## CryptoDB

### Yu Long Chen

#### Publications

Year
Venue
Title
2022
CRYPTO
This paper provides the first analysis of reflection ciphers such as PRINCE from a provable security viewpoint. As a first contribution, we initiate the study of key-alternating reflection ciphers in the ideal permutation model. Specifically, we prove the security of the two-round case and give matching attacks. The resulting security bound takes form $O(qp^2/2^{2n}+q^2/2^n)$, where q is the number of construction evaluations and p is the number of direct adversarial queries to the underlying permutation. Since the two-round construction already achieves an interesting security lower bound, this result can also be of interest for the construction of reflection ciphers based on a single public permutation. Our second contribution is a generic key-length extension method for reflection ciphers. It provides an attractive alternative to the FX construction, which is used by PRINCE and other concrete key-alternating reflection ciphers. We show that our construction leads to better security with minimal changes to existing designs. The security proof is in the ideal cipher model and relies on a reduction to the two-round Even-Mansour cipher with a single round key. In order to obtain the desired result, we sharpen the bad-transcript analysis and consequently improve the best-known bounds for the single-key Even-Mansour cipher with two rounds. This improvement is enabled by a new sum-capture theorem that is of independent interest.
2021
ASIACRYPT
A growing number of lightweight block ciphers are proposed for environments such as the Internet of Things. An important contribution to the reduced implementation cost is a block length n of 64 or 96 bits rather than 128 bits. As a consequence, encryption modes and message authentication code (MAC) algorithms require security beyond the 2^{n/2} birthday bound. This paper provides an extensive treatment of MAC algorithms that offer beyond birthday bound PRF security for both nonce-respecting and nonce-misusing adversaries. We study constructions that use two block cipher calls, one universal hash function call and an arbitrary number of XOR operations. We start with the separate problem of generically identifying all possible secure n-to-n-bit pseudorandom functions (PRFs) based on two block cipher calls. The analysis shows that the existing constructions EDM, SoP, and EDMD are the only constructions of this kind that achieve beyond birthday bound security. Subsequently we deliver an exhaustive treatment of MAC algorithms, where the outcome of a universal hash function evaluation on the message may be entered at any point in the computation of the PRF. We conclude that there are a total amount of nine schemes that achieve beyond birthday bound security, and a tenth construction that cannot be proven using currently known proof techniques. For these former nine MAC algorithms, three constructions achieve optimal n-bit security in the nonce-respecting setting, but are completely insecure if the nonce is reused. The remaining six constructions have 3n/4-bit security in the nonce-respecting setting, and only four out of these six constructions still achieve beyond the birthday bound security in the case of nonce misuse.
2021
ASIACRYPT
We improve upon the security of (tweakable) correlation-robust hash functions, which are essential components of garbling schemes and oblivious-transfer extension schemes. We in particular focus on constructions from permutations, and improve upon the work by Guo etal. (IEEE S\&P '20) in terms of security and efficiency. We present a tweakable one-call construction which matches the security of the most secure two-call construction -- the resulting security bound takes form O((p+q)q/2^n), where q is the number of construction evaluations and p is the number of direct adversarial queries to the underlying n-bit permutation, which is modeled as random. Moreover, we present a new two-call construction with much better security degradation -- in particular, for applications of interest, where only a constant number of evaluations per tweak are made, the security degrades as O((\sqrt{q} p+q^2)/2^n). Our security proof relies on on the sum-capture theorems (Babai ’02; Steinberger ’12, Cogliati and Seurin ’18), as well as on new balls-into-bins combinatorial lemmas for limited independence ball-throws. Of independent interest, we also provide a self-contained concrete security treatment of oblivious transfer extension.
2020
TOSC
With the trend to connect more and more devices to the Internet, authenticated encryption has become a major backbone in securing the communication, not only between these devices and servers, but also the direct communication among these devices. Most authenticated encryption algorithms used in practice are developed to perform well on modern high-end devices, but are not necessarily suited for usage on resource-constrained devices. We present a lightweight authenticated encryption scheme, called Elephant. Elephant retains the advantages of GCM such as parallelism, but is tailored to the needs of resource-constrained devices. The two smallest instances of Elephant, Dumbo and Jumbo, are based on the 160-bit and 176-bit Spongent permutation, respectively, and are particularly suited for hardware; the largest instance of Elephant, Delirium, is based on 200-bit Keccak and is developed towards software use. All three instances are parallelizable, have a small state size while achieving a high level of security, and are constant time by design.
2019
CRYPTO
Pseudorandom functions are traditionally built upon block ciphers, but with the trend of permutation based cryptography, it is a natural question to investigate the design of pseudorandom functions from random permutations. We present a generic study of how to build beyond birthday bound secure pseudorandom functions from public random permutations. We first show that a pseudorandom function based on a single permutation call cannot be secure beyond the $2^{n/2}$ birthday bound, where n is the state size of the function. We next consider the Sum of Even-Mansour (SoEM) construction, that instantiates the sum of permutations with the Even-Mansour construction. We prove that SoEM achieves tight $2n{/}3$-bit security if it is constructed from two independent permutations and two randomly drawn keys. We also demonstrate a birthday bound attack if either the permutations or the keys are identical. Finally, we present the Sum of Key Alternating Ciphers (SoKAC) construction, a translation of Encrypted Davies-Meyer Dual to a public permutation based setting, and show that SoKAC achieves tight $2n{/}3$-bit security even when a single key is used.
2018
ASIACRYPT
Length doublers are cryptographic functions that transform an n-bit cryptographic primitive into an efficient and secure cipher that length-preservingly encrypts strings of length in $[n,2n-1]$. All currently known constructions are only proven secure up to the birthday bound, and for all but one construction this bound is known to be tight. We consider the remaining candidate, $\mathrm {LDT}$ by Chen et al. (ToSC 2017(3)), and prove that it achieves beyond the birthday bound security for the domain [n, 3n / 2). We generalize the construction to multiple rounds and demonstrate that by adding one more encryption layer to $\mathrm {LDT}$, beyond the birthday bound security can be achieved for all strings of length in $[n,2n-1]$: security up to around $2^{2n/3}$ for the encryption of strings close to n and security up to around $2^{n}$ for strings of length close to 2n. The security analysis of both schemes is performed in a modular manner through the introduction and analysis of a new concept called “harmonic permutation primitives.”
2017
TOSC
We present a length doubler, LDT, that turns an n-bit tweakable block cipher into an efficient and secure cipher that can encrypt any bit string of length [n..2n − 1]. The LDT mode is simple, uses only two cryptographic primitive calls (while prior work needs at least four), and is a strong length-preserving pseudorandom permutation if the underlying tweakable block ciphers are strong tweakable pseudorandom permutations. We demonstrate that LDT can be used to neatly turn an authenticated encryption scheme for integral data into a mode for arbitrary-length data.

#### Coauthors

Tim Beyne (2)
Christoph Dobraunig (1)
Eran Lambooij (1)
Atul Luykx (1)
Bart Mennink (5)
Mridul Nandi (1)
Bart Preneel (2)
Stefano Tessaro (1)