How to convert an SSH2 public Key to OpenSSH format

Converting an SSH2 public key to OpenSSH format is necessary when using keys from systems or applications that follow the SSH2 standard. The OpenSSH format is the default for many Linux systems and is required for compatibility with most SSH servers and clients.

SSH2 public keys are typically formatted according to the RFC4716 standard, which differs from the OpenSSH format. Converting these keys ensures compatibility in environments that rely on OpenSSH. This conversion can be done using the ssh-keygen utility.

Follow these steps to convert an SSH2 public key to OpenSSH format.

1. Launch the terminal on your local machine.
2. Locate your SSH2 public key file.

   $ ls ~/path/to/your/ssh2_key.pub

   SSH2 public keys are often stored with a .pub extension.

3. Check the format of your SSH2 public key file.

   $ file ~/path/to/your/ssh2_key.pub

   The file should be in RFC4716 format, which is typical for SSH2 keys.

4. Convert the SSH2 public key to OpenSSH format.

   $ ssh-keygen -i -f ~/path/to/your/ssh2_key.pub > ~/path/to/your/openssh_key.pub

   The -i option tells ssh-keygen to read an SSH2 key and convert it to OpenSSH format.

5. Verify the conversion by viewing the contents of the converted public key file.

   $ cat ~/path/to/your/openssh_key.pub

   The output should be in the standard OpenSSH format, typically starting with `ssh-rsa` or `ssh-ed25519`.

6. If needed, extract a public key from a private key directly to a file in OpenSSH format.

   $ ssh-keygen -y -f ~/path/to/your/private_key > ~/path/to/your/openssh_key.pub

   This command generates the public key from a private key and saves it in OpenSSH format.

7. Check the format of the generated OpenSSH public key file.

   $ file ~/path/to/your/openssh_key.pub

   Verify that the public key file is in OpenSSH format.

8. Optionally, copy the converted public key to the remote server's authorized keys file to enable key-based authentication.

   $ ssh-copy-id -i ~/path/to/your/openssh_key.pub user@remotehost

   This command adds the converted public key to the authorized_keys file on the remote server.