Calculate the Fingerprint from an RSA Public Key

Updated July 5th, 2017.
SSH is a great protocol that encrypts traffic between the client and the server (among many other things that it does). But it doesn't provide authentication. That can leave you open to a Man in the Middle (MiTM) attack. You have probably started an SSH session to a switch, router or server and seen this dialog (or a similar one depending you the OS you are using):

The message reinforces the fact that SSH doesn't provide authentication. So what to do? 

In a previous blog Discovering SSH host keys with NMAP I showed you how to use NMAP to pull the fingerprint or full SSH key from a Cisco device. The problem here is that you still can't be sure that the device you scanned is actually the device you want to connect to.  

With newer Cisco IOS versions you can easily display the full RSA key of the device. If you are connected with a console cable there is no doubt that you are connected to the correct device. So you can record the key right after you generate it during the initial setup with a console cable.

Once you have the full key it's easy to get the fingerprint using OpenSSH on Linux/MAC. For windows users Didier Stevens over at the SANS InfoSec forums wrote a Python script you can use. His post can be found at SSH Fingerprints Are Important.

Here is the output on the switch

3750x#sh ip ssh

SSH Enabled - version 2.0

Authentication methods:publickey,keyboard-interactive,password

Encryption Algorithms:aes128-ctr,aes192-ctr,aes256-ctr,aes128-cbc,3des-cbc,aes192-cbc,aes256-cbc

MAC Algorithms:hmac-sha1,hmac-sha1-96

Authentication timeout: 120 secs; Authentication retries: 3

Minimum expected Diffie Hellman key size : 1024 bits

IOS Keys in SECSH format(ssh-rsa, base64 encoded):

ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAAAgQDQbH8FnvSpDpEsL3OzzWal1DCFBQmiImg4WWhSwlCu9mdHb53YigPEXJOlsVdX9KTKIACvwyHu5+yRQXa6+7Ahp7f98c1ac8wRW4Q8EP35kI00l+40LoC5G8eMiZ5Pz8o8UyAD/zmY1vWJzxs8k4mtdvgI4Pf5C3mmJ8r2nu0Zjw==

Some things to note here 

SSH Enabled - Version 2.0. It is critical that you don't allow your devices to use ANY version of SSH except 2.0. You do this by issuing "IP SSH Version 2" from global config mode. If you see version 1.99 then your device will accept SSH 1.0 connections and any version before 2.0 is broken.

The device will accept 3des-cbc encryption. 3des is an outdated encryption algorithm.  

Finally, the Hash Message Authentication Code (HMAC) is SHA1 which has been deprecated for SSL since the end of 2015.

Continuing on:

Copy everything from ssh-rsa to ==.

From a terminal:

Create an empty file to paste the key into. I used nano to create a file in the .ssh folder in my home directory. I called the file ios.pub. Obviously a more descriptive name would be used in production.

nano ~/.ssh/ios.pub 

Paste the key into the file. MAKE sure that the key is a continuos string. You will probably have to remove a couple line feeds after you paste the key in. Save the file.

ssh-keygen uses arguments to select the action to perform. To calculate the fingerprint we will use the following argurments:

-f filename - Filename of the key file.

-l - Show fingerprint of key file. (lowercase L)

Run this command:

ssh-keygen -lf ~/.ssh/ios.pub

1024 73:e9:e6:e3:f6:52:22:05:fc:d7:5d:d8:d7:ef:12:4d /home/mhubbard/.ssh/ios.pub (RSA)

You can see that the fingerprint matches the one in the dialog so I know for sure that I am connecting to the correct device and no MiTM is present.

This will work for any rsa .pub file. For example, you can run ssh-keygen -lf /etc/ssh/ssh_host_rsa_key >> $HOME/keys.txt on your Linux/Mac rig and keys.txt will contain the fingerprints for your rig.

cat keys.txt

1024 c1:c8:63:45:ce:56:66:c3:1e:7c:58:d9:c2:8f:28:1e  root@1S1K-DO (DSA)

256 1d:24:82:44:4f:56:40:4e:53:44:92:7a:1b:e5:43:e0  root@1S1K-DO (ECDSA)

2048 2c:84:a5:cd:81:eb:71:0d:42:10:68:c0:fe:28:3c:d4  root@1S1K-DO (RSA)

Didier Stevens from SANS wrote a cool Python 2 script for calculating the fingerprint from the key - Calculating a SSH Fingerprint From a (Cisco) Public Key | Didier Stevens. His blog explains the various parts of the key, worth reading for sure.

