How to Use SSH Keys for Authentication

Technical writer

Linux Servers SSH

30.01.2025

Reading time: 10 min

Many cloud applications are built on the popular SSH protocol—it is widely used for managing network infrastructure, transferring files, and executing remote commands.

SSH stands for Secure Socket Shell, meaning it provides a shell (command-line interface) around the connection between multiple remote hosts, ensuring that the connection is secure (encrypted and authenticated).

SSH connections are available on all popular operating systems, including Linux, Ubuntu, Windows, and Debian. The protocol establishes an encrypted communication channel within an unprotected network by using a pair of public and private keys.

Keys: The Foundation of SSH

SSH operates on a client-server model. This means the user has an SSH client (a terminal in Linux or a graphical application in Windows), while the server side runs a daemon, which accepts incoming connections from clients.

In practice, an SSH channel enables remote terminal management of a server. In other words, after a successful connection, everything entered in the local console is executed directly on the remote server.

The SSH protocol uses a pair of keys for encrypting and decrypting information: public key and private key.

These keys are mathematically linked. The public key is shared openly, resides on the server, and is used to encrypt data. The private key is confidential, resides on the client, and is used to decrypt data.

Of course, keys are not generated manually but with special tools—keygens. These utilities generate new keys using encryption algorithms fundamental to SSH technology.

More About How SSH Works

Exchange of Public Keys

SSH relies on symmetric encryption, meaning two hosts wishing to communicate securely generate a unique session key derived from the public and private data of each host.

For example, host A generates a public and private key pair. The public key is sent to host B. Host B does the same, sending its public key to host A.

Using the Diffie-Hellman algorithm, host A can create a key by combining its private key with the public key of host B. Likewise, host B can create an identical key by combining its private key with the public key of host A.

This results in both hosts independently generating the same symmetric encryption key, which is then used for secure communication. Hence, the term symmetric encryption.

Message Verification

To verify messages, hosts use a hash function that outputs a fixed-length string based on the following data:

The symmetric encryption key
The packet number
The encrypted message text

The result of hashing these elements is called an HMAC (Hash-based Message Authentication Code). The client generates an HMAC and sends it to the server. The server then creates its own HMAC using the same data and compares it to the client's HMAC. If they match, the verification is successful, ensuring that the message is authentic and hasn't been tampered with.

Host Authentication

Establishing a secure connection is only part of the process. The next step is authenticating the user connecting to the remote host, as the user may not have permission to execute commands.

There are several authentication methods:

Password Authentication: The user sends an encrypted password to the server. If the password is correct, the server allows the user to execute commands.
Certificate-Based Authentication: The user initially provides the server with a password and the public part of a certificate. Once authenticated, the session continues without requiring repeated password entries for subsequent interactions.

These methods ensure that only authorized users can access the remote system while maintaining secure communication.

Encryption Algorithms

A key factor in the robustness of SSH is that decrypting the symmetric key is only possible with the private key, not the public key, even though the symmetric key is derived from both. Achieving this property requires specific encryption algorithms.

There are three primary classes of such algorithms: RSA, DSA, and algorithms based on elliptic curves, each with distinct characteristics:

RSA: Developed in 1978, RSA is based on integer factorization. Since factoring large semiprime numbers (products of two large primes) is computationally difficult, the security of RSA depends on the size of the chosen factors. The key length ranges from 1024 to 16384 bits.
DSA: DSA (Digital Signature Algorithm) is based on discrete logarithms and modular exponentiation. While similar to RSA, it uses a different mathematical approach to link public and private keys. DSA key length is limited to 1024 bits.
ECDSA and EdDSA: These algorithms are based on elliptic curves, unlike DSA, which uses modular exponentiation. They assume that no efficient solution exists for the discrete logarithm problem on elliptic curves. Although the keys are shorter, they provide the same level of security.

Key Generation

Each operating system has its own utilities for quickly generating SSH keys.

In Unix-like systems, the command to generate a key pair is:

ssh-keygen -t rsa

Here, the type of encryption algorithm is specified using the -t flag. Other supported types include:

dsa
ecdsa
ed25519

You can also specify the key length with the -b flag. However, be cautious, as the security of the connection depends on the key length:

ssh-keygen -b 2048 -t rsa

After entering the command, the terminal will prompt you to specify a file path and name for storing the generated keys. You can accept the default path by pressing Enter, which will create standard file names: id_rsa (private key) and id_rsa.pub (public key).

Thus, the public key will be stored in a file with a .pub extension, while the private key will be stored in a file without an extension.

Next, the command will prompt you to enter a passphrase. While not mandatory (it is unrelated to the SSH protocol itself), using a passphrase is recommended to prevent unauthorized use of the key by a third-party user on the local Linux system. Note that if a passphrase is used, you must enter it each time you establish the connection.

To change the passphrase later, you can use:

ssh-keygen -p

Or, you can specify all parameters at once with a single command:

ssh-keygen -p old_password -N new_password -f path_to_files

For Windows, there are two main approaches:

Using ssh-keygen from OpenSSH: The OpenSSH client provides the same ssh-keygen command as Linux, following the same steps.
Using PuTTY: PuTTY is a graphical application that allows users to generate public and private keys with the press of a button.

