I want to generate an
RSA key in
GPG and use it in
SSH login. Is this even possible? If so, how?
I'm doing some research about this topic and I can give you some hints, but I've not found a way to make it work yet.
Monkeysphere seems a very interesting project, but I've not been able to compile it under Mac OS X without clogging my little free diskspace with MacPorts.
The first way I suggest you to try is to generate a compatible authorized_keys entry from your key id (e.g. BFB2E5E3) with
Here I added it to my localhost since I ran an ssh server for testing purposes, but of course you should add this to the target host ~/.ssh/authorized_keys. Next you need to tell SSH to use the private portion of this key during authentication, but simply exporting an ASCII armored version of the keypair doesn't work
gpg-agent has the option "--enable-ssh-support" that allows it to use it as a drop-in replacement for the well known ssh-agent. I've read of some people trying to add via ssh-add their GPG key after launching gpg-agent this way: gpg-agent --enable-ssh-support --daemon gpg --armor --export-secret-key BFB2E5E3! |tee ~/.gnupg/exported-keys/BFB2E5E3_sec.asc ssh-add ~/.gnupg/exported-keys/BFB2E5E3_sec.asc
But I don't think this will ever work. The manpage says: "SSH Keys, which are to be used through the agent, need to be added to the gpg-agent initially through the ssh-add utility. When a key is added, ssh-add will ask for the password of the provided key file and send the unprotected key material to the agent; this causes the gpg-agent to ask for a passphrase, which is to be used for encrypting the newly received key and storing it in a gpg-agent specific directory." So it seems that gpg-agent should be used as an additional measure to protect your SSH keys with a GPG encryption.
Converting a GPG key to OpenSSH
Jérôme Pouiller in his blog writes that Gpgsm utility can exports keys and certificate in PCSC12 and than used by OpenSSH using gpgsm:
But I haven't find a way to make gpgsm accept my gpg keypairs.
Other things you can try
SSH has -I option to specify the PKCS#11 shared library ssh should use to communicate with a PKCS#11 token providing the user's private RSA key. ssh-keygen can use RFC4716/SSH2 public or private key, PEM PKCS8 public keys, and PEM public keys to generate an OpenSSH compatible private (or public) key using -i and -m options. Still I miss a way to put it all toghether
No, they are not interchangeable. Yes, it is possible to use GPG keys for authentication – the Monkeysphere package has tools to extract the raw RSA keypair from your GPG certificate.
Somewhat relevant: this gnupg-users thread.
With the information from the answers on this question and the help of the gnupg-users mailinglist I was able to figure out how to use my GPG key for SSH authentication. As already mentioned by Claudio Floreani in his answer, there are a few possible methods to do this.
I have written a blogpost about some possible solutions: http://budts.be/weblog/2012/08/ssh-authentication-with-your-pgp-key
To summarize: Either you use GnuPG 2.1, which is currently in beta. When using this version, you can simply start gpg-agent with the --enable-ssh-support option and add the keygrip for you GPG key (or subkey) into ~/.gnupg/sshcontrol.
When you are using the current stable GnuPG version (2.0.x) you can use monkeysphere to add your key to gpg-agent (again, after starting gpg-agent with the --enable-ssh-support option).
It is also possible to use GNOME keyring (or even the regular ssh-agent) with the help of monkeysphere. The only problem in this case is that you will have to re-add your key when logging on again (into Gnome or XFCE). To solve this you can manually export your key and convert it.