Written for the failure you are actually looking at, not the happy path the gateway documented. Every worked example is computed by the same code behind the tools, so the output on the page is what the tool would give you.
A tunnel points the internet at your laptop and loses everything sent after you close it. Capturing first and forwarding outward keeps the deliveries, replays them on demand, and never exposes a port.
Checkout, payment links and subscriptions are keyed with your API secret. Webhooks are keyed with the webhook secret. Mixing them up is the most common cause of a signature that will never match.
Four causes account for nearly every INVALID_WEBHOOK_SIGNATURE: a re-serialised body, middleware ordering, a rotated secret on a retried event, and comparing the wrong encoding.
ISO 8583 has no delimiters, so a single wrong length silently misaligns every field that follows. The fix is finding the first field that is wrong, not the first field that looks wrong.
UPI apps disagree about how strictly to parse a upi://pay URI. A literal space, a raw plus or an unescaped ampersand is tolerated by some and truncates the link in others.
Cashfree's scheme looks like Stripe's and differs in three details that each produce a total mismatch: no dot between timestamp and body, base64 rather than hex, and a timestamp that lives in its own header.
Field order, empty udf slots and amount formatting all change the digest. The intermittent failures are usually additionalCharges silently switching PayU to a different formula.
An EMVCo payload's checksum includes the literal characters 6304 that introduce it. Compute it over the payload without them and you get a plausible, wrong answer every time.
The XOR carries no integrity check, so the wrong PAN produces a valid-looking length and four numeric digits. Here is why, and the one field that tells you it happened.
Five bytes of hex that answer 'what did the terminal find'. Read the wrong byte and you debug the wrong thing — and a set reserved bit almost always means you are a byte out.
One page for all three schemes — which secret signs, which string is signed, and which encoding comes out. Every other comparison covers Stripe, GitHub and Shopify; these are the ones you actually integrate in India.
The framework, not the gateway. request.json() and even request.text() can hand you something that is no longer byte-identical to what was signed, and the mismatch looks exactly like a wrong secret.