Xiaoyun Wang

Bio: Xiaoyun Wang is an academic researcher from Tsinghua University. The author has contributed to research in topics: Block cipher & Hash function. The author has an hindex of 21, co-authored 73 publications receiving 5276 citations. Previous affiliations of Xiaoyun Wang include University of Hong Kong & Shandong University.

Automatic Search of Meet-in-the-Middle Preimage Attacks on AES-like Hashing

Abstract: The Meet-in-the-Middle (MITM) preimage attack is highly effective in breaking the preimage resistance of many hash functions, including but not limited to the full MD5, HAVAL, and Tiger, and reduced SHA-0/1/2. It was also shown to be a threat to hash functions built on block ciphers like AES by Sasaki in 2011. Recently, such attacks on AES hashing modes evolved from merely using the freedom of choosing the internal state to also exploiting the freedom of choosing the message state. However, detecting such attacks especially those evolved variants is difficult. In previous works, the search space of the configurations of such attacks is limited, such that manual analysis is practical, which results in sub-optimal solutions. In this paper, we remove artificial limitations in previous works, formulate the essential ideas of the construction of the attack in well-defined ways, and translate the problem of searching for the best attacks into optimization problems under constraints in Mixed-Integer-Linear-Programming (MILP) models. The MILP models capture a large solution space of valid attacks; and the objectives of the MILP models are attack configurations with the minimized computational complexity. With such MILP models and using the off-the-shelf solver, it is efficient to search for the best attacks exhaustively. As a result, we obtain the first attacks against the full (5-round) and an extended (5.5-round) version of Haraka-512 v2, and 8-round AES-128 hashing modes, as well as improved attacks covering more rounds of Haraka-256 v2 and other members of AES and Rijndael hashing modes.

New Linear Cryptanalytic Results of Reduced-Round of CAST-128 and CAST-256

Abstract: This paper presents a linear cryptanalysis for reduced round variants of CAST-128 and CAST-256 block ciphers. Compared with the linear relation of round function with the bias 2? 17 by J. Nakahara et al., we found the more heavily biased linear approximations for 3 round functions and the highest one is 2? 12.91. We can mount the known-plaintext attack on 6-round CAST-128 and the ciphertext-only attack on 4-round CAST-128. Moreover the known-plaintext attack on 24-round CAST-256 with key size 192 and 256 bits has been given, and the ciphertext-only attack on 21-round CAST-256 with key size 192 and 256 bits can be performed. At the same time, we also present the attack on 18-round CAST-256 with key size 128 bits.

Meet-in-the-Middle Attacks Revisited: Key-Recovery, Collision, and Preimage Attacks

Abstract: At EUROCRYPT 2021, Bao et al. proposed an automatic method for systematically exploring the configuration space of meet-in-the-middle (MITM) preimage attacks. We further extend it into a constraint-based framework for finding exploitable MITM characteristics in the context of key-recovery and collision attacks by taking the subtle peculiarities of both scenarios into account. Moreover, to perform attacks based on MITM characteristics with nonlinear constrained neutral words, which have not been seen before, we present a procedure for deriving the solution spaces of neutral words without solving the corresponding nonlinear equations or increasing the overall time complexities of the attack. We apply our method to concrete symmetric-key primitives, including SKINNY, ForkSkinny, Romulus-H, Saturnin, Grostl, WHIRLPOOL, and hashing modes with AES-256. As a result, we identify the first 23-round key-recovery attack on SKINNY-n-3n and the first 24-round key-recovery attack on ForkSkinny-n-3n in the single-key model. Moreover, improved (pseudo) preimage or collision attacks on round-reduced WHIRLPOOL, Grostl, and hashing modes with AES-256 are obtained. In particular, employing the new representation of the AES key schedule due to Leurent and Pernot (EUROCRYPT 2021), we identify the first preimage attack on 10-round AES-256 hashing.

Quantum Key-recovery Attack on Feistel Structures.

