## CryptoDB

### Michael Klooß

#### Publications

**Year**

**Venue**

**Title**

2024

ASIACRYPT

Practical Blind Signatures in Pairing-Free Groups
Abstract

Blind signatures have garnered significant attention in recent years, with several efficient constructions in the random oracle model relying on well-understood assumptions. However, this progress does not apply to pairing-free cyclic groups: fully secure constructions over cyclic groups rely on pairings, remain inefficient, or depend on the algebraic group model or strong interactive assumptions. To address this gap, Chairattana-Apirom, Tessaro, and Zhu (CTZ, Crypto 2024) proposed a new scheme based on the CDH assumption. Unfortunately, their construction results in large signatures and high communication complexity.
In this work, we propose a new blind signature construction in the random oracle model that significantly improves upon the CTZ scheme. Compared to CTZ, our scheme reduces communication complexity by a factor of more than 10 and decreases the signature size by a factor of more than 45, achieving a compact signature size of only 224~Bytes. The security of our scheme is based on the DDH assumption over pairing-free cyclic groups, and we show how to generalize it to the partially blind setting.

2023

CRYPTO

Publicly Verifiable Zero-Knowledge and Post-Quantum Signatures From VOLE-in-the-Head
Abstract

We present a new method for transforming zero-knowledge protocols in the designated verifier setting into public-coin protocols, which can be made non-interactive and publicly verifiable.
Our transformation applies to a large class of ZK protocols based on oblivious transfer.
In particular, we show that it can be applied to recent, fast protocols based on vector oblivious linear evaluation (VOLE), with a technique we call VOLE-in-the-head, upgrading these protocols to support public verifiability.
Our resulting ZK protocols have linear proof size, and are simpler, smaller and faster than related approaches based on MPC-in-the-head.
To build VOLE-in-the-head while supporting both binary circuits and large finite fields, we develop several new technical tools.
One of these is a new proof of security for the SoftSpokenOT protocol (Crypto 2022), which generalizes it to produce certain types of VOLE correlations over large fields.
Secondly, we present a new ZK protocol that is tailored to take advantage of this form of VOLE, which leads to a publicly verifiable VOLE-in-the-head protocol with only 2x more communication than the best, designated-verifier VOLE-based protocols.
We analyze the soundness of our approach when made non-interactive using the Fiat-Shamir transform, using round-by-round soundness.
As an application of the resulting NIZK, we present FAEST, a post-quantum signature scheme based on AES.
FAEST is the first AES-based signature scheme to be smaller than SPHINCS+, with signature sizes between 5.6 and 6.6kB at the 128-bit security level.
Compared with the smallest version of SPHINCS+ (7.9kB), FAEST verification is slower, but the signing times are between 8x and 40x faster.

2023

ASIACRYPT

Universally Composable Auditable Surveillance
Abstract

User privacy is becoming increasingly important in our digital society. Yet, many applications face legal requirements or regulations that prohibit unconditional anonymity guarantees, e.g., in electronic payments where surveillance is mandated to investigate suspected crimes.
As a result, many systems have no effective privacy protections at all, or have backdoors, e.g., stored at the operator side of the system, that can be used by authorities to disclose a user’s private information (e.g., lawful interception). The problem with such backdoors is that they also enable silent mass surveillance within the system. To prevent such misuse, various approaches have been suggested which limit possible abuse or ensure it can be detected. Many works consider auditability of surveillance actions but do not enforce that traces are left when backdoors are retrieved. A notable exception which offers retrospective and silent surveillance is the recent work on misuse-resistant surveillance by Green et al. (EUROCRYPT’21). However, their approach relies on extractable witness encryption, which is a very strong primitive with no known efficient and secure implementations.
In this work, we develop a building block for auditable surveillance. In our protocol, backdoors or escrow secrets of users are protected in multiple ways: (1) Backdoors are short-term and user-specific; (2) they are shared between trustworthy parties to avoid a single point of failure; and (3) backdoor access is given conditionally. Moreover (4) there are audit trails and public statistics for every (granted) backdoor request; and (5) surveillance remains silent, i.e., users do not know they are surveilled. Concretely, we present an abstract UC-functionality which can be used to augment applications with auditable surveillance capabilities. Our realization makes use of threshold encryption to protect user secrets, and is concretely built in a blockchain context with committee-based YOSO MPC. As a consequence, the committee can verify that the conditions for backdoor access are given, e.g., that law enforcement is in possession of a valid surveillance warrant (via a zero-knowledge proof). Moreover, access leaves an audit trail on the ledger, which allows an auditor to retrospectively examine surveillance decisions.
As a toy example, we present an Auditably Sender-Traceable Encryption scheme, a PKE scheme where the sender can be deanonymized by law enforcement. We observe and solve problems posed by retrospective surveillance via a special non-interactive non-committing encryption scheme which allows zero-knowledge proofs over message, sender identity and (escrow) secrets.

