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Switching Password Hashing Algorithm in Spring Boot (The Right Way)
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Password hashing is a critical part of securing user credentials in any application. Over time, however, cryptographic standards evolve. What was once considered a secure hashing algorithm (like sha256) may eventually need to be replaced by a more robust alternative (like Argon2 or PBKDF2) due to security, or compliance requirements.
But what happens when your Spring Boot application is already in production, and user passwords are securely hashed and stored using the older algorithm?
Unlike encryption, hashing is a one-way operation — there’s no way to reverse a hash back to the original password. This means you can’t simply “re-hash” existing password hashes using the new algorithm. Forcing all users to reset their passwords isn’t user-friendly either.
So how do you migrate to a new password hashing algorithm without breaking the login experience for existing users?
In this article, we’ll explore how to gradually migrate your users’ password hashes to a newer algorithm as they log in, using Spring Boot. We’ll walk through:
- Why switching algorithms is necessary
- How to design a backward-compatible login mechanism
- How to transparently rehash passwords using the new algorithm on successful login
