International Association
for Cryptologic Research

In this paper, we propose Garrison, which is a payment channel with watchtower for Bitcoin. For this scheme, the storage requirements of both channel parties and their watchtower would be O(log(N)) with N being the maximum number of channel updates. Furthermore, using properties of the adaptor signature, Garrison avoids state duplication, meaning that both parties store the same version of transactions for each state and hence the number of transactions does not exponentially increase with the number of applications on top of the channel.

This paper proposes Addax, a fast, verifiable, and private online ad exchange. When a user visits an ad-supported site, Addax runs an auction similar to those of leading exchanges; Addax requests bids, selects the winner, collects payment, and displays the ad to the user. A key distinction is that bids in Addax’s auctions are kept private and the outcome of the auction is publicly verifiable. Addax achieves these properties by adding public verifiability to the affine aggregatable encodings in Prio (NSDI’17) and by building an auction protocol out of them. Our implementation of Addax over WAN with hundreds of bidders can run roughly half the auctions per second as a non-private and non-verifiable exchange, while delivering ads to users in under 600 ms with little additional bandwidth requirements. This efficiency makes Addax the first architecture capable of bringing transparency to this otherwise opaque ecosystem.

Approximated homomorphic encryption (HE) schemes such as CKKS are commonly used to perform computations over encrypted real numbers. It is commonly assumed that these schemes are not “exact” and thus they cannot execute circuits with unbounded depth over discrete sets, such as binary or integer numbers, without error overflows. These circuits are usually executed using BGV and B/FV for integers and TFHE for binary numbers. This artificial separation can cause users to favor one scheme over another for a given computation, without even exploring other, perhaps better, options.

We show that by treating step functions as “clean-up” utilities and by leveraging the SIMD capabilities of CKKS, we can extend the homomorphic encryption toolbox with efficient tools. These tools use CKKS to run unbounded circuits that operate over binary and small-integer elements and even combine these circuits with fixed-point real numbers circuits. We demonstrate the results using the Turing-complete Conway’s Game of Life. In our evaluation, for boards of size 128x128, these tools achieved an order of magnitude faster latency than previous implementations using other HE schemes. We argue and demonstrate that for large enough real-world inputs, performing binary circuits over CKKS, while considering it as an “exact” scheme, results in comparable or even better performance than using other schemes tailored for similar inputs.

The advent of quantum computers brought a large interest in post-quantum cryptography and in the migration to quantum-resistant systems. Protocols for Self-Sovereign Identity (SSI) are among the fundamental scenarios touched by this need. The core concept of SSI is to move the control of digital identity from third-party identity providers directly to individuals. This is achieved through Verificable Credentials (VCs) supporting anonymity and selective disclosure. In turn, the implementation of VCs requires cryptographic signature schemes compatible with a proper Zero-Knowledge Proof (ZKP) framework. We describe the two main ZKP VCs schemes based on classical cryptographic assumptions, that is, the signature scheme with efficient protocols of Camenisch and Lysyanskaya, which is based on the strong RSA assumption, and the BBS+ scheme of Boneh, Boyen and Shacham, which is based on the strong Diffie-Hellman assumption. Since these schemes are not quantum-resistant, we select as one of the possible post-quantum alternatives a lattice-based scheme proposed by Jeudy, Roux-Langlois, and Sander, and we try to identify the open problems for achieving VCs suitable for selective disclosure, non-interactive renewal mechanisms, and efficient revocation.

Motivated by applications to cold-storage solutions for ECDSA-based cryptocurrencies, we present a new ECDSA protocol between $n$ ``online'' parties and a single ``offline'' party. Our protocol tolerates all-but-one adaptive corruptions, and it achieves full proactive security. Our protocol improves as follows over the state of the art.

** The preprocessing phase for calculating preprocessed data for future signatures is lightweight and non-interactive; it consists of each party sending a single independently-generated short message per future signature per online party (approx.~300B for typical choice of parameters).

