up to 24 rounds when k = 2n. In the case of expanding Feistels, we consider two scenarios: one, where different nonlinear functions without particular structures are used in the round function, and a more practical one, where a single nonlinear is used but different linear functions are introduced in the state update. In the former case, we propose generic attacks on 13 rounds when k = n, and up to 21 rounds when k = 2n. In the latter case, 16 rounds can be attacked for k = n, and 24 rounds for k = 2n.