feat(internal): add basic auth HTTP middleware

Signed-off-by: Xe Iaso <me@xeiaso.net>

internal/basicauth.go

@@ -0,0 +1,52 @@
+package internal
+
+import (
+	"crypto/sha256"
+	"crypto/subtle"
+	"fmt"
+	"log/slog"
+	"net/http"
+)
+
+// BasicAuth wraps next in HTTP Basic authentication using the provided
+// credentials. If either username or password is empty, next is returned
+// unchanged and a debug log line is emitted.
+//
+// Credentials are compared in constant time to avoid leaking information
+// through timing side channels.
+func BasicAuth(realm, username, password string, next http.Handler) http.Handler {
+	if username == "" || password == "" {
+		slog.Debug("skipping middleware, basic auth credentials are empty")
+		return next
+	}
+
+	expectedUser := sha256.Sum256([]byte(username))
+	expectedPass := sha256.Sum256([]byte(password))
+	challenge := fmt.Sprintf("Basic realm=%q, charset=\"UTF-8\"", realm)
+
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		user, pass, ok := r.BasicAuth()
+		if !ok {
+			unauthorized(w, challenge)
+			return
+		}
+
+		gotUser := sha256.Sum256([]byte(user))
+		gotPass := sha256.Sum256([]byte(pass))
+
+		userMatch := subtle.ConstantTimeCompare(gotUser[:], expectedUser[:])
+		passMatch := subtle.ConstantTimeCompare(gotPass[:], expectedPass[:])
+
+		if userMatch&passMatch != 1 {
+			unauthorized(w, challenge)
+			return
+		}
+
+		next.ServeHTTP(w, r)
+	})
+}
+
+func unauthorized(w http.ResponseWriter, challenge string) {
+	w.Header().Set("WWW-Authenticate", challenge)
+	http.Error(w, "Unauthorized", http.StatusUnauthorized)
+}

internal/basicauth_test.go

@@ -0,0 +1,138 @@
+package internal
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+func okHandler() http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write([]byte("ok"))
+	})
+}
+
+func TestBasicAuth(t *testing.T) {
+	t.Parallel()
+
+	const (
+		realm    = "test-realm"
+		username = "admin"
+		password = "hunter2"
+	)
+
+	for _, tt := range []struct {
+		name       string
+		setAuth    bool
+		user       string
+		pass       string
+		wantStatus int
+		wantBody   string
+		wantChall  bool
+	}{
+		{
+			name:       "valid credentials",
+			setAuth:    true,
+			user:       username,
+			pass:       password,
+			wantStatus: http.StatusOK,
+			wantBody:   "ok",
+		},
+		{
+			name:       "missing credentials",
+			setAuth:    false,
+			wantStatus: http.StatusUnauthorized,
+			wantChall:  true,
+		},
+		{
+			name:       "wrong username",
+			setAuth:    true,
+			user:       "nobody",
+			pass:       password,
+			wantStatus: http.StatusUnauthorized,
+			wantChall:  true,
+		},
+		{
+			name:       "wrong password",
+			setAuth:    true,
+			user:       username,
+			pass:       "wrong",
+			wantStatus: http.StatusUnauthorized,
+			wantChall:  true,
+		},
+		{
+			name:       "empty supplied credentials",
+			setAuth:    true,
+			user:       "",
+			pass:       "",
+			wantStatus: http.StatusUnauthorized,
+			wantChall:  true,
+		},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			h := BasicAuth(realm, username, password, okHandler())
+
+			req := httptest.NewRequest(http.MethodGet, "/", nil)
+			if tt.setAuth {
+				req.SetBasicAuth(tt.user, tt.pass)
+			}
+			rec := httptest.NewRecorder()
+			h.ServeHTTP(rec, req)
+
+			if rec.Code != tt.wantStatus {
+				t.Errorf("status = %d, want %d", rec.Code, tt.wantStatus)
+			}
+
+			if tt.wantBody != "" && rec.Body.String() != tt.wantBody {
+				t.Errorf("body = %q, want %q", rec.Body.String(), tt.wantBody)
+			}
+
+			chall := rec.Header().Get("WWW-Authenticate")
+			if tt.wantChall {
+				if chall == "" {
+					t.Error("WWW-Authenticate header missing on 401")
+				}
+				if !strings.Contains(chall, realm) {
+					t.Errorf("WWW-Authenticate = %q, want realm %q", chall, realm)
+				}
+			} else if chall != "" {
+				t.Errorf("unexpected WWW-Authenticate header: %q", chall)
+			}
+		})
+	}
+}
+
+func TestBasicAuthPassthrough(t *testing.T) {
+	t.Parallel()
+
+	for _, tt := range []struct {
+		name     string
+		username string
+		password string
+	}{
+		{name: "empty username", username: "", password: "hunter2"},
+		{name: "empty password", username: "admin", password: ""},
+		{name: "both empty", username: "", password: ""},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			h := BasicAuth("realm", tt.username, tt.password, okHandler())
+
+			req := httptest.NewRequest(http.MethodGet, "/", nil)
+			rec := httptest.NewRecorder()
+			h.ServeHTTP(rec, req)
+
+			if rec.Code != http.StatusOK {
+				t.Errorf("status = %d, want %d (passthrough expected)", rec.Code, http.StatusOK)
+			}
+			if rec.Body.String() != "ok" {
+				t.Errorf("body = %q, want %q", rec.Body.String(), "ok")
+			}
+		})
+	}
+}