How to Convert an SSH-RSA Key to SSH2 Format

Secure Shell (SSH) keys are vital for maintaining secure communications between servers and clients. Most SSH users are familiar with the SSH-RSA key format, but in some cases, you may encounter a situation where you need to convert your key to SSH2 format. In this blog post, I’ll guide you through the process of converting your SSH-RSA key to SSH2 format using common tools like ssh-keygen and PuTTYgen.

Understanding SSH-RSA and SSH2

Before we dive into the technical details, let’s quickly cover what these formats represent:

SSH-RSA: This is a widely used format for SSH public keys. It starts with ssh-rsa and is often followed by a base64-encoded string and an optional comment (usually the username and hostname).

SSH2: This format is typically used by certain SSH clients and servers that adhere to the SSH2 protocol specification (RFC 4716). While SSH2 itself is the protocol, some clients require a specific key format associated with it.


In most cases, the key types are the same; it’s the formatting that may differ slightly based on the SSH client or server you’re using.

Why Convert SSH-RSA to SSH2?

You may need to convert your SSH-RSA key to SSH2 format when working with specific clients, such as older SSH tools or certain security appliances, which require SSH2 format.

Converting SSH-RSA to SSH2 Format Using ssh-keygen

The simplest way to convert an SSH-RSA public key to SSH2 format is by using the ssh-keygen tool. Here’s how you can do it:

Step 1: Open Your Terminal

If you are using Linux or macOS, open your terminal. On Windows, you can use a tool like Git Bash or Windows Subsystem for Linux (WSL) to access the terminal.

Step 2: Run the ssh-keygen Command

Assuming you have the SSH-RSA public key file (id_rsa.pub), you can convert it using this command:

ssh-keygen -f id_rsa.pub -e -m RFC4716 > id_rsa_ssh2.pub

Let’s break this command down:

-f id_rsa.pub: Specifies the file you want to convert (your SSH-RSA public key).

-e: Indicates that you want to export the key.

-m RFC4716: This flag converts the key to SSH2 format (RFC 4716).

> id_rsa_ssh2.pub: Saves the converted key to a new file.


The result will be an SSH2-compatible public key, which looks like this:

—- BEGIN SSH2 PUBLIC KEY —-
Comment: “2048-bit RSA, user@example.com”
AAAAB3NzaC1yc2EAAAABIwAAAQEAr0…
—- END SSH2 PUBLIC KEY —-

Converting SSH-RSA to SSH2 Using PuTTYgen (Windows)

If you are a Windows user, you can convert SSH keys using PuTTYgen, which is part of the PuTTY suite. Here’s how:

Step 1: Open PuTTYgen

Download and install PuTTYgen from the PuTTY official website if you haven’t already. Then, open the application.

Step 2: Load Your SSH-RSA Key

Click on Load.

Navigate to where your SSH-RSA public key file is located, and load it into PuTTYgen.


Step 3: Save the SSH2 Public Key

Once your key is loaded, click Save public key.

This will export your key in an SSH2-compatible format.


Verifying Your SSH2 Key

Once you’ve converted your key, it’s a good idea to verify that it works as expected with your SSH client or server. You can do this by copying the new SSH2 key to the appropriate location (such as ~/.ssh/authorized_keys on a remote server) and testing the connection.

Conclusion

Converting an SSH-RSA key to SSH2 format is straightforward, especially with tools like ssh-keygen or PuTTYgen. Whether you’re switching to a new SSH client or meeting a specific protocol requirement, these steps will help you get your keys in the right format.