How HMAC Request Signing with Timestamps Hardens API Security

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Bearer API keys alone let an intercepted request be replayed indefinitely, tampered with, or reused against other endpoints. The article proposes request signing, where each call carries an HMAC-SHA256 signature computed over the method, path, body hash, and a timestamp. Servers reject requests whose timestamps fall outside a 30-second window and verify signatures with constant-time comparison to prevent timing attacks. The author provides Python client and FastAPI server implementations and stresses that deterministic JSON serialization, such as sorted keys and fixed separators, is necessary for signatures to verify correctly. The approach mirrors techniques used by AWS SigV4, Stripe webhooks, and GitHub webhooks.

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