References
SSH Mastery: OpenSSH, PuTTY, Tunnels and Keys (ebook) - An excellent reference on SSH.
What is a SSH key fingerprint and how is it generated?

Discovering SSH Host Keys with NMAP

As network engineers we use SSH daily (hopefully, ssh and not telnet!) and with all the uproar over duplicate SSH keys lately I thought it would be a good time to do a blog about NMAP’s SSH Host-Key script. Last month, John Matherly, founder of Shodan published this blog:

Duplicate SSH Keys Everywhere

If you aren’t familiar with www.shodan.com it’s a search engine for Internet connected devices. There is a monthly fee but if you are interested in hacking it’s well worth the cost.

Digitial Ocean is a Cloud Hosting Provider that sells “Droplets”. You spin up a droplet using their control panel ($5 per month for 512MB RAM, 20GB SSD and a TB of bandwidth!). Ubuntu is a very popular OS for a droplet and they have instructions explaining how duplicate SSH keys can be accidentally created. Here are their instructions - Avoid Duplicate SSH Host Keys. Well worth reading if you run any version of Linux and enable SSH. You can use lasdigital (Linux Action Show) as a code to get two months of hosting for free.

Background

The script returns the SSH Host key of the target. There are some script arguments depending on whether you want to see the fingerprint or the full key. With no options the script returns the Fingerprint. This is a quick way to verify that the device has a key created and to see the fingerprint if you are looking for duplicates.

I have used the script when I get a “Connection Refused” message when connecting to a network device that should have SSH running. If SSH is enabled but you get a connection refused message the device may not have a host key installed. 

Let’s check the keys!

In this example I configured SSH on a Cisco 3750 switch. I kicked off "cypto key generate rsa usage-keys mod 1024” and immediately ran the script. I was lucky enough to catch it right before it created the keys:

nmap --script ssh-hostkey 192.168.10.50

Starting Nmap 6.47 ( http://nmap.org ) at 2015-03-14 20:22 Pacific Daylight Time
Nmap scan report for 192.168.10.50
Host is up (0.053s latency).
Not shown: 997 closed ports
PORT    STATE SERVICE
22/tcp  open  ssh
|_ssh-hostkey:
23/tcp  open  telnet
443/tcp open  https

MAC Address: 00:1B:90:9F:FF:C0 (Cisco Systems)

When the keys finished I re-ran the script with no options to get the fingerprint:

nmap --script ssh-hostkey 192.168.10.50

Starting Nmap 6.47 ( http://nmap.org ) at 2015-03-14 20:24 Pacific Daylight Time
Nmap scan report for 192.168.10.50
Host is up (0.047s latency).
Not shown: 997 closed ports
PORT    STATE SERVICE
22/tcp  open  ssh
| ssh-hostkey:
|_  1024 5b:b1:f3:00:b9:09:4a:04:75:10:ce:f5:11:e7:48:73 (RSA)
23/tcp  open  telnet
443/tcp open  https

MAC Address: 00:1B:90:9F:FF:C0 (Cisco Systems) 

A Quick Policy Assessment

You arrive at a customer site and are asked to do log into the switches/routers and make sure that they meet the corporate security policy:
SSH key length at least 1024 bits
telnet disabled
HTTP disabled

You complete the assignment almost before the customer finishes describing it to you!

root@YHS-kali:~# nmap --script ssh-hostkey 10.99.16.231-254
Starting Nmap 6.47 ( http://nmap.org ) at 2015-03-17 13:50 PDT

Nmap scan report for 10.99.16.240
Host is up (0.0043s latency).
Not shown: 998 closed ports
PORT   STATE SERVICE
23/tcp open  telnet
80/tcp open  http

Nmap scan report for 10.99.16.241
Host is up (0.0044s latency).
Not shown: 998 closed ports
PORT   STATE SERVICE
23/tcp open  telnet
80/tcp open  http

Nmap scan report for 10.99.16.249
Host is up (0.0053s latency).
Not shown: 998 closed ports
PORT    STATE SERVICE
22/tcp  open  ssh
| ssh-hostkey:
|_  1024 6a:7a:5f:54:ba:ce:ce:1e:7a:2c:4e:d3:65:fb:51:bf (RSA)
443/tcp open  https

Nmap scan report for 10.99.16.250
Host is up (0.0076s latency).
Not shown: 998 closed ports
PORT    STATE SERVICE
22/tcp  open  ssh
| ssh-hostkey:
|_  1024 b5:75:9d:d4:0b:f2:29:97:d1:ce:c4:2f:13:aa:c7:c7 (RSA)
443/tcp open  https