Abstract: Post-quantum cryptography has drawn considerable attention from cryptologists on a global scale. At Asiacrypt 2017, Leander and May combined Grovers and Simons quantum algorithms to break the FX-based block ciphers, which were introduced by Kilian and Rogaway to strengthen DES. In this study, we investigate the Feistel constructions using Grovers and Simons algorithms to generate new quantum key-recovery attacks on different rounds of Feistel constructions. Our attacksrequire $2^{0.25nr-0.75n}$ quantum queries to break an $r$-round Feistel construction.The time complexity of our attacks is less than that observed for quantum brute-force search by a factor of $2^{0.75n}$. When compared with the best classical attacks, i.e., Dinur et al.s attacks at CRYPTO 2015, the time complexity is reduced by a factor of $2^{0.5n}$ without incurring any memory cost.

The knowledge complexity of interactive proof-systems

Abstract: Usually, a proof of a theorem contains more knowledge than the mere fact that the theorem is true. For instance, to prove that a graph is Hamiltonian it suffices to exhibit a Hamiltonian tour in it; however, this seems to contain more knowledge than the single bit Hamiltonian/non-Hamiltonian.In this paper a computational complexity theory of the “knowledge” contained in a proof is developed. Zero-knowledge proofs are defined as those proofs that convey no additional knowledge other than the correctness of the proposition in question. Examples of zero-knowledge proof systems are given for the languages of quadratic residuosity and 'quadratic nonresiduosity. These are the first examples of zero-knowledge proofs for languages not known to be efficiently recognizable.

Finding collisions in the full SHA-1

Abstract: In this paper, we present new collision search attacks on the hash function SHA-1. We show that collisions of SHA-1 can be found with complexity less than 269 hash operations. This is the first attack on the full 80-step SHA-1 with complexity less than the 280 theoretical bound.

How to break MD5 and other hash functions

Abstract: MD5 is one of the most widely used cryptographic hash functions nowadays. It was designed in 1992 as an improvement of MD4, and its security was widely studied since then by several authors. The best known result so far was a semi free-start collision, in which the initial value of the hash function is replaced by a non-standard value, which is the result of the attack. In this paper we present a new powerful attack on MD5 which allows us to find collisions efficiently. We used this attack to find collisions of MD5 in about 15 minutes up to an hour computation time. The attack is a differential attack, which unlike most differential attacks, does not use the exclusive-or as a measure of difference, but instead uses modular integer subtraction as the measure. We call this kind of differential a modular differential. An application of this attack to MD4 can find a collision in less than a fraction of a second. This attack is also applicable to other hash functions, such as RIPEMD and HAVAL.

A Survey of Attacks and Countermeasures in Mobile Ad Hoc Networks

Abstract: Security is an essential service for wired and wireless network communications. The success of mobile ad hoc network (MANET) will depend on people ’s confidence in its security. However, the characteristics of MANET pose both challenges and opportunities in achieving security goals, such as confidentiality, authentication, integrity, availability, access control, and non-repudiation. We provide a survey of attacks and countermeasures in MANET in this chapter. The countermeasures are features or functions that reduce or eliminate security vulnerabilities and attacks. First, we give an overview of attacks according to the protocol layers, and to security attributes and mechanisms. Then we present preventive approaches following the order of the layered protocol layers. We also put forward an overview of MANET intrusion detection systems (IDS), which are reactive approaches to thwart attacks and used as a second line of defense.

Traitor Tracing with constant transmission rate

Abstract: An important open problem in the area of Traitor Tracing is designing a scheme with constant expansion of the size of keys (users' keys and the encryption key) and of the size of ciphertexts with respect to the size of the plaintext. This problem is known from the introduction of Traitor Tracing by Chor, Fiat and Naor. We refer to such schemes as traitor tracing with constant transmission rate. Here we present a general methodology and two protocol constructions that result in the first two public-key traitor tracing schemes with constant transmission rate in settings where plaintexts can be calibrated to be sufficiently large. Our starting point is the notion of copyrighted function which was presented by Naccache, Shamir and Stern. We first solve the open problem of discrete-log-based and public-key-based copyrighted function. Then, we observe the simple yet crucial relation between (public-key) copyrighted encryption and (public-key) traitor tracing, which we exploit by introducing a generic design paradigm for designing constant transmission rate traitor tracing schemes based on copyrighted encryption functions. Our first scheme achieves the same expansion efficiency as regular ElGamal encryption. The second scheme introduces only a slightly larger (constant) overhead, however, it additionally achieves efficient black-box traitor tracing (against any pirate construction).