Federating Free-Tier Cloud Storage with Zero-Knowledge Encryption: The X3Sync Architecture

X3Sync is a research proof-of-concept for federated cloud storage aggregation across multiple free-tier providers (Google Drive, Dropbox, Koofr). Files are chunked, compressed (zstd), and encrypted client-side using AES-256-GCM before distribution. The system introduces a dual-mode decryption architecture: Sovereign Mode, where ciphertext is relayed to the client for local decryption, and Edge Mode, where an ephemeral X25519 key exchange enables worker-side decryption. The backend runs on Cloudflare Workers with Neon PostgreSQL for metadata storage. This paper details the system architecture, security model, provider abstraction layer, and a commutative storage model for heterogeneous provider aggregation.