Does OpenPGP key expiration add to security?

Question

I've created a new OpenPGP key to sign a software package in a source repository with an expiration date three years from now. It seemed like a good security measure, because if the key is compromised or stolen the damage will be limited.

But then I thought about the day when I will need to sign my new key. Signing the new key with the old key seems equivalent to keeping the old key, and thus adds nothing to security.

Does setting an expiration date improve key security? If so, what's the best expiration/key replacement policy?

Answer 1

tl;dr: the expiry date is no reasonable mechanism to protect the primary key, and you should have a revocation certificate at hand.

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The slightly longer version is, that the effect of the expiration date differs between primary and subkeys, and also what you aim to prevent.

Subkeys

For subkeys, the effect is rather simple: after a given time frame, the subkey will expire. This expiry date can only be changed using the primary key. If an attacker gets hold of your subkey (and only this), it will automatically be inactivated after the expiry date.

The expiry date of a subkey is a great tool to announce you switch your subkeys on a regular base, and that it's time for others to update your key after a given time.

Primary Keys

For primary keys, the situation is different. If you have access to the private key, you can change the expiry date as you wish. This means, if an attacker gets access to your private key, he can extend the validity period arbitrarily. Worst case, you lose access to the private key at the same time, then even you cannot revoke the public key any more (you do have a printed or otherwise offline and safely stored revocation certificate, do you?). An expiry date might help in the case you just lose control over the key (while no attacker has control over it). The key will automatically expire after a given time, so there wouldn't be an unusable key with your name on it sitting forever on the key servers.

Recovering Weak Unrevoked Keys

Even worse, expiry dates might provide a false sense of security. The key on the key servers expired, so why bother to revoke it? There is a large number of well-connected RSA 512 bit keys on the key server network, and probably a comparabily large number of weak DSA keys (because of the Debian RNG problems). With faster processors and possibly new knowledge on algorithm weaknesses, an attacker might in future be able to crack the expired, but non-revoked key and use it!

Keeping a Revocation Certificate Safe Instead

If you've got a revocation certificate and are sure you never might lose access to both your private key and revocation certificate at the same time (consider fire, (physical) theft, official institutions searching your house), there is absolutely no use in setting an expiry date apart from possible confusion and more work extending it.

Answer 2

Note that an expiration date on the master key doesn't really do anything for security, as anyone who compromised that could always just extend it anyway. See http://madduck.net/blog/2006.06.20:expiring-gpg/ (archived version).

(Expiring subkeys, on the other hand, can be useful, and the option at key creation sets the date on all of the keys.)

Answer 3

Does setting an expiration date improves key security?

Yes. You've already said why, too:

It seemed like a good security measure, because if the key is compromised or stolen - the damage will be limited.

Assuming you are compromised and the key is stolen, clearly, your first action would be to revoke the key and issue a new one. However, not everyone will check for revoked keys; they'll continue blindly using it until it becomes invalid.

Likewise, if you're compromised, somebody can only pretend to be you for a fixed window.

If so, what's the best expiration\key replacement policy?

There really isn't one - the question is more "what is the best I can do given constraints X Y Z". For example, if you set the Window too small, you inconvenience users; however, too high and you open up the risk if compromised. It really comes down to a judgement call - how sensitive exactly is what you're protecting and how often/inconvenient is it to replace keys frequently?