In order to guard against a brute-force attack, every key must have an expiration date after which it is no longer valid.

The expiration date is stored in the public key of a Digital ID. Every Web browser or e-mail application checks the validity of a Digital ID by making sure the date you receive the Digital ID (and the information it is protecting) is within the valid dates. This means that when your own key expires, everything you signed with it will no longer be valid.

After expiration, the you need to renew your Digital ID using the Digital ID Center.