SSH Key Authentication in your Linux VPS Server

Table of Contents

SSH Key Authentication in your Linux VPS Server

Why SSH Key Authentication

SSH Key authentication is a more secure alternative to password authentication in Linux/Unix SSH server. Password authentication is vulnerable to brute force cracking attacks where an attacker will try a number of user/password combinations until he finds one that allows access to the system. SSH Keys have a number of over password as they are less likely to loose than passwords to forget and they are strings of characters longer than passwords, thus making them much more secure than password authentication. SSH private key can be encrypted with a user known password so it’s use is limited to the person knowing the decryption passphrase.

RSA or DSA SSH Keys ? Or ECDSA ?

The internet is full of debates over what ssh key type is more secure, faster when it comes to encryption, decryption and signature marking/checking, but almost everyone agrees that RSA keys are safe whereas ECDSA are supposed to provide smaller key sizes at faster operations and similar degree of security as RSA and DSA crypto algorithms. This article focuses on RSA key type, but before continuing make sure you read the part of man ssh-keygen if you are looking for FIPS compliancy:

     -b bits
             Specifies the number of bits in the key to create.  For RSA keys, the minimum size is 768 bits and the default is 2048 bits.  Generally, 2048
             bits is considered sufficient.  DSA keys must be exactly 1024 bits as specified by FIPS 186-2.  For ECDSA keys, the -b flag determines the key
             length by selecting from one of three elliptic curve sizes: 256, 384 or 521 bits.  Attempting to use bit lengths other than these three values
             for ECDSA keys will fail.

Generate SSH Key Pair with SSH-KEYGEN

$ ssh-keygen -b 2048 -C "My first ssh key" -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/home/Andrei/.ssh/id_rsa): /home/Andrei/.ssh/my_ssh_key
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/Andrei/.ssh/my_ssh_key.
Your public key has been saved in /home/Andrei/.ssh/
The key fingerprint is:
f0:61:d9:9a:7f:b6:c3:f0:d5:59:78:71:2e:81:db:04 My first ssh key
The key's randomart image is:
+--[ RSA 2048]----+
|            Eo   |
|         o  . o..|
|      . + .  + +o|
|       + +  . + +|
|        S      +o|
|         ..   ...|
|          .+o.   |
|           o+.   |
|            ..   |
$ cat .ssh/
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDA1SL7dsAc5Wxpp/9h0zuDwGeN5H0vb4JFZ+n2+Js/B8Egqzw5Xb3iwZzHiXCCp71UgVrrPoYBeqhqDACWqJjj57D7vEDiKVmdyAcw2DfAT09uLe6wWeoX+s11Tadap11fVSYyARd3ih7OdoW9Hq1t0FyNgLXFvff60A0XTIeoM/6Y9/l0a7KoN8F5dy3kGvV4iNfCUsNEu0jrOPILRV39Bw8GKTxM7GoHAdAk/n4dOl9u+KgEictyVE1AXigipK6RtTClFO5xgGJBathXr0irkFCXRA+0FX79wql588bkmm/vrltdnZqT1rlzxp0q2cdsbbwJz/UItm5Un2t2MV1n My first ssh key
The generation of the ssh keys is done. Few things to note here: – Key length is set with the -b option. – The public key will contains the chosen comment by the -C option. – Type is RSA Whether or not the private key is encrypted and the passphrase protecting it is entirely optional, but for higher security, it is recommended to set one and do not forget it. The entered passphrase is not shown on the terminal. En encrypted private key ensures that a potential loss of the key does not post an immediate threat to the systems using that key. The destination directory of the keys is optional, but it is a good practice to generate them in home directory under the .ssh directory. Now that both public and private keys are available, it is important to know that the private key is private and must only be in the possession of the legally appointed administrator. The public ssh key will be used on the destination Linux VPS servers that will accept the private key.

Deploy the Public SSH Key to destination systems

The easiest way to accomplish this is to copy the contents of the public key, login to destination Linux/Unix system and paste it in the authorized_keys under the .ssh directory by default. Once you have done so, save and exit the file. SSH Key authentication should now work.

(Optional) SSH Key authentication – Final steps

Some SSH deployments will have Public key authentication disabled by default so you need to make sure that the following options are set to “yes” in /etc/ssh/sshd_config (or equivalent SSH daemon configuration file):
RSAAuthentication yes
PubkeyAuthentication yes
Depending on whether or not password authentication is still desired, the following options should be set to “yes” or “no”:
PasswordAuthentication yes
If regular users should be allowed to login via ssh using password, but root user should be disallowed to use it’s password, change the following line to:
 PermitRootLogin   without-password
Save, exit and restart/reload ssh so above changes to take effect and login securely.

Deploy new Linux VPS servers with SSH Public key automatically

With VPSie, you can add the SSH Public key in the web administration tool and new Linux based VPS servers will have the key added at creation time. The image below provides a preview of this feature: Adding SSH Public key to new Linux VPS servers automatically

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