Why does the attempt to replicate the Github Webhook -> Jenkins request on Postman result in failure due to an unmatched signature?

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The unmatched signature error occurs because Github uses a HMAC SHA-1 hash to verify the authenticity of the webhook payload. When the webhook is sent from Github to Jenkins, the signature is included in the HTTP headers. In order to replicate this webhook request on Postman, you must also generate the same HMAC SHA-1 hash value for the payload data and include it in the HTTP headers. If the signature in the HTTP headers does not match the one generated by Postman, Github will reject the request and return an unmatched signature error.