Stamping with interrupted signing

[spapas]

Hello, I want to use interrupted signing and also want to stamp my file. I have created something similar to this:

import base64
from pyhanko.pdf_utils.incremental_writer import IncrementalPdfFileWriter
import os
from asyncio import run
from hashlib import sha256

import pyhanko
import requests
from asn1crypto import algos, x509
from cryptography.hazmat.primitives import hashes
from django.conf import settings
from pyhanko import stamp
from pyhanko.pdf_utils import images, text
from pyhanko.pdf_utils.font import opentype
from pyhanko.pdf_utils.writer import BasePdfFileWriter
from pyhanko.sign import Signer, signers, timestamps
from pyhanko.sign.signers.pdf_cms import ExternalSigner
from pyhanko_certvalidator.registry import SimpleCertificateStore



def prep_document(ww: BasePdfFileWriter, cert, chain):
    pdf_signer = signers.PdfSigner(
        signature_meta=signers.PdfSignatureMetadata(field_name="Signature", md_algorithm="sha256"),
        signer=signers.ExternalSigner(
            signing_cert=cert,
            cert_registry=SimpleCertificateStore.from_certs(chain),
            signature_value=bytes(256),
        ),
        timestamper=timestamps.HTTPTimeStamper("http://pki.byte.gr/tsa/server/basic"),
        stamp_style=stamp.TextStampStyle(
            border_width=4,
            background_layout=pyhanko.pdf_utils.layout.SimpleBoxLayoutRule(
                margins=pyhanko.pdf_utils.layout.Margins(),
                x_align=pyhanko.pdf_utils.layout.AxisAlignment(2),
                y_align=pyhanko.pdf_utils.layout.AxisAlignment(2),
            ),
            stamp_text="Υπογραφή από: %(signer)s\nΧρόνος υπογραφής: %(ts)s",
            background=images.PdfImage(os.path.join(settings.BASE_DIR, "etc", "logo.png")),
            text_box_style=text.TextBoxStyle(
                font=opentype.GlyphAccumulatorFactory(os.path.join(settings.BASE_DIR, "etc", "GFSDidotBold.ttf"))
            ),
            background_opacity=0.1,
        ),
    )

    prep_digest, tbs_document, output_handle = pdf_signer.digest_doc_for_signing(ww)

    psi = tbs_document.post_sign_instructions

    ext_signer = ExternalSigner(signing_cert=cert, cert_registry=SimpleCertificateStore.from_certs(chain))
    signed_attrs = run(
        ext_signer.signed_attrs(
            prep_digest.document_digest,
            "sha256",
        )
    )

    return prep_digest, signed_attrs, psi, output_handle


def finish_signing(sig_value: bytes, prep_digest, signed_attrs, psi, output_handle, cert, chain):  # noqa
    timestamper = timestamps.HTTPTimeStamper("http://pki.byte.gr/tsa/server/basic")
    ext_signer = ExternalSigner(
        signing_cert=cert, cert_registry=SimpleCertificateStore.from_certs(chain), signature_value=sig_value
    )
    sig_cms = ext_signer.sign_prescribed_attributes("sha256", signed_attrs=signed_attrs, timestamper=timestamper)

    prep_digest.fill_with_cms(output_handle, sig_cms)


def multi_sign(username, password):
    chain = retrieve_certs(username, password)
    cert = chain[0]

    with open("a.pdf", "rb") as inf:
        ww = IncrementalPdfFileWriter(inf, strict=False)
        prep_digest, signed_attrs, psi, output_handle = prep_document(ww, cert, chain)
        signature = get_signature(prep_digest)
        finish_signing(signature, prep_digest, signed_attrs, psi, output_handle, cert, chain)
        with open("b.pdf", "wb") as fou:
            fou.write(output_handle.getvalue())

Although the sign field is created in the resulting pdf it is invisible, so nothing is displayed on the PDF! I'd like to see the stamp (see my prep_document) Can you please help?

Thank you

[MatthiasValvekens]

I don't see you defining where on the page the signature needs to go. In order to create visible signatures, you need to set up an empty signature field with coordinates before passing the writer off to PdfSigner (see https://pyhanko.readthedocs.io/en/latest/lib-guide/sig-fields.html#general-api-design; see in particular box and on_page). Or work with a file that already includes a signature field.

[spapas]

Hello @MatthiasValvekens thanks for the help! I added this to the top of my prep_document function and it displays the signature now!

    fields.append_signature_field(
        ww, sig_field_spec=fields.SigFieldSpec(
            'Signature', box=(0, 0, 100, 50)
        )
    )