## CryptoDB

### Ferdinand Sibleyras

#### Publications

**Year**

**Venue**

**Title**

2023

TOSC

Key Committing Security of AEZ and More
Abstract

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.

2022

EUROCRYPT

Beyond quadratic speedups in quantum attacks on symmetric schemes
📺
Abstract

In this paper, we report the first quantum key-recovery attack on a symmetric block cipher design, using classical queries only, with a more than quadratic time speedup compared to the best classical attack.
We study the 2XOR-Cascade construction of Ga{\v{z}}i and Tessaro (EUROCRYPT~2012). It is a key length extension technique which provides an n-bit block cipher with 5n/2 bits of security out of an n-bit block cipher with 2n bits of key, with a security proof in the ideal model. We show that the offline-Simon algorithm of Bonnetain et al. (ASIACRYPT~2019) can be extended to, in particular, attack this construction in quantum time $\widetilde{\mathcal{O}}{2^n}$, providing a 2.5 quantum speedup over the best classical attack.
Regarding post-quantum security of symmetric ciphers, it is commonly assumed that doubling the key sizes is a sufficient precaution. This is because Grover's quantum search algorithm, and its derivatives, can only reach a quadratic speedup at most. Our attack shows that the structure of some symmetric constructions can be exploited to overcome this limit. In particular, the 2XOR-Cascade cannot be used to generically strengthen block ciphers against quantum adversaries, as it would offer only the same security as the block cipher itself.

2022

TOSC

Cryptanalysis of Rocca and Feasibility of Its Security Claim
Abstract

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.

2022

ASIACRYPT

Key-Reduced Variants of 3kf9 with Beyond-Birthday-Bound Security
📺
Abstract

3kf9 is a three-key CBC-type MAC that enhances the standardized integrity algorithm f9 (3GPP-MAC). It has beyond-birthday-bound security and is expected to be a possible candidate in constrained environments when instantiated with lightweight blockciphers. Two variants 2kf9 and 1kf9 were proposed to reduce key size for efficiency, but recently, Leurent et al. (CRYPTO'18) and Shen et al. (CRYPTO'21) pointed out critical flaws on these two variants and invalidated their security proofs with birthday-bound attacks.
In this work, we revisit previous constructions of key-reduced variants of 3kf9 and analyze what went wrong in security analyzes. Interestingly, we find that a single doubling at the end can not only fix 2kf9 to go beyond the birthday bound, but also help 1kf9 to go beyond the birthday bound. We then propose two new key-reduced variants of 3kf9, called n2kf9 and n1kf9. By leveraging previous attempts, we prove that n2kf9 is secure up to 2^{2n/3} queries, and prove that n1kf9 is secure up to
2^{2n/3} queries when the message space is prefix-free. We also provide beyond-birthday analysis of n2kf9 in the multi-user setting. Note that compared to EMAC and CBC-MAC, the additional cost to provide a higher security guarantee is expected to be minimal for n2kf9 and n1kf9. It only requires one additional blockcipher call and one doubling.

2021

JOFC

Internal Symmetries and Linear Properties: Full-permutation Distinguishers and Improved Collisions on Gimli
Abstract

$$\mathsf {Gimli}$$ Gimli is a family of cryptographic primitives (both a hash function and an AEAD scheme) that has been selected for the second round of the NIST competition for standardizing new lightweight designs. The candidate $$\mathsf {Gimli}$$ Gimli is based on the permutation $$\mathsf {Gimli}$$ Gimli , which was presented at CHES 2017. In this paper, we study the security of both the permutation and the constructions that are based on it. We exploit the slow diffusion in $$\mathsf {Gimli}$$ Gimli and its internal symmetries to build, for the first time, a distinguisher on the full permutation of complexity $$2^{64}$$ 2 64 . We also provide a practical distinguisher on 23 out of the full 24 rounds of $$\mathsf {Gimli}$$ Gimli that has been implemented. Next, we give (full state) collision and semi-free start collision attacks on $$\mathsf {Gimli}$$ Gimli -Hash, reaching, respectively, up to 12 and 18 rounds. On the practical side, we compute a collision on 8-round $$\mathsf {Gimli}$$ Gimli -Hash. In the quantum setting, these attacks reach 2 more rounds. Finally, we perform the first study of linear trails in $$\mathsf {Gimli}$$ Gimli , and we find a linear distinguisher on the full permutation.

