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Adding automatic key rotation to an application ensures your app always has fresh, unguessable secrets. They can be used anywhere in your application where some sort of entropy is needed. Secure Key promotes best practices by keeping these secrets in your environment (rather than checked into source control) and by rotating them periodically.
You always have access to the current key and the previous key. This way when the keys are rotated, you can still validate thing such as cookies signed with the previous key, while signing new things with the new key.