** The signing phase is asymmetric in the following sense; to calculate the signature, it is enough for the offline party to receive a single short message from the online ``world'' (approx.~300B).

We note that all previous ECDSA protocols require many rounds of interaction between all parties, and thus all previous protocols require extensive ``interactive time'' from the offline party. In contrast, our protocol requires minimal involvement from the offline party, and it is thus ideal for MPC-based cold storage.

Our main technical innovation for achieving the above is twofold: First, building on recent protocols, we design a two-party protocol that we non-generically compile into a highly efficient $(n+1)$-party protocol. Second, we present a new batching technique for proving in zero-knowledge that the plaintext values of practically any number of Paillier ciphertexts lie in a given range. The cost of the resulting (batched) proof is very close to the cost of the underlying single-instance proof of MacKenzie and Reiter (CRYPTO'01, IJIS'04). We prove security in the UC framework, in the global random oracle model, assuming Strong RSA, semantic security of Paillier encryption, DDH, and enhanced existential unforgeability of ECDSA; these assumptions are widely used in the threshold-ECDSA literature and many commercially-available MPC-based wallets.

Lightning Network (LN), the most widely deployed payment channel for Bitcoin, requires channel parties to generate and store distinct revocation keys for all n payments of a channel to resolve fraudulent channel closures. To reduce the required storage in a payment channel, eltoo introduces a new signature type for Bitcoin to enable payment versioning. This allows a channel party to revoke all old payments by using a payment with a higher version number, reducing the storage complexity from O(n) to O(1). However, eltoo fails to achieve bounded closure, enabling a dishonest channel party to significantly delay the channel closure process. Eltoo also lacks a punishment mechanism, which may incentivize profit-driven channel parties to close a payment channel with an old state, to their own advantage.

This paper introduces Daric, a payment channel with unlimited lifetime for Bitcoin that achieves optimal storage and bounded closure. Moreover, Daric implements a punishment mechanism and simultaneously avoids the methods other schemes commonly use to enable punishment: 1) state duplication which leads to exponential increase in the number of transactions with the number of applications on top of each other or 2) dedicated design of adaptor signatures which introduces compatibility issues with BLS or most post-quantum resistant digital signatures. We also formalise Daric and prove its security in the Universal Composability model.

Recent works of Roughgarden (EC'21) and Chung and Shi (Highlights Beyond EC'22) initiate the study of a new decentralized mechanism design problem called transaction fee mechanism design (TFM). Unlike the classical mechanism design literature, in the decentralized environment, even the auctioneer (i.e., the miner) can be a strategic player, and it can even collude with a subset of the users facilitated by binding side contracts. Chung and Shi showed two main impossibility results that rule out the existence of a dream TFM. First, any TFM that provides incentive compatibility for individual users and miner-user coalitions must always have zero miner revenue, no matter whether the block size is finite or infinite. Second, assuming finite block size, no non-trivial TFM can simultaenously provide incentive compatibility for any individual user, and for any miner-user coalition.

In this work, we explore what new models and meaningful relaxations can allow us to circumvent the impossibility results of Chung and Shi. Besides today’s model that does not employ cryptography, we introduce a new MPC-assisted model where the TFM is implemented by a joint multi-party computation (MPC) protocol among the miners. We prove several feasibility and infeasibility results for achieving strict and approximate incentive compatibility, respectively, in the plain model as well as the MPC-assisted model. We show that while cryptography is not a panacea, it indeed allows us to overcome some impossibility results pertaining to the plain model, leading to non-trivial mechanisms with useful guarantees that are otherwise impossible in the plain model. Our work is also the first to characterize the mathematical landscape of transaction fee mechanism design under approximate incentive compatibility, as well as in a cryptography-assisted model.

