AttributeError: 'builtin_function_or_method' object has no attribute 'algorithm'

[Akhil-Turnkey]

Hey everyone, I am a relatively new developer and this is the first time I am working with something like PDF signings.

I use a few bash commands to generate a .pfx file of my own for developing & testing purposes.

I am encountering an AttributeError: 'builtin_function_or_method' object has no attribute 'algorithm'
Chatgpt explained to me that this error is being caused because the public key in my .pfx file does not have an 'algorithm' attribute but it further goes to say that since the key or hashing is an RSA algorithm, the pyhanko library handles that automatically.

I have been hard stuck on this for 2 days and I am not able to progress further. I hope someone on this forum can help me.
I have attached my entire code and the stack trace as well.
Thanks in advance.

from pyhanko.sign import PdfSigner
from pyhanko.sign.fields import SigFieldSpec
from pyhanko.sign.signers import SimpleSigner
from pyhanko.sign.signers.pdf_signer import PdfSignatureMetadata
from cryptography.hazmat.primitives.asymmetric import padding
from cryptography.hazmat.backends import default_backend
from cryptography.hazmat.primitives.serialization import pkcs12


def sign_pdf(input_pdf_path, signed_pdf_path, private_key, cert):
    # Create a SimpleSigner object
    simple_signer = SimpleSigner(
        signing_cert=cert, signing_key=private_key, cert_registry=None
    )

    # Create PdfSignatureMetadata object
    sig_meta = PdfSignatureMetadata(
        field_name="Signature1",  
        reason="Document signed",  
        location="Hyderabad",  
        contact_info="you@example.com",  
        name="A Y",  
    )

    # Create PdfSigner instance
    signer = PdfSigner(
        signature_meta=sig_meta,
        signer=simple_signer,
    )

    # Sign the PDF
    with open(input_pdf_path, "rb") as doc:
        with open(signed_pdf_path, "wb") as signed_doc:
            signer.sign_pdf(
                doc,
                signed_doc,
                SigFieldSpec(sig_field_name="Signature1"),
                simple_signer,
            )


from pyhanko.sign.validation import validate_pdf_signature


def verify_signature(pdf_path):
    with open(pdf_path, "rb") as f:
        validation_result = validate_pdf_signature(f)

    if validation_result.intact and validation_result.valid:
        print("Signature is valid.")
    else:
        print(f"Signature is invalid. Reason: {validation_result.summary()}")


input_pdf_path = r"C:\Users\local\eSignService\test.pdf"
signed_pdf_path = r"C:\Users\local\eSignService\signed_test.pdf"
dsc_pfx_path = r"C:\Users\local\eSignService\certificate.pfx"
pfx_password = "Password"


def load_dsc(pfx_path, password):
    with open(pfx_path, "rb") as f:
        pfx_data = f.read()
    private_key, cert, additional_certs = pkcs12.load_key_and_certificates(
        pfx_data, password.encode(), backend=default_backend()
    )
    return private_key, cert


private_key, cert = load_dsc(dsc_pfx_path, pfx_password)

sign_pdf(input_pdf_path, signed_pdf_path, private_key, cert)
verify_signature(signed_pdf_path)

Traceback (most recent call last):
  File "C:\Users\local\eSignService\checkpoint.py", line 72, in <module>
    sign_pdf(input_pdf_path, signed_pdf_path, private_key, cert)
  File "C:\Users\local\eSignService\checkpoint.py", line 26, in sign_pdf
    signer = PdfSigner(
             ^^^^^^^^^^
  File "C:\Users\local\eSignService\myenv\Lib\site-packages\pyhanko\sign\signers\pdf_signer.py", line 1078, in __init__
    mech = self.signer.get_signature_mechanism_for_digest(None)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\local\eSignService\myenv\Lib\site-packages\pyhanko\sign\signers\pdf_cms.py", line 407, in get_signature_mechanism_for_digest
    algo = self.signing_cert.public_key.algorithm
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'builtin_function_or_method' object has no attribute 'algorithm'

[stollcri]

I'm not sure this exactly applies to your case, but maybe this will point you in a good direction. There are different implementations of certificate objects, each with their own structures. In my case I needed to convert cryptography x509 certs/keys into asn1crypto x509 certs/keys

from asn1crypto import pem
from asn1crypto import x509 as asn1x509
from pyhanko.keys import load_private_key_from_pemder_data

if pem.detect(self.signing_crt):
    _, _, self.signing_crt = pem.unarmor(self.signing_crt)
asn1_crt = asn1x509.Certificate.load(self.signing_crt)
asn1_key = load_private_key_from_pemder_data(self.signing_key, None)