OpenSSH 7.9's new key revocation support is welcome but can't be a full fix

November 11, 2018

I was reading the OpenSSH 7.9 release notes, as one does, when I ran across a very interesting little new feature (or combination of features):

  • sshd(8), ssh-keygen(1): allow key revocation lists (KRLs) to revoke keys specified by SHA256 hash.

  • ssh-keygen(1): allow creation of key revocation lists directly from base64-encoded SHA256 fingerprints. This supports revoking keys using only the information contained in sshd(8) authentication log messages.

Any decent security system designed around Certificate Authorities needs a way of revoking CA-signed keys to make them no longer valid. In a disturbingly large number of these systems as people actually design and implement them, you need a fairly decent amount of information about a signed key in order to revoke it (for instance, its full public key). In theory, of course you'll have this information in your CA system's audit records because you'll capture all of it in your audit system when you sign a key. In practice there are many things that can go wrong even if you haven't been compromised.

Fortunately, OpenSSH was never one of these systems; as covered in ssh-keygen(1)'s 'Key Revocation Lists', you could specify keys in a variety of ways that didn't require a full copy of the key's certificate (by serial number or serial number range, by 'key id', or by its SHA1 hash). What's new in OpenSSH 7.9 is that they've reduced the amount of things you need to know in practice, as now you can revoke a key given only the information in your ordinary log messages. This includes but isn't limited to CA-signed SSH keys (as I noticed recently).

(This took both the OpenSSH 7.9 change and an earlier change to log the SHA256 of keys, which happened in OpenSSH 6.8.)

This OpenSSH 7.9 new feature is a very welcome change; it's now much easier to go from a log message about a bad login to blocking all future use of that key, including and especially if that key is a CA-signed key and so you don't (possibly) have a handy copy of the full public key in someone's ~/.ssh/authorized_keys. However, this isn't and can't be a full fix for the tradeoff of having a local CA. The tradeoff is still there, it's just somewhat easier to deal with either a compromised signed key or the disaster scenario of a compromised CA (or a potentially compromised one).

With a compromised key, you can immediately push it into your system for distributing revocation lists (and you should definitely build such a system if you're going to use a local CA); you don't have to go to your CA audit records first to fish out the full key and other information. With a potentially compromised CA, it buys you some time to roll over your CA certificate, distribute the new one, re-issue keys, and so on, without being in a panic situations where you can't do anything but revoke the CA certificate immediately and invalidate everyone's keys. Of course, you may want to do that anyway and deal with the fallout, but at least now you have more options.

(If you believe that your attacker was courteous enough to use unique serial numbers, you can also do the brute force approach of revoking every serial number range except the ones that you're using for known, currently valid keys. Whether or not you want to use consecutive serial numbers or random ones is a good question, though, and if you use random ones, this probably isn't too feasible.)

PS: I continue to believe that if you use a local CA, you should be doing some sort of (offline) auditing to look for use of signed keys or certificates that are not in your CA audit log. You don't even have to be worried that your CA has been compromised, because CA software (and hardware) can have bugs, and you want to detect them. Auditing used keys against issued keys is a useful precaution, and it shouldn't need to be expensive at most people's scale.

Written on 11 November 2018.
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Last modified: Sun Nov 11 17:31:47 2018
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