Should You Share Your RSA Public Key via API or Embed It in the Frontend?

Published on January 24, 2025

When implementing encryption in web applications, particularly for end-to-end encryption, a common question arises: Should the RSA public key be shared dynamically via an API call, or can it be embedded directly in the frontend? The answer depends on how often the public key changes and the operational constraints of your system. Let’s break this down to help you decide.


What Is an RSA Public Key, and Why Is It Safe to Share?

In an RSA encryption system, the public key is used to encrypt data, while the private key is used to decrypt it. By design, the public key is meant to be shared openly and securely. Even if an attacker intercepts the public key, they cannot decrypt the data because decryption requires the private key, which is securely stored on the server.

This makes sharing the public key a straightforward process—but how you share it matters.


Factors to Consider: Key Changes

The primary factor influencing whether to embed the key or share it dynamically is the likelihood of the public key changing frequently.

Case 1: The Public Key Rarely Changes

In most systems, the RSA public key is stable and changes only under specific circumstances, such as:

  • Key rotation policies (e.g., every 6-12 months).
  • Security incidents (e.g., private key compromise).
  • Major application updates or deployments.

If these events are rare, embedding the public key in the frontend is a viable and efficient solution. For example:

  • Hardcode the public key directly in the frontend code.
  • Store it in a configuration file that is bundled during the build process.

Advantages:

  • No additional API call is required, reducing latency and server overhead.
  • Simpler architecture with fewer dependencies.

How to Embed:

const PUBLIC_KEY = `
-----BEGIN PUBLIC KEY-----
MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA...
-----END PUBLIC KEY-----
`;

When This Works Best:

  • The key rotation frequency is predictable (e.g., every 6 months).
  • Deployment processes ensure that the public key can be updated easily if needed.

Case 2: The Public Key Changes Frequently

In some systems, the public key may change more often due to:

  • Automated key rotation policies (e.g., every week or month).
  • High-security environments requiring frequent updates.
  • Dynamic environments where servers may generate new key pairs on deployment.

In such cases, embedding the key in the frontend is impractical because:

  • Frequent frontend updates would be required, increasing maintenance overhead.
  • There’s a higher risk of the frontend using outdated keys if deployments aren’t perfectly synced.

Solution: Share the Public Key via an API Call

You can expose an endpoint to allow clients to fetch the current public key dynamically:

Example Endpoint:

[ApiController]
[Route("api/[controller]")]
public class KeyExchangeController : ControllerBase
{
    private readonly KeyManagementService _keyService;

    public KeyExchangeController(KeyManagementService keyService)
    {
        _keyService = keyService;
    }

    [HttpGet("public-key")]
    public IActionResult GetPublicKey()
    {
        var publicKey = _keyService.GetPublicKey();
        return Ok(publicKey);
    }
}

Advantages:

  • Ensures the client always gets the latest public key.
  • Supports dynamic environments where the key changes frequently.

How to Optimize:

  • Cache the public key on the client-side to reduce repeated API calls.
  • Use a versioning system to track key updates (e.g., include a key ID in the response).

Best Practices for Key Management

Regardless of the method you choose, follow these best practices:

  1. Use HTTPS: Always serve the public key over HTTPS to prevent man-in-the-middle (MITM) attacks.
  2. Monitor Key Usage: Track when and how keys are accessed to detect unusual activity.
  3. Plan for Key Rotation: Have a clear policy for rotating RSA key pairs to maintain security.
  4. Fallback Mechanism: Implement error handling on the client side to fetch a new key if decryption fails due to a key mismatch.

Conclusion: When to Embed vs. Share Dynamically

  • Embed in Frontend: If the public key rarely changes and your deployment process can handle updates when needed.
  • Share via API: If the public key changes frequently or your system has dynamic key management requirements.

Your understanding is correct: if the public key rarely changes, embedding it in the frontend is efficient and simplifies the architecture. However, if the key changes often, an API-based approach is more reliable and scalable.

By evaluating your system’s requirements and key rotation frequency, you can decide the best approach to securely distribute the public key while maintaining performance and simplicity.

Software Architecture and Design