International Association
for Cryptologic Research

We present a new block cipher mode of operation for authenticated encryption (AE), dubbed XOCB, that has the following features: (1) beyond-birthday-bound (BBB) security based on standard pseudorandom assumption of the internal block cipher if the maximum block length is sufficiently smaller than the birthday bound, (2) rate-1 computation, and (3) supporting any block cipher with any key length. Namely, XOCB has effectively the same efficiency as the seminal OCB while having stronger quantitative security without any change in the security model or the required primitive in OCB. Although numerous studies have been conducted in the past, our XOCB is the first mode of operation to achieve these multiple goals simultaneously.

This paper presents a provably secure, higher-order, and leakage-resilient (LR) rekeying scheme named LR Rekeying with Random oracle Repetition (LR4), along with a quantitative security evaluation methodology. Many existing LR primitives are based on a concept of leveled implementation, which still essentially require a leak-free sanctuary (i.e., differential power analysis (DPA)-resistant component(s)) for some parts. In addition, although several LR pseudorandom functions (PRFs) based on only bounded DPA-resistant components have been developed, their validity and effectiveness for rekeying usage still need to be determined. In contrast, LR4 is formally proven under a leakage model that captures the practical goal of side-channel attack (SCA) protection (e.g., masking with a practical order) and assumes no unbounded DPA-resistant sanctuary. This proof suggests that LR4 resists exponential invocations (up to the birthday bound of key size) without using any unbounded leak-free component, which is the first of its kind. Moreover, we present a quantitative SCA success rate evaluation methodology for LR4 that combines the bounded leakage models for LR cryptography and a state-of-the-art information-theoretical SCA evaluation method. We validate its soundness and effectiveness as a DPA countermeasure through a numerical evaluation; that is, the number of secure calls of a symmetric primitive increases exponentially by increasing a security parameter under practical conditions.

For an Authenticated Encryption with Associated Data (AEAD) scheme, the key committing security refers to the security notion of whether the adversary can produce a pair of distinct input tuples, including the key, that result in the same output. While the key committing security of various nonce-based AEAD schemes is known, the security analysis of Robust AE (RAE) is largely unexplored. In particular, we are interested in the key committing security of AEAD schemes built on the Encode-then-Encipher (EtE) approach from a wide block cipher. We first consider AEZ v5, the classical and the first dedicated RAE that employs the EtE approach. We focus our analysis on the core part of AEZ to show our best attacks depending on the length of the ciphertext expansion. In the general case where the Tweakable Block Cipher (TBC) is assumed to be ideal, we show a birthday attack and a matching provable security result. AEZ adopts a simpler key schedule and the prove-then-prune approach in the full specification, and we show a practical attack against it by exploiting the simplicity of the key schedule. The complexity is 227, and we experimentally verify the correctness with a concrete example. We also cover two AEAD schemes based on EtE. One is built on Adiantum, and the other one is built on HCTR2, which are two wide block ciphers that are used in real applications. We present key committing attacks against these schemes when used in EtE and matching proofs for particular cases.

Rocca is an authenticated encryption with associated data scheme for beyond 5G/6G systems. It was proposed at FSE 2022/ToSC 2021(2), and the designers make a security claim of achieving 256-bit security against key-recovery and distinguishing attacks, and 128-bit security against forgery attacks (the security claim regarding distinguishing attacks was subsequently weakened in the full version in ePrint 2022/116). A notable aspect of the claim is the gap between the privacy and authenticity security. In particular, the security claim regarding key-recovery attacks allows an attacker to obtain multiple forgeries through the decryption oracle. In this paper, we first present a full key-recovery attack on Rocca. The data complexity of our attack is 2128 and the time complexity is about 2128, where the attack makes use of the encryption and decryption oracles, and the success probability is almost 1. The attack recovers the entire 256-bit key in a single-key and nonce-respecting setting, breaking the 256-bit security claim against key-recovery attacks. We then extend the attack to various security models and discuss several countermeasures to see the feasibility of the security claim. Finally, we consider a theoretical question of whether achieving the security claim of Rocca is possible in the provable security paradigm. We present both negative and positive results to the question.

We present practical attacks on OCB2. This mode of operation of a blockcipher was designed with the aim to provide particularly efficient and provably secure authenticated encryption services, and since its proposal about 15 years ago it belongs to the top performers in this realm. OCB2 was included in an ISO standard in 2009. An internal building block of OCB2 is the tweakable blockcipher obtained by operating a regular blockcipher in $${\text {XEX}}^*$$ XEX ∗  mode. The latter provides security only when evaluated in accordance with certain technical restrictions that, as we note, are not always respected by OCB2. This leads to devastating attacks against OCB2’s security promises: We develop a range of very practical attacks that, amongst others, demonstrate universal forgeries and full plaintext recovery. We complete our report with proposals for (provably) repairing OCB2. As a direct consequence of our findings, OCB2 is currently in a process of removal from ISO standards. Our attacks do not apply to OCB1 and OCB3, and our privacy attacks on OCB2 require an active adversary.

Tweakable block ciphers (TBCs) have proven highly useful to boost the security guarantees of authentication schemes. In 2017, Cogliati et al. proposed two MACs combining TBC and universal hash functions: a nonce-based MAC called NaT and a deterministic MAC called HaT. While both constructions provide high security, their properties are complementary: NaT is almost fully secure when nonces are respected (i.e., n-bit security, where n is the block size of the TBC, and no security degradation in terms of the number of MAC queries when nonces are unique), while its security degrades gracefully to the birthday bound (n/2 bits) when nonces are misused. HaT has n-bit security and can be used naturally as a nonce-based MAC when a message contains a nonce. However, it does not have full security even if nonces are unique.This work proposes two highly secure and efficient MACs to fill the gap: NaT2 and eHaT. Both provide (almost) full security if nonces are unique and more than n/2-bit security when nonces can repeat. Based on NaT and HaT, we aim at achieving these properties in a modular approach. Our first proposal, Nonce-as-Tweak2 (NaT2), is the sum of two NaT instances. Our second proposal, enhanced Hash-as-Tweak (eHaT), extends HaT by adding the output of an additional nonce-depending call to the TBC and prepending nonce to the message. Despite the conceptual simplicity, the security proofs are involved. For NaT2 in particular, we rely on the recent proof framework for Double-block Hash-then-Sum by Kim et al. from Eurocrypt 2020.

We present practical attacks on OCB2. This mode of operation of a blockcipher was designed with the aim to provide particularly efficient and provably-secure authenticated encryption services, and since its proposal about 15 years ago it belongs to the top performers in this realm. OCB2 was included in an ISO standard in 2009.An internal building block of OCB2 is the tweakable blockcipher obtained by operating a regular blockcipher in $$ \text {XEX} ^*$$ mode. The latter provides security only when evaluated in accordance with certain technical restrictions that, as we note, are not always respected by OCB2. This leads to devastating attacks against OCB2’s security promises: We develop a range of very practical attacks that, amongst others, demonstrate universal forgeries and full plaintext recovery. We complete our report with proposals for (provably) repairing OCB2. To our understanding, as a direct consequence of our findings, OCB2 is currently in a process of removal from ISO standards. Our attacks do not apply to OCB1 and OCB3, and our privacy attacks on OCB2 require an active adversary.