A practical demo of the zero-downtime migration pattern: run old + new password hash formats side-by-side, migrate users on login, and remove legacy safely.

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Milan Jovanović

Password hashing algorithms need upgrading over time, but hashing is one-way so you can't simply convert existing hashes. The solution is a zero-downtime migration pattern: register both old (PBKDF2) and new (Argon2) hashers using .NET keyed services, attempt verification with the new algorithm first, fall back to legacy on failure, and when legacy succeeds, immediately re-hash with the new algorithm and update the database. This migrate-on-login approach gradually transitions active users without breaking authentication. Production improvements include algorithm prefixes for efficient routing and feature flags to control migration rollout under load. After most users migrate, force password resets for remaining legacy accounts and remove the old code path.

A Practical Demo of Zero-Downtime Migrations Using Password Hashing