feat(middleware): add constant-time bearer token auth for machine routes

backend/internal/middleware/bearer_token.go

@@ -0,0 +1,38 @@
+package middleware
+
+import (
+	"crypto/subtle"
+	"strings"
+
+	"github.com/gin-gonic/gin"
+
+	"marktvogt.de/backend/internal/pkg/apierror"
+)
+
+// RequireBearerToken validates Authorization: Bearer <token> against the given
+// secret using constant-time compare. An empty secret disables the route (all
+// requests are rejected) — a safety default for dev environments that haven't
+// set the token.
+func RequireBearerToken(secret string) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		if secret == "" {
+			apiErr := apierror.Unauthorized("endpoint disabled")
+			c.AbortWithStatusJSON(apiErr.Status, apierror.NewResponse(apiErr))
+			return
+		}
+		auth := c.GetHeader("Authorization")
+		const prefix = "Bearer "
+		if !strings.HasPrefix(auth, prefix) {
+			apiErr := apierror.Unauthorized("missing bearer token")
+			c.AbortWithStatusJSON(apiErr.Status, apierror.NewResponse(apiErr))
+			return
+		}
+		provided := strings.TrimPrefix(auth, prefix)
+		if subtle.ConstantTimeCompare([]byte(provided), []byte(secret)) != 1 {
+			apiErr := apierror.Unauthorized("invalid bearer token")
+			c.AbortWithStatusJSON(apiErr.Status, apierror.NewResponse(apiErr))
+			return
+		}
+		c.Next()
+	}
+}

backend/internal/middleware/bearer_token_test.go

@@ -0,0 +1,57 @@
+package middleware
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+)
+
+func TestRequireBearerToken(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	secret := "s3cret"
+	mw := RequireBearerToken(secret)
+
+	tests := []struct {
+		name       string
+		header     string
+		wantStatus int
+	}{
+		{"missing", "", http.StatusUnauthorized},
+		{"wrong scheme", "Basic abcd", http.StatusUnauthorized},
+		{"wrong token", "Bearer wrong", http.StatusUnauthorized},
+		{"correct", "Bearer s3cret", http.StatusOK},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			w := httptest.NewRecorder()
+			_, r := gin.CreateTestContext(w)
+			r.Use(mw)
+			r.GET("/t", func(c *gin.Context) { c.Status(http.StatusOK) })
+			req := httptest.NewRequest(http.MethodGet, "/t", nil)
+			if tc.header != "" {
+				req.Header.Set("Authorization", tc.header)
+			}
+			r.ServeHTTP(w, req)
+			if w.Code != tc.wantStatus {
+				t.Errorf("status=%d want %d", w.Code, tc.wantStatus)
+			}
+		})
+	}
+}
+
+func TestRequireBearerToken_EmptySecretRejectsAll(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	mw := RequireBearerToken("") // feature disabled → all requests 401
+	w := httptest.NewRecorder()
+	_, r := gin.CreateTestContext(w)
+	r.Use(mw)
+	r.GET("/t", func(c *gin.Context) { c.Status(http.StatusOK) })
+	req := httptest.NewRequest(http.MethodGet, "/t", nil)
+	req.Header.Set("Authorization", "Bearer anything")
+	r.ServeHTTP(w, req)
+	if w.Code != http.StatusUnauthorized {
+		t.Errorf("expected 401 when secret is empty, got %d", w.Code)
+	}
+}