Ring signatures allow signers to produce verifiable signatures and remain anonymous within a set of signers (i.e., the ring) while doing so. They are well-suited to protocols that target anonymity as a primary goal, for example, anonymous cryptocurrencies. However, standard ring signatures do not ensure that signers are held accountable if they act maliciously. Fraser and Quaglia (CANS'21) introduced a ring signature variant that they called report and trace ring signatures which balances the anonymity guarantee of standard ring signatures with the need to hold signers accountable. In particular, report and trace ring signatures introduce a reporting system whereby ring members can report malicious message/signature pairs. A designated tracer can then revoke the signer's anonymity if, and only if, a ring member submits a report to the tracer. Fraser and Quaglia present a generic construction of a report and trace ring signature scheme and outline an instantiation for which it is claimed that the complexity of signing is linear in the size of the ring $|R|$. In this paper, we introduce a new instantiation of Fraser and Quaglia's generic report and trace ring signature construction. Our instantiation uses a pairing-based variant of ElGamal that we define. We demonstrate that our instantiation is more efficient. In fact, we highlight that the efficiency of Fraser and Quaglia's instantiation omits a scaling factor of $\lambda$ where $\lambda$ is a security parameter. As such, the complexity of signing for their instantiation grows linearly in $\lambda \cdot |R|$. Our instantiation, on the other hand, achieves signing complexity linear in $|R|$. We also introduce a new pairing-free report and trace ring signature construction reaching a similar signing complexity. Whilst this construction requires some additional group exponentiations, it can be instantiated over any prime order group for which the Decisional Diffie-Hellman assumption holds.

A Bloom filter is a data structure that maintains a succinct and probabilistic representation of a set $S\subseteq U$ of elements from a universe $U$. It supports approximate membership queries. The price of the succinctness is allowing some error, namely false positives: for any $x\notin S$, it might answer `Yes' but with a small (non-negligible) probability. When dealing with such data structures in adversarial settings, we need to define the correctness guarantee and formalize the requirement that bad events happen infrequently and those false positives are appropriately distributed. Recently, several papers investigated this topic, suggesting different robustness definitions. In this work we unify this line of research and propose several robustness notions for Bloom filters that allow the adaptivity of queries. The goal is that a robust Bloom filter should behave like a random biased coin even against an adaptive adversary. The robustness definitions are expressed by the type of test that the Bloom filter should withstand. We explore the relationships between these notions and highlight the notion of Bet-or-Pass as capturing the desired properties of such a data structure.

The paper introduces a new AEAD-mode called sMGM (strong Multilinear Galois Mode). The proposed construction can be treated as an extension of the Russian standardized MGM mode and its modification MGM2 mode presented at the CTCrypt'21 conference. The distinctive feature of the new mode is that it provides an interface allowing one to choose specific security properties required for a certain application case. Namely, the mode has additional parameters allowing to switch on/off misuse-resistance or re-keying mechanisms. The sMGM mode consists of two main "building blocks" that are a CTR-style gamma generation function with incorporated re-keying and a multilinear function that lies in the core of the original MGM mode. Different ways of using these functions lead to achieving different sets of security properties. Such an approach to constructing parameterizable AEAD-mode allows for reducing the code size which can be crucial for constrained devices. We provide security bounds for the proposed mode. We focus on proving the misuse-resistance of the sMGM mode, since the standard security properties were already analyzed during the development of the original MGM and MGM2 modes.

TCC 2022 will take place in Chicago, USA on November 7-10 2022.

The registration site is now open: https://tcc.iacr.org/2022/registration.php

With the advancement of blockchain technology, hundreds of cryptocurrencies have been deployed. The bloom of heterogeneous blockchain platforms brings a new emerging problem: typically, various blockchains are isolated systems, how to securely identify and/or transfer digital properties across blockchains? There are three main kinds of cross-chain approaches: sidechains/relays, notaries, and hashed time-lock contracts. Among them, notary-based cross-chain solutions have the best compatibility and user-friendliness, but they are typically centralized. To resolve this issue, we present Bool Network -- an open, distributed, secure cross-chain notary platform powered by MPC-based distributed key management over evolving hidden committees. More specifically, to protect the identities of the committee members, we propose a Ring verifiable random function (Ring VRF) protocol, where the real public key of a VRF instance can be hidden among a ring, which may be of independent interest to other cryptographic protocols. Furthermore, all the key management procedures are executed in the TEE, such as Intel SGX, to ensure the privacy and integrity of partial key components. A prototype of the proposed Bool Network is implemented in Rust language, using Polkadot Substrate.

