How to configure SSH Public Key Authentication for login to the ESA without a password

Introduction

This document describes how to generate a private secure shell (SSH) key and use that for username and authentication when logging into the command line interface (CLI) on the Cisco Email Security Appliance (ESA).

How to configure SSH Public Key Authentication for login to the ESA without a password

Public-key authentication (PKI) is an authentication method that relies on a generated public/private keypair. With PKI, a special "key" is generated which has a very useful property: Anyone who can read the public half of the key is able encrypt data which can then only be read by a person who has access to the private half of the key. In this way, having access to the public half of a key allows you to send secret information to anyone with the private half, and to also verify that a person does in fact have access to the private half. It's easy to see how this technique could be used to authenticate.

As a user, you can generate a keypair and then place the public half of the key on a remote system, such as your ESA. That remote system is then able to authenticate your user ID, and allow you to login just by having you demonstrate that you have access to the private half of the keypair. This is done at the protocol level inside SSH and happens automatically.

It does, however, mean that you need to protect the privacy of the private key. On a shared system where you do not have root this can be accomplished by encrypting the private key with a passphrase, which functions similarly to a password. Before SSH can read your private key in order to perform the public key authentication you'll be asked to supply the passphrase so that the private key can be decrypted. On more secure systems (like a machine where you are the only user, or a machine at your home where no strangers will have physical access) you can simplify this process either by creating an unencrypted private key (with no passphrase) or by entering your passphrase once and then caching the key in memory for the duration of your time at the computer. OpenSSH contains a tool called ssh-agent which simplifies this process.

ssh-keygen example for Linux/Unix

Complete the following steps to set up your a linux/unix workstation (or server) to connect to the ESA without a password.  In this example, we will not specify as passphrase.

1) On your workstation (or server), generate a private key using the Unix command ssh-keygen:

$ ssh-keygen -b 2048 -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/home/[USERID]/.ssh/id_rsa): 
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /home/[USERID]/.ssh/id_rsa.
Your public key has been saved in /home/[USERID]/.ssh/id_rsa.pub.
The key fingerprint is:
00:11:22:77:f6:a9:1e:19:f0:ca:28:9c:ff:00:11:22 [USERID]@hostname.com
The key's randomart image is:
+--[ RSA 2048]----+
|           +... +|
|            o= o+|
|           o o ..|
|       . ..o . + |
|       . ES. o + |
|         o + . . |
|           o . . |
|             o o |
|             . . |
+-----------------+

(*the above was generated from an Ubuntu 14.04.1) 

2) Open the public key file (id_rsa.pub) created in #1 and copy the output:

$ cat .ssh/id_rsa.pub 
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDJg9W3DeGf83m+E/PLGzUFPalSoJz5F
 t54Wl2wUS36NLxm4IO4Xfrrb5bA97I+ZA4YcB1l/HsFLZcoljAK4uBbmpY5kXg96A6Wf
 mIYMnl+nV2vrhrODgbcicEAdMcQN3wWHXiEWacV+6u+FlHlonkSAIDEug6vfnd+bsbcP
 Zz2uYnx1llxbVtGftbWVssBK3LkFp9f0GwDiYs7LsXvQbTkixrECXqeSrr+NLzhU5hf6
 eb9Kn8xjytf+eFbYAslam/NEfl9i4rjide1ebWN+LnkdcE5eQ0ZsecBidXv0KNf45RJa
 KgzF7joke9niLfpf2sgCTiFxg+qZ0rQludntknw [USERID]@hostname.com

3) Login to your appliance and configure your ESA to recognize your workstation (or server) using the public SSH key that you created in #1, and commit the changes.  Notice the password prompt during login:

$ ssh admin@192.168.0.199
******************************
CONNECTING to myesa.local
Please stand by...
******************************

Password:[PASSWORD]
Last login: Mon Aug 18 14:11:40 2014 from 192.168.0.200
Copyright (c) 2001-2013, Cisco Systems, Inc.


AsyncOS 8.5.6 for Cisco C100V build 074

Welcome to the Cisco C100V Email Security Virtual Appliance

myesa.local> sshconfig

Currently installed keys for admin:

Choose the operation you want to perform:
- NEW - Add a new key.
- USER - Switch to a different user to edit.
[]> new

Please enter the public SSH key for authorization.
Press enter on a blank line to finish.
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDJg9W3DeGf83m+E/PLGzUFPalSoJz5F
 t54Wl2wUS36NLxm4IO4Xfrrb5bA97I+ZA4YcB1l/HsFLZcoljAK4uBbmpY5kXg96A6Wf
 mIYMnl+nV2vrhrODgbcicEAdMcQN3wWHXiEWacV+6u+FlHlonkSAIDEug6vfnd+bsbcP
 Zz2uYnx1llxbVtGftbWVssBK3LkFp9f0GwDiYs7LsXvQbTkixrECXqeSrr+NLzhU5hf6
 eb9Kn8xjytf+eFbYAslam/NEfl9i4rjide1ebWN+LnkdcE5eQ0ZsecBidXv0KNf45RJa
 KgzF7joke9niLfpf2sgCTiFxg+qZ0rQludntknw [USERID]@hostname.com

