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

Question

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 1

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()

Answer 2

You should try this Python-ASN1 encoder and decoder . Works for Python 2.6+ and 3.3+. Short example on page:

https://pypi.org/project/asn1/

Make sure to install pip install future before pip install asn1

Answer 3

PyOpenSSL is being deprecated, so might want to consider using the cryptography module instead

import cryptography
der = open('...', 'rb').read()
cert = cryptography.x509.load_der_x509_certificate(der)
cert.serial_number