ssl.rst

:mod:`ssl` --- SSL wrapper for socket objects
====================================================================

.. module:: ssl
   :synopsis: SSL wrapper for socket objects

.. moduleauthor:: Bill Janssen <bill.janssen@gmail.com>
.. sectionauthor::  Bill Janssen <bill.janssen@gmail.com>


.. index:: single: OpenSSL; (use in module ssl)

.. index:: TLS, SSL, Transport Layer Security, Secure Sockets Layer

This module provides access to Transport Layer Security (often known
as "Secure Sockets Layer") encryption and peer authentication
facilities for network sockets, both client-side and server-side.
This module uses the OpenSSL library. It is available on all modern
Unix systems, Windows, Mac OS X, and probably additional
platforms, as long as OpenSSL is installed on that platform.

.. note::

   Some behavior may be platform dependent, since calls are made to the operating
   system socket APIs.  The installed version of OpenSSL may also cause
   variations in behavior.

This section documents the objects and functions in the ``ssl`` module;
for more general information about TLS, SSL, and certificates, the
reader is referred to the documents in the "See Also" section at
the bottom.

This module provides a class, :class:`ssl.SSLSocket`, which is
derived from the :class:`socket.socket` type, and provides
a socket-like wrapper that also encrypts and decrypts the data
going over the socket with SSL.  It supports additional
:meth:`read` and :meth:`write` methods, along with a method, :meth:`getpeercert`,
to retrieve the certificate of the other side of the connection, and
a method, :meth:`cipher`, to retrieve the cipher being used for the
secure connection.

Functions, Constants, and Exceptions
------------------------------------

.. exception:: SSLError

   Raised to signal an error from the underlying SSL implementation.  This 
   signifies some problem in the higher-level
   encryption and authentication layer that's superimposed on the underlying
   network connection.  This error is a subtype of :exc:`socket.error`, which
   in turn is a subtype of :exc:`IOError`.

.. function:: wrap_socket (sock, keyfile=None, certfile=None, server_side=False, cert_reqs=CERT_NONE, ssl_version={see docs}, ca_certs=None, do_handshake_on_connect=True, suppress_ragged_eofs=True)

   Takes an instance ``sock`` of :class:`socket.socket`, and returns an instance of :class:`ssl.SSLSocket`, a subtype
   of :class:`socket.socket`, which wraps the underlying socket in an SSL context.
   For client-side sockets, the context construction is lazy; if the underlying socket isn't
   connected yet, the context construction will be performed after :meth:`connect` is called
   on the socket.  For server-side sockets, if the socket has no remote peer, it is assumed
   to be a listening socket, and the server-side SSL wrapping is automatically performed
   on client connections accepted via the :meth:`accept` method.  :func:`wrap_socket` may
   raise :exc:`SSLError`.

   The ``keyfile`` and ``certfile`` parameters specify optional files which