SSH into Kali 2.0 on ARM (Odroid, Banana Pi, etc)

My latest pen testing tool is an Odroid UX4 from Ameridriod.com. The XU4 is an 8 core ARM based Single Board Computer (SBC). The team at Offensive Security have ported Kali 2.0 to run on the Odroid. I flashed Kali 2.0 onto the eMMC card and the image works great. My next step was to setup SSH for remote access.

Kali 2.0 follows the Debian model of not allowing Root logins without a key.  This is definitely the most secure way to use SSH for remote access and it easy to set up.

If you are running headless you will need to use the USB/UART that Hardkernel sells for the UX4. I have found the Logitech MK620 wireless keyboard/mouse works great with the Odroid XU4. The wireless keyboard does have security issues (see Keysniffer.net) but I don't use it anywhere but in my lab.

Generating the RSA private/pubic keys

If you need to create RSA keys on Windows please see my blog Authenticating to Cisco devices using SSH and your RSA Public Key on using puttygen to create the keys.

On a Linux box

You should always move the default SSH keys on your distribution to a backup folder and create new keys before using SSH. See Duplicate SSH Keys Everywhere
If you need to generate an RSA key pair:
ssh-keygen
select the default location
enter a pass phrase to protect the keys.

Here's what it looked like on my Ubuntu laptop:

mhubbard@1S1K-SYS76:~$ ssh-keygen
Generating public/private rsa key pair.
Enter file in which to save the key (/home/mhubbard/.ssh/id_rsa):
Created directory '/home/mhubbard/.ssh'.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/mhubbard/.ssh/id_rsa.
Your public key has been saved in /home/mhubbard/.ssh/id_rsa.pub.
The key fingerprint is:
3a:1f:82:3a:60:b5:99:e0:4e:9e:91:92:12:05:4d:fc mhubbard@1S1K-SYS76

Here are the keys in the hidden .ssh folder:
mhubbard@1S1K-SYS76:~$ ls -la ~/.ssh
total 20
drwx------  2 mhubbard mhubbard 4096 Aug 30 11:13 .
drwxr-xr-x 18 mhubbard mhubbard 4096 Aug 30 11:13 ..
-rw-------  1 mhubbard mhubbard 1766 Aug 30 11:05 id_rsa
-rw-r--r--  1 mhubbard mhubbard  401 Aug 30 11:05 id_rsa.pub
-rw-r--r--  1 mhubbard mhubbard  222 Aug 30 11:13 known_hosts

To display the fingerprint in the future:
mhubbard@1S1K-SYS76:~$ ssh-keygen -lf ~/.ssh/id_rsa
2048 3a:1f:82:3a:60:b5:99:e0:4e:9e:91:92:12:05:4d:fc  mhubbard@1S1K-SYS76 (RSA)

This pass phrase will be required the first time you connect to a device but is it much safer than not using one. If you lose control of your private key and didn't have a pass phrase set it could be used to log into any device you set up.

Once you have the key pair generated you need to copy the public key and paste it into the authorized_keys file on the Odroid.

mhubbard@1S1K-SYS76:cd ~/.ssh
mhubbard@1S1K-SYS76:~/.ssh$ cat id_rsa.pub
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCUUfkp+xPLQDmGaPokHXxK6yhxG1a+pmdCHDzHFBSqsGJIMuVSSwzUksfJU7XG5PEcE8Yt9qU2WVieZXsjMcdZDphhNzdT4RSW63m12ov7jo35KBGFEHnNGv/m6cUEOq9oGfRD5sE7g2wF+QQh74Y4m1ld5VnOicGaR/qyYSazpBJlZKHP2lYfb2coDhHHsnoyBedZbtFxNK3lPKggd4mywmJ19XSZ9Bm2/HurcibYJodHdDn0BoP9Qlvc5vRiZqT6XAS9tIhzeydgkmhYQ8UnAYeclo/WiTR98FYEXzm+aGV0/bH4IZB6mlth53YyR/hxeeKzn7WqTc6/WKx9qrSB mhubbard@1S1K-SYS76

Copy the entire key including the ssh-rsa and username@system

On the Odriod

cd ~
mkdir .ssh
nano /root/.ssh/authorized_keys


ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCUUfkp+xPLQDmGaPokHXxK6yhxG1a+pmdCHDzHFBSqsGJIMuVSSwzUksfJU7XG5PEcE8Yt9qU2WVieZXsjMcdZDphhNzdT4RSW63m12ov7jo35KBGFEHnNGv/m6cUEOq9oGfRD5sE7g2wF+QQh74Y4m1ld5VnOicGaR/qyYSazpBJlZKHP2lYfb2coDhHHsnoyBedZbtFxNK3lPKggd4mywmJ19XSZ9Bm2/HurcibYJodHdDn0BoP9Qlvc5vRiZqT6XAS9tIhzeydgkmhYQ8UnAYeclo/WiTR98FYEXzm+aGV0/bH4IZB6mlth53YyR/hxeeKzn7WqTc6/WKx9qrSB mhubbard@1S1K-SYS76

CTRL+x to exit. Be sure to answer y and save the file.

Note: Make sure the key is on one line! This isn't an issue on Linux/Mac OX but if you copy the key from a Windows box it will probably have line feeds in it. MS can never follow the standards set by Unix decades ago!

On your favorite ssh client 

I use Perl Auto Connect (PAC) on Linux and Putty on Windows. If you are a Windows user my previous blog covers using an RSA key pair with Putty.

On PAC
Create a new connection
Click the Private Key Radio button
Browse to the ~/.ssh folder and select your private key
Enter root as the user:
Enter the pass phrase for the private key.
Click "Save and Close".

You can now log into Kali with your RSA key pair.

References
Kali Linux 2.0 Top 10 Post Install Tips
Kali SSH problems

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?