Isogeny-based cryptography suffers from a long-running time due to its requirement of a great amount of large integer arithmetic. The Residue Number System (RNS) can compensate for that drawback by making computation more efficient via parallelism. However, performing a modular reduction by a large prime which is not part of the RNS base is very expensive. In this paper, we propose a new fast and efficient modular reduction algorithm using RNS. Our method reduces the number of required multiplications by 40\% compared to RNS Montgomery modular reduction algorithm. Also, we evaluate our modular reduction method by realizing a cryptoprocessor for isogeny-based SIDH key exchange. On a Xilinx Ultrascale+ FPGA, the proposed cryptoprocessor consumes 151,009 LUTs, 143,171 FFs and 1,056 DSPs. It achieves 250 MHz clock frequency and finishes the key exchange for SIDH in 3.8 and 4.9 ms.

We give round-optimal {\em black-box} constructions of two-party and multiparty protocols in the common random/reference string (CRS) model, with security against malicious adversaries, based on any two-round oblivious transfer (OT) protocol in the same model. Specifically, we obtain two types of results.

\begin{enumerate} \item {\bf Two-party protocol.} We give a (two-round) {\it two-sided NISC} protocol that makes black-box use of two-round (malicious-secure) OT in the CRS model. In contrast to the standard setting of non-interactive secure computation (NISC), two-sided NISC allows communication from both parties in each round and delivers the output to both parties at the end of the protocol. Prior black-box constructions of two-sided NISC relied on idealized setup assumptions such as OT correlations, or were proven secure in the random oracle model. \item {\bf Multiparty protocol.} We give a three-round secure multiparty computation protocol for an arbitrary number of parties making black-box use of a two-round OT in the CRS model. The round optimality of this construction follows from a black-box impossibility proof of Applebaum et al. (ITCS 2020). Prior constructions either required the use of random oracles, or were based on two-round malicious-secure OT protocols that satisfied additional security properties. \end{enumerate}

Let $P(x, y)$ be a bivariate polynomial with coefficients in $\mathbb{C}$. Form the $n \times n$ matrices $L_n$ whose elements are defined by $P(i, j)$. Define the matrices $M_n = I_n - L_n $.

We show that $\mu_n = \det(M_n)$ is a polynomial in $n$, thus answering a conjecture of Naccache and Yifrach.

ZEBRA is an Anonymous Credential (AC) scheme, supporting auditability and revocation, that provides practical on-chain verification for the first time. It realizes efficient access control on permissionless blockchains while achieving both privacy and accountability. In all prior solutions, users either pay exorbitant fees or lose privacy since authorities granting access can map users to their wallets. Hence, ZEBRA is the first to enable DeFi platforms to remain compliant with imminent regulations without compromising user privacy.

We evaluate ZEBRA and show that it reduces the gas cost incurred on the Ethereum Virtual Machine (EVM) by 11.8x when compared to Coconut [NDSS 2019], the state-of-the-art AC scheme for blockchains. This translates to a reduction in transaction fees from 94 USD to 8 USD on Ethereum in August 2022. However, 8 USD is still high for most applications, and ZEBRA further drives down credential verification costs through batched verification. For a batch of 512 layer-1 and layer-2 wallets, the gas cost is reduced by 35x and 641x on EVM, and the transaction fee is reduced to just 0.23 USD and 0.0126 USD on Ethereum, respectively. For perspective, these costs are comparable to the minimum transaction costs on Ethereum.

Registration-based encryption (Garg, Hajiabadi, Mahmoody, Rahimi, TCC'18) aims to offer what identity-based encryption offers without the key-escrow problem, which refers to the ability of the private-key generator to obtain parties' decryption keys at wish. In RBE, parties generate their own secret and public keys and register their public keys to the key curator (KC) who updates a compact public parameter after each registration. The updated public parameter can then be used to securely encrypt messages to registered identities.