2023

TCC

Composable Long-Term Security with Rewinding
Abstract

Long-term security, a variant of Universally Composable (UC) security introduced by Müller-Quade and Unruh (TCC ’07, JoC ’10), allows to analyze the security of protocols in a setting where all hardness assumptions no longer hold after the protocol execution has finished. Such a strict notion is highly desirable when properties such as input privacy need to be guaranteed for a long time, e.g. with zero-knowledge proofs for secure electronic voting. Strong impossibility results rule out so-called long-term-revealing setups, e.g. a common reference string (CRS), to achieve long-term security, with known constructions for long-term security requiring hardware assumptions, e.g. signature cards.
We circumvent these impossibility results with new techniques, enabling rewinding-based simulation in a way that universal composability is achieved. This allows us to construct a long-term-secure composable commitment scheme in the CRS-hybrid model, which is provably impossible in the notion of Müller-Quade and Unruh. We base our construction on a statistically hiding commitment scheme in the CRS-hybrid model with CCA-like properties. To provide a CCA oracle, we cannot rely on super-polynomial extraction techniques and instead extract the value committed to via rewinding. To this end, we incorporate rewinding-based commitment extraction into the UC framework via a helper in analogy to Canetti, Lin and Pass (FOCS 2010), allowing both adversary and environment to extract statistically hiding commitments.
Our new framework provides the first setting in which a commitment scheme that is both statistically hiding and universally composable can be constructed from standard polynomial-time hardness assumptions and a CRS only. We also prove that our CCA oracle is k-robust extractable. This asserts that extraction is possible without rewinding a concurrently executed k-round protocol. Consequently any k-round (standard) UC-secure protocol remains secure in the presence of our helper.
Finally, we prove that building long-term-secure oblivious transfer (and thus general two-party computations) from long-term-revealing setups remains impossible in our setting. Still, our long-term-secure commitment scheme suffices for natural applications, such as long-term secure and composable (commit-and-prove) zero-knowledge arguments of knowledge.

2023

JOFC

Fiat–Shamir Transformation of Multi-Round Interactive Proofs (Extended Version)
Abstract

The celebrated Fiat–Shamir transformation turns any public-coin interactive proof into a non-interactive one, which inherits the main security properties (in the random oracle model) of the interactive version. While originally considered in the context of 3-move public-coin interactive proofs, i.e., so-called $$\varSigma $$ Σ -protocols, it is now applied to multi-round protocols as well. Unfortunately, the security loss for a $$(2\mu + 1)$$ ( 2 μ + 1 ) -move protocol is, in general, approximately $$Q^\mu $$ Q μ , where Q is the number of oracle queries performed by the attacker. In general, this is the best one can hope for, as it is easy to see that this loss applies to the $$\mu $$ μ -fold sequential repetition of $$\varSigma $$ Σ -protocols, but it raises the question whether certain (natural) classes of interactive proofs feature a milder security loss. In this work, we give positive and negative results on this question. On the positive side, we show that for $$(k_1, \ldots , k_\mu )$$ ( k 1 , … , k μ ) -special-sound protocols (which cover a broad class of use cases), the knowledge error degrades linearly in Q , instead of $$Q^\mu $$ Q μ . On the negative side, we show that for t -fold parallel repetitions of typical $$(k_1, \ldots , k_\mu )$$ ( k 1 , … , k μ ) -special-sound protocols with $$t \ge \mu $$ t ≥ μ (and assuming for simplicity that t and Q are integer multiples of $$\mu $$ μ ), there is an attack that results in a security loss of approximately $$\frac{1}{2} Q^\mu /\mu ^{\mu +t}$$ 1 2 Q μ / μ μ + t .

