[relay] Improve relay messages (#2574)

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

relay/auth/hmac/v2/generator.go

@@ -0,0 +1,45 @@
+package v2
+
+import (
+	"crypto/hmac"
+	"fmt"
+	"hash"
+	"strconv"
+	"time"
+)
+
+type Generator struct {
+	algo       func() hash.Hash
+	algoType   AuthAlgo
+	secret     []byte
+	timeToLive time.Duration
+}
+
+func NewGenerator(algo AuthAlgo, secret []byte, timeToLive time.Duration) (*Generator, error) {
+	algoFunc := algo.New()
+	if algoFunc == nil {
+		return nil, fmt.Errorf("unsupported auth algorithm: %s", algo)
+	}
+	return &Generator{
+		algo:       algoFunc,
+		algoType:   algo,
+		secret:     secret,
+		timeToLive: timeToLive,
+	}, nil
+}
+
+func (g *Generator) GenerateToken() (*Token, error) {
+	expirationTime := time.Now().Add(g.timeToLive).Unix()
+
+	payload := []byte(strconv.FormatInt(expirationTime, 10))
+
+	h := hmac.New(g.algo, g.secret)
+	h.Write(payload)
+	signature := h.Sum(nil)
+
+	return &Token{
+		AuthAlgo:  g.algoType,
+		Signature: signature,
+		Payload:   payload,
+	}, nil
+}