Webhook HMAC Signature Generator & Verifier

Webhooks form the asynchronous nervous system of the modern cloud, transmitting real-time state changes between decoupled microservices, payment gateways, and developer platforms. When a customer completes a checkout on Stripe, pushes code to a GitHub repository, or creates an order on Shopify, the provider transmits an HTTP POST request containing a JSON payload directly to your application public endpoint. Because webhook endpoints are exposed to the open internet, accepting unauthenticated payloads creates severe security vulnerabilities, allowing malicious actors to spoof payment confirmations, inject unauthorized state changes, or orchestrate server disruption. To guarantee both message authenticity and payload integrity without transmitting sensitive API keys over HTTP, industry-standard webhook architectures rely on Hash-based Message Authentication Codes (HMAC). An HMAC combines a shared cryptographic secret with the exact byte content of the payload using an iterative cryptographic hash function, most commonly SHA-256 (RFC 2104). The receiving server recomputes the HMAC digest using its copy of the secret and verifies that the resulting signature matches the signature transmitted in the HTTP request header. This Webhook HMAC Signature Generator and Tester delivers a comprehensive, zero-transmission cryptographic tool for developers building, debugging, and testing webhook integrations. The tool operates in dual modes: Signer Mode generates provider-exact HTTP signature headers and signed basestrings from any raw payload and shared secret to simulate third-party events; Verifier Mode ingests received signature headers, inspects cryptographic digests against the payload, flags timestamp replay drift, and verifies message authenticity. Supported providers include Stripe with its timestamped basestring (t=...,v1=...), GitHub with sha256= prefixes, Shopify with Base64 encoding, and Slack with v0 versioned basestrings. It simulates constant-time comparison to demonstrate defense against timing side-channel attacks and includes production code snippets for Node.js, Python, and PHP.