Tutorial
========

X.509 certificates are used to authenticate clients on servers. The most common
use case is for web servers using HTTPS.

Creating a Certificate Signing Request (CSR)
--------------------------------------------

When obtaining a certificate from a certificate authority (CA), the usual
flow is:

1. You generate a private/public key pair.
2. You create a request for a certificate, which is signed by your key (to
   prove that you own that key).
3. You give your CSR to a CA (but *not* the private key).
4. The CA validates that you own the resource (e.g. domain) you want a
   certificate for.
5. The CA gives you a certificate, signed by them, which identifies your public
   key, and the resource you are authenticated for.
6. You configure your server to use that certificate, combined with your
   private key, to server traffic.

If you want to obtain a certificate from a typical commercial CA, here's how.
First, you'll need to generate a private key, we'll generate an RSA key (these
are the most common types of keys on the web right now):

.. code-block:: pycon

    >>> 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=2048,
    ...     backend=default_backend()
    ... )
    >>> # Write our key to disk for safe keeping
    >>> with open("path/to/store/key.pem", "wb") as f:
    ...     f.write(key.private_bytes(
    ...         encoding=serialization.Encoding.PEM,
    ...         format=serialization.PrivateFormat.TraditionalOpenSSL,
    ...         encryption_algorithm=serialization.BestAvailableEncryption(b"passphrase"),
    ...     ))

If you've already generated a key you can load it with
:func:`~cryptography.hazmat.primitives.serialization.load_pem_private_key`.

Next we need to generate a certificate signing request. A typical CSR contains
a few details:

* Information about our public key (including a signature of the entire body).
* Information about who *we* are.
* Information about what domains this certificate is for.

.. code-block:: pycon

    >>> from cryptography import x509
    >>> from cryptography.x509.oid import NameOID
    >>> from cryptography.hazmat.primitives import hashes
    >>> # Generate a CSR
    >>> csr = x509.CertificateSigningRequestBuilder().subject_name(x509.Name([
    ...     # Provide various details about who we are.
    ...     x509.NameAttribute(NameOID.COUNTRY_NAME, u"US"),
    ...     x509.NameAttribute(NameOID.STATE_OR_PROVINCE_NAME, u"CA"),
    ...     x509.NameAttribute(NameOID.LOCALITY_NAME, u"San Francisco"),
    ...     x509.NameAttribute(NameOID.ORGANIZATION_NAME, u"My Company"),
    ...     x509.NameAttribute(NameOID.COMMON_NAME, u"mysite.com"),
    ... ])).add_extension(
    ...     x509.SubjectAlternativeName([
    ...         # Describe what sites we want this certificate for.
    ...         x509.DNSName(u"mysite.com"),
    ...         x509.DNSName(u"www.mysite.com"),
    ...         x509.DNSName(u"subdomain.mysite.com"),
    ...     ]),
    ...     critical=False,
    ... # Sign the CSR with our private key.
    ... ).sign(key, hashes.SHA256(), default_backend())
    >>> # Write our CSR out to disk.
    >>> with open("path/to/csr.pem", "wb") as f:
    ...     f.write(csr.public_bytes(serialization.Encoding.PEM))

Now we can give our CSR to a CA, who will give a certificate to us in return.
