refactor(sign): 抽取公共签名原语 signutil，三套签名串格式保持不变

商户侧 /api/client/v1、源头侧 /api/open/v1 与乐玩开放平台各有一套签名实现，
其中语义完全一致的部分重复了多份，任一处修复都容易漏改另外两处。

- 新增 internal/pkg/signutil，只收敛四类一致性原语：HMAC-SHA256→hex、
  body SHA256 摘要、时间戳偏差校验、常数时间比较
- 中间件与 lewan 改为调用该包，删除 middleware 里重复的 hmacSHA256Hex 与 abs64
- 刻意不统一「签名串构造」：三者拼接方式是对外契约且互不相同
  （商户：字典序 & + body 摘要；源头：字典序 & + body 原文；乐玩：换行 6 行），
  强行合并会直接改变握手结果
- 保留各处原有细节语义：商户侧十六进制签名大小写不敏感、乐玩签名原样比较、
  VerifyWebhook 在 allowedSkew<=0 时仍要求时间戳可解析

验证：三套方案既有端到端测试全部通过（含乐玩官方测试向量、大写签名、重放拒绝）；
新增 signutil 已知向量与偏差/比较边界测试，以及 VerifyWebhook 边界语义测试。

backend/internal/middleware/open_auth.go

@@ -2,17 +2,14 @@ package middleware
 
 import (
 	"bytes"
-	"crypto/hmac"
-	"crypto/sha256"
-	"encoding/hex"
 	"io"
-	"strconv"
 	"strings"
 	"time"
 
 	"affiliate_dash/internal/model"
 	"affiliate_dash/internal/pkg/openlog"
 	"affiliate_dash/internal/pkg/response"
+	"affiliate_dash/internal/pkg/signutil"
 	"affiliate_dash/internal/service"
 
 	"github.com/gin-gonic/gin"
@@ -71,8 +68,8 @@ func OpenAuth(cfg OpenAuthConfig) gin.HandlerFunc {
 			c.Abort()
 			return
 		}
-		ts, err := strconv.ParseInt(timestamp, 10, 64)
-		if err != nil || abs64(time.Now().Unix()-ts) > cfg.SkewSeconds {
+		// 时间戳非法或超出偏差窗口都视为过期；解析与偏差判断统一由 signutil 负责。
+		if !signutil.TimestampWithinSkew(timestamp, time.Duration(cfg.SkewSeconds)*time.Second) {
 			openlog.Warn(c, "open_auth expired app_key=%s ts=%s skew=%d", openlog.MaskKey(appKey), timestamp, cfg.SkewSeconds)
 			response.Unauthorized(c, "请求已过期或 X-Timestamp 格式错误")
 			c.Abort()
@@ -113,10 +110,9 @@ func OpenAuth(cfg OpenAuthConfig) gin.HandlerFunc {
 		method := strings.ToUpper(c.Request.Method)
 		path := c.Request.URL.Path
 		expected := BuildOpenV1Sign(secret, appKey, timestamp, nonce, method, path, bodyBytes)
-		if !hmac.Equal([]byte(strings.ToLower(sign)), []byte(expected)) {
-			bodyHash := sha256.Sum256(bodyBytes)
+		if !signutil.EqualHexSignatures(sign, expected) {
 			openlog.Warn(c, "open_auth sign_mismatch app_key=%s method=%s path=%s body_sha256=%s sign=%s expected=%s",
-				openlog.MaskKey(appKey), method, path, hex.EncodeToString(bodyHash[:]),
+				openlog.MaskKey(appKey), method, path, signutil.SHA256Hex(bodyBytes),
 				openlog.MaskSign(sign), openlog.MaskSign(expected))
 			response.Unauthorized(c, "签名校验失败")
 			c.Abort()
@@ -189,16 +185,15 @@ func GetMerchantID(c *gin.Context) uint {
 // 与上游 BuildSignString 风格一致；body 以 SHA256 摘要参与签名（避免大 body 与特殊字符问题）。
 // 参与签名的参数固定顺序为：app_key, body_sha256, method, nonce, path, timestamp。
 func BuildOpenV1Sign(secret, appKey, timestamp, nonce, method, path string, body []byte) string {
-	bodyHash := sha256.Sum256(body)
 	content := strings.Join([]string{
 		"app_key=" + appKey,
-		"body_sha256=" + hex.EncodeToString(bodyHash[:]),
+		"body_sha256=" + signutil.SHA256Hex(body),
 		"method=" + strings.ToUpper(method),
 		"nonce=" + nonce,
 		"path=" + path,
 		"timestamp=" + timestamp,
 	}, "&")
-	return hmacSHA256Hex(secret, content)
+	return signutil.HMACSHA256Hex(secret, content)
 }
 
