SSH/Fingerprint

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Increasing security by verifying the identity of the machine that you connect to with ssh by eye and with SSHFP in DNS.

ssh allows you to connect to a remote machine with the data between the two machines being encrypted. You can then securely do: command line login, file copy, tunnel other protocols (eg: rsync, X11), ... You authenticate by either typing a password or key exchange.

A big vulnerability is a man in the middle attack where some malevolent snooping entity (Mallory) between you and the remote machine intercepts all traffic, decrypts it & reencrypts before sending it on. Mallory can do this fast enough so that you are not aware of his existance.

To protect you against this your ssh program checks the remote ssh server's fingerprint with the fingerprint saved from the last time it connected. If the fingerprint has changed you will be warned and asked if you wish to continue. In openssh (the ssh used on most Linux systems) this fingerprint is stored in $HOME/.ssh/known_hosts.

The fingerprint is a short version of the server's public key; it is easier for you to verify than the full key. It is very hard to spoof another public key with the same fingerprint.

Das Problem

When you connect to a machine for the first time you do not have the fingerprint in your known_hosts, so ssh has nothing to compare it to, so it asks you. This page describes ways in which you can do more than blindly say yes.

Checking a ssh server key fingerprint by eye

When you connect to a machine for the first time you will be told that the authenticity can't be established and presented with a key fingerprint to check. Something like this: 

The authenticity of host 'mint.phcomp.co.uk (78.32.209.33)' can't be established.
RSA key fingerprint is 6a:de:e0:af:56:f8:0c:04:11:5b:ef:4d:49:ad:09:23.
Are you sure you want to continue connecting (yes/no)? no

You might find that the fingerprint is shown in a different format — just keep reading.

To be able to check the fingerprint you need to know what it is. You will probably find the .pub files in /etc/ssh/ that contain RSA & DSA keys. This will generate the fingerprints that you can check: 

cd /etc/ssh
for file in *sa_key.pub
do   ssh-keygen -lf $file
done

There is little point in doing this after you have logged in, a sufficiently ingenious Mallory could modify what the above generates on the fly so that you see what he wants you to see. This needs to be done before hand and you can then check it with what you see. You might print the fingerprints out and keep them in your briefcase or wallet.

Displaying fingerprints in other formats

You might find that the fingerprint is generated in a different format from what you have. This section tells you how, when connecting, you get the ssh client to show them in different formats and, on the server, have ssh-keygen generate different format references.

You may need to mix and match depending on what you have in front of you and where you are able to run commands.

Forcing MD5 hash in hexadecimal

This is the format shown above. You can force ssh to display this thus: 

$ ssh -o FingerprintHash=md5 host.example.org

SHA256 in base64

This new format looks as below: 

The authenticity of host 'mint.phcomp.co.uk (78.32.209.33)' can't be established.
RSA key fingerprint is SHA256:jP0pfKJ9OAXt2F+LM7j3+BMalQ/2Koihl5eH/kli6A4.
Are you sure you want to continue connecting (yes/no)?

You can request this with: 

$ ssh -o FingerprintHash=sha256 host.example.org

You can get ssh-keygen to display it: 

 ssh-keygen -l -E sha256 -f $file

This is used in OpenSSH 6.8 & later. The FingerprintHash is not available in old versions.

ASCII Art Visual Host Key

This displays the host key in a box and is, hopefully, easier to recognise than a string of numbers. It can be used to display both MD5 and SHA256 keys. It looks like this: 

The authenticity of host 'mint.phcomp.co.uk (78.32.209.33)' can't be established.
RSA key fingerprint is SHA256:jP0pfKJ9OAXt2F+LM7j3+BMalQ/2Koihl5eH/kli6A4.
+---[RSA 2048]----+
|                 |
|                 |
|         .     . |
|       +. .   =  |
|      . S=   o + |
|       .oo+.. o o|
|      E.=***.= + |
|      .=*=BoXo+  |
|      .o+=o=+*o. |
+----[SHA256]-----+
Are you sure you want to continue connecting (yes/no)?

You can request ssh to display it thus: 

$ ssh -o VisualHostKey=yes host.example.org

You can request MD5 or SHA256 keys, eg: 

$ ssh -o VisualHostKey=yes -o FingerprintHash=sha256 host.example.org

On the server ssh-keygen will display when given the -v option. Combine this with -E md5 or -E sha256 for the fingerprint hash algorithm: 

 ssh-keygen -lv -E sha256 -f $file

Checking of a ssh server key via DNS

You can put the server keys fingerprint in DNS (Domain Name System) and get ssh to tell you if what it the two fingerprints match. This is not a guarantee but it makes Mallory's job harder since he needs to spoof DNS as well as ssh, which can be done as few domains yet implement DNSSEC.

