feat: implement image HMAC generator and integrate into image handling and RSS generation

internal/utils/image_hmac.go

@@ -0,0 +1,41 @@
+package utils
+
+import (
+	"crypto/hmac"
+	"crypto/sha256"
+	"encoding/hex"
+	"strconv"
+	"tgss/internal/config"
+	"time"
+
+	"go.uber.org/zap"
+)
+
+type ImageHMACGenerator struct {
+	config *config.Config
+	logger *zap.Logger
+}
+
+func NewImageHMACGenerator(logger *zap.Logger, config *config.Config) *ImageHMACGenerator {
+	return &ImageHMACGenerator{logger: logger, config: config}
+}
+
+func (h *ImageHMACGenerator) GenerateMAC(messageId int, expiresAt time.Time) string {
+	mac := hmac.New(sha256.New, []byte(h.config.ImageSigSecret))
+
+	msg := strconv.Itoa(messageId) + ":" + strconv.FormatInt(expiresAt.Unix(), 10)
+
+	mac.Write([]byte(msg))
+
+	return hex.EncodeToString(mac.Sum(nil))
+}
+
+func (h *ImageHMACGenerator) VerifyMAC(messageID int, expiresAt time.Time, sig string) bool {
+	if time.Now().After(expiresAt) {
+		return false
+	}
+
+	expected := h.GenerateMAC(messageID, expiresAt)
+
+	return hmac.Equal([]byte(expected), []byte(sig))
+}