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Scenario: I have multiple signing subkeys, each for every portable device (i.e laptop). None of them have private master key (it is deleted following the tips in Alex Cabal's post. One of the devices is stolen, I download the public key from the server (like https://pgp.mit.edu/), revoke the public version with master key and upload the revoked key back to server. Same procedure goes with encryption key.

And now my questions:

  1. Can I revoke the signing subkey using the public key or do I need a copy of a private key as well?
  2. Should I delete the key from my keyring to have a clean list of keys or should I keep the revoked key (and why)?
  3. How can I inform friends/colleagues about it? Do I email every one of them or is there a more massive approach? If they don't receive the information, will they see for example my emails as "TRUSTED Good signature from ..."?
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  1. Can I revoke the signing subkey using the public key or do I need a copy of a private key as well?

No, you always need the private primary key for all key management operations (subkeys, user IDs, ... are all bound by signatures of the private primary key). Consider it was possible to do this using the public primary key, everybody would be able to revoke your key (as the public key is -- well, public).

  1. Should I delete the key from my keyring to have a clean list of keys or should I keep the revoked key (and why)?

Leave it there. The public key will get resynchronized anyway as soon as you update your key from the key servers. And the public key also carries the revocation information.

  1. How can I inform friends/colleagues about it? Do I email every one of them or is there a more massive approach? If they don't receive the information, will they see for example my emails as "TRUSTED Good signature from ..."?

"It depends". Some clients will automatically fetch the updated key when they receive the first mail signed with an unknown (sub)key, others won't.

If you're just doing key escrow and exchange keys in advance, consider creating the new key some weeks in advance to allow people fetching them up. I wouldn't bother people when creating new subkeys (especially signing subkeys) without a real revocation reason, they will realize they're missing the new key as soon as they receive a signed message. As you're taking advantage of subkeys, if they trusted your old key, they still trust your new one (trust and validity are assigned to primary keys, subkey validity is derived from the primary key).

If you suspect or are sure your private signing subkey got into the hands of an attacker, I'd indeed go out and mail those I signed/signed me and/or publish this information at relevant places (for example, if you're signing software).

  • Ad.1 I thought you need the PRIMARY to revoke the subkey? So the public subkey will not suffice. Ad.2 The lesson is: one shall have at least one backup copy of every key *both public and private) ever created. Ad.3 No, I don't bother people when creating new subkeys, I worry that that - if not informed - they can receive message from a thief and see it as "TRUSTED" – mDfRg Apr 5 '16 at 22:50
  • 1: I slightly extended the answer to be more clear. 2. Doesn't hurt to have one. Some people decided to dedicate some old computer not used any more otherwise for this purpose, which is also the only place the private primary key is stored ("offline primary key"). 3. If you're not just escrowing keys periodically, but suspect it to be in the hands of an attacker, notifying other peers of course is a good thing to do. I mostly wanted to say "if you're just doing periodic key escrow without an immediate reason, just trust in others to fetch the key anyway". – Jens Erat Apr 6 '16 at 6:10

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