LibPDF: Treat Encyption's Length item as optional

With the StandardSecurityHandler the Length item in the Encryption
dictionary is optional, and needs to be given only if the encryption
algorithm (V) is other than 1; otherwise we can assume a length of 40
bits for the encryption key.

Userland/Libraries/LibPDF/CommonNames.h

@@ -134,6 +134,7 @@
     A(UCR)                        \
     A(UseBlackPTComp)             \
     A(UserUnit)                   \
+    A(V)                          \
     A(W)                          \
     A(WhitePoint)                 \
     A(Width)                      \

Userland/Libraries/LibPDF/Encryption.cpp

@@ -63,7 +63,21 @@ PDFErrorOr<NonnullRefPtr<StandardSecurityHandler>> StandardSecurityHandler::crea
     auto o = TRY(encryption_dict->get_string(document, CommonNames::O))->string();
     auto u = TRY(encryption_dict->get_string(document, CommonNames::U))->string();
     auto p = encryption_dict->get_value(CommonNames::P).get<int>();
-    auto length = encryption_dict->get_value(CommonNames::Length).get<int>() / 8;
+
+    // V, number: [...] 1 "Algorithm 1 Encryption of data using the RC4 or AES algorithms" in 7.6.2,
+    // "General Encryption Algorithm," with an encryption key length of 40 bits, see below [...]
+    // Lenght, integer: (Optional; PDF 1.4; only if V is 2 or 3) The length of the encryption key, in bits.
+    // The value shall be a multiple of 8, in the range 40 to 128. Default value: 40.
+    int length_in_bits;
+    auto v = encryption_dict->get_value(CommonNames::V).get<int>();
+    if (encryption_dict->contains(CommonNames::Length))
+        length_in_bits = encryption_dict->get_value(CommonNames::Length).get<int>();
+    else if (v == 1)
+        length_in_bits = 40;
+    else
+        return Error(Error::Type::Parse, "Can't determine length of encryption key");
+    auto length = length_in_bits / 8;
+
     bool encrypt_metadata = true;
     if (encryption_dict->contains(CommonNames::EncryptMetadata))
         encryption_dict->get_value(CommonNames::EncryptMetadata).get<bool>();