Signing Subkey Cross-Certification

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or, what does the warning "signing subkey is not cross-certified" mean?

Short Answer:

• If you receive this warning when verifying a signature from someone else, please point them to this page. The signature that you verified has not been tampered with, but might not have been issued by the person who sent it to you.
• If you have been pointed to this page by someone who received a warning when verifying one of your signatures, your key does not contain a subkey cross-certification. You can easily add this cross-certification using GnuPG 1.4.3 or later. To do this, simply run "gpg --edit-key (yourkey)" and then enter "cross-certify". You'll need to type your passphrase, and GnuPG will add the necessary cross-certification. Once this is done, you should distribute your key however you like (send it to a keyserver, post on a web page, etc). If you have already done this and people are still receiving the warning, make sure they have updated their copy of your key from the keyserver or web page.

Full Explanation:

An OpenPGP key is made up of several keys: a primary key, which is used to digitally sign things, and optionally, subkeys that can be used to sign or encrypt things. To prove that the subkeys "belong" to the key and were not inserted by an attacker, they are digitally signed by the primary key.

Most users use their primary key for signing, and a subkey for encryption, and this is the default key style in both GnuPG and PGP. Some, generally more advanced users use two subkeys: one for encryption, and another one for signing. This allows the user more flexibility in managing their keys, as the primary key can then be kept off of their computer altogether.

There is a subtle weakness in the OpenPGP design for signing subkeys. Recall that subkeys are signed by the primary key to show they belong to the primary key. However, the signing subkey does not sign the primary to show that it is owned by the primary. This allows an attacker to take a signing subkey and attach it to their own key.

This does not mean that an attacker can issue signatures pretending to be someone else: the attacker cannot issue any signatures from that subkey, as all they have is the public half. The only thing this allows an attacker to do is to take an existing signature issued by a signing subkey, and claim that it was issued by the attacker's own key. The end result is that the signature can be verified by both the actual signer's key and the attacker's key. In practice, this can be detected by asking the attacker to issue a new signature for you on data that you supply (a "challenge"). The attacker, as they cannot issue signatures from the key, will not be able to do so.

The OpenPGP Working Group has defined a fix so this problem is stopped immediately, before the attacker can even try and claim a signature as their own.

Subkey cross-certification (sometimes called "back signing") involves the subkey issuing a signature on the primary key, just like the primary key signature on the subkey. Since only the true owner of the subkey can issue such a signature, the attacker's key cannot be cross-certified.

GnuPG has code for adding this cross-certification to signing subkeys that were issued before this change to the OpenPGP design. Just run "gpg --edit-key (yourkey)" and then enter "cross-certify". You'll need to type your passphrase, and GnuPG will add the cross-certification.