Withdrawable Signature: How to Call off a Signature

Digital signatures are a cornerstone of security and trust in cryptography, providing authenticity, integrity, and non-repudiation. Despite their benefits, traditional digital signature schemes suffer from inherent immutability, offering no provision for a signer to retract a previously issued signature. This paper introduces the concept of a withdrawable signature scheme, which allows for the retraction of a signature without revealing the signer\u27s private key or compromising the security of other signatures the signer created before. This property, defined as ``withdrawability\u27\u27, is particularly relevant in decentralized systems, such as e-voting, blockchain-based smart contracts, and escrow services, where signers may wish to revoke or alter their commitment. The core idea of our construction of a withdrawable signature scheme is to ensure that the parties with a withdrawable signature are not convinced whether the signer signed a specific message. This ability to generate a signature while preventing validity from being verified is a fundamental requirement of our scheme, epitomizing the property of withdrawability. After formally defining security notions for withdrawable signatures, we present two constructions of the scheme based on the pairing and the discrete logarithm. We provide proofs that both constructions are unforgeable under insider corruption and satisfy the criteria of withdrawability. We anticipate our new type of signature will significantly enhance flexibility and security in digital transactions and communications

Certificateless Designated Verifier Proxy Signature

Proxy signature (PS) is a kind of digital signature, in which an entity called original signer can delegate his signing rights to another entity called proxy signer. Designated verifier signature (DVS) is a kind of digital signature where the authenticity of any signature can be verified by only one verifier who is designated by the signer when generating it. Designated verifier proxy signature (DVPS) combines the idea of DVS with the concept of proxy signature (PS) and is suitable for being applied in many scenarios from e-tender, e-voting, e-auction, e-health and e-commerce, etc. Many DVPS schemes have been proposed and Identity-based DVPS (IBDVPS) schemes have also been discussed. Certificateless public-key cryptography (CL-PKC) is acknowledged as an appealing paradigm because there exists neither the certificate management issue as in traditional PKI nor private key escrow problem as in Identity-based setting. A number of certificateless designated verifier signature (CLDVS) schemes and many certificateless proxy signature (CLPS) schemes have been proposed. However, to the best of our knowledge, the concept of Certificateless Designated Verifier Proxy Signature (CLDVPS) has not been appeared in the literature. In this paper, we formalize the definition and the security model of CLDVPS schemes. We then construct the first CLDVPS scheme and prove its security