2022

TCC

Fiat-Shamir Transformation of Multi-Round Interactive Proofs
Abstract

The celebrated Fiat-Shamir transformation turns any public-coin interactive proof into a non-interactive one, which inherits the main security properties (in the random oracle model) of the interactive version. While originally considered in the context of 3-move public-coin interactive proofs, i.e., so-called $\Sigma$-protocols, it is now applied to multi-round protocols as well. Unfortunately, the security loss for a $(2\mu + 1)$-move protocol is, in general, approximately $Q^\mu$, where $Q$ is the number of oracle queries performed by the attacker. In general, this is the best one can hope for, as it is easy to see that this loss applies to the $\mu$-fold sequential repetition of $\Sigma$-protocols, but it raises the question whether certain (natural) classes of interactive proofs feature a milder security loss.
In this work, we give positive and negative results on this question. On the positive side, we show that for $(k_1, \ldots, k_\mu)$-special-sound protocols (which cover a broad class of use cases),
the knowledge error degrades linearly in $Q$, instead of $Q^\mu$. On the negative side, we show that for $t$-fold \emph{parallel repetitions} of typical $(k_1, \ldots, k_\mu)$-special-sound protocols with $t \geq \mu$ (and assuming for simplicity that $t$ and $Q$ are integer multiples of $\mu$),
there is an attack that results in a security loss of approximately~$\frac12 Q^\mu /\mu^{\mu+t}$.

2021

EUROCRYPT

Efficient Range Proofs with Transparent Setup from Bounded Integer Commitments
📺
Abstract

We introduce a new approach for constructing range proofs. Our approach is modular, and leads to highly competitive range proofs under standard assumption, using less communication and (much) less computation than the state of the art methods, and without relying on a trusted setup. Our range proofs can be used as a drop-in replacement in a variety of protocols such as distributed ledgers, anonymous transaction systems, and many more, leading to significant reductions in communication and computation for these applications.
At the heart of our result is a new method to transform any commitment over a finite field into a commitment scheme which allows to commit to and efficiently prove relations about bounded integers. Combining these new commitments with a classical approach for range proofs based on square decomposition, we obtain several new instantiations of a paradigm which was previously limited to RSA-based range proofs (with high communication and computation, and trusted setup). More specifically, we get:
- Under the discrete logarithm assumption, we obtain the most compact and efficient range proof among all existing candidates (with or without trusted setup). Our proofs are 12% to 20% shorter than the state of the art Bulletproof (Bootle et al., CRYPTO'18) for standard choices of range size and security parameter, and are more efficient (both for the prover and the verifier) by more than an order of magnitude.
- Under the LWE assumption, we obtain range proofs that improve over the state of the art in a batch setting when at least a few dozen range proofs are required. The amortized communication of our range proofs improves by up to two orders of magnitudes over the state of the art when the number of required range proofs grows.
- Eventually, under standard class group assumptions, we obtain the first concretely efficient standard integer commitment scheme (without bounds on the size of the committed integer) which does not assume trusted setup.

