[kijin]

In asymmetric encryption, the key for encryption is different from the key for decryption. You can encrypt something without being able to decrypt it, or vice versa.

You can encrypt your "master" key with another key derived from your password. When you change your password, you just decrypt and re-encrypt the master key so that it doesn't have to change.

The combination of these two techniques in one form or another is responsible for much of modern computer security, including the encryption used on this very website.