 // BuildSignString 保留旧接口的字典序签名算法，供上游发货兼容客户端和测试使用。
@@ -215,18 +210,5 @@ func BuildSignString(apiKey, timestamp, nonce, method, path, body string) string
 
 // BuildOpenSign 供旧发货系统兼容使用。
 func BuildOpenSign(apiKey, apiSecret, timestamp, nonce, method, path, body string) string {
-	return hmacSHA256Hex(apiSecret, BuildSignString(apiKey, timestamp, nonce, method, path, body))
-}
-
-func hmacSHA256Hex(secret, content string) string {
-	mac := hmac.New(sha256.New, []byte(secret))
-	_, _ = mac.Write([]byte(content))
-	return hex.EncodeToString(mac.Sum(nil))
-}
-
-func abs64(value int64) int64 {
-	if value < 0 {
-		return -value
-	}
-	return value
+	return signutil.HMACSHA256Hex(apiSecret, BuildSignString(apiKey, timestamp, nonce, method, path, body))
 }

backend/internal/middleware/source_open_auth.go

@@ -2,15 +2,13 @@ package middleware
 
 import (
 	"bytes"
-	"crypto/hmac"
 	"io"
-	"strconv"
-	"strings"
 	"time"
 
 	"affiliate_dash/internal/model"
 	"affiliate_dash/internal/pkg/openlog"
 	"affiliate_dash/internal/pkg/response"
+	"affiliate_dash/internal/pkg/signutil"
 
 	"github.com/gin-gonic/gin"
 	"gorm.io/gorm"
@@ -73,8 +71,7 @@ func SourceOpenAuth(cfg SourceOpenAuthConfig) gin.HandlerFunc {
 			c.Abort()
 			return
 		}
-		ts, err := strconv.ParseInt(timestamp, 10, 64)
-		if err != nil || abs64(time.Now().Unix()-ts) > cfg.SkewSeconds {
+		if !signutil.TimestampWithinSkew(timestamp, time.Duration(cfg.SkewSeconds)*time.Second) {
 			openlog.Warn(c, "source_open_auth expired ts=%s skew=%d", timestamp, cfg.SkewSeconds)
 			response.Unauthorized(c, "请求已过期或 X-Timestamp 格式错误")
 			c.Abort()
@@ -92,7 +89,7 @@ func SourceOpenAuth(cfg SourceOpenAuthConfig) gin.HandlerFunc {
 		c.Set(openlog.CtxBody, string(bodyBytes))
 
 		expected := BuildOpenSign(apiKey, cfg.APISecret, timestamp, nonce, c.Request.Method, c.Request.URL.Path, string(bodyBytes))
-		if !hmac.Equal([]byte(strings.ToLower(sign)), []byte(expected)) {
+		if !signutil.EqualHexSignatures(sign, expected) {
 			openlog.Warn(c, "source_open_auth sign_mismatch method=%s path=%s body=%s sign=%s expected=%s",
 				c.Request.Method, c.Request.URL.Path, openlog.Truncate(string(bodyBytes), 200),
 				openlog.MaskSign(sign), openlog.MaskSign(expected))