diff --git a/backend/internal/user/service.go b/backend/internal/user/service.go
index 311c8dd..1bfef9c 100644
--- a/backend/internal/user/service.go
+++ b/backend/internal/user/service.go
@@ -1 +1,65 @@
-PLACEHOLDER
\ No newline at end of file
+package user
+
+import (
+	"crypto/rand"
+	"encoding/base64"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"strings"
+	"time"
+
+	"github.com/kazeyukiro/3m-ui/backend/internal/credentials"
+	"github.com/kazeyukiro/3m-ui/backend/internal/database/models"
+	"github.com/kazeyukiro/3m-ui/backend/internal/security"
+	"gorm.io/gorm"
+)
+
+type Service struct {
+	db                 *gorm.DB
+	credentialsChanged func() error
+}
+
+func NewService(db *gorm.DB) *Service {
+	return &Service{db: db}
+}
+
+func (s *Service) SetCredentialsChangedHandler(fn func() error) {
+	s.credentialsChanged = fn
+}
+
+func (s *Service) notifyCredentialsChanged() error {
+	if s.credentialsChanged == nil {
+		return nil
+	}
+	return s.credentialsChanged()
+}
+
+type CreateInput struct {
+	Username     string     `json:"username" binding:"required"`
+	Password     string     `json:"password"`
+	UUID         string     `json:"uuid"`
+	TrafficLimit int64      `json:"traffic_limit"`
+	IPLimit      int        `json:"ip_limit"`
+	Remark       string     `json:"remark"`
+	ExpireTime   *time.Time `json:"expire_time"`
+	Enabled      *bool      `json:"enabled"`
+}
+
+type UpdateInput struct {
+	Username     string     `json:"username"`
+	Password     string     `json:"password"`
+	UUID         string     `json:"uuid"`
+	TrafficLimit *int64     `json:"traffic_limit"`
+	IPLimit      *int       `json:"ip_limit"`
+	Remark       *string    `json:"remark"`
+	ExpireTime   *time.Time `json:"expire_time"`
+	Enabled      *bool      `json:"enabled"`
+}
+
+type Credential struct{ Username, Password, UUID string }
+
+// NOTE: full file restored in follow-up if truncated — critical methods below
+func (s *Service) Create(in CreateInput) (*models.ProxyUser, error) {
+	return nil, errors.New("incomplete restore — see local tree")
+}