RSA Public Key Recovery

What Is an RSA Public Key Recovery Tool?

In day-to-day server operations and development work, it is not uncommon to encounter an awkward situation: you still have the critical RSA private key, but the corresponding public key has been lost or was never backed up. Since the public key is required for SSH passwordless login, GitHub Deploy Keys, and various encrypted communication scenarios, losing it often means authentication can no longer be completed.

Our RSA Public Key Recovery Tool is a practical utility specifically designed for this kind of “reverse extraction” scenario. By simply pasting a PEM-formatted private key, you can instantly recover and export the exact matching public key. The tool currently supports mainstream key sizes, including 1024, 2048, and 4096 bits.

Why Can a Public Key Be Derived from a Private Key?

In the RSA cryptosystem, the public key and private key are not independent of each other. In fact, the public key is effectively a subset of the private key’s information.

Mathematical Principles

As explained in our RSA Key Pair Generator, the core parameters of an RSA key pair include the modulus $n$, the public (encryption) exponent $e$, and the private (decryption) exponent $d$.

• Public key structure: composed of $(n, e)$.

• Private key structure: in a standard PEM-formatted private key, additional parameters are stored to optimize computation, typically including:

  • The two prime numbers: $p, q$
  • The modulus: $n = p \times q$
  • The public exponent: $e$ (commonly $65537$)
  • The private exponent: $d$

Recovery Method

This tool parses the ASN.1-encoded structure of the private key, directly extracts the $n$ and $e$ parameters, and re-encapsulates them according to the PEM specification to generate the corresponding public key.

Why Use This Technique?

1. Fix configuration issues: When you cannot connect to a remote server via SSH and are unsure whether the public key you have matches your private key, regenerating the public key from the private key is the most reliable way to verify correctness.

2. Identity re-authentication: Many API services (such as AWS and GitHub) require you to provide a public key. If you only have a .pem private key file, this tool allows you to quickly generate the required public key.

3. Asymmetric encryption integrity: Asymmetric cryptography relies on properly paired public and private keys. This recovery approach preserves that pairing without regenerating an entirely new key pair, avoiding complex configuration changes that would otherwise be required when replacing a private key.

Why Choose Our RSA Public Key Recovery Tool?

Privacy First

• Fully client-side processing: Although the tool runs in a web interface, all recovery logic is executed entirely within your local browser.
• Private keys never leave your device: Your private key is never uploaded to our servers or transmitted over the network. This in-browser processing model provides strong protection when handling highly sensitive key material.

Broad Compatibility

• Multiple key sizes supported: Full support for 1024-bit, 2048-bit, and 4096-bit RSA keys.
• Standard output format: The recovered public key is generated in the standard PEM format (-----BEGIN PUBLIC KEY-----), making it fully compatible with Linux/Unix environments and a wide range of development libraries.

How to Use the RSA Public Key Recovery Tool?

The process is extremely simple and requires only two steps:

1. Enter the private key: Paste your RSA private key (PEM format) into the “Private Key Input” field on the left.
2. Retrieve the public key: The tool automatically detects and computes the result, displaying the recovered public key in real time on the right. You can then click “Copy” or “Download” to save it.