ZKIM File Format: A Three-Layer Encrypted Binary Format Specification

Abstract

This paper presents the ZKIM File Format, a production-ready binary format specification featuring three-layer encryption (Platform/User/Content), XChaCha20-Poly1305 authenticated encryption, Merkle tree integrity verification with BLAKE3, and searchable encryption with OPRF (Oblivious Pseudorandom Function) on Ristretto255. The format addresses the challenges of privacy-preserving search, multi-level access control, and content integrity in decentralized messaging platforms. We describe the wire format specification, encryption layers, key management strategies, and search capabilities. The format achieves privacy-preserving search without data access, perfect forward secrecy through per-file random keys, and tamper detection through Merkle tree verification. Performance evaluation demonstrates efficient encryption/decryption operations with minimal overhead while maintaining strong security guarantees.