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Using GPG To Sign SSH Keys

Often to access a remote server by SSH the administrator of the server will ask for your public ssh_dsa key so that he knows it is really your computer that is trying to access his server. The administrator may ask you to first sign the ssh_dsa key using GPG so that he knows the ssh_dsa key comes from you and has not been intercepted. This guide will show you how to generate your SSH and GPG keys and then how to use them to perform a secure transaction between two parties.

Ubuntu Releases

This guide should work on any Gnu/Linux operating system. This guide assumes you have already installed open-ssh and gnupg.

Generate The SSH DSA Keys

Run all commands as a regular user.

# ssh-keygen 

This will create your public and private SSH-DSA keys. The public key that the administraitor needs should be located here: ~/.ssh/id_dsa.pub.

Generate The GPG Keys

This is the output from generating a new key.

# gpg --gen-key
gpg (GnuPG) 1.2.5; Copyright (C) 2004 Free Software Foundation, Inc.
This program comes with ABSOLUTELY NO WARRANTY.
This is free software, and you are welcome to redistribute it
under certain conditions. See the file COPYING for details.

gpg: keyring `/home/alex/.gnupg/secring.gpg' created
Please select what kind of key you want:

#->  (1) DSA and ElGamal (default)
  (2) DSA (sign only)
  (4) RSA (sign only)
Your selection? 1
DSA keypair will have 1024 bits.
About to generate a new ELG-E keypair.
             minimum keysize is  768 bits
             default keysize is 1024 bits
   highest suggested keysize is 2048 bits
What keysize do you want? (1024)

#->Requested keysize is 1024 bits
Please specify how long the key should be valid.
        0 = key does not expire
     <n>  = key expires in n days
     <n>w = key expires in n weeks
     <n>m = key expires in n months
     <n>y = key expires in n years

#->Key is valid for? (0)
Key does not expire at all
Is this correct (y/n)? y

You need a User-ID to identify your key; the software constructs the user id
from Real Name, Comment and Email Address in this form:
   "Heinrich Heine (Der Dichter) <[email protected]>"

Real name: mr bo jangles
Email address: [email protected]
Comment: comment
You selected this USER-ID:
   "mr bo jangles (comment) <[email protected]>"

Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? O
You need a Passphrase to protect your secret key.

#-> passphrase:

We need to generate a lot of random bytes. It is a good idea to perform
some other action (type on the keyboard, move the mouse, utilize the
disks) during the prime generation; this gives the random number
generator a better chance to gain enough entropy.
+++++.+++++++++++++++.+++++++++++++++.+++++.++++++++
We need to generate a lot of random bytes. It is a good idea to perform
some other action (type on the keyboard, move the mouse, utilize the
disks) during the prime generation; this gives the random number
generator a better chance to gain enough entropy.
+++++++++++++++++++++++++.++++++++++..+++++++++++++
public and secret key created and signed.
key marked as ultimately trusted.

pub  1024D/5F6D1662 2005-03-26 mr bo jangles (comment) <[email protected]>
    Key fingerprint = D1BC 6822 0ACB 0025 8902  6DE7 87EA 4324 5F6D 1662
sub  1024g/0572A97F 2005-03-26

Your public and private GPG keys should now be located in your ~/.gnupg directory. Put your private key on a cd-rom or a floppy disc or somewhere very safe. Do not lose it or you will be unable to sign any documents. Never give it to anyone under any circumstances. If you have given anyone your private key then you must revoke the key immediately and generate a new set.

Exchange Public Keys

It is good practice to put your public GPG key on a public key server where others can access it easily. [[1]] is a public key server. In order to put your public key on biglumber you will first need to go though a verification process. Go to [[2]] and put your public key on their server. While you are at Biglumber you will need to find the public key of the administrator to whom you are planning to send your digitally signed and encrypted message. Once you have done that, you must then import the pubic key of that administrators into your keyring.

# gpg --import Administrator.pub

Now get the Administrators key ID, and your key ID as well:

# gpg --list-keys
pub  1024D/ABCABCAB 2005-03-26 Administrator_Email <[email protected]>
pub  1024D/XYZXYZXY 2005-03-26 Your_Email_Address <[email protected]>

Aministrator ID: ABCABCAB Your ID: XYZXYZXY

Make a Secure Transaction

GPG will use your secret key (~/.gnupg/secring.gpg) to sign and encrypt your public ssh key (~/.ssh/id_dsa.pub). Only the administrator will be able to decrypt the file because you are also using his public key to encrypt it. In turn, he will only be able to decrypt it if he has your public key on his key ring. Sign the key:

# gpg -u XYZXYZXY -r ABCABCAB --armor --sign --encrypt ~/.ssh/id_dsa.pub

Send the result (id_dsa.pub.gpg) to the administrator along with a link to where you keep your public key on [[3]]. He will verify the your information and then allow you to access his system by SSH. In an ideal world you are only supposed to exchange public keys directly and in person. This way you know 100% that the public key truly belongs to the correct person.

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