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I would like to change my pgp key because I have the same one 2 years ago. But, I want to warn my freinds that the owner of the old key and the owner of the new one is the same person (me). hwat is the way I can use to prove I am the owner of the new pgp key ? Include it on the revocation certificate of the new one ?

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Has the private key been compromised in some way? Being two years of age doesn't seem to be a sufficient reason in itself to warrant a change. If it has been compromised however, the answers could change as the attacker could also impersonate you for the revocation. –  Yolanda Ruiz Feb 19 at 18:26
    
No, it isn’t compromised at all. I whant only change it for a better key. –  fauve Feb 19 at 18:49
    
gpg --gen-key --cert-digest-algo sha256 - this changes the self-signature to be SHA256 (you could also choose SHA512 for very slightly lower compatibility with some other OpenPGP products). –  Anti-weakpasswords Feb 20 at 2:57

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The main thing you need to do is revoke any usage of your old certificate. To do this you probably want to both upload your revokation certificate to any key servers you used to share your public key.

In addition it wouldn't go amiss to send this to all your contacts you use PGP with, so that they can manually revoke/remove your old key.

There is no way to prove to your contacts using the revocation certificate that the new public key is yours. This is to be expected and you really should be aware that the very ability to issue a recovcation certificate indicates access to the private key, something an attacker could just as well have (then using social engineering, convince users like you're trying to do to use their public key).

You need to negotiate distributing your new key manually, by other secure means. The most convenient way is probably uploading your new public key to a key server and then calling your friends with the fingerprint ID to confirm they pick the right one.

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Then the best think I can do is to add a message in the revocation certificate and to confirm with non-GPG ways to all my contacts that the owner of the new key is really me ? –  fauve Feb 19 at 19:03
    
No. You should never include instructions for a new public key in a revocation certificate. The reason is simple - a certificate can be revoked by an attacker who has gained access to your private key. So you do not want people to assign trust to a public key purely on the basis it's in the revocation certificate - an attacker could have created it! –  deed02392 Feb 20 at 10:29
    
But an attacker could have created the revocation key like he could have crypted and sign a lot of the messages sent in my name! Anyway, this idea was just in addition to other tings not the main evidence. In conclusion, I have to recall my contacts? –  fauve Feb 26 at 0:34
    
@fauve that is exactly my point - if an attacker can convince your friends to accept his new key, he's not only compromised your existing key but now has the trust of all your friends. So yes, in conclusion, the only way to switch to a new key is to revoke your old one only (do not send any more messages with it, signed) and then recall your contacts. –  deed02392 Feb 26 at 9:30

I would personally probably sign my new public key with the old private key and send it to my friends with an e-mail explaining the change. They could then verify I was the holder of the previous key and are made aware of the change. I'd then post a revocation of the old certificate and include the details of the new one if supported. (I'm not that particularly familiar with PGP, just general PKI.)

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Your friends probably would have to use another channel to confirm that the old key was not compromised. Otherwise it could be an attacker impersonating you, changing to a new key that you won't have access to. –  Yolanda Ruiz Feb 19 at 18:22
    
That's true, they would possibly still want to validate it with you. It might not hurt to also talk to one of them that is well known and have them sign that the update is legit as well. –  AJ Henderson Feb 19 at 18:24
    
But, if I say in the revocation’s message that the old key haven’t been compressed. They can understand I just whant to change it. No? –  fauve Feb 19 at 18:51
    
@fauve - problem is, if I compromised your private key, I can send a revocation of the old key and say that you still have control of it. That said, there is just as much reason to trust that the signature of a new certificate is valid as there is to trust that the key actually hasn't been compromised any other time, but revocation would be a way to prevent you from being able to still use your key and limit it to my new one. –  AJ Henderson Feb 19 at 19:10
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Then you think I have to kip my old key some times and warn my friends I change the key by I mail with the old key signature? –  fauve Feb 19 at 23:15

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