Programming Tutorials

  Few things in FinTech integration are more frustrating than a generic   401 Unauthorized   or   9124 Token Validation Failed   error. You have your mTLS certificates configured, your API Key is correct, and your logic seems sound. Yet, the Visa Developer Platform (VDP) refuses your requests. The culprit is almost always the  X-Pay-Token . This custom header relies on a Shared Secret HMAC-SHA256 hash. If your computed hash differs from Visa's internal calculation by even a single byte—due to timestamp formatting, URL encoding, or string concatenation order—the request fails immediately. This guide provides a root cause analysis of why this failure occurs and a production-ready Node.js solution to generate the token correctly. The Root Cause: Why The Hash Mismatch Occurs The  X-Pay-Token  protects the integrity of the request payload. It ensures that the parameters and body haven't been tampered with during transit. Visa calculates the token using...