Verify that a private key matches a certificate with PyOpenSSL

Verify that a private key matches a certificate using PyOpenSSL and PyCrypto:

[code language="python"]
import OpenSSL.crypto
from Crypto.Util import asn1

c=OpenSSL.crypto

# The certificate - an X509 object
cert=...

# The private key - a PKey object
priv=...

pub=cert.get_pubkey()

# Only works for RSA (I think)
if pub.type()!=c.TYPE_RSA or priv.type()!=c.TYPE_RSA:
raise Exception('Can only handle RSA keys')

# This seems to work with public as well
pub_asn1=c.dump_privatekey(c.FILETYPE_ASN1, pub)
priv_asn1=c.dump_privatekey(c.FILETYPE_ASN1, priv)

# Decode DER
pub_der=asn1.DerSequence()
pub_der.decode(pub_asn1)
priv_der=asn1.DerSequence()
priv_der.decode(priv_asn1)

# Get the modulus
pub_modulus=pub_der[1]
priv_modulus=priv_der[1]

if pub_modulus==priv_modulus:
print('Match')
else:
print('Oops')
[/code]

The idea is to get the modulus from the two DER structures and compare them. They should be the same.

Note: You can use the above under the MIT license. If it doesn’t fit your needs let me know. My intention is to make this usable by anyone for any kind of use with no obligation.