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It's ok that the various linux distributions use gpg to verify e.g. new packages downloaded with http. But what happens if the server that holds the packages gets hacked? OK, they need a new gpg key, but how do they guarantee that the new gpg keys are the valid gpg keys? There is no valid gpg key until the new one gets downloaded to the client, so there is a window when the packages [containing the new gpg keys] are not "validated" with gpg keys. Is there any method to give the new gpg keys to the clients in an "authoritative"/secure way?

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If only the server holding the packages is compromised, and not the private signing key, there's not much to do. The old key remains valid, and packages modified by the attacker will be detected as such. I assume your question is what happens if the signing key is compromised.

As long as the old key hasn't expired, people will continue to download the old packages, blissfully unaware that the key has been compromised. Ideally the package management tool should check that the key hasn't been revoked (I don't know if apt, yum and friends do it). However, the first step in responding to a compromise would be to stop distributing packages signed with the old key, and start distributing packages signed with the new key. So any maliciously modified package would linger only in mirrors before they are updated.

When people start receiving packages signed with the new key, they'll get an error message telling them that the packages are unsigned. This will hopefully prompt them into inquiring what is happening and trying to obtain the new key.

The compromise will also be announced in security mailing lists and in various industry news channels. So if you follow these, you'll get notice. Of course you should be wary of these as well: an attacker might compromise the list server or a developer's account and send a fake key compromise warning with a new public key that is actually for his own private key.

There's no magic bullet for distributing the new key. You need a trusted channel to distribute the key in the first place, or more precisely to establish trust in the new key. This is exactly as difficult as establishing trust in the old key. (In other words, most people will get it from the HTTP website or from an unsigned CD image.) You can get the new key from an HTTPS website, if you trust that the website (and the CA that made the site's certificate, and the browser you're using and its trusted base!) hasn't been compromised. Or if you know and trust someone who has the key, you can ask them for it.

Note that above, I'm using “package” in a loose sense, assuming a simple model where the packages are directly signed with the distribution signing key. In fact, in some distributions (e.g. all the ones that use APT), what is signed is files containing a list of cryptographic checksums of packages, and there's a two-stage process whereby the installer checks that the package has the expected checksum, and that the list has the expected signature. The principle is the same: the attacker who compromised the key would inject both malicious packages and list files with the checksums for the malicious packages, signed with the compromised key. The resolution requires restoring both the list files and the packages.

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It may be worth differentiating between how apt and friends does it, and how rpm does it. Apt signs release files, and rpm signs the actual packages. The difference means that potential attack vectors would be different. –  Faheem Mitha Mar 31 '11 at 21:19
Debian, at least, distributes archive keys in a package, namely debian-archive-keyring, which is covered by the old key. As I noted above, Debian does not actually sign the individual packages. –  Faheem Mitha Mar 31 '11 at 21:23
@FaheemMitha: Indeed, the attack vectors are different, but the distributor and user reactions would be similar, which is why I deliberately didn't make a difference. But you're right, I should be more explicit. –  Gilles Mar 31 '11 at 21:25
yes, sorry! i wanted to say: "what happens if the signing key is compromised" :\ –  LanceBaynes Mar 31 '11 at 22:33
@johnny8888: When you realize your question is unclear, go and improve it. For example, here, edit your question and make it explicit that you're worried about a signing key compromise. And as a more general matter, please try to make your questions more readable. Even if your native language isn't English (neither is mine, by the way), surely you can do better. Even tiny things help: for example, start every sentence with a capital letter. –  Gilles Mar 31 '11 at 22:45
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The standard process to give new "authoritative" key is through a package that is signed with the old key. If you trust the old key (and you trust GPG) then the new one is distributed in a package signed with the old one. Now, you can begin to use the new one.

The packages can be hold on an insecure server as each package will be checked for correct signature.

A problem occurs only is the packaging private key is compromised. I.e. the master key that sign all packages. (Debian SecureApt, Ubuntu SecureApt, Fedora). If it would happens once, it would be really ugly.

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Your answer is very confusing, which is especially bad since it's talking about security. If the old key is compromised, you can't use it in any useful way to sign the new key! –  Gilles Mar 31 '11 at 20:01
@Gilles: where did you see in the question that the old key is compromised? –  shellholic Mar 31 '11 at 20:23
The question is not clearly written, the asker doesn't distinguish between a package server compromise and a signing server compromise. But if “they need a new gpg key”, it would be because the key is compromised. –  Gilles Mar 31 '11 at 20:25
@Gilles: he write about the server that hold the packages, not the server that hold the private key. I hope he would have written that the private key was compromised before searching about potentially altered packages. But yes, if the private key was compromised, it is a big deal, but not the deal I can decipher from this question. –  shellholic Mar 31 '11 at 20:33
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