2020

TOSC

Release of Unverified Plaintext: Tight Unified Model and Application to ANYDAE
📺
Abstract

Authenticated encryption schemes are usually expected to offer confidentiality and authenticity. In case of release of unverified plaintext (RUP), an adversary gets separated access to the decryption and verification functionality, and has more power in breaking the scheme. Andreeva et al. (ASIACRYPT 2014) formalized RUP security using plaintext awareness, informally meaning that the decryption functionality gives no extra power in breaking confidentiality, and INT-RUP security, covering authenticity in case of RUP. We describe a single, unified model, called AERUP security, that ties together these notions: we prove that an authenticated encryption scheme is AERUP secure if and only if it is conventionally secure, plaintext aware, and INT-RUP secure. We next present ANYDAE, a generalization of SUNDAE of Banik et al. (ToSC 2018/3). ANYDAE is a lightweight deterministic scheme that is based on a block cipher with block size n and arbitrary mixing functions that all operate on an n-bit state. It is particularly efficient for short messages, it does not rely on a nonce, and it provides maximal robustness to a lack of secure state. Whereas SUNDAE is not secure under release of unverified plaintext (a fairly simple attack can be mounted in constant time), ANYDAE is. We make handy use of the AERUP security model to prove that ANYDAE achieves both conventional security as RUP security, provided that certain modest conditions on the mixing functions are met. We describe two simple instances, called MONDAE and TUESDAE, that conform to these conditions and that are competitive with SUNDAE, in terms of efficiency and optimality.

2020

TOSC

On the Security Margin of TinyJAMBU with Refined Differential and Linear Cryptanalysis
📺
Abstract

This paper presents the first third-party security analysis of TinyJAMBU, which is one of 32 second-round candidates in NIST’s lightweight cryptography standardization process. TinyJAMBU adopts an NLFSR based keyed-permutation that computes only a single NAND gate as a non-linear component per round. The designers evaluated the minimum number of active AND gates, however such a counting method neglects the dependency between multiple AND gates. There also exist previous works considering such dependencies with stricter models, however those are known to be too slow. In this paper, we present a new model that provides a good balance of efficiency and accuracy by only taking into account the first-order correlation of AND gates that frequently occurs in TinyJAMBU. With the refined model, we show a 338-round differential with probability 2−62.68 that leads to a forgery attack breaking 64-bit security. This implies that the security margin of TinyJAMBU with respect to the number of unattacked rounds is approximately 12%. We also show a differential on full 384 rounds with probability 2−70.64, thus the security margin of full rounds with respect to the data complexity, namely the gap between the claimed security bits and the attack complexity, is less than 8 bits. Our attacks also point out structural weaknesses of the mode that essentially come from the minimal state size to be lightweight.

2020

ASIACRYPT

New results on Gimli: full-permutation distinguishers and improved collisions
📺 ★
Abstract

Gimli is a family of cryptographic primitives (both a hash function and an AEAD scheme) that has been selected for the second round of the NIST competition for standardizing new lightweight designs. The candidate Gimli is based on the permutation Gimli, which was presented at CHES 2017. In this paper, we study the security of both the permutation and the constructions that are based on it. We exploit the slow diffusion in Gimli and its internal symmetries to build, for the first time, a distinguisher on the full permutation of complexity $2^{64}$. We also provide a practical distinguisher on 23 out of the full 24 rounds of Gimli that has been implemented.
Next, we give (full state) collision and semi-free-start collision attacks on Gimli-Hash, reaching respectively up to 12 and 18 rounds. On the practical side, we compute a collision on 8-round Gimli-Hash. In the quantum setting, these attacks reach 2 more rounds. Finally, we perform the first study of linear trails in the permutation, and we propose differential-linear cryptanalysis that reach up to 17 rounds of Gimli.