Configuring DNS

Generate the SSHFP fingerprint information to go into DNS: 

cd /etc/ssh
for file in *sa_key.pub
do   ssh-keygen -r freshmint.phcomp.co.uk -f $file -g
done
freshmint.phcomp.co.uk IN TYPE44 \# 22 02 01 e8fe15b374207a2e6ee99bbbadc87ecd068c17f8
freshmint.phcomp.co.uk IN TYPE44 \# 22 01 01 e5091496bda76d015e89db0bf925ffb1b9d8facb

The above output should be used for bind versions earlier than 9.5.0a1, after that the SSHFP RR type is understood: 

freshmint.phcomp.co.uk IN SSHFP 02 01 e8fe15b374207a2e6ee99bbbadc87ecd068c17f8
freshmint.phcomp.co.uk IN SSHFP 01 01 e5091496bda76d015e89db0bf925ffb1b9d8facbEnter the above 2 RR (resource records) into the DNS for the machine. The complete record for the machine looks like: freshmint      IN      A       78.32.209.33
               IN      AAAA    2001:4d48:ad51:2f00::2:2
               IN      MX  10  freshmint
               IN      SSHFP 1 1 b4b8f2f051a16f57f69590c7c06aeaad039a3882
               IN      SSHFP 2 1 ea35c2064a5fc2ec9f51da2e7c790966f9844e59
mint           IN      CNAME   freshmintCheck that this DNS update is correct with host -a your.machine.name. You should see the SSHFP lines as above, although with old versions of host it may not display properly but like:freshmint.phcomp.co.uk. 259200 IN TYPE44 \# 22 0101b4b8f2f051a16f57f69590c7c06aeaad039a3882
freshmint.phcomp.co.uk. 259200 IN TYPE44 \# 22 0201ea35c2064a5fc2ec9f51da2e7c790966f9844e59

Using the DNS SSHFP RR

Configure your local ssh client, do this for just you be editing $HOME/.ssh/config or system wide /etc/ssh/ssh_config (on Minix: /usr/pkg/etc/ssh/ssh_config):VerifyHostKeyDNS ask

You can also specify this on the command line when connecting. Note how you are told how the DNS fingerprint matches: 

ssh -o VerifyHostKeyDNS=ask freshmint.phcomp.co.uk
The authenticity of host 'freshmint.phcomp.co.uk (2001:4d48:ad51:2f00::2:2)' can't be established.
RSA key fingerprint is 6a:de:e0:af:56:f8:0c:04:11:5b:ef:4d:49:ad:09:23.
Matching host key fingerprint found in DNS.
Are you sure you want to continue connecting (yes/no)?If the fingerprint in the DNS does not match you will see the following — you may have seen similar before, but this is on a first connection:ssh -o VerifyHostKeyDNS=ask freshmint.phcomp.co.uk
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that the RSA host key has just been changed.
The fingerprint for the RSA key sent by the remote host is
6a:de:e0:af:56:f8:0c:04:11:5b:ef:4d:49:ad:09:23.
Please contact your system administrator.
Update the SSHFP RR in DNS with the new host key to get rid of this message.
The authenticity of host 'freshmint.phcomp.co.uk (2001:4d48:ad51:2f00::2:2)' can't be established.
RSA key fingerprint is 6a:de:e0:af:56:f8:0c:04:11:5b:ef:4d:49:ad:09:23.
No matching host key fingerprint found in DNS.
Are you sure you want to continue connecting (yes/no)?

Other things of interest

Passwordless login with SSH (https://www.phcomp.co.uk/Tutorials/Unix-And-Linux/ssh-passwordless-login.html)

References

  1. OpenSSH/Cookbook (https://en.wikibooks.org/wiki/OpenSSH/Cookbook/Public_Key_Authentication#Downloading_keys)
  2. ssh man page
  3. ssh-keygen man page
  4. ssh_config man page

Technical bits

You don't really need to understand this bit to use the above; however if you are a technical nerd you will want to know.

Key types, these are the first number in the SSHFP RR:

  1. RSA — 1 — a public key encryption algorithm invented by Ron Rivest, Adi Shamir and Leonard Adleman (http://en.wikipedia.org/wiki/RSA_%28algorithm%29)
  2. DSA — 2 — Digital Signature Algorithm (http://en.wikipedia.org/wiki/Digital_Signature_Algorithm)
  3. ECDSA — 3 — Elliptic Curve Digital Signature Algorithm (http://en.wikipedia.org/wiki/Elliptic_Curve_DSA)


Where the key types are used:

  1. SSH version 1 — only uses RSA. You should not be using SSH version 1, it is no longer considered to be secure
  2. SSH version 2 — RSA & DSA
  3. DNS SSHFP RR — RSA, DSA & recently (2012) ECDSA (http://www.iana.org/assignments/dns-sshfp-rr-parameters/dns-sshfp-rr-parameters.xml)
  4. The second number in the SSHFP RR is the fingerprint type:
  5. SHA-1 — 1 — secure hash algorithm 1 a 160-bit message digest (http://en.wikipedia.org/wiki/Sha-1)
  6. SHA-256 — 2 — secure hash algorithm 2 family a 256-bit message digest (http://en.wikipedia.org/wiki/Sha-256)
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