Create a self-signed x509 certificate with python cryptography library

selfsigned.py

# Copyright 2018 Simon Davy
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def generate_selfsigned_cert(hostname, public_ip, private_ip):
    from cryptography import x509
    from cryptography.x509.oid import NameOID
    from cryptography.hazmat.primitives import hashes
    from cryptography.hazmat.backends import default_backend
    from cryptography.hazmat.primitives import serialization
    from cryptography.hazmat.primitives.asymmetric import rsa
    
    # Generate our key
    key = rsa.generate_private_key(
        public_exponent=65537,
        key_size=1024,
        backend=default_backend()
    )
    
    name = x509.Name([
        x509.NameAttribute(NameOID.COMMON_NAME, hostname)
    ])
    alt_names = x509.SubjectAlternativeName([
        # best practice seem to be to include the hostname in the SAN, which *SHOULD* mean COMMON_NAME is ignored.
        x509.DNSName(hostname),
        # allow addressing by IP, for when you don't have real DNS (common in most testing scenarios)
        # openssl wants DNSnames for ips...
        x509.DNSName(public_ip),
        x509.DNSName(private_ip),
        # ... whereas golang's crypto/tls is stricter, and needs IPAddresses 
        x509.IPAddress(public_ip),
        x509.IPAddress(private_ip),
    ])
    # path_len=0 means this cert can only sign itself, not other certs.
    basic_contraints = x509.BasicConstraints(ca=True, path_length=0)
    now = datetime.utcnow()
    cert = (
        x509.CertificateBuilder()
        .subject_name(name)
        .issuer_name(name)
        .public_key(key.public_key())
        .serial_number(1000)
        .not_valid_before(now)
        .not_valid_after(now + timedelta(days=10*365))
        .add_extension(basic_contraints, False)
        .add_extension(alt_names, False)
        .sign(key, hashes.SHA256(), default_backend())
    )
    cert_pem = cert.public_bytes(encoding=serialization.Encoding.PEM)
    key_pem = key.private_bytes(
        encoding=serialization.Encoding.PEM,
        format=serialization.PrivateFormat.TraditionalOpenSSL,
        encryption_algorithm=serialization.NoEncryption(),
    )

    return cert_pem, key_pem