Commitments from Quantum One-Wayness

One-way functions are central to classical cryptography. They are both necessary for the existence of non-trivial classical cryptosystems, and sufficient to realize meaningful primitives including commitments, pseudorandom generators and digital signatures. At the same time, a mounting body of evidence suggests that assumptions even weaker than one-way functions may suffice for many cryptographic tasks of interest in a quantum world, including bit commitments and secure multi-party computation. This work studies one-way state generators [Morimae-Yamakawa, CRYPTO 2022], a natural quantum relaxation of one-way functions. Given a secret key, a one-way state generator outputs a hard to invert quantum state. A fundamental question is whether this type of quantum one-wayness suffices to realize quantum cryptography. We obtain an affirmative answer to this question, by proving that one-way state generators with pure state outputs imply quantum bit commitments and secure multiparty computation. Along the way, we build an intermediate primitive with classical outputs, which we call a (quantum) one-way puzzle. Our main technical contribution is a proof that one-way puzzles imply quantum bit commitments.Comment: 68 page

Commitments from Quantum One-Wayness

One-way functions are central to classical cryptography. They are both necessary for the existence of non-trivial classical cryptosystems, and sufficient to realize meaningful primitives including commitments, pseudorandom generators and digital signatures. At the same time, a mounting body of evidence suggests that assumptions even weaker than one-way functions may suffice for many cryptographic tasks of interest in a quantum world, including bit commitments and secure multi-party computation. This work studies one-way state generators [Morimae-Yamakawa, CRYPTO 2022], a natural quantum relaxation of one-way functions. Given a secret key, a one-way state generator outputs a hard to invert quantum state. A fundamental question is whether this type of quantum one-wayness suffices to realize quantum cryptography. We obtain an affirmative answer to this question by proving that one-way state generators with pure state outputs imply quantum bit commitments and secure multiparty computation. Along the way, we build an intermediate primitive with classical outputs, which we call a (quantum) one-way puzzle. Our main technical contribution is a proof that one-way puzzles imply quantum bit commitments

Making Existential-Unforgeable Signatures Strongly Unforgeable in the Quantum Random-Oracle Model

Strongly unforgeable signature schemes provide a more stringent security guarantee than the standard existential unforgeability. It requires that not only forging a signature on a new message is hard, it is infeasible as well to produce a new signature on a message for which the adversary has seen valid signatures before. Strongly unforgeable signatures are useful both in practice and as a building block in many cryptographic constructions. This work investigates a generic transformation that compiles any existential-unforgeable scheme into a strongly unforgeable one, which was proposed by Teranishi et al. and was proven in the classical random-oracle model. Our main contribution is showing that the transformation also works against quantum adversaries in the quantum random-oracle model. We develop proof techniques such as adaptively programming a quantum random-oracle in a new setting, which could be of independent interest. Applying the transformation to an existential-unforgeable signature scheme due to Cash et al., which can be shown to be quantum-secure assuming certain lattice problems are hard for quantum computers, we get an efficient quantum-secure strongly unforgeable signature scheme in the quantum random-oracle model.Comment: 15 pages, to appear in Proceedings TQC 201

Mobile qualified electronic signatures for secure mobile brokerage

Despite a legal framework being in place for several years, the market share of qualified electronic signatures is disappointingly low. Mobile Signatures provide a new and promising opportunity for the deployment of an infrastructure for qualified electronic signatures. We that SIM-based signatures are the most secure and convenient solution. However, using the SIM-card as a secure signature creation device (SSCD) raises new challenges, because it would contain the user’s private key as well as the subscriber identification. Combining both functions in one card raises the question who will have the control over the keys and certificates. We propose a protocol called Certification on Demand (COD) that separates certification services from subscriber identification information and allows consumers to choose their appropriate certification services and service providers based on their needs. This infrastructure could be used to enable secure mobile brokerage services that can ommit the necessity of TAN lists and therefore allow a better integration of information and transaction services

Mobile qualified electronic signatures and certification on demand

Despite a legal framework being in place for several years, the market share of qualified electronic signatures is disappointingly low. Mobile Signatures provide a new and promising opportunity for the deployment of an infrastructure for qualified electronic signatures. We analyzed two possible signing approaches (server based and client based signatures) and conclude that SIM-based signatures are the most secure and convenient solution. However, using the SIM-card as a secure signature creation device (SSCD) raises new challenges, because it would contain the user’s private key as well as the subscriber identification. Combining both functions in one card raises the question who will have the control over the keys and certificates. We propose a protocol called Certification on Demand (COD) that separates certification services from subscriber identification information and allows consumers to choose their appropriate certification services and service providers based on their needs. We also present some of the constraints that still have to be addressed before qualified mobile signatures are possible