Public Key to Private Key: Is it Possible?

TL;DR

No, you generally cannot convert a public key into its corresponding private key. Public keys are designed to be shared freely, while private keys must remain secret. If someone could easily derive the private key from the public key, encryption would be broken.

Why You Can’t Convert

Public-key cryptography (like RSA or ECC) relies on mathematical problems that are easy to perform in one direction but extremely difficult to reverse without knowing a secret piece of information – the private key. Think of it like making scrambled eggs: easy to scramble, very hard to unscramble back into the original egg.

Understanding Public and Private Keys

Before we go further, let’s quickly recap:

• Public Key: This can be shared with anyone. It’s used for encrypting messages that only the owner of the private key can decrypt, or verifying digital signatures.
• Private Key: This must be kept secret. It’s used to decrypt messages encrypted with its corresponding public key, and to create digital signatures.

The relationship is one-way. The private key generates the public key, but not vice versa.

What if I have a Public Key?

1. You can’t get the original Private Key: As stated before, this isn’t possible with current technology and algorithms.
2. Generate a new key pair: The best course of action is to create a new private/public key pair. This will give you a fresh, secure key that you control.

   ssh-keygen -t rsa -b 4096

   This command creates an RSA key pair with a bit length of 4096 (a good standard for security). You’ll be prompted to enter a filename and passphrase.

3. If the original Private Key is lost: If you’ve lost your private key, there’s no way to recover it from the public key. You need to generate a new one.

   Important: Losing your private key means you can no longer decrypt messages encrypted with that specific public key or sign documents using that key.

Common Scenarios and What to Do

• Lost SSH Key: If you’ve lost the private part of an SSH key pair, generate a new one as shown above. Update your authorized_keys file on any servers where you used the old public key with the new public key.

  ssh-copy-id -i ~/.ssh/id_rsa.pub user@server

• Compromised Key: If you suspect your private key has been compromised (e.g., stolen), revoke it immediately and generate a new one.

  Note: How to revoke depends on how the key was used (e.g., for SSH, GPG signing, etc.).

Security Considerations

• Never share your Private Key: This is the most important rule!
• Protect your Private Key file: Use strong permissions (e.g., 600) to ensure only you can read and write it.

  chmod 600 ~/.ssh/id_rsa

• Use a passphrase: Protect your private key with a strong passphrase for added security.