Understanding ssh-keygen -p -m PEM -f: Changing Passphrases and Converting Key Formats

When working with SSH keys, developers often encounter compatibility issues between different tools, platforms, and key formats. One command that helps bridge these gaps is:

ssh-keygen -p -m PEM -f <path-to-private-key>

This single line can save hours of frustration when connecting to remote servers or integrating SSH with software such as MySQL Workbench, PuTTY, Jenkins, or legacy systems. Let’s break down what this command does and why it matters.

Breaking Down the Command

• ssh-keygen
  The built-in OpenSSH utility used for generating, managing, and converting SSH keys.
• -p (passphrase)
  This option allows you to change the passphrase of an existing private key.
  • If the key is already encrypted with a passphrase, you’ll be asked for the current one.
  • You can then set a new passphrase or press Enter to remove it entirely.
  • Why it matters: passphrases add an extra layer of security in case your private key is stolen. However, in automated systems (scripts, CI/CD), you might need a passphrase-free key.
• -m PEM (format)
  This converts the private key into PEM format.
  • Modern OpenSSH versions generate keys in the OPENSSH PRIVATE KEY format by default.
  • Why it matters: Without converting, you may see errors like “invalid key format” or “unsupported key type” when trying to use your SSH key outside of OpenSSH.
• -f <file>
  This specifies the path to the private key file you want to modify. The command will overwrite the file in-place.

Many third-party tools still expect PEM, which is the older, Base64-encoded format that starts with:

-----BEGIN RSA PRIVATE KEY-----

What Happens When You Run It

1. You’ll be prompted for the current passphrase (if the key has one).
2. You’ll be asked to enter a new passphrase, or press Enter twice to remove it.
3. The key will be rewritten in PEM format and saved back to the same file.

In short: the command both re-encrypts the key with a new passphrase (or none) and ensures it is stored in a compatible format.

When Should You Use It?

• Compatibility issues:
  If a tool like MySQL Workbench, PuTTY, or certain libraries rejects your key with an “invalid format” error.
• Legacy systems:
  Some older servers and applications still only recognize PEM-formatted keys.
• Automation scenarios:
  When you need to remove a passphrase for scripts or CI/CD pipelines.
• Security hardening:
  If you need to update your passphrase periodically as part of security policy.

Important Considerations

• Passphrase trade-offs:
  • With a passphrase → safer if the key is stolen.
  • Without a passphrase → easier for automation but less secure. Consider using an SSH agent instead of removing the passphrase.
• Don’t confuse formats:
  • PEM → starts with -----BEGIN RSA PRIVATE KEY-----
  • OpenSSH → starts with -----BEGIN OPENSSH PRIVATE KEY-----
  • They are both valid, but tools vary in what they support.
• Exporting for PuTTY:
  If you need a .ppk file, you’ll still have to use PuTTYgen after converting to PEM.

File permissions: Ensure that your private key file is restricted:

chmod 600 <private-key>

This prevents unauthorized users from accessing it.

Backup first: Always make a copy of your key before converting, in case something goes wrong. For example:

cp id_rsa id_rsa.backup

Finally

The command ssh-keygen -p -m PEM -f is more than just a trick—it’s a lifesaver when dealing with cross-platform SSH key compatibility. By understanding what it does—changing passphrases and converting to PEM format—you’ll avoid the common headaches of “invalid format” errors and keep your workflows smooth.

In today’s mixed environment of modern and legacy systems, knowing how to manipulate SSH keys is an essential skill. Always remember to balance security with convenience, and choose the right format based on the tools you are working with.

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