How to ignore page rotation while signing pdf using pyHanko?

[ihegde]

hi, I'm trying to use pyHanko to add a visible signature to a PDF document. One of the documents has its page rotated by 90.

I'm using the following code from the samples. I don't need background, just plain text.
The problem is, the page is rotated ( but it's visually alright, they've just done it to get more horizontal space) But the signature is appearing vertical! Is there any way I can ask pyHanko to sign straight ignoring the page rotation?

fields.append_signature_field(
    w, sig_field_spec=fields.SigFieldSpec(
        'Signature', box=(330, 229, 530, 289)  # width 200 , height 60
    )
)

meta = signers.PdfSignatureMetadata(field_name='Signature')
pdf_signer = signers.PdfSigner(
    meta, signer=signer, stamp_style=stamp.TextStampStyle(
        # the 'signer' and 'ts' parameters will be interpolated by pyHanko, if present
        stamp_text='Digitally signed by\n%(signer)s\nTime: %(ts)s',
        text_box_style=text.TextBoxStyle(
            font=opentype.GlyphAccumulatorFactory('NotoSans-Regular.ttf')
        ),
        #background=images.PdfImage('stamp.png')
    ),
)

[MatthiasValvekens]

Hi @ihegde,

Per the PDF standard, the default behaviour of the Rotate entry in a page dictionary is to rotate all annotations along with the page content. That also applies to (visible) signature fields, and I'm pretty sure that's the cause of the behaviour you're seeing.

The behaviour you want can be achieved by setting the NoRotate flag on the annotation, but unfortunately there's no convenient API to set that bit for you in pyHanko. It's not hard to add an option to SigFieldSpec to toggle that one, though. I'll see what I can do about that and get back to you.

In the meantime, you can work around the issue as follows:

• Set up a signature field using pyhanko.sign.fields.append_signature_field(...), in much the same way as you do now.
• Change the field's annotation flags manually (see snippet below for an example)
• Sign.

Flag snippet:

flags = w.root['/AcroForm']['/Fields'][0]['/F']   # assuming there's only one field
w.root['/AcroForm']['/Fields'][0]['/F'] = generic.NumberObject(flags | 0b10000)

(warning: untested code)

Hope that helps.

[ihegde]

Oh! does it mean it's not a hack after all and that's the only way? Adding multiple signatures using adobe reader works ok,
though we need to save it multiple times:

1. Sign the document using first signature, save as signed1.pdf
2. Open the Signed1.pdf, sign with second signature and save as Signed2.pdf
   I could do the same steps programmatically, hope it will not make the first signature invalid. That's how I'm adding signatures to multiple placeholders in the same document currently.

[MatthiasValvekens]

Oh! does it mean it's not a hack after all and that's the only way?

The "hack" I mentioned further up in the thread was about embedding the rotation in the page content stream (thus removing the need to worry about /Rotate going forward). That will always invalidate existing signatures. This workaround is different: it rotates the signature appearance only. That's fine and shouldn't cause any issues with prior signatures.

Open the Signed1.pdf, sign with second signature and save as Signed2.pdf. I could do the same steps programmatically, hope it will not make the first signature invalid. That's how I'm adding signatures to multiple placeholders in the same document currently.

Yeah, that's expected and independent of the whole visual appearance/rotation discussion. PDF signatures are always serial like that. Here's some background from the docs (well, there are many explanations out there on the internet, you don't have to take my word for it :) )

[ihegde]

Please pardon my ignorance on PDF intricacies. When I saw cm with matrix I thought both are the same. I'll study some more and understand. I was totally new to PDF when I started a couple of weeks ago. Thanks for your guidance and pointers without which I wouldn't have come this far :)

[MatthiasValvekens]

No problem, I should've been more clear in my explanation :).

[ihegde]

Hi @MatthiasValvekens,
Update on this: I was finally able to get the appearance of signature working. Since I was not able to find a generic solution, I have hardcoded cm and Td in a couple of places. I have limited variations of font sizes, box sizes, and also rotation (0 and 90) so works for my requirements.
Thanks!