2019

CRYPTO

Low-Memory Attacks Against Two-Round Even-Mansour Using the 3-XOR Problem
📺
Abstract

The iterated Even-Mansour construction is an elegant construction that idealizes block cipher designs such as the AES. In this work we focus on the simplest variant, the 2-round Even-Mansour construction with a single key. This is the most minimal construction that offers security beyond the birthday bound: there is a security proof up to $$2^{2n/3}$$ evaluations of the underlying permutations and encryption, and the best known attacks have a complexity of roughly $$2^n/n$$ operations.We show that attacking this scheme with block size n is related to the 3-XOR problem with element size $$\ell = 2n$$, an important algorithmic problem that has been studied since the nineties. In particular the 3-XOR problem is known to require at least $$2^{\ell /3}$$ queries, and the best known algorithms require around $$2^{\ell /2}/\ell $$ operations: this roughly matches the known bounds for the 2-round Even-Mansour scheme.Using this link we describe new attacks against the 2-round Even-Mansour scheme. In particular, we obtain the first algorithms where both the data and the memory complexity are significantly lower than $$2^{n}$$. From a practical standpoint, previous works with a data and/or memory complexity close to $$2^n$$ are unlikely to be more efficient than a simple brute-force search over the key. Our best algorithm requires just $$\lambda n$$ known plaintext/ciphertext pairs, for some constant $$0< \lambda < 1$$, $$2^n/\lambda n$$ time, and $$2^{\lambda n}$$ memory. For instance, with $$n=64$$ and $$\lambda = 1/2$$, the memory requirement is practical, and we gain a factor 32 over brute-force search. We also describe an algorithm with asymptotic complexity $$\mathcal {O}(2^{n} \ln ^2 n/n^2)$$, improving the previous asymptotic complexity of $$\mathcal {O}(2^n/n)$$, using a variant of the 3-SUM algorithm of Baran, Demaine, and Pǎtraşcu.

2018

CRYPTO

Generic Attacks Against Beyond-Birthday-Bound MACs
📺
Abstract

In this work, we study the security of several recent MAC constructions with provable security beyond the birthday bound. We consider block-cipher based constructions with a double-block internal state, such as SUM-ECBC, PMAC+, 3kf9, GCM-SIV2, and some variants (LightMAC+, 1kPMAC+). All these MACs have a security proof up to $$2^{2n/3}$$ queries, but there are no known attacks with less than $$2^{n}$$ queries.We describe a new cryptanalysis technique for double-block MACs based on finding quadruples of messages with four pairwise collisions in halves of the state. We show how to detect such quadruples in SUM-ECBC, PMAC+, 3kf9, GCM-SIV2 and their variants with $$\mathcal {O}(2^{3n/4})$$ queries, and how to build a forgery attack with the same query complexity. The time complexity of these attacks is above $$2^n$$, but it shows that the schemes do not reach full security in the information theoretic model. Surprisingly, our attack on LightMAC+ also invalidates a recent security proof by Naito.Moreover, we give a variant of the attack against SUM-ECBC and GCM-SIV2 with time and data complexity $$\tilde{\mathcal {O}}(2^{6n/7})$$. As far as we know, this is the first attack with complexity below $$2^n$$ against a deterministic beyond-birthday-bound secure MAC.As a side result, we also give a birthday attack against 1kf9, a single-key variant of 3kf9 that was withdrawn due to issues with the proof.

#### Coauthors

- Xavier Bonnetain (1)
- Donghoon Chang (1)
- Yu Long Chen (1)
- Nilanjan Datta (1)
- Avijit Dutta (1)
- Antonio Flórez-Gutiérrez (1)
- Antonio Flórez Gutiérrez (2)
- Akinori Hosoyamada (1)
- Akiko Inoue (2)
- Ryoma Ito (2)
- Tetsu Iwata (2)
- Gaëtan Leurent (5)
- Bart Mennink (1)
- Kazuhiko Mimematsu (1)
- Kazuhiko Minematsu (1)
- Nicky Mouha (1)
- Yusuke Naito (1)
- Mridul Nandi (2)
- María Naya-Plasencia (2)
- Léo Perrin (2)
- Dhiman Saha (1)
- Somitra Sanadhya (1)
- Yu Sasaki (1)
- André Schrottenloher (3)
- Yaobin Shen (1)
- Danping Shi (1)
- Siwei Sun (1)
- Yosuke Todo (2)
- Yingjie Zhang (1)