2021

TCC

On expected polynomial runtime in cryptography
📺
Abstract

A common definition of black-box zero-knowledge considers strict polynomial time (PPT) adversaries but expected polynomial time (EPT) simulation. This is necessary for constant round black-box zero-knowledge in the plain model, and the asymmetry between simulator and adversary an accepted consequence. Consideration of EPT adversaries naturally leads to designated adversaries, i.e. adversaries which are only required to be efficient in the protocol they are designed to attack. They were first examined in Feige’s thesis [Fei90],
where obstructions to proving security are shown. Prior work on (designated) EPT adversaries by Katz and Lindell (TCC’05) requires superpolynomial hardness assumptions, whereas the work of Goldreich (TCC’07) postulates “nice” behaviour under rewinding.
In this work, we start from scratch and revisit the definition of efficient algorithms. We argue that the standard runtime classes, PPT and EPT, behave “unnatural” from a cryptographic perspective. Namely, algorithms can have indistinguishable runtime distributions, yet one is considered efficient while the other is not. Hence, classical runtime classes are not “closed under indistinguishability”, which causes problems. Relaxations of PPT which
are “closed” are (well-)known and used.
We propose computationally expected polynomial time (CEPT), the class of runtimes which are (computationally) indistinguishable from EPT, which is “closed”. We analyze CEPT in the setting of uniform complexity (following Goldreich (JC’93)) with designated adversaries, and provide easy-to-check criteria for zero-knowledge protocols with blackbox simulation in the plain model, which show that many (all known?) such protocols
handle designated CEPT adversaries in CEPT.

2019

EUROCRYPT

(R)CCA Secure Updatable Encryption with Integrity Protection
Abstract

An updatable encryption scheme allows a data host to update ciphertexts of a client from an old to a new key, given so-called update tokens from the client. Rotation of the encryption key is a common requirement in practice in order to mitigate the impact of key compromises over time. There are two incarnations of updatable encryption: One is ciphertext-dependent, i.e. the data owner has to (partially) download all of his data and derive a dedicated token per ciphertext. Everspaugh et al. (CRYPTO’17) proposed CCA and CTXT secure schemes in this setting. The other, more convenient variant is ciphertext-independent, i.e., it allows a single token to update all ciphertexts. However, so far, the broader functionality of tokens in this setting comes at the price of considerably weaker security: the existing schemes by Boneh et al. (CRYPTO’13) and Lehmann and Tackmann (EUROCRYPT’18) only achieve CPA security and provide no integrity protection. Arguably, when targeting the scenario of outsourcing data to an untrusted host, plaintext integrity should be a minimal security requirement. Otherwise, the data host may alter or inject ciphertexts arbitrarily. Indeed, the schemes from BLMR13 and LT18 suffer from this weakness, and even EPRS17 only provides integrity against adversaries which cannot arbitrarily inject ciphertexts. In this work, we provide the first ciphertext-independent updatable encryption schemes with security beyond CPA, in particular providing strong integrity protection. Our constructions and security proofs of updatable encryption schemes are surprisingly modular. We give a generic transformation that allows key-rotation and confidentiality/integrity of the scheme to be treated almost separately, i.e., security of the updatable scheme is derived from simple properties of its static building blocks. An interesting side effect of our generic approach is that it immediately implies the unlinkability of ciphertext updates that was introduced as an essential additional property of updatable encryption by EPRS17 and LT18.

#### Coauthors

- Thomas Attema (2)
- Carsten Baum (1)
- Robin Berger (1)
- Lennart Braun (1)
- Brandon Broadnax (1)
- Geoffroy Couteau (1)
- Serge Fehr (2)
- Valerie Fetzer (1)
- Cyprien Delpech de Saint Guilhem (1)
- Michael Klooß (9)
- Anja Lehmann (1)
- Huang Lin (1)
- Jeremias Mechler (1)
- Jörn Müller-Quade (2)
- Emmanuela Orsini (1)
- Astrid Ottenhues (1)
- Markus Raiber (2)
- Michael Reichle (2)
- Lawrence Roy (1)
- Andy Rupp (2)
- Peter Scholl (1)
- Benedikt Wagner (1)