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I always find that I get this message when I ssh into a new machine:

12:f8:7e:78:61:b4:bf:e2:de:24:15:96:4e:d4:72:53

What does it stand for? Will every machine have the same fingerprint every time?

How are these fingerprints generated? What parameters do they depend on?

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3 Answers

up vote 6 down vote accepted

The fingerprint is based on the Host's Public key, usually based on "/etc/ssh/ssh_host_rsa_key.pub" Generally its for easy identification/verification of the host you are connecting to.

If the fingerprint changes, the machine you are connecting to has changed their public key. This may not be a bad thing(happens from re-installing ssh), but it could also indicate that you are connecting to a different machine at the same domain/IP(happens when you are connecting through something like load balancer) or that you are being targeted with a man-in-the-middle attack, where the attacker is somehow intercepting/rerouting your ssh connection to connect to a different host which could be snooping your user/pw.

Bottom line: if you get warned of a changed fingerprint, we cautious and double check that your actually connecting to the correct host over a secure connection. Though most of the time this is harmless, it can be an indication of an issue

See: http://www.lysium.de/blog/index.php?/archives/186-How-to-get-ssh-server-fingerprint-information.html
and: http://en.wikipedia.org/wiki/Public_key_fingerprint

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You can generate a fingerprint for a public key using ssh-keygen like so:

ssh-keygen -lf /path/to/key.pub

Concrete example (if you use an RSA public key):

$ ssh-keygen -lf ~/.ssh/id_rsa.pub
2048 00:11:22:33:44:55:66:77:88:99:aa:bb:cc:dd:ee:ff /Users/username/.ssh/id_rsa.pub (RSA)

The first part (2048) is the key length in bits, second part (00:11:22:33:44:55:66:77:88:99:aa:bb:cc:dd:ee:ff) is the fingerprint of the public key and the third part is location of the public key file itself.

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do u know how to translate into 12:f8:7e:78:61:b4:bf:e2:de:24:15:96:4e:d4:72:53 this format from that public key? –  Kit Ho Jul 24 '12 at 16:37
    
@KitHo I'm not sure if I understand your question. I updated the example, as I think ssh-keygen -lf will do what you want. –  Benjamin Oakes Jul 24 '12 at 18:53
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The fingerprint is the MD5 of the Base64-encoded public key.

$ ssh-keygen -f foo
Generating public/private rsa key pair.
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in foo.
Your public key has been saved in foo.pub.
The key fingerprint is:
65:30:38:96:35:56:4f:64:64:e8:e3:a4:7d:59:3e:19 andrew@localhost
The key's randomart image is:
+--[ RSA 2048]----+
|       +*..+*    |
|      =. +.=     |
|     . . .o .    |
|         o+   E  |
|        S= . + o |
|        . o o +  |
|           .   . |
|                 |
|                 |
+-----------------+
$ cat foo.pub
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDEbKq5U57fhzQ3SBbs3NVmgY2ouYZfPhc6cXBNEFpRT3T100fnbkYw+EHi76nwsp+uGxk08kh4GG881DrgotptrJj2dJxXpWp/SFdVu5S9fFU6l6dCTC9IBYYCCV8PvXbBZ3oDZyyyJT7/vXSaUdbk3x9MeNlYrgItm2KY6MdHYEg8R994Sspn1sE4Ydey5DfG/WNWVrzFCI0sWI3yj4zuCcUXFz9sEG8fIYikD9rNuohiMenWjkj6oLTwZGVW2q4wRL0051XBkmfnPD/H6gqOML9MbZQ8D6/+az0yF9oD61SkifhBNBRRNaIab/Np7XD61siR8zNMG/vCKjFGICnp andrew@localhost
$ echo 'AAAAB3NzaC1yc2EAAAADAQABAAABAQDEbKq5U57fhzQ3SBbs3NVmgY2ouYZfPhc6cXBNEFpRT3T100fnbkYw+EHi76nwsp+uGxk08kh4GG881DrgotptrJj2dJxXpWp/SFdVu5S9fFU6l6dCTC9IBYYCCV8PvXbBZ3oDZyyyJT7/vXSaUdbk3x9MeNlYrgItm2KY6MdHYEg8R994Sspn1sE4Ydey5DfG/WNWVrzFCI0sWI3yj4zuCcUXFz9sEG8fIYikD9rNuohiMenWjkj6oLTwZGVW2q4wRL0051XBkmfnPD/H6gqOML9MbZQ8D6/+az0yF9oD61SkifhBNBRRNaIab/Np7XD61siR8zNMG/vCKjFGICnp' \
    | base64 -D | md5
6530389635564f6464e8e3a47d593e19

The md5sum 6530389635564f6464e8e3a47d593e19 is the fingerprint displayed when the key is generated, only without the separating colons.

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