Currently installed keys for admin:
1. ssh-rsa AAAAB3NzaC1yc2EAA...rQludntknw ([USERID]@hostname.com)

Choose the operation you want to perform:
- NEW - Add a new key.
- DELETE - Remove a key.
- PRINT - Display a key.
- USER - Switch to a different user to edit.
[]> 

myesa.local> commit

4) Exit out of the appliance, and re-login.  Notice the password prompt is removed, and access is directly granted:

myesa.local> exit

Connection to 192.168.0.199 closed.
robert@ubuntu:~$ ssh admin@192.168.0.199
******************************
CONNECTING to myesa.local
Please stand by...
******************************

Last login: Mon Aug 18 14:14:50 2014 from 192.168.0.200
Copyright (c) 2001-2013, Cisco Systems, Inc.


AsyncOS 8.5.6 for Cisco C100V build 074

Welcome to the Cisco C100V Email Security Virtual Appliance
myesa.local> 

ssh-keygen example for Windows

Complete the following steps to set up your a Windows workstation (or server) to connect to the ESA without a password.  In this example, we will not specify as passphrase.  

Note: There are a variation on console application used from Windows.  You will need to research and find the solution that works best for your console application.  This example will use PuTTy and PuTTyGen.

1) Open PuttyGen.

2) For Type of key to generate, select SSH-2 RSA.

3) Click the Generate button.

4) Move your mouse in the area below the progress bar. When the progress bar is full, PuTTYgen generates your key pair.

5) Type a passphrase in the Key passphrase field. Type the same passphrase in the Confirm passphrase field. You can use a key without a passphrase, but this is not recommended.

6) Click the Save private key button to save the private key.

Note: You must save the private key. You will need it to connect to your machine.

7) Right-click in the text field labeled Public key for pasting into OpenSSH authorized_keys file and choose Select All.

8) Right-click again in the same text field and choose Copy.

9) Using PuTTY, login to your appliance and configure your ESA to recognize your Windows workstation (or server) using the public SSH key that you saved and copied from #6 - #8, and commit the changes.  Notice the password prompt during login:

login as: admin
Using keyboard-interactive authentication.
Password: [PASSWORD]
Last login: Mon Aug 18 11:46:17 2014 from 192.168.0.201
Copyright (c) 2001-2013, Cisco Systems, Inc.


AsyncOS 8.5.6 for Cisco C100V build 074

Welcome to the Cisco C100V Email Security Virtual Appliance
myesa.local> sshconfig

Currently installed keys for admin:

Choose the operation you want to perform:
- NEW - Add a new key.
- USER - Switch to a different user to edit.
[]> new

Please enter the public SSH key for authorization.
Press enter on a blank line to finish.
ssh-rsa AAAAB3NzaC1yc2EAAAABJQAAAQEAj6ReI+gqLU3W1uQAMUG0620B+tpdkjkgBn
 5NfYc+qrtyB93stG38O1T4s0zHnhuKJLTdwBg/JHdFuNO77BY+21GYGS27dMp3UT9/VuQ
 TjP8DmWKOa+8Mpc9ePdCBZp1C4ct9oroidUT3V3Fbl5M9rL8q4/gonSi+7iFc9uOaqgDM
 /h+RxhYeFdJLechMY5nN0adViFloKGmV1tz3K9t0p+jEW5l9TJf+fl5X6yxpBBDoNcaB9
 jNwQ5v7vcIZBv+fl98OcXD9SNt08G0XaefyD2VuphtNA5EHwx+f6eeA8ftlmO+PgtqnAs
 c2T+i3BAdC73xwML+1IG82zY51pudntknw rsa-key-20140818

Currently installed keys for admin:
1. ssh-rsa AAAAB3NzaC1yc2EAA...51pudntknw (rsa-key-20140818)

Choose the operation you want to perform:
- NEW - Add a new key.
- DELETE - Remove a key.
- PRINT - Display a key.
- USER - Switch to a different user to edit.
[]>

myesa.local> commit  

10) From the PuTTy configuration window, and your pre-existing Saved Session for your ESA, choose Connection > SSH > Auth and in the Private key file for authentication field, click Browse and find your saved private key from step #6.

11) Save the Session (profile) in PuTTY, and click Open.  Login with the username, if not already saved or specified from the pre-configured Session.  Notice the inclusion of "Authenticating with public key "[FILE NAME OF SAVED PRIVATE KEY]" when logging in:

login as: admin
Authenticating with public key "rsa-key-20140818"
Last login: Mon Aug 18 11:56:49 2014 from 192.168.0.201
Copyright (c) 2001-2013, Cisco Systems, Inc.


AsyncOS 8.5.6 for Cisco C100V build 074

Welcome to the Cisco C100V Email Security Virtual Appliance
myesa.local>

Related Information

• Cisco Email Security Appliance - End-User Guides
• Technical Support & Documentation - Cisco Systems