Installing the Client and Server Components

The primary tool for an SSH connection on Linux platforms (both client and server) is OpenSSH. While it is typically pre-installed on most operating systems, there may be situations (such as with Ubuntu) where manual installation is necessary.

The general command for installing SSH, followed by entering the superuser password, is:

sudo apt-get install ssh

However, in some operating systems, SSH may be divided into separate components for the client and server.

For the Client

To check whether the SSH client is installed on your local machine, simply run the following command in the terminal:

ssh

If SSH is supported, the terminal will display a description of the command. If nothing appears, you’ll need to install the client manually:

sudo apt-get install openssh-client

You will be prompted to enter the superuser password during installation. Once completed, SSH connectivity will be available.

For the Server

Similarly, the server-side part of the OpenSSH toolkit is required on the remote host.

To check if the SSH server is available on your remote host, try connecting locally via SSH:

ssh localhost

If the SSH daemon is running, you will see a message indicating a successful connection. If not, you’ll need to install the SSH server:

sudo apt-get install openssh-server

As with the client, the terminal will prompt you to enter the superuser password. After installation, you can check whether SSH is active by running:

sudo service ssh status

Once connected, you can modify SSH settings as needed by editing the configuration file:

./ssh/sshd_config

For example, you might want to change the default port to a custom one. Don’t forget that after making changes to the configuration, you must manually restart the SSH service to apply the updates:

sudo service ssh restart

Copying an SSH Key to the Server

On Hostman, you can easily add SSH keys to your servers using the control panel.

Using a Special Copy Command

After generating a public SSH key, it can be used as an authorized key on a server. This allows quick connections without the need to repeatedly enter a password.

The most common way to copy the key is by using the ssh-copy-id command:

ssh-copy-id -i ~/.ssh/id_rsa.pub name@server_address

This command assumes you used the default paths and filenames during key generation. If not, simply replace ~/.ssh/id_rsa.pub with your custom path and filename.

Replace name with the username on the remote server.
Replace server_address with the host address. If the usernames on both the client and server are the same, you can shorten the command:

ssh-copy-id -i ~/.ssh/id_rsa.pub server_address

If you set a passphrase during the SSH key creation, the terminal will prompt you to enter it. Otherwise, the key will be copied immediately.

In some cases, the server may be configured to use a non-standard port (the default is 22). If that’s the case, specify the port using the -p flag:

ssh-copy-id -i ~/.ssh/id_rsa.pub -p 8129 name@server_address

Semi-Manual Copying

There are operating systems where the ssh-copy-id command may not be supported, even though SSH connections to the server are possible. In such cases, the copying process can be done manually using a series of commands:

ssh name@server_address 'mkdir -pm 700 ~/.ssh; echo ' $(cat ~/.ssh/id_rsa.pub) ' >> ~/.ssh/authorized_keys; chmod 600 ~/.ssh/authorized_keys'

This sequence of commands does the following:

Creates a special .ssh directory on the server (if it doesn’t already exist) with the correct permissions (700) for reading and writing.
Creates or appends to the authorized_keys file, which stores the public keys of all authorized users. The public key from the local file (id_rsa.pub) will be added to it.
Sets appropriate permissions (600) on the authorized_keys file to ensure it can only be read and written by the owner.

If the authorized_keys file already exists, it will simply be appended with the new key.

Once this is done, future connections to the server can be made using the same SSH command, but now the authentication will use the public key added to authorized_keys:

ssh name@server_address

Manual Copying

Some hosting platforms offer server management through alternative interfaces, such as a web-based control panel. In these cases, there is usually an option to manually add a public key to the server. The web interface might even simulate a terminal for interacting with the server.

Regardless of the method, the remote host must contain a file named ~/.ssh/authorized_keys, which lists all authorized public keys.

Simply copy the client’s public key (found in ~/.ssh/id_rsa.pub by default) into this file.

If the key pair was generated using a graphical application (typically PuTTY on Windows), you should copy the public key directly from the application and add it to the existing content in authorized_keys.

Connecting to a Server

To connect to a remote server on a Linux operating system, enter the following command in the terminal:

ssh name@server_address

Alternatively, if the local username is identical to the remote username, you can shorten the command to:

ssh server_address

The system will then prompt you to enter the password. Type it and press Enter. Note that the terminal will not display the password as you type it.

Just like with the ssh-copy-id command, you can explicitly specify the port when connecting to a remote server:

ssh client@server_address -p 8129

Once connected, you will have control over the remote machine via the terminal; any command you enter will be executed on the server side.

Conclusion

Today, SSH is one of the most widely used protocols in development and system administration. Therefore, having a basic understanding of its operation is crucial.

This article aimed to provide an overview of SSH connections, briefly explain the encryption algorithms (RSA, DSA, ECDSA, and EdDSA), and demonstrate how public and private key pairs can be used to establish secure connections with a personal server, ensuring that exchanged messages remain inaccessible to third parties.

We covered the primary commands for UNIX-like operating systems that allow users to generate key pairs and grant clients SSH access by copying the public key to the server, enabling secure connections.