Exercises

Using SSH public/private Keys for Authentication
1. Munging a Document and Comparing Message Digests
2. Generate a public/private Key Pair for SSH
3. Copy Your Public Key to Your Neighbor's admin Account

Notes

1. The "#" and "$" characters before commands represents your system prompt and is not part of the command itself. "#" indicates a command issued as root while "$" indicates a command issued as a normal user.

1.) Munging a Document and Comparing Message Digests [Top]

To do this exercise you will need to be root.

On your machine type:

# cat /etc/motd

Now run the file through the sha1 one-way hashing function:

# sha1 /etc/motd

# sha1 /etc/motd > /tmp/motd-hash

Example:

# vi /etc/motd

# sha1 /etc/motd >> /tmp/motd-hash

Compare the results with your neighbor, or with your previous sha1 message digest. They should be very different.

As discused the sha1 hashing algorithm is no longer considered reliable. You can do this same exercises using sha256 instead.

Note: In Linux the equivalent hashing functions are named sha1sum and sha256sum.

2.) Generate Public/Private Key Pair for SSH [Top]

Note: Please be sure that you are logged in and using your afnog account for this exercise - not root.

We will now generate a single RSA SSH protocol 2 key of 2048 bits. To do this, issue the following commands:

$ cd
$ ssh-keygen -t rsa -b 2048

This command output should look like:

Generating public/private rsa key pair.
Enter file in which to save the key (/home/afnog/.ssh/id_rsa):   [PRESS ENTER]
Created directory '/home/afnog/.ssh'.
Enter passphrase (empty for no passphrase):     [TYPE IN PASSPHRASE]
Enter the same passphrase again:                [TYPE IN SAME PASSPHRASE]
...

You will see something like this:

Your identification has been saved in /home/afnog/.ssh/id_rsa.
Your public key has been saved in /home/afnog/.ssh/id_rsa.pub.
The key fingerprint is:
d9:99:7c:ad:80:90:df:8c:1b:7e:79:a4:bb:c3:89:a1 afnog@pc10.sae.ws.afnog.org
The key's randomart image is:
+--[ RSA 2048]----+
|      E.         |
|       ..        |
|         .       |
|        +        |
|     o oSo .     |
|      = o.o .    |
|     . o *.o.    |
|        = *o.    |
|         =**     |
+-----------------+

3.) Copy Your Public Key to Your Neighbor's afnog Account [Top]

To avoid problems please use this neighbor list for this exercise:

Neighbor List

Note: "pcX" refers to your neighbor's machine. If your neighbor is pc10, then pcX would be pc10, etc.

This exercise can be confusing. To make thing easier open two terminal windows on your desktop. In one window make sure you are the afnog user on your machine. We will call this your local window.

In another window type:

$ ssh afnog@pcX

You have already generated your public/private ssh key pair. In your local window do the following:

$ cd ~/.ssh
$ scp id_rsa.pub afnog@pcX:/tmp/.

$ cd ~/.ssh
$ cat /tmp/id_rsa.pub >> authorized_keys
$ rm /tmp/id_rsa.pub

In your local window connect to your neighbor's machine as afnogusing ssh:

$ ssh afnog@pcX

If you remember our discussion in the presentation this is cool.

Remember to log out of your neighbor's machine in both your local window and your remote window by doing:

# exit

Hervey Allen