ZERO-KNOWLEDGE PROOF SYSTEMS FOR PRIVACY-PRESERVING APPLICATIONS IN DECENTRALIZED WEB INFRASTRUCTURES

In this work, we deal with the study and application of zero-knowledge proofs (ZKPs), a cryptographic paradigm that enables proving the validity of statements without revealing any additional information. The first chapter introduces the theoretical background of zero-knowledge proofs, analyzing early interactive protocols such as graph isomorphism and presenting the fundamental notions of completeness, soundness, and zero-knowledge. The discussion is framed within the theory of interactive proof systems, with connections to complexity classes such as NP, IP, and PSPACE. The chapter also explores Sigma protocols, special soundness, Schnorr and Chaum--Pedersen schemes, Pedersen commitments, and OR-proofs, concluding with an electronic voting system as a unifying example. The second chapter examines advanced non-interactive constructions, with a focus on zk-SNARKs and zk-STARKs. Their structure, differences, and trade-offs are analyzed, together with their main applications in blockchain ecosystems. Issues such as scalability, verifier efficiency, gas costs, and privacy-preserving authentication are discussed, along with open challenges and research directions. The third chapter shifts to the practical implementation of zero-knowledge proof systems. It presents the design goals and architecture of a system based on Circom circuits and the snarkjs library, covering circuit compilation, proof generation, verification, and integration with a React frontend. The chapter emphasizes data flow, performance optimization, and scalability considerations. The fourth chapter develops a running example based on the previous architecture: a zero-knowledge access-control system for an NFT gallery. This case study demonstrates how zk-SNARKs and commitment schemes can be used to guarantee user authentication, privacy, and robustness against dishonest parties, while maintaining scalability and usability in a real-world scenario.