fix: encrypted PDFs with EncryptMetadata false fail with invalid password

read.go

@@ -982,6 +982,11 @@ func (r *Reader) initEncrypt(password string) error {
 	h.Write([]byte(O))
 	h.Write([]byte{byte(P), byte(P >> 8), byte(P >> 16), byte(P >> 24)})
 	h.Write([]byte(ID))
+	if R >= 4 {
+		if encryptMetadata, ok := encrypt["EncryptMetadata"].(bool); ok && !encryptMetadata {
+			h.Write([]byte{0xff, 0xff, 0xff, 0xff})
+		}
+	}
 	key := h.Sum(nil)
 
 	if R >= 3 {

read_test.go

@@ -2,6 +2,8 @@ package pdf
 
 import (
 	"bytes"
+	"context"
+	"io"
 	"strings"
 	"testing"
 )
@@ -151,3 +153,26 @@ func TestNewReaderMaliciousPDF(t *testing.T) {
 		t.Fatal("expected error from malicious PDF, got nil")
 	}
 }
+
+// Regression test for https://github.com/ledongthuc/pdf/issues/82:
+// AES-128 encrypted PDFs (V=4, R=4) with /EncryptMetadata false and an empty
+// user password must open successfully instead of returning ErrInvalidPassword.
+func TestOpenEncryptedMetadataFalse(t *testing.T) {
+	f, r, err := Open("testdata/encrypted_metadata_false.pdf")
+	if err != nil {
+		t.Fatalf("Open: %v", err)
+	}
+	defer f.Close()
+
+	b, err := r.GetPlainText(context.Background())
+	if err != nil {
+		t.Fatalf("GetPlainText: %v", err)
+	}
+	text, err := io.ReadAll(b)
+	if err != nil {
+		t.Fatalf("ReadAll: %v", err)
+	}
+	if !strings.Contains(string(text), "hello world") {
+		t.Fatalf("unexpected text: %q", text)
+	}
+}