Extract public-key modulus from PrivateKey/CertificateRequest/Certificate in Python , with the same format as OpenSSL

Question

using OpenSSL, i can extract the modulus of the public key from various objects with these commands:

openssl rsa -noout -modulus -in {KEY}
openssl req -noout -modulus -in {CSR}
openssl x509 -noout -modulus -in {CERT}

I am trying to replicate this within python, using cryptography or the pyopenssl package.

I can extract the public key from all of these objects in Python, but I can not figure out how to encode the modulus to match the OpenSSL commandline output -- which appears to be a base64 encoded version of a format that I can't figure out from the docs or source code of any of the 3 projects.

Answer 1

The RSAPublicNumbers class (link) in cryptography has what you are looking for. You can get it using to_cryptography_key method (link) of PKey class in pyopenssl.

from OpenSSL.crypto import load_certificate
from OpenSSL.crypto import FILETYPE_PEM

with open(certfile, 'rb') as fp:
    cert = load_certificate(FILETYPE_PEM, fp.read())

# This gives you the modulus in integer form
modn = cert.get_pubkey().to_cryptography_key().public_numbers().n

# Convert it to hex
print('{:X}'.format(modn))