When your key expires, it becomes invalid. This means:
It can no longer be used to encrypt and decrypt messages.
It cannot sign other keys or receive signatures from other keys.
Depending on how long you want your key to be valid, you can set an expiration date for it. By default, Passwords and Keys sets all keys to be valid forever. That is, the keys never expire.
The expiration date on a key can be changed anytime, even after the key has expired. However, if you want to stop using the expired key, you should delete or revoke it.
If you have any messages encrypted with an expired key, you must change the expiration date on the key to decrypt such messages before deleting or revoking it.
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