flatrender/services/payment/internal/signing/signing.go

// Package signing provides the HMAC-SHA256 helpers the broker uses to (a) verify
// inbound client requests and (b) sign outbound webhooks + return redirects so a
// client site can trust a result came from the broker untampered.
package signing

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/hex"
)

// Sign returns the lowercase hex HMAC-SHA256 of message keyed by secret.
func Sign(secret string, message []byte) string {
	mac := hmac.New(sha256.New, []byte(secret))
	mac.Write(message)
	return hex.EncodeToString(mac.Sum(nil))
}

// Verify compares an expected hex signature against the computed one in constant time.
func Verify(secret string, message []byte, provided string) bool {
	expected := Sign(secret, message)
	// hmac.Equal is constant-time; compare the raw bytes after decoding.
	pb, err := hex.DecodeString(provided)
	if err != nil {
		return false
	}
	eb, _ := hex.DecodeString(expected)
	return hmac.Equal(eb, pb)
}