How to create a PGP/GPG-key free of SHA-1

If you've read my last blog entry, you saw that I was struggling a bit with the fact that I was unable to create a PGP key without SHA-1. This is a bit tricky, as there are various places where hash functions are used within a pgp key:

1. The key self-signatures and signatures on other keys. Every key has user IDs that are signed with the master key itself. This is to proofe that the names and mail adresses in the key belong to the keyholder itself and can't be replaced my a malicous attacker.
2. The signatures on messages, for example E-Mails.
3. The preference in side the key - this indicates to other people what sigature algorithms you would prefer if they send messages to you.
4. The fingerprint.

1 is controlled by the setting cert-digest-algo in the file gpg.conf (for both self-signatures and signatures to other keys). 2 is controlled by the setting personal-digest-preferences. So you should add these two lines to your gpg.conf, preferrably before you create your own key (if you intend to create one, don't bother if you want to stick with your current one):

personal-digest-preferences SHA256
cert-digest-algo SHA256

3 defaults to SHA256 if you generate your key with a recent GnuPG version. You can check it with gpg --edit-key [your key ID] and then showpref. For 4, I think it can't be changed at all (though I think it doesn't mean a security threat for collission attacks - still it should be changed at some point).

It is also not really trivial to check the used algorithms. For message signatures, if you verify them with gpg -v --verify [filename]. For key signatures, I found no option to do that - but a workaround: Export the key whose signatures you'd like to check gpg --export --armor [key ID] > filename.asc. Then parse the exported file with gpg -vv filename.asc. It'll show you blocks like this:

:signature packet: algo 1, keyid A5880072BBB51E42
	version 4, created 1294258192, md5len 0, sigclass 0x13
	digest algo 8, begin of digest 3e c3

The digest algo 8 is what you're looking for. 1 means MD5, 2 means SHA1, 8 means SHA256. Other values can be looked up in include/cipher.h in the source code. No, that's not user friendly. But I found no easier way.

The big question remains: Why is this so complicated and why isn't gnupg just defaulting to SHA256? I don't know the answer.

(Please also have a look at this blog entry from Debian about the topic)