fix(crypt): R2–R4 passwords with non-ASCII characters open (PDFDocEncoding) (#905)

Revisions 2–4 of the standard security handler take the password in
PDFDocEncoding (ISO 32000-2 §7.6.4.3.2, Algorithm 2). PdfCraft authenticates
an encrypted file twice, and neither did that:
- pdfcraft-crypt took Latin-1 and dropped every other character, so "ı"
  (0x9A) and "€" (0xA0) vanished and "ılık€" became "lk";
- the renderer (vendored hayro-syntax, "TODO: Convert to PDFDocEncoding")
  used the UTF-8 bytes, so any non-ASCII password failed, even "Çok güzel",
  which Latin-1 and PDFDocEncoding write alike;
- the engine handed the renderer an owner-recovered user password as Latin-1
  characters, which the renderer then read as UTF-8 again.
Files that qpdf or Acrobat protected with such passwords could not be
opened with the right password.

- pdfcraft-crypt encodes R2–R4 passwords in PDFDocEncoding and, for a
  password with characters PDFDocEncoding lacks (ğ, ş, İ), also tries its
  UTF-8 bytes, which qpdf writes for those. The PDFDocEncoding table moves
  here from pdfcraft-cos (which depends on this crate) and cos uses it.
- hayro-syntax (vendored patch 14) tries an R2–R4 password as given, then
  in PDFDocEncoding.
- The engine passes a recovered user password to the renderer as UTF-8 when
  its bytes are UTF-8 and as PDFDocEncoding otherwise, so the renderer gets
  exactly those bytes back.

Co-authored-by: Brandon Thomas <[email protected]>

crates/engine/src/tests.rs

@@ -2817,3 +2817,39 @@ fn oversized_exports_clamp_and_report_their_dpi_unless_strict() {
     let err = export::Exporter::from_source(strict).png(0, 600.0).unwrap_err();
     assert!(err.contains("exceeds renderer limits"), "{err}");
 }
+
+#[test]
+fn legacy_passwords_with_turkish_characters_open() {
+    // RC4-128 (R3): user "ılık€" in PDFDocEncoding (ı = 0x9A, € = 0xA0), owner "Çağlar"
+    // (PDFDocEncoding has no ğ: written without it, as pdfcraft-crypt writes R2–R4 passwords).
+    let mut cos = pdfcraft_cos::Document::open(Arc::new(fixture(1))).unwrap();
+    let params = pdfcraft_cos::NewEncryption {
+        algorithm: pdfcraft_cos::Algorithm::Rc4_128,
+        user_password: "ılık€",
+        owner_password: "Çağlar",
+        permissions: -1,
+        encrypt_metadata: true,
+        seed: [7; 32],
+    };
+    cos.set_encryption(&params).unwrap();
+    let bytes = Arc::new(pdfcraft_cos::write_full(&cos, &pdfcraft_cos::SaveOptions::default()).unwrap());
+    let mut s = Session::new();
+    // The renderer used to try only the UTF-8 bytes, and the owner fallback handed it the
+    // recovered user password as Latin-1: both came back as a wrong password.
+    for pw in ["ılık€", "Çağlar"] {
+        let id = s.open("x.pdf", None, bytes.clone(), Some(pw)).unwrap_or_else(|e| panic!("{pw}: {e:?}"));
+        assert_eq!(page_texts(&s, id), ["Page 1"], "{pw}");
+        assert!(s.get(id).unwrap().info.encrypted, "{pw}");
+    }
+    assert!(matches!(s.open("x.pdf", None, bytes, Some("ilik€")), Err(OpenError::WrongPassword)));
+}
+
+#[test]
+fn recovered_user_passwords_reach_the_renderer_byte_for_byte() {
+    assert_eq!(crate::renderer_password(b"pw"), "pw");
+    // UTF-8 bytes (qpdf writes "şifre" so; its \xC5\x9F has no PDFDocEncoding reading).
+    assert_eq!(crate::renderer_password("şifre".as_bytes()), "şifre");
+    // PDFDocEncoding bytes: ı = 0x9A, € = 0xA0, Ç = 0xC7.
+    assert_eq!(crate::renderer_password(&[0x9A, b'l', 0x9A, b'k', 0xA0]), "ılık€");
+    assert_eq!(crate::renderer_password(&[0xC7, b'o', b'k']), "Çok");
+}