How To Retrieve the Public Key from an SSH Private Key

SSH, or Secure Shell, is a protocol used to securely log onto remote systems. It is the most common way to access remote Linux and Unix-like servers. SSH uses public-key cryptography to authenticate the remote computer and allow the remote computer to authenticate the user, if necessary.

In this article, we will discuss how to retrieve the public key from an SSH private key.

Overview of SSH Keys

SSH keys are a pair of cryptographic keys that can be used to authenticate to an SSH server as an alternative to password-based logins. A private key, which is secret, is used to decrypt data and a public key, which is shared, is used to encrypt data.

Retrieving Your Public Key

In some cases, you might have the private key but have lost the public key. You can easily retrieve the public key if you have the private key. Here’s how:

Method 1: Using ssh-keygen

The ssh-keygen tool is a part of the OpenSSH package and is used to create new key pairs. You can also use it to extract the public key from the private key.

Here is the command you can use:

ssh-keygen -y -f ~/.ssh/id_rsa > ~/.ssh/id_rsa.pub

Let’s break down this command:

• ssh-keygen is the command for creating new authentication key pairs.
• -y option is used to read a private SSH key file and print an associated public key.
• -f option specifies the filename of the key file.
• ~/.ssh/id_rsa is the default filename of the private key. You can replace this with your actual private key file path.
• > ~/.ssh/id_rsa.pub is used to redirect the output to the id_rsa.pub file. This will be your public key.

Method 2: Using a Text Editor

You can also use a text editor to retrieve the public key from the private key. Here’s how:

1. Open the private key file (id_rsa) in a text editor.
2. Copy the content of the file, which starts with ssh-rsa and ends with your username or a comment.
3. Create a new file called id_rsa.pub in the ~/.ssh/ directory (if it doesn’t exist).
4. Paste the copied content into the id_rsa.pub file.
5. Save the file.

Additional Steps for Windows Users

If you are using Windows and need to SSH into remote machines, you can follow these additional steps:

1. Download and install the puttygen tool from here.
2. Open puttygen and load your private key file (*.pem).
3. Copy the public key from the “Public key for pasting into OpenSSH authorized_keys file” section.
4. Create or open the authorized_keys file on the remote server.
5. Paste the copied public key into the authorized_keys file.
6. Save the file.

Remember to set the correct permissions for the ~/.ssh/ directory and the authorized_keys file on the remote server. You can use the chmod command for this.

Conclusion

In this article, we have discussed how to retrieve the public key from an SSH private key. We have discussed two methods using ssh-keygen and a text editor. We have also discussed additional steps for Windows users.

Remember, your private key is like your password. You should keep it secure and never share it with anyone. If you believe your private key has been compromised, you should generate a new pair of SSH keys.

If you have any further questions or need more information, you can refer to the OpenSSH manual or the PuTTYgen manual.