Signing keys

A SigningKey is a built-in secret type for the key that sessions and tokens are signed with: at least 32 characters, with no spaces. Authentication reads it as signing_key: config.session_signing_key. When your app sets a revocation delay, it signs sessions and tokens with that key (The application and revocation).

A SigningKey setting reads its variable and, when that is set, <NAME>_PREVIOUS as well: session_signing_key reads SESSION_SIGNING_KEY and SESSION_SIGNING_KEY_PREVIOUS. New tokens are signed with the current key, and tokens signed with either key are accepted. So to rotate a key, set the new one and move the old one to _PREVIOUS. Remove the old one once the longest token lifetime has passed. Both keys are checked when the app starts, and neither is ever printed.

An EncryptionKey setting rotates the same way, with one difference: what it sealed stays sealed with it until rewritten, so _PREVIOUS can go only once nothing needs it. Rotating the key says how the application's field_encryption_key: does that.