refactoring, added documentation

sign.go

@@ -0,0 +1,31 @@
+package roots
+
+import (
+	"encoding/hex"
+	"fmt"
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcec/v2/schnorr"
+)
+
+// SignEvent generates a Schnorr signature for the given event ID using the
+// provided private key. Returns the signature as 128 lowercase hex characters.
+func SignEvent(eventID, privateKeyHex string) (string, error) {
+	skBytes, err := hex.DecodeString(privateKeyHex)
+	if err != nil {
+		return "", ErrMalformedPrivKey
+	}
+
+	idBytes, err := hex.DecodeString(eventID)
+	if err != nil {
+		return "", ErrMalformedID
+	}
+
+	// discard public key return value
+	sk, _ := btcec.PrivKeyFromBytes(skBytes)
+	sig, err := schnorr.Sign(sk, idBytes)
+	if err != nil {
+		return "", fmt.Errorf("schnorr signature error: %w", err)
+	}
+
+	return hex.EncodeToString(sig.Serialize()), nil
+}