International Association
for Cryptologic Research

Considering security against quantum adversaries, while it is important to consider the traditional existential unforgeability (EUF-CMA security), it is desirable to consider security against adversaries making quantum queries to the signing oracle: Plus-one security (PO security) and blind unforgeability (BU security) proposed by Boneh and Zhandry (Crypto 2013) and Alagic et al. (EUROCRYPT 2020), respectively. Hash-and-sign is one of the most common paradigms for constructing EUF-CMA-secure signature schemes in the quantum random oracle model, employing a trapdoor function and a hash function. It is known that its derandomized version is PO- and BU-secure. A variant of hash-and-sign, known as hash-and-sign with retry (HSwR), formulated by Kosuge and Xagawa (PKC 2024), is widespread since it allows for weakening the security assumptions of a trapdoor function. Unfortunately, it has not been known whether HSwR can achieve PO- and BU-secure even with derandomization. In this paper, we apply a derandomization with bounded loops to HSwR. We demonstrate that HSwR can achieve PO and BU security through this approach. Since derandomization with bounded loops offers advantages in some implementations, our results support its wider adoption, including in NIST PQC candidates.

A hash-and-sign signature based on a preimage-sampleable function (Gentry et al., STOC 2008) is secure in the quantum random oracle model if the preimage-sampleable function is collision-resistant (Boneh et al., ASIACRYPT 2011) or one-way (Zhandry, CRYPTO 2012). However, trapdoor functions in code-based and multivariate-quadratic-based signatures are not preimage-sampleable functions; for example, underlying trapdoor functions of the Courtois-Finiasz-Sendrier, Unbalanced Oil and Vinegar (UOV), and Hidden Field Equations (HFE) signatures are not surjections. Thus, such signature schemes adopt probabilistic hash-and-sign with retry. While Sakumoto et al. in PQCRYPTO 2011 showed the security of this paradigm in the classical random oracle model, their proof contains an error. Also, there is currently no known security proof for the probabilistic hash-and-sign with retry in the quantum random oracle model. We correct the proof in the random oracle model and give the first security proof in the quantum random oracle model for the probabilistic hash-and-sign with retry, assuming that the underlying trapdoor function is non-invertible, that is, it is hard to find a preimage of a given random value in the range. Our reduction from the non-invertibility assumption is tighter than the existing ones that apply only to signature schemes based on preimage-sampleable functions. We apply the security proof to code-based and multivariate-quadratic-based signatures. Additionally, we extend the proof into the multi-key setting and propose a generic method that provides security reduction without any security loss in the number of keys.