441

When you generate a key, you get "randomart" from newer versions of OpenSSH. I am unable to find an explanation of why, and what I'm supposed to use it for.

Generating public/private rsa key pair.
The key fingerprint is:
05:1e:1e:c1:ac:b9:d1:1c:6a:60:ce:0f:77:6c:78:47 you@i
The key's randomart image is:
+--[ RSA 2048]----+
|       o=.       |
|    o  o++E      |
|   + . Ooo.      |
|    + O B..      |
|     = *S.       |
|      o          |
|                 |
|                 |
|                 |
+-----------------+

Generating public/private dsa key pair.
The key fingerprint is:
b6:dd:b7:1f:bc:25:31:d3:12:f4:92:1c:0b:93:5f:4b you@i
The key's randomart image is:
+--[ DSA 1024]----+
|            o.o  |
|            .= E.|
|             .B.o|
|              .= |
|        S     = .|
|       . o .  .= |
|        . . . oo.|
|             . o+|
|              .o.|
+-----------------+
6
  • 4
    You can find an in-depth analysis of the VisualHostKey randomart in the short paper The Drunken Bishop.
    – loomi
    Sep 4, 2012 at 11:35
  • 11
    Another question I would like to ask is; is it safe to share your randomart image with others? In other words, given a randomart image such as the above, is it possible to reverse-engineer this back to the key?
    – AndyJ0076
    Mar 24, 2019 at 3:36
  • 4
    @AndyJ0076 randomart is a visualisation of the PUBLIC key's fingerprint, so it is safe :-)
    – opyate
    Jul 3, 2020 at 20:05
  • 1
    I was wondering what the key pair should be to see the Mona Lisa Dec 27, 2020 at 7:27
  • 1
    @ShadiNamrouti Something to be avoided in the future where everyone uses images. Mona Lisa would be equivalent of 1234 and asdf. ;)
    – JoonasD6
    Sep 4, 2021 at 20:36

4 Answers 4

317

The randomart is meant to be an easier way for humans to validate keys.

Validation is normally done by a comparison of meaningless strings (i.e. the hexadecimal representation of the key fingerprint), which humans are pretty slow and inaccurate at comparing. Randomart replaces this with structured images that are faster and easier to compare.

This paper "Hash Visualization: a New Technique to improve Real-World Security", Perrig A. and Song D., 1999, International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)" explains some techniques and advantages.

9
  • 94
    If you could just explain why humans validate keys, that could help, because frankly I tend to just put my public key in my authorized_keys file and be done with it.
    – dlamblin
    Sep 6, 2009 at 5:10
  • 57
    @dlamblin: You generally would not verify your own keys with this. It would, however, be useful for verifying the host key of a remote machine. One idea is that if you login to a particular machine from various locations (or you do not save its key into your known_hosts file), you will be able to recognize the “art” of the host's key. If that art suddenly changed you should be wary of typing in your password because it might mean that a man-in-the-middle attack is in progress on your connection (or it might mean that the host has just changed its keys for some other reason). Oct 27, 2009 at 5:43
  • 49
    Uhm, When could I see the hosts art? (I think I never did.) I only saw such an image after generating my key pair. And to what would I have to compare it to recognize 'sudden' change.
    – DerMike
    Feb 9, 2011 at 10:15
  • 15
    I'd wager the randomart adheres to a similar principle as hashes for integrity checks, namely: a small difference in the input generates a wildly different output. That would mean you would just have to memorize the rough shape of the expected randomart to be able to notice something is amiss. Of course this doesn't work in practice when SSH et al don't show you the randomart of the host you're connecting to (they should do so even when the host is known).
    – Alan Plum
    Feb 25, 2011 at 16:22
  • 7
    9 years later (I'm old) I have too many different hosts I connect to. Recognizing a change is unlikely.
    – dlamblin
    Jan 22, 2018 at 7:12
248

Add

-o VisualHostKey=yes 

to your command line, or put

VisualHostKey=yes 

in your ~/.ssh/config.

You'll see the randomart of the box you are logging onto. If you log on one day and the random art is different (your brain should go Hey! I don't recognise that!), then maybe someone is hacking, or something.

The idea is that you don't consciously need to do it. One of the keys for one of our machines kinda looks like a butterfly. Another one kinda looks like a dick (yes, our brains are primitive). If you log on every day, you get accustomed to the images without even trying.

7
  • 10
    Not great. If you've logged in before, much better for the computer to do the recognition for you using a stored fingerprint. The feature's only meant to be used for logging into new machines. May 21, 2012 at 13:55
  • 67
    Way late to this answer, but it's worth pointing out that this would be immensely useful if you were logging in from a different machine that didn't have all of your known_hosts. In that case, the computer wouldn't be able to verify that it's known, but the user should be able to see "That looks way different than normal!" and abort.
    – Xkeeper
    Oct 3, 2012 at 19:56
  • 11
    Letting your computer do the recognition is vulnerable to your own computer's known hosts being hacked. Much like you shouldn't let your computer enter passwords for you, you would be better off validating the host's key yourself. May 25, 2015 at 17:54
  • 6
    @MarkoTopolnik: if you can't trust your own local machine that you're connecting from, you shouldn't be using that machine to do anything, much less using SSH. For all you know, the ssh executable itself might've been replaced with a sinister version.
    – Lie Ryan
    Dec 11, 2019 at 3:58
  • @LieRyan good point, how do we protect against that? Since I'm unable to decipher the bits manually we'd need some tools that fail "when tampered with". Do you have insight how we could spot this? To spot when someone is using hacked firmware/hardware from government involvement?
    – paul23
    Jun 19, 2020 at 12:34
44

Official announcement: OpenSSH 5.1 released

Introduce experimental SSH Fingerprint ASCII Visualisation to ssh(1) and ssh-keygen(1). Visual fingerprinnt display is controlled by a new ssh_config(5) option "VisualHostKey". The intent is to render SSH host keys in a visual form that is amenable to easy recall and rejection of changed host keys. This technique inspired by the graphical hash visualisation schemes known as "random art[*]", and by Dan Kaminsky's musings at 23C3 in Berlin.

Fingerprint visualisation in is currently disabled by default, as the algorithm used to generate the random art is still subject to change.

1
32

The Randomart displayed after the ssh-keygen generation is a graphic representation of the key you have just generated. Then:

  • the Randomart is not really useful for the user who generated the ssh-key

  • the Randomart can be very useful for a user using a connection via SSH to connect often to the same server: if he added the "-o VisualHostKey=yes " option to his SSH command:

    ssh user@domainname.com -o VisualHostKey=yes

the Randomart corresponding to the public key of the server will be displayed.

To see an example, you can try:

ssh git@github.com -o VisualHostKey=yes

In the case where the user often connects to the same server, then he can quickly and easily check if he recognizes the Randomart corresponding to the public-key of this server or not. Which is easier and faster than checking the string of characters of the public-key itself !

1

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.