Recipe 6.4 Authenticating by Public Key (OpenSSH)

6.4.1 Problem

You want to set up public-key authentication between an OpenSSH client and an OpenSSH server.

6.4.2 Solution

OpenSSH public keys go into the file ~/.ssh/authorized_keys. Older versions of OpenSSH, however, require SSH-2 protocol keys to be in ~/.ssh/authorized_keys2.

6.4.3 Discussion

Public-key authentication lets you prove your identity to a remote host using a cryptographic key instead of a login password. SSH keys are more secure than passwords because keys are never transmitted over the network, whereas passwords are (albeit encrypted). Also, keys are stored encrypted, so if someone steals yours, it's useless without the passphrase for decrypting it. A stolen password, on the other hand, is immediately usable.

An SSH "key" is actually a matched pair of keys stored in two files. The private or secret key remains on the client machine, encrypted with a passphrase. The public key is copied to the remote (server) machine. When establishing a connection, the SSH client and server perform a complex negotiation based on the private and public key, and if they match (in a cryptographic sense), your identity is proven and the connection succeeds.

To set up public-key authentication, first create an OpenSSH key pair, if you don't already have one:

$ ssh-keygen -t dsa
Generating public/private dsa key pair.
Enter file in which to save the key (/home/smith/.ssh/id_dsa): <RETURN>
Enter passphrase (empty for no passphrase): *******
Enter same passphrase again: *******
Your identification has been saved in id_dsa
Your public key has been saved in id_dsa.pub.
The key fingerprint is: 76:00:b3:e8:99:1c:07:9b:84:af:67:69:b6:b4:12:17 smith@mymachine

Copy the public key to the remote host using password authentication:

$ scp ~/.ssh/id_dsa.pub remoteuser@remotehost:
Password: *********
id_dsa.pub      100% |*****************************|     736    00:03

Log into the remote host using password authentication:

$ ssh -l remoteuser remotehost
Password: ********

If your local and remote usernames are the same, you can omit the -l remoteuser part and just type ssh remotehost.

On the remote host, create the ~/.ssh directory if it doesn't already exist and set its mode appropriately:

remotehost$ mkdir -p ~/.ssh
remotehost$ chmod 700 ~/.ssh

Then append the contents of id_dsa.pub to ~/.ssh/authorized_keys:

remotehost$ cat id_dsa.pub >> ~/.ssh/authorized_keys    (Appending)
remotehost$ chmod 600 ~/.ssh/authorized_keys

Log out of the remote host and log back in. This time you'll be prompted for your key passphrase instead of your password:

$ ssh -l remoteuser remotehost
Enter passphrase for key '/home/smith/.ssh/id_dsa': *******

and you're done! If things aren't working, rerun ssh with the -v option (verbose) to help diagnose the problem.

The SSH server must be configured to permit public-key authentication, which is the default:

/etc/ssh/sshd_config:
PubkeyAuthentication yes           If no, change it and restart sshd

For more convenience, you can eliminate the passphrase prompt using ssh-agent [Recipe 6.9] and create host aliases in ~/.ssh/config. [Recipe 6.12]

6.4.4 See Also

ssh(1), scp(1), ssh-keygen(1).

SSH-2 Key File Formats The two major implementations of SSH—OpenSSH and SSH Secure Shell ("SSH2")—use different file formats for SSH-2 protocol keys. (Their SSH-1 protocol keys are compatible.) OpenSSH public keys for the SSH-2 protocol begin like this: ssh-dss A9AAB3NzaC1iGMqHpSCEliaouBun8FF9t8p... or: ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAIEA3DIqRox... SSH Secure Shell public keys for the SSH-2 protocol look like this: ---- BEGIN SSH2 PUBLIC KEY ---- AAAAB3NzaC1kc3MAAACBAM4a2KKBE6zhPBgRx4q6Dbjxo5hXNKNWYIGkX/W/k5PqcCH0J6 ... ---- END SSH2 PUBLIC KEY ---- These keys are installed differently too. For OpenSSH, you insert your public keys into the file ~/.ssh/authorized_keys. For SSH Secure Shell, you copy your public key files into the directory ~/.ssh2 and reference them in the file ~/.ssh2/authorization by name: Key public_key_filename As for private keys, OpenSSH has no special requirements for installation, but SSH Secure Shell does. You must reference them in the file ~/.ssh2/identification by name: IdKey private_key_filename