Bifurcated Signatures: Folding the Accountability vs. Anonymity Dilemma into a Single Private Signing Scheme

The 3-volume-set LNCS 12696 ⁰́₃ 12698 constitutes the refereed proceedings of the 40th Annual International Conference on the Theory and Applications of Cryptographic Techniques, Eurocrypt 2021, which was held in Zagreb, Croatia, during October 17-21, 2021. The 78 full papers included in these proceedings were accepted from a total of 400 submissions. They were organized in topical sections as follows: Part I: Best papers; public-key cryptography; isogenies; post-quantum cryptography; lattices; homomorphic encryption; symmetric cryptanalysis; Part II: Symmetric designs; real-world cryptanalysis; implementation issues; masking and secret-sharing; leakage, faults and tampering; quantum constructions and proofs; multiparty computation; Part III: Garbled circuits; indistinguishability obfuscation; non-malleable commitments; zero-knowledge proofs; property-preserving hash functions and ORAM; blockchain; privacy and law enforcement Bifurcated Signatures: Folding the Accountability vs. Anonymity Dilemma into a Single Private Signing Scheme 2 Introduction 3 Our Contributions 4 Technical Overview 5 Related Work 8 Preliminaries 10 R-Lossy Public-Key Encryption 10 Homomorphic Equivocal Commitments 13 Bifurcated Anonymous Signatures 14 Syntax 14 Branch-Hiding and Privacy 17 Branch-Soundness and Security 19 Generic Construction 23 Branch-Soundness and Security 26 Branch-Hiding and Privacy 28 Definition of Dual-Mode NIZK 32 Branch-Hiding: Formal Definition 33 Stronger Anonymity Notion 34 Deferred Proofs for the Generic Construction 35 Proof of Lemma 2 35 Proof of Lemma 3 36 Proof of Lemma 4 37 Material for Lattice-Based Instantiations 39 Lattices 39 Randomness Extraction 40 Fully Homomorphic Commitments and the GSW FHE 41 An RNEQ-Lossy PKE Scheme from LWE 42 Efficiency Estimations for the Pairing-Based Instantiation 46 New primitive,privacy,anonymity,accountability,group signatures,conditional traceability,predicate-based privacy Computer Science [cs]/Cryptography and Security [cs.CR] The 3-volume-set LNCS 12696 0́3 12698 constitutes the refereed proceedings of the 40th Annual International Conference on the Theory and Applications of Cryptographic Techniques, Eurocrypt 2021, which was held in Zagreb, Croatia, during October 17-21, 2021. The 78 full papers included in these proceedings were accepted from a total of 400 submissions. They were organized in topical sections as follows: Part I: Best papers; public-key cryptography; isogenies; post-quantum cryptography; lattices; homomorphic encryption; symmetric cryptanalysis; Part II: Symmetric designs; real-world cryptanalysis; implementation issues; masking and secret-sharing; leakage, faults and tampering; quantum constructions and proofs; multiparty computation; Part III: Garbled circuits; indistinguishability obfuscation; non-malleable commitments; zero-knowledge proofs; property-preserving hash functions and ORAM; blockchain; privacy and law enforcement