A major drawback of RBE, compared with IBE, is that in order to decrypt, parties might need to periodically request so-called decryption updates from the KC. Current RBE schemes require $\Omega(\log n)$ number of updates after $n$ registrations, while the public parameter is of length $\text{poly}(\log n)$. Clearly, it would be highly desirable to have RBEs with only, say, a constant number of updates. This leads to the following natural question: are so many (logarithmic) updates necessary for RBE schemes, or can we decrease the frequency of updates significantly?

In this paper, we prove an almost tight lower bound for the number of updates in RBE schemes, as long as the times that parties receive updates only depend on the registration time of the parties, which is a natural property that holds for all known RBE constructions. More generally, we prove a trade-off between the number of updates in RBEs and the length of the public parameter for any scheme with fixed update times. Indeed, we prove that for any such RBE scheme, if there are $n \geq \binom{k+d}{d+1}$ identities that receive at most $d$ updates, the public parameter needs to be of length $\Omega(k)$. As a corollary, we find that RBE systems with fixed update times and public parameters of length $\text{poly} (\log n)$, require $\Omega(\log n/\text{loglog}\ n)$ decryption updates, which is optimal up to a $O(\text{loglog}\ n)$ factor.

We propose a novel variant of functional encryption which supports ciphertext updates, dubbed ciphertext updatable functional encryption (CUFE). Such a feature further broadens the practical applicability of the functional encryption paradigm and is carried out via so-called update tokens. However, allowing update tokens requires some care for the security definition as we want that updates can be done by any semi-trusted third party and only on ciphertexts. Our contribution is three-fold: a) We define our new primitive with a security notion in the indistinguishability setting. Within CUFE, functional decryption keys and ciphertexts are labeled with tags such that only if the tag of the decryption key and the ciphertext match, then decryption succeeds. Furthermore, we allow ciphertexts to switch their tags to any other tag via update tokens. Such tokens are generated by the holder of the main secret key and can only be used in the desired direction. b) We present a generic construction of CUFE for any functionality as well as predicates different from equality testing on tags, which relies on the existence of (probabilistic) indistinguishability obfuscation (iO). c) We present a practical construction of CUFE for the inner-product functionality from standard assumptions (i.e., LWE) in the random-oracle model. On the technical level, we build on the recent functional encryption schemes with fine-grained access control and linear operations on encrypted data (Abdalla et al., AC'20) and introduce an additional ciphertext updatability feature. Proving security for such a construction turned out to be non-trivial, particularly when revealing keys for the updated challenge ciphertext is allowed. Overall, such construction enriches the set of known inner-product functional-encryption schemes with the additional updatability feature of ciphertexts.

This note describes the implementation of the Castryck-Decru key recovery attack on SIDH using the computer algebra system, SageMath. We describe in detail alternate computation methods for the isogeny steps of the original attack ($(2,2)$-isogenies from a product of elliptic curves and from a Jacobian), using explicit formulas to compute values of these isogenies at given points, motivated by both performance considerations and working around SageMath limitations. A performance analysis is provided, with focus given to the various algorithmic and SageMath specific improvements made during development, which in total accumulated in approximately an eight-fold performance improvement compared with a naïve reimplementation of the proof of concept.

Recent works on key rank estimation methods claim that algorithmic key rank estimation is too slow, and suggest two new ideas: replacing repeat attacks with simulated attacks (PS-TH-GE rank estimation), and a shortcut rank estimation method that works directly on distinguishing vector distributions (GEEA). We take these ideas and provide a comprehensive comparison between them and a performant implementation of a classical, algorithmic ranking approach, as well as some earlier work on estimating distinguisher distributions. Our results show, in contrast to the recent work, that the algorithmic ranking approach outperforms GEEA, and that simulation based ranks are unreliable.