Decentralized Threshold Signatures for Blockchains with Non-Interactive and Transparent Setup

Threshold signatures are digital signatures that support the multi-party signature generation such that a number of parties initially share a signing key and more than a threshold number of parties gather to generate a signature. In this paper, we propose a non-interactive decentralized threshold signature (NIDTS) scheme that supports the non-interactive and transparent key setup based on BLS signatures. Our NIDTS scheme has the following properties. 1) The key setup process is completely non-interactive and does not require message exchange between parties since the transfer of the register keys of parties is enough for a combiner to generate a compact verification key. 2) The register key of a party is compact since the size is independent of the number of group parties. 3) The signing process of parties is non-interactive. 4) The final threshold signature as well as partial signatures are succinct. We prove the security of our NIDTS scheme under computational assumptions in the random oracle model. Furthermore, we implement our NIDTS scheme in Rust and compare its performance with other scheme to show that the key setup of our scheme is more efficient. For example, in the unweighted setting of 1000 parties, the key setup process of the NIDTS scheme takes 164 seconds, which is 5.9 times faster than the key setup process of the multiverse threshold signature (MTS) scheme

Signcryption schemes with threshold unsigncryption, and applications

The final publication is available at link.springer.comThe goal of a signcryption scheme is to achieve the same functionalities as encryption and signature together, but in a more efficient way than encrypting and signing separately. To increase security and reliability in some applications, the unsigncryption phase can be distributed among a group of users, through a (t, n)-threshold process. In this work we consider this task of threshold unsigncryption, which has received very few attention from the cryptographic literature up to now (maybe surprisingly, due to its potential applications). First we describe in detail the security requirements that a scheme for such a task should satisfy: existential unforgeability and indistinguishability, under insider chosen message/ciphertext attacks, in a multi-user setting. Then we show that generic constructions of signcryption schemes (by combining encryption and signature schemes) do not offer this level of security in the scenario of threshold unsigncryption. For this reason, we propose two new protocols for threshold unsigncryption, which we prove to be secure, one in the random oracle model and one in the standard model. The two proposed schemes enjoy an additional property that can be very useful. Namely, the unsigncryption protocol can be divided in two phases: a first one where the authenticity of the ciphertext is verified, maybe by a single party; and a second one where the ciphertext is decrypted by a subset of t receivers, without using the identity of the sender. As a consequence, the schemes can be used in applications requiring some level of anonymity, such as electronic auctions.Peer ReviewedPostprint (author's final draft

Cryptanalysis of a One-Time Code-Based Digital Signature Scheme

We consider a one-time digital signature scheme recently proposed by Persichetti and show that a successful key recovery attack can be mounted with limited complexity. The attack we propose exploits a single signature intercepted by the attacker, and relies on a statistical analysis performed over such a signature, followed by information set decoding. We assess the attack complexity and show that a full recovery of the secret key can be performed with a work factor that is far below the claimed security level. The efficiency of the attack is motivated by the sparsity of the signature, which leads to a significant information leakage about the secret key.Comment: 5 pages, 1 figur