• Generate Ssh Host Key Fingerprint Windows 10
• Get Fingerprint Of Ssh Key
• Ssh Generate Key Ubuntu

Index

What is this all about ?

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

A host key is a cryptographic key used for authenticating computers in the SSH protocol. Host keys are key pairs, typically using the RSA, DSA, or ECDSA algorithms. Public host keys are stored on and/or distributed to SSH clients, and private keys are stored on SSH servers. Simple: It is the fingerprint of a key that is verified when you try to login to a remote computer using SSH. When you log into an SSH server for the first time, you'll see something like that shown in Figure A. You have to accept the fingerprint before the SSH connection can be made.

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.

Generate Ssh Host Key Fingerprint Windows 10

The 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:

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:

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:

SHA256 in base64

This new format looks as below:

You can request this with:

You can get ssh-keygen to display it:

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:

You can request ssh to display it thus:

You can request MD5 or SHA256 keys, eg:

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:

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:

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

• Enter the above 2 RR (resource records) into the DNS for the machine. The complete record for the machine looks like:

• Check 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:

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):

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

• 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:

Other things of interest

References

• ssh man page
• ssh-keygen man page
• 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:

  • RSA — 1 — a public key encryption algorithm invented by Ron Rivest, Adi Shamir and Leonard Adleman
  • DSA — 2 — Digital Signature Algorithm
  • ECDSA — 3 — Elliptic Curve Digital Signature Algorithm

• Where the key types are used:

  • SSH version 1 — only uses RSA. You should not be using SSH version 1, it is no longer considered to be secure
  • SSH version 2 — RSA & DSA
  • DNS SSHFP RR — RSA, DSA & recently (2012) ECDSA

• The second number in the SSHFP RR is the fingerprint type:

  • SHA-1 — 1 — secure hash algorithm 1 a 160-bit message digest
  • SHA-256 — 2 — secure hash algorithm 2 family a 256-bit message digest

Thanks

Thanks to Katie Foster <Katie.Foster@auspost.com.au> for reporting an error in a previous version of this page.

License and copyright

All description & sample files copyright (c) 2012, 2016 Parliament Hill Computers. Author: Alain D D Williams.

You may used these files as the basis your own (or organisation's/company's) project(s) (under whatever licence that you see fit). You may not claim ownership or copyright of any substantially unmodified files. Acknowledgement would be appreciated, but is not necessary.

These demonstrations are made available in the hope that they are useful. There may be errors: there is no warranty at all, use at your own risk.

Return to tutorial home.

If you want any help using the above, or have any comments or suggestions, please contact us.

Get Fingerprint Of Ssh Key

Introduction

Establishing an SSH (Secure Shell) connection is essential to log in and effectively manage a remote server. Encrypted keys are a set of access credentials used to establish a secure connection.

This guide will walk you how to generate SSH keys on Ubuntu 18.04. We will also cover setting up SSH key-based authentication to connect to a remote server without requiring a password.

• A server running Ubuntu 18.04
• A user account with sudo privileges
• Access to a terminal window / command line (Ctrl-Alt-T)

If you are already running an Ubuntu 18.04 server, you can skip this step. If you are configuring your server for the first time, you may not have SSH installed.

1. Start by installing the tasksel package:

The system will first ask for confirmation before proceeding:

2. Next, use tasksel to install the ssh-server:

3. Load the SSH server service, and set it to launch at boot:

On your client system – the one you’re using to connect to the server – you need to create a pair of key codes.

To generate a pair of SSH key codes, enter the commands:

This will create a hidden directory to store your SSH keys, and modify the permissions for that directory. The ssh-keygen command creates a 2048-bit RSA key pair.

For extra security, use RSA4096:

If you’ve already generated a key pair, this will prompt to overwrite them, and those old keys will not work anymore.

The system will ask you to create a passphrase as an added layer of security. Input a memorable passphrase, and press Enter.

This process creates two keys. One is a public key, which you can hand out to anyone – in this case, you’ll save it to the server. The other one is a private key, which you will need to keep secure. The secure private key ensures that you are the only person who can encrypt the data that is decrypted by the public key.

Step 2- Copy Public Key to the Ubuntu Server

First, get the IP address of the Ubuntu server you want to connect to.

In a terminal window, enter:

The system’s IP address is listed in the second entry:

On the client system, use the ssh-copy-id command to copy the identity information to the Ubuntu server:

Replace server_IP with the actual IP address of your server.

If this is the first time you’re connecting to the server, you may see a message that the authenticity of the host cannot be established:

Ssh Generate Key Ubuntu

Type yes and press Enter.

The system will check your client system for the id_rsa.pub key that was previously generated. Then it will prompt you to enter the password for the server user account. Type it in (the system won’t display the password), and press Enter.

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The system will copy the contents of the ~/.ssh/id_rsa.pub from the client system into the ~/.ssh/authorized_keys directory of the server system.

The system should display:

If your system does not have the ssh-copy-id command, you can copy the key manually over the SSH.

Use the following command:

To log in to a remote server, input the command:

The system should not ask for a password as it is negotiating a secure connection using the SSH keys. If you used a security passphrase, you would be prompted to enter it. After you do so, you are logged in.

If this is the first time you’ve logged into the server, you may see a message similar to the one in part two. It will ask if you are sure you want to connect – type yes and press Enter.

Step 4- Disable Password Authentication

This step creates an added layer of security. If you’re the only person logging into the server, you can disable the password. The server will only accept a login with your private key to match the stored public key.

Edit the sshd_config file:

Search the file and find the PasswordAuthentication option.

Edit the file and change the value to no:

Save the file and exit, then restart the SSH service:

Verify that SSH is still working, before ending the session:

If everything works, you can close out and resume work normally.

By following the instructions in this tutorial, you have setup SSH-key-based authentication on an Ubuntu 18.04 server.

The connection is now highly secure as it uses a set of unique, encrypted SSH keys.

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