Nmap scan report for 10.99.16.252
Host is up (0.0060s latency).
Not shown: 998 closed ports
PORT    STATE SERVICE
22/tcp  open  ssh
| ssh-hostkey:
|_  1024 08:62:de:73:1a:b6:66:cc:9b:62:f5:86:64:a7:68:a3 (RSA)
443/tcp open  https

Nmap scan report for 10.99.16.253
Host is up (0.0033s latency).
Not shown: 998 closed ports
PORT    STATE SERVICE
22/tcp  open  ssh
| ssh-hostkey:
|_  1024 c9:3a:9c:0f:79:7f:62:73:dd:3d:dd:16:c3:85:08:4c (RSA)
443/tcp open  https

Nmap scan report for 10.99.16.254
Host is up (0.0025s latency).
Not shown: 997 closed ports
PORT    STATE SERVICE
22/tcp  open  ssh
| ssh-hostkey:
|_  1024 07:f1:5d:51:1d:93:96:ae:72:96:e0:fd:95:2f:b9:b6 (RSA)
23/tcp  open  telnet
443/tcp open  https

Grabbing the banner from the services running on the device

NMAP also comes with a Banner Grab script. This one is handy if you are verifying that none of your devices are running SSH V1. Remember, NMAP can scan a range of IP addresses, not just one host!

nmap --script banner 192.168.10.50

Starting Nmap 6.47 ( http://nmap.org ) at 2015-03-14 21:19 Pacific Daylight Time
Nmap scan report for 192.168.10.50
Host is up (0.036s latency).
Not shown: 997 closed ports
PORT    STATE SERVICE
22/tcp  open  ssh
|_banner: SSH-2.0-Cisco-1.25
23/tcp  open  telnet
|_banner: \xFF\xFB\x01\xFF\xFB\x03\xFF\xFD\x18\xFF\xFD\x1F
443/tcp open  https
MAC Address: 00:1B:90:9F:FF:C0 (Cisco Systems)

Nmap done: 1 IP address (1 host up) scanned in 70.09 seconds

The "Full" script argument

This time the script was run against my SonicWall firewall

sudo nmap --script ssh-hostkey --script-args ssh_hostkey=full 192.168.10.254

Starting Nmap 6.40 ( http://nmap.org ) at 2015-03-14 19:09 PDT

Nmap scan report for 192.168.10.254

Host is up (0.0051s latency).

Not shown: 997 closed ports

PORT STATE SERVICE

22/tcp open ssh

|_ssh-hostkey: ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAAAgQDSypZN2N3s3ACk+yOF72YCB4mmHcvF/kIjLuoJz2C991orznolsbBWIdKWcGjcil6mvIT5nj93miiaLMrS746Y9r7GBvrzV2xiNSYsK1Gs61sWn13tyTFbDaUeHK5SW2DYFbXDKHHtiVkTlXtc4bVPrvNpSmgmbCsD2Wo+ZZdlzQ==

80/tcp open http

443/tcp open https

MAC Address: 00:17:C5:67:BE:CC (SonicWALL)

Configuring SSH on Recent Versions of Cisco IOS

It only takes a few steps to setup SSH on an IOS device. You should add it to your standard build script! Note: The Cisco IOS image used must be a k9(crypto) image in order to support SSH.

Switch(config)#ip ssh version 2

Switch(config)#ip domain-name hubbardonnetworking.com

Switch(config)#crypto key generate rsa usage-keys modulus 1024

The name for the keys will be: Switch.example.com

% The key modulus size is 1024 bits
% Generating 1024 bit RSA keys, keys will be non-exportable...
[OK] (elapsed time was 1 seconds)
% Generating 1024 bit RSA keys, keys will be non-exportable...
[OK] (elapsed time was 1 seconds)
exit

#sh ip ssh
SSH Enabled - version 2.0
Authentication methods:publickey,keyboard-interactive,password
Authentication timeout: 120 secs; Authentication retries: 3
Minimum expected Diffie Hellman key size : 1024 bits
IOS Keys in SECSH format(ssh-rsa, base64 encoded):
ssh-rsa

References

SSH Fingerprints are Important - Python Script to calculate Fingerprint

Nmap 6 Cookbook: The Fat-Free Guide to Network Scanning [Kindle Edition]

Mastering the Nmap Scripting Engine [Kindle Edition]

NMAP Banner Grabber Script

NMAP ssh-hostkey Script

A nice guide on Configuring SSH

Cisco Secure Shell Commands

ASA - Configuring Management Access

Kali Linux remote SSH

UNSECURED SSH – THE CHALLENGE OF MANAGING SSH KEYS AND ASSOCIATIONS