MuSig-L: Lattice-Based Multi-Signature With Single-Round Online Phase

Multi-signatures are protocols that allow a group of signers to jointly produce a single signature on the same message. In recent years, a number of practical multi-signature schemes have been proposed in the discrete-log setting, such as MuSigT (CRYPTO\u2721) and DWMS (CRYPTO\u2721). The main technical challenge in constructing a multi-signature scheme is to achieve a set of several desirable properties, such as (1) security in the plain public-key (PPK) model, (2) concurrent security, (3) low online round complexity, and (4) key aggregation. However, previous lattice-based, post-quantum counterparts to Schnorr multi-signatures fail to satisfy these properties. In this paper, we introduce MuSigL, a lattice-based multi-signature scheme simultaneously achieving these design goals for the first time. Unlike the recent, round-efficient proposal of Damgård et al. (PKC\u2721), which had to rely on lattice-based trapdoor commitments, we do not require any additional primitive in the protocol, while being able to prove security from the standard module-SIS and LWE assumptions. The resulting output signature of our scheme therefore looks closer to the usual Fiat--Shamir-with-abort signatures

Floppy-Sized Group Signatures from Lattices

We present the first lattice-based group signature scheme whose cryptographic artifacts are of size small enough to be usable in practice: for a group of $2^{25}$ users, signatures take 910 kB and public keys are 501 kB. Our scheme builds upon two recently proposed lattice-based primitives: the verifiable encryption scheme by Lyubashevsky and Neven (Eurocrypt 2017) and the signature scheme by Boschini, Camenisch, and Neven (IACR ePrint 2017). To achieve such short signatures and keys, we first re-define verifiable encryption to allow one to encrypt a function of the witness, rather than the full witness. This definition enables more efficient realizations of verifiable encryption and is of independent interest. Second, to minimize the size of the signatures and public keys of our group signature scheme, we revisit the proof of knowledge of a signature and the proofs in the verifiable encryption scheme provided in the respective papers

Relaxed Lattice-Based Signatures with Short Zero-Knowledge Proofs

Higher-level cryptographic privacy-enhancing protocols such as anonymous credentials, voting schemes, and e-cash are often constructed by suitably combining signature, commitment, and encryption schemes with zero-knowledge proofs. Indeed, a large body of protocols have been constructed in that manner from Camenisch-Lysyanskaya signatures and generalized Schnorr proofs. In this paper, we build a similar framework for lattice-based schemes by presenting a signature and commitment scheme that are compatible with Lyubashevsky\u27s Fiat-Shamir proofs with abort, currently the most efficient zero-knowledge proofs for lattices. To cope with the relaxed soundness guarantees of these proofs, we define corresponding notions of relaxed signature and commitment schemes. We demonstrate the flexibility and efficiency of our new primitives by constructing a new lattice-based anonymous attribute token scheme and providing concrete parameters to securely instantiate this scheme

On Access Control Encryption without Sanitization

Access Control Encryption (ACE) allows to control information flow between parties by enforcing a policy that specifies which user can send messages to whom. The core of the scheme is a sanitizer, i.e., an entity that \u27\u27sanitizes\u27\u27 all messages by essentially re-encrypting the ciphertexts under its key. In this work we investigate the natural question of whether it is still possible to achieve some meaningful security properties in scenarios when such a sanitization step is not possible. We answer positively by showing that it is possible to limit corrupted users to communicate only through insecure subliminal channels, under the necessary assumption that parties do not have pre-shared randomness. Moreover, we show that the bandwidth of such channels can be limited to be O(log(n)) by adding public ciphertext verifiability to the scheme under computational assumptions. In particular, we rely on a new security definition for obfuscation, Game Specific Obfuscation (GSO), which is a weaker definition than VBB, as it only requires the obfuscator to obfuscate programs in a specific family of programs, and limited to a fixed security game

That’s not my signature! Fail-stop signatures for a post-quantum world

The Snowden\u27s revelations kick-started a community-wide effort to develop cryptographic tools against mass surveillance. In this work, we propose to add another primitive to that toolbox: Fail-Stop Signatures (FSS) [EC\u2789]. FSS are digital signatures enhanced with a forgery-detection mechanism that can protect a PPT signer from more powerful attackers. Despite the fascinating concept, research in this area stalled after the \u2790s. However, the ongoing transition to post-quantum cryptography, with its hiccups due to the novelty of underlying assumptions, has become the perfect use case for FSS. This paper aims to reboot research on FSS with practical use in mind: Our framework for FSS includes ``fine-grained\u27\u27 security definitions (that assume a powerful, but bounded adversary e.g: can break $128$-bit of security, but not $256$-bit). As an application, we show new FSS constructions for the post-quantum setting. We show that FSS are equivalent to standard, provably secure digital signatures that do not require rewinding or programming random oracles, and that this implies lattice-based FSS. Our main construction is an FSS version of SPHINCS, which required building FSS versions of all its building blocks: WOTS, XMSS, and FORS. In the process, we identify and provide generic solutions for two fundamental issues arising when deriving a large number of private keys from a single seed, and when building FSS for Hash-and-Sign-based signatures

Lattice-based protocols for privacy

Privacy and control over data have become a public concern. Simultaneously, the increasing likelihood of the construction of a general purpose quantum computer has led companies and governments to demand for quantum safe alternatives to the protocols used today. New schemes have been elaborated, whose conjectured security against a quantum computer relies on the hardness to solve different mathematical problems, such as problems defined over lattices. However, while quantum-safe alternatives are known, they tend to output tokens whose size is too large to be considered practical. The goal of this dissertation is to address these concerns by building privacy-preserving signatures whose security is based on the hardness of solving some problems over ideal lattices, and whose token sizes are an improvement over the state of the art. Our first result is a toolbox of primitives (signatures, commitment and NIZK proofs) that are composable and allow building privacy-preserving protocols, such as Anonymous Attribute Tokens. The core building block are non-interactive zero-knowledge proofs with relaxed extractability that we obtained extending the construction in [Lyubashevsky, 2012]. In a second work, we combine them with a verifiable encryption scheme to construct a group signature whose keys and signatures require less that 2MB of storage. Finally, we give efficient statistical zero-knowledge proofs (SNARKs) for Module/Ring LWE and Module/Ring SIS relations, providing the remaining ingredient for building efficient cryptographic protocols from lattice-based hardness assumptions. We apply our approach to the example use case of partially dynamic group signatures and obtain a lattice-based group signature that protects users against corrupted issuers, and that produces signatures smaller than the state of the art. The results contained in this dissertation were published at international conferences

Progressive and efficient verification for digital signatures

Digital signatures are widely deployed to authenticate the source of incoming information, or to certify data integrity. Common signature verification procedures return a decision (accept/reject) only at the very end of the execution. If interrupted prematurely, however, the verification process cannot infer any meaningful information about the validity of the given signature. We notice that this limitation is due to the algorithm design solely, and it is not inherent to signature verification.In this work, we provide a formal framework to handle interruptions during signature verification. In addition, we propose a generic way to devise alternative verification procedures that progressively build confidence on the final decision. Our transformation builds on a simple but powerful intuition and applies to a wide range of existing schemes considered to be post-quantum secure including the NIST finalist Rainbow.While the primary motivation of progressive verification is to mitigate unexpected interruptions, we show that verifiers can leverage it in two innovative ways. First, progressive verification can be used to intentionally adjust the soundness of the verification process. Second, progressive verifications output by our transformation can be split into a computationally intensive offline set-up (run once) and an efficient online verification that is progressive