How do I create and sign certificates with Python's pyOpenSSL?

Question

I would like to use python to create a CA certificate, and client certificates that I sign with it. I will be using these with OpenVPN. After several days of research, and trial and error, this is what I've come up with:

#!/usr/bin/env python

import os
import sys
import random
from OpenSSL import crypto

###########
# CA Cert #
###########

ca_key = crypto.PKey()
ca_key.generate_key(crypto.TYPE_RSA, 2048)

ca_cert = crypto.X509()
ca_cert.set_version(2)
ca_cert.set_serial_number(random.randint(50000000,100000000))

ca_subj = ca_cert.get_subject()
ca_subj.commonName = "My CA"

ca_cert.add_extensions([
    crypto.X509Extension("subjectKeyIdentifier", False, "hash", subject=ca_cert),
])

ca_cert.add_extensions([
    crypto.X509Extension("authorityKeyIdentifier", False, "keyid:always", issuer=ca_cert),
])

ca_cert.add_extensions([
    crypto.X509Extension("basicConstraints", False, "CA:TRUE"),
    crypto.X509Extension("keyUsage", False, "keyCertSign, cRLSign"),
])

ca_cert.set_issuer(ca_subj)
ca_cert.set_pubkey(ca_key)
ca_cert.sign(ca_key, 'sha256')

ca_cert.gmtime_adj_notBefore(0)
ca_cert.gmtime_adj_notAfter(10*365*24*60*60)

# Save certificate
with open("ca.crt", "wt") as f:
    f.write(crypto.dump_certificate(crypto.FILETYPE_PEM, ca_cert))

# Save private key
with open("ca.key", "wt") as f:
    f.write(crypto.dump_privatekey(crypto.FILETYPE_PEM, ca_key))

###############
# Client Cert #
###############

client_key = crypto.PKey()
client_key.generate_key(crypto.TYPE_RSA, 2048)

client_cert = crypto.X509()
client_cert.set_version(2)
client_cert.set_serial_number(random.randint(50000000,100000000))

client_subj = client_cert.get_subject()
client_subj.commonName = "Client"

client_cert.add_extensions([
    crypto.X509Extension("basicConstraints", False, "CA:FALSE"),
    crypto.X509Extension("subjectKeyIdentifier", False, "hash", subject=client_cert),
])

client_cert.add_extensions([
    crypto.X509Extension("authorityKeyIdentifier", False, "keyid:always", issuer=ca_cert),
    crypto.X509Extension("extendedKeyUsage", False, "clientAuth"),
    crypto.X509Extension("keyUsage", False, "digitalSignature"),
])

client_cert.set_issuer(ca_subj)
client_cert.set_pubkey(client_key)
client_cert.sign(ca_key, 'sha256')

client_cert.gmtime_adj_notBefore(0)
client_cert.gmtime_adj_notAfter(10*365*24*60*60)

# Save certificate
with open("client.crt", "wt") as f:
    f.write(crypto.dump_certificate(crypto.FILETYPE_PEM, client_cert))

# Save private key
with open("client.key", "wt") as f:
    f.write(crypto.dump_privatekey(crypto.FILETYPE_PEM, client_key))

This generates the certificates and private keys, but unfortunately, I must be doing something wrong because they do not verify:

$ openssl verify -verbose -CAfile ca.crt client.crt
client.crt: CN = Client
error 7 at 0 depth lookup:certificate signature failure
139823049836448:error:04091068:rsa routines:INT_RSA_VERIFY:bad signature:rsa_sign.c:293:
139823049836448:error:0D0C5006:asn1 encoding routines:ASN1_item_verify:EVP lib:a_verify.c:241:

What am I doing wrong?

Answer 1

You are setting notBefore and notAfter after you already signed the certificate and thus change the already signed certificate - which makes the signature not match the certificate anymore:

client_cert.sign(ca_key, 'sha256')  

client_cert.gmtime_adj_notBefore(0)
client_cert.gmtime_adj_notAfter(10*365*24*60*60)

# Save certificate
...

If you move the signing part to be the last step, i.e. directly before writing the file, then the verification will be successful:

client_cert.gmtime_adj_notBefore(0)
client_cert.gmtime_adj_notAfter(10*365*24*60*60)

client_cert.sign(ca_key, 'sha256')  

# Save certificate
...