[relay] Improve relay messages (#2574)

Co-authored-by: Zoltán Papp <zoltan.pmail@gmail.com>

relay/auth/hmac/v2/token.go

@@ -0,0 +1,39 @@
+package v2
+
+import "errors"
+
+type Token struct {
+	AuthAlgo  AuthAlgo
+	Signature []byte
+	Payload   []byte
+}
+
+func (t *Token) Marshal() []byte {
+	size := 1 + len(t.Signature) + len(t.Payload)
+
+	buf := make([]byte, size)
+
+	buf[0] = byte(t.AuthAlgo)
+	copy(buf[1:], t.Signature)
+	copy(buf[1+len(t.Signature):], t.Payload)
+
+	return buf
+}
+
+func UnmarshalToken(data []byte) (*Token, error) {
+	if len(data) == 0 {
+		return nil, errors.New("invalid token data")
+	}
+
+	algo := AuthAlgo(data[0])
+	sigSize := algo.Size()
+	if len(data) < 1+sigSize {
+		return nil, errors.New("invalid token data: insufficient length")
+	}
+
+	return &Token{
+		AuthAlgo:  algo,
+		Signature: data[1 : 1+sigSize],
+		Payload:   data[1+sigSize:],
+	}, nil
+}