prow/github/hmac.go
func PayloadSignature(payload []byte, key []byte) string {
	mac := hmac.New(sha1.New, key)
	mac.Write(payload)
	sum := mac.Sum(nil)
	return "sha1=" + hex.EncodeToString(sum)
}
func PayloadSignature(payload []byte, key []byte) string {
	mac := hmac.New(sha1.New, key)
	mac.Write(payload)
	sum := mac.Sum(nil)
	return "sha1=" + hex.EncodeToString(sum)
}
// PayloadSignature returns the signature that matches the payload.