Mutual WebService Authentication

TL;DR

Secure communication between two WebServices using API keys and a shared secret for verification. This guide covers key generation, exchange, and validation.

Step-by-step Guide

1. Generate API Keys: Each service needs its own unique API key (a public identifier) and a shared secret (kept private). Don’t use passwords; generate random strings.
   • For example, using OpenSSL:

     openssl rand -base64 32

     This will create a 32-byte random string encoded in Base64.

2. Key Exchange: Securely exchange the API keys between the two services.
   • Important: Never transmit secrets over unencrypted channels (like HTTP). Use HTTPS, SSH, or a secure configuration management system.
   • Consider using an out-of-band method for initial key distribution (e.g., phone call, in-person meeting) to avoid man-in-the-middle attacks during setup.
3. Authentication Header: Define a standard header for authentication requests.
   • A common approach is using the X-API-Key header:

     X-API-Key: <service_api_key>

4. Request Validation (Service A validating Service B): When Service A receives a request from Service B, it performs these checks.
   • Extract the API Key: Read the X-API-Key header from the incoming request.
   • Verify the Key: Check if the received API key matches the expected API key for Service B stored in its configuration.
   • Generate a Challenge (Optional but Recommended): Create a unique, random challenge string.

     openssl rand -base64 16

   • Sign the Challenge: Combine the API key and the challenge using your shared secret with a hashing algorithm (like HMAC-SHA256).

     echo -n '<api_key><challenge>' | openssl dgst -sha256 -hmac "<shared_secret>"

   • Receive and Verify the Signature: Service B signs the challenge with its shared secret and sends the signature back to Service A. Service A verifies this signature against the expected value calculated using its stored shared secret.

     echo -n '<api_key><challenge>' | openssl dgst -sha256 -hmac "<shared_secret>"

5. Request Validation (Service B validating Service A): Repeat step 4, but with the roles reversed. Service B validates requests from Service A using its own API key and shared secret.
   • Ensure both services have a clear understanding of which service is initiating the request to use the correct keys.
6. Error Handling: Implement robust error handling for authentication failures.
   • Return appropriate HTTP status codes (e.g., 401 Unauthorized) and informative error messages.
   • Log all authentication attempts, both successful and failed, for auditing purposes.
7. Key Rotation: Regularly rotate API keys and shared secrets to minimize the impact of potential compromises.
   • Establish a key rotation schedule (e.g., every 90 days).
   • Implement a mechanism for gracefully transitioning to new keys without disrupting service operation.

Security Considerations

• HTTPS: Always use HTTPS to encrypt communication between the services.
• Shared Secret Protection: Protect the shared secret with utmost care. Store it securely (e.g., using a secrets management system like HashiCorp Vault).
• Rate Limiting: Implement rate limiting to prevent brute-force attacks against the authentication endpoint.
• Input Validation: Validate all input data to prevent injection vulnerabilities.