Shell

Working with SSH Keys

SSH Keys are used for authentication to a remote system. Having SSH keys will relief you from typing in your password every time you log in to a remote system if properly set up.

Quick Start

1. Log in to a Sun Ray Terminal at the Institute.

2. Type in

   ssh-keygen -t rsa -b 2048

   ssh-keygen will ask some questions. Just press ENTER. This will create a Private/Public key pair without passphrase.

3. Still in the terminal, type in

   cat $HOME/.ssh/id_rsa.pub >> $HOME/.ssh/authorized_keys

   and

   chmod 600 $HOME/.ssh/authorized_keys

Now, you are able to log in to host you are granted ssh access without providing a password. For more information, read the sections below.

Private/Public Key Creation

On Unix systems, the command ssh-keygen is used to create the Private/Public key pair, as shown in the example below

$ ssh-keygen -t rsa -b 2048
Generating public/private rsa key pair.
Enter file in which to save the key (/home/<USER>/.ssh/id_rsa):
Enter passphrase (empty for no passphrase): 
Enter same passphrase again:
Your identification has been saved in /home/<USER>/.ssh/id_rsa.
Your public key has been saved in /home/<USER>/.ssh/id_rsa.pub.
The key fingerprint is:
ae:73:55:6a:22:68:2e:1e:10:df:e6:a3:5d:57:07:2a <USER>@<HOST>
$

The output may differ, depending on the system you invoke the command. The example will create a RSA private/public key (-t rsa) with 2048 bits (-b 2048).

When asked for the file in which to save the key, it is usually safe to go with the default, i.e. just pressing the ENTER key.

When asked for the passphrase, you have two choices:

1. just pressing the ENTER key, hence not setting any passphrase, which in turn will leave your private key unprotected
2. entering a pass phrase and thus encrypting the private key (see below for a discussion of the pros and cons of setting a pass phrase)

After You need to create the private/public key pair only once.

Using the Private/Public Key Pair for Authentication

After creating the key pair, you have to set up your account to make use of them. This is straight forward by adding your public key you created previously to the .ssh/authorized_keys file, e.g.

$ cat $HOME/.ssh/id_rsa.pub >> $HOME/.ssh/authorized_keys

You have to make sure this file has the proper permissions, else ssh refuses to authenticate using the private/public key mechanism. Therefore, do

$ chmod 600 $HOME/.ssh/authorized_keys

Now, you are able to login to any system you are granted access without having to type in your password when using SSH. This is especially true if you did not set a pass phrase.

If you have set a passphrase, a dialog will pop up asking you for the pass phrase. You have to enter it only once, and it will be remembered for the duration of your session, thus any further ssh connections will work without the need to enter a pass phrase.

Using Private/Public Key Authentication from Outside IMATH

The above is all good and fine if you are using one of the Sun Ray Terminals, or SGD at IMATH. If you want to access the IT infrastructure of IMATH from the outside using SSH, follow those steps

1. Create a SSH private/public key pair as described above for your private account (not your IMATH account).

2. Copy the public key to your IMATH account for example by using scp.

3. Log in to the IMATH IT infrastructure and type on in a terminal

   cat <publickey> >> $HOME/.ssh/authorized_keys

   Where <publickey> is the file name of the public key you copied in step 2.

Why using a Passphrase?

The passphrase is used to encrypt and decrypt your private key. If somehow some gets possession of your private key, he won't be able to use it unless he also knows the passphrase.

In case the private key is not protected by a passphrase, and the private key gets 'stolen', the thief is able to use the private key in order to log in on any computer you can, with your credentials.

So, the bottom line is to use a strong passphrase (>15 characters) to protect the private key. The security benefit outweighs the discomfort caused by the need to provide the passphrase the first time only the key is used.

Problems

no connection

The SSH Server is protected with an IP blacklist.

An IP address will show up on the blacklist, if more than 3 logins (with an unknown user id) or 5 logins (with a known user id and a bad password) will occur during 5 days.

A successfull login will reset the counter.

If you think your IP address have been blocked (indication: no password prompt appears), you can free your IP address easily:

http://www.math.uzh.ch/my > SSH > Open