Every hashing and encryption algorithm will eventually become vulnerable and obsolete. Before that happens, you need to stay ahead and use something known to protect your sensitive data.
When the algorithm you are using becomes vulnerable, you must act quickly and replace it as soon as possible to minimize the exposure window. Replacing your current algorithm might be a daunting task if you did not prepare for change.
We must design systems for change.
Recently I had to upgrade an encryption library while maintaining the old library for backward compatibility. The new library gave us some flexibility; the old one did not have. However, the underlying algorithms stayed the same.
The idea was that whenever we needed to decrypt an old piece of data, we would re-encrypt using the new library. That way, we could gradually migrate to the new library without interruption. We needed a way to encode what library we were using with each piece of data to decide what library we needed to use to decrypt and if we needed to migrate that data to the new library format.
When presented with this challenge, I remembered how Django encoded passwords for database storage.
They encode the algorithm and other needed information to recreate the password hash into a string. Then, when a user enters a password, you can quickly parse this string and get all the required information to validate the hash and decide if the hash needs an upgrade.
It’s a straightforward idea that works great.
Quick aside to clarify: If you have a known vulnerable hash or encryption algorithm you will want to do something about it immediately and not wait for user action.
Specifically, in case of a vulnerable password hash the recommended technique is to rehash your current weak hashes with a secure hashing algorithm. Then, when the user logs in you can migrate their password hash to the new setup.
Finally, I wrote this post to remind you and me that we should add a version prefix whenever we need to store a password hash or encrypted data. That way, when we need to migrate, we are ready to do it.