Shell/Fingerprints

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To verify a fingerprint you need to know the correct one first. These to-be-verified correct fingerprints can be shown after you're logged in on the target server. Of course this is a classic chicken-and-egg problem. You need to get the correct fingerprints through an independent communication channel in beforehand. In our case that is the list below:

Current fingerprints as of October 2017

~$ for F in /etc/ssh/*.pub ; do  echo -e "\n$F:"; ssh-keygen -l -E sha256  -f $F; done

/etc/ssh/ssh_host_ecdsa.pub:
256 SHA256:L+FCMj2bm8x/BfR8AdaaLnqTmFD35D0EYNlFG7a2dt8 root@nfsadm (ECDSA)

/etc/ssh/ssh_host_ed25519_key.pub:
256 SHA256:H4FLNG2aNYRZ3jxepIx5E0s0a2ZvtZbbmVLt56b+nK0 root@nfsadm (ED25519)

/etc/ssh/ssh_host_rsa_key.pub:
2048 SHA256:DpP5/EfbApVUwseVeQOVpAFvGiZIJmYmjUyC4Cnuatk root@nfsadm (RSA)
~$ for F in /etc/ssh/*.pub ; do  echo -e "\n$F:"; ssh-keygen -l -E md5  -f $F; done

/etc/ssh/ssh_host_ecdsa.pub:
256 MD5:07:84:c9:e1:59:4f:03:75:69:b1:e4:d0:b4:1f:9a:cd root@nfsadm (ECDSA)

/etc/ssh/ssh_host_ed25519_key.pub:
256 MD5:93:11:29:c4:a2:03:e1:2d:b1:82:05:74:dd:a5:3b:9a root@nfsadm (ED25519)

/etc/ssh/ssh_host_rsa_key.pub:
2048 MD5:de:db:6e:72:52:de:30:73:db:bb:6e:79:df:f9:2c:0d root@nfsadm (RSA)

Actually compare a fingerprint when establishing a session

Older implementations (for example from debian Jessie) use md5 - without explicitly stating this fact:

~$ ssh  -o VisualHostKey=yes shell.informatik.uni-goettingen.de
The authenticity of host 'shell.informatik.uni-goettingen.de (134.76.81.100)' can't be established.
ECDSA key fingerprint is 07:84:c9:e1:59:4f:03:75:69:b1:e4:d0:b4:1f:9a:cd.
+---[ECDSA 256]---+
|     ..++o=o=o   |
|     .++ o *+o   |
|      o . ..+ .  |
|         .   * . |
|        S . o E  |
|         .       |
|                 |
|                 |
|                 |
+-----------------+

Newer version (e.g. from Ubuntu Xenial) use sha256 by default:

~$ ssh  -o VisualHostKey=yes shell.informatik.uni-goettingen.de 
The authenticity of host 'shell.informatik.uni-goettingen.de (134.76.81.100)' can't be established.
ECDSA key fingerprint is SHA256:L+FCMj2bm8x/BfR8AdaaLnqTmFD35D0EYNlFG7a2dt8.
+---[ECDSA 256]---+
|          o+o==  |
|         .o.o.o+ |
|         . o ++. |
|     .  . o *.o. |
|    o +.S. * +o .|
|     +.= o. =.o.o|
|      +.o+.+   .E|
|     o ++.=      |
|      =..o .     |
+----[SHA256]-----+

On those newer ssh-clients you can request a specific algorithm to be used:

~$ ssh  -o VisualHostKey=yes -o FingerprintHash=md5 shell.informatik.uni-goettingen.deHost 
key fingerprint is MD5:07:84:c9:e1:59:4f:03:75:69:b1:e4:d0:b4:1f:9a:cd
+---[ECDSA 256]---+
|     ..++o=o=o   |
|     .++ o *+o   |
|      o . ..+ .  |
|         .   * . |
|        S . o E  |
|         .       |
|                 |
|                 |
|                 |
+------[MD5]------+

Note that older implementations just don't know "-o FingerprintHash" and will generate an error message only. That's the reason we list both old and new fingerprints above...


Windows

PuTTY on windows looks like this:

Putty-login+fingerprint.png


See also

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