How do I read(open) an ASN.1 file in python

I want to get a certificates serial number using python:

der = open('/Users/me/MyApp/Payload/codesign0').read()```
cert = OpenSSL.crypto.load_certificate(OpenSSL.crypto.FILETYPE_ASN1, der
cert.get_serial_number()

Unfortunately it fails in the first line:

UnicodeDecodeError: 'utf-8' codec can't decode byte 0x82 in position 1: invalid start byte

How do I read an ASN.1 file format (DER) in Python?

Answer

You are opening the file as a text file, which means read tries to decode the data using UTF-8 in order to return a str object.

Instead, open it as a binary file, so that read simply returns a bytes object without trying to decode the data at all.

 der = open('...', 'rb').read()