Why set an expiration date for a key?

When you create or edit a key, the default option in GPG is not to set any expiration date. I set all my keys to expire after 1 year though. Here’s why.

The short explanation

• Expiration dates are easy to set up and change (even after a key has expired).
• Expiration dates can give you peace of mind about old, forgotten keys. This is especially important if you’re still learning about and experimenting with GPG.

The long explanation

Suppose the following scenario:

• Alice no longer wants anyone to use an old public key of hers.
• Bob has the public key.
• She hasn’t sent him a revocation certificate.

There can be many reasons why Alice hasn’t sent Bob a revocation certificate:

• Maybe she has forgotten to.
• Maybe she couldn’t reach Bob.
• Maybe she couldn’t generate a revocation certificate (because she has forgotten her passphrase or lost the secret key completely).
• Maybe she doesn’t want to message Bob.
• Maybe she doesn’t know Bob (because Bob got her public key from her website).

There can also be many reasons why Alice no longer wants to use the key:

• Maybe she doesn’t use the email address anymore (and doesn’t want to change the UID).
• Maybe the secret key has been compromised.
• Maybe she has forgotten her passphrase or lost the secret key completely.
• Maybe she just doesn’t feel like using the key any more.

If Alice has lost her secret key and Bob sends her an encrypted message, it’s just an inconvenience that she can’t decrypt the message. But if the secret key has been compromised and Bob wrongly believes that only Alice can decrypt his secret message, that could have grave consequences.

Whatever the reason, Alice doesn’t want anyone (including Bob) to use the key any more. But as long as the initial scenario persists, Bob will never know that, unless Alice has set an expiration date for her key.

The expiration date of a key tells Bob to no longer use the key after that date. If a key has expired or is soon to expire, Bob can check if Alice has issued her key again with a new expiration date or if she has issued a new key entirely.

If Alice sets all her keys to expire after 1 year, Bob has to check every year if Alice still uses the key if he wants to use it. And after 1 year, Alice can be sure that if there are any old public keys of hers out there that she has completely forgotten about, everyone knows not to use them any more (as long as the secret key hasn’t been compromised, because then someone could set a new expiration date).

So much for the advantages. The only disadvantage is that it’s a bit of work for you (you have to set a new expiration date if you still want to use the key after a year) and for others (they have to check if you still want to use the key after a year). But better to be safe than sorry. And don’t worry: you can always set a new expiration date for a key if you still have the secret key.

=> Set a new expiration date for a key

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