DSB: A Local-First Offline Architecture for Secure Backup of Cryptocurrency Seed Phrases

Cryptocurrency seed phrases represent the ultimate recovery mechanism for self-custodied digital assets. Despite their critical importance, many users continue to store recovery phrases in plain-text files, cloud storage, screenshots, password managers, or other formats, which substantially increases the risk of unauthorized disclosure. Existing encryption solutions, while cryptographically sound, are generally designed as general-purpose file containers rather than workflows specifically optimized for seed phrase protection. This paper introduces Digital Seed Phrase Backup (DSB), a local-first architecture that securely protects cryptocurrency recovery phrases without requiring cloud services, user accounts, network connectivity, or server-side infrastructure. The proposed architecture combines password-based key derivation using Argon2id with authenticated encryption based on AES-256-GCM, and stores encrypted data within a compact authenticated binary container. Two independent implementations are presented: DSB-App, a desktop application developed in Python, and DSB-Web, a dependency-free browser implementation using the Web Crypto API. Both implementations perform all cryptographic operations entirely on the user's device while maintaining compatibility with modern operating systems and web browsers. The DSB architecture emphasizes simplicity, portability, metadata minimization, authenticated encryption, offline usability, and open-source transparency. Rather than proposing a new cryptographic primitive, the work demonstrates how established cryptographic standards can be integrated into a dedicated workflow that minimizes operational risks associated with cryptocurrency backup management. The resulting system offers a practical approach for secure offline storage of recovery phrases while remaining accessible to non-technical users.