diff --git a/backend-old/internal/http/handler/handler.go b/backend-old/internal/http/handler/handler.go
index 5fc15d1..63439c3 100644
--- a/backend-old/internal/http/handler/handler.go
+++ b/backend-old/internal/http/handler/handler.go
@@ -123,12 +123,12 @@ func (h *Handler) Login(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	identifier := req.Email
-	if identifier == "" {
-		identifier = req.Username
+	if req.Username == "" {
+		writeError(w, http.StatusBadRequest, errors.New("username is required"))
+		return
 	}
 
-	token, exp, err := h.svc.Authenticate(r.Context(), identifier, req.Password)
+	token, exp, err := h.svc.Login(r.Context(), req.Username, req.Password)
 	if err != nil {
 		writeError(w, http.StatusUnauthorized, err)
 		return
diff --git a/backend-old/internal/usecase/auth_usecase.go b/backend-old/internal/usecase/auth_usecase.go
new file mode 100644
index 0000000..460f386
--- /dev/null
+++ b/backend-old/internal/usecase/auth_usecase.go
@@ -0,0 +1,41 @@
+package usecase
+
+import (
+	"context"
+	"errors"
+	"strings"
+	"time"
+
+	"github.com/golang-jwt/jwt/v5"
+	"golang.org/x/crypto/bcrypt"
+)
+
+// Login validates credentials using username and returns a signed JWT.
+func (s *Service) Login(ctx context.Context, username, password string) (string, time.Time, error) {
+	if strings.TrimSpace(username) == "" {
+		return "", time.Time{}, errors.New("username is required")
+	}
+
+	user, err := s.repo.GetUserByUsername(ctx, username)
+	if err != nil {
+		return "", time.Time{}, errors.New("invalid credentials")
+	}
+
+	if err := bcrypt.CompareHashAndPassword([]byte(user.PasswordHash), []byte(s.pepperPassword(password))); err != nil {
+		return "", time.Time{}, errors.New("invalid credentials")
+	}
+
+	expiresAt := time.Now().Add(s.tokenTTL)
+	claims := jwt.MapClaims{
+		"user_id":    user.ID.String(),
+		"company_id": user.CompanyID.String(),
+		"role":       user.Role,
+	}
+
+	signed, err := s.signToken(claims, expiresAt)
+	if err != nil {
+		return "", time.Time{}, err
+	}
+
+	return signed, expiresAt, nil
+}