[Bug]: File Attachments do not open when they are encrypted

[Aditi-1400]

Attach (recommended) or Link to PDF file

encryption.pdf

Web browser and its version

141.0 (aarch64)

Operating system and its version

MacOS 15.5

PDF.js version

v5.4.54

Is the bug present in the latest PDF.js version?

Yes

Is a browser extension

No

Steps to reproduce the problem

1. Open the attached PDF.
2. Enter the password 000000.
3. Try opening the attached file.

What is the expected behavior?

The attachment should open as expected.

What went wrong?

The attachments does not open.

Link to a viewer

No response

Additional context

No response

[wooorm]

Here’s some more research!

I made two smaller repros of this issue, with help from @Aditi-1400:

• The main PDF just shows “Example”, the attached attachment.pdf shows “Attachment” but is encrypted.
• Both have a password of 000000. The difference is that one has /AuthEvent /EFOpen\n, the other nothing.

Acrobat works with both. The difference is that it prompts for a password on-attachment-open vs on-main-open.

pdf.js works with neither. That is, it always prompts for password on-main-open, and it’s never possible to see the encrypted attachment (a Warning: Filter "Crypt" is not supported in the main window console, infinite load in the attachment window).

(Preview/QuickLook/Safari on macos are similar: always password prompt on-main-open, they don’t support attachments at all)

(Chrome does not prompt for a password, it just displays things, and it does not support attachments, so it indeed never needs the password)

So, output-no-auth-event.pdf is like encryption.pdf in the OP but smaller.
output-auth-event-ef-open.pdf is included to confirm that this is unrelated to /AuthEvent (GH-20140).

And, some relevant quotes from the PDF spec:

If passwords or access restrictions are specified, the document shall be encrypted […]. Documents in which only file attachments are encrypted shall use the same password as the user and owner password.

AuthEvent can also be EFOpen, which indicates the presence of an embedded file that is encrypted with a crypt filter that may be different from the crypt filters used by default to encrypt strings and streams in the document.

Test files:

• output-auth-event-ef-open.pdf
• output-no-auth-event.pdf

example.py

from pypdf import PdfReader, PdfWriter
from pypdf.generic import ArrayObject, DictionaryObject, NameObject, NullObject


def _prepend_crypt_filter(stream):
    crypt_params = DictionaryObject({NameObject("/Name"): NameObject("/StdCF")})
    old_filter = stream.get("/Filter")
    old_decode = stream.get("/DecodeParms")

    if old_filter is None:
        stream[NameObject("/Filter")] = ArrayObject([NameObject("/Crypt")])
        stream[NameObject("/DecodeParms")] = ArrayObject([crypt_params])
        return

    if isinstance(old_filter, ArrayObject):
        filters = ArrayObject([NameObject("/Crypt")] + list(old_filter))
        if isinstance(old_decode, ArrayObject):
            decode_parms = ArrayObject([crypt_params] + list(old_decode))
        elif old_decode is None:
            decode_parms = ArrayObject([crypt_params] + [NullObject()] * len(old_filter))
        else:
            decode_parms = ArrayObject([crypt_params, old_decode])
    else:
        filters = ArrayObject([NameObject("/Crypt"), old_filter])
        decode_parms = ArrayObject(
            [crypt_params, old_decode if old_decode is not None else NullObject()]
        )

    while len(decode_parms) < len(filters):
        decode_parms.append(NullObject())

    stream[NameObject("/Filter")] = filters
    stream[NameObject("/DecodeParms")] = decode_parms


reader = PdfReader("example.pdf")
writer = PdfWriter()
writer.append_pages_from_reader(reader)

with open("attachment.pdf", "rb") as f:
    writer.add_attachment("attachment.pdf", f.read())

try:
    writer.encrypt(
        user_password="000000",
        owner_password="000000",
        algorithm="AES-256",
    )
except TypeError:
    writer.encrypt(user_password="000000", owner_password="000000")

encryption = writer._encryption
encryption.StmF = "/Identity"
encryption.StrF = "/Identity"
encryption.EFF = "/AESV3"

encrypt_dict = writer._encrypt_entry
std_cf = encrypt_dict["/CF"]["/StdCF"]
std_cf[NameObject("/AuthEvent")] = NameObject("/EFOpen")
std_cf[NameObject("/CFM")] = NameObject("/AESV3")
encrypt_dict[NameObject("/EFF")] = NameObject("/StdCF")
encrypt_dict[NameObject("/StmF")] = NameObject("/Identity")
encrypt_dict[NameObject("/StrF")] = NameObject("/Identity")

attachment_stream = (
    writer._root_object["/Names"]["/EmbeddedFiles"]["/Names"][1]
    .get_object()["/EF"]["/F"]
    .get_object()
)
_prepend_crypt_filter(attachment_stream)
attachment_ref = attachment_stream.indirect_reference
crypt_filter = encryption._make_crypt_filter(
    attachment_ref.idnum, attachment_ref.generation
)
attachment_stream._data = crypt_filter.ef_crypt.encrypt(attachment_stream._data)

with open("output.pdf", "wb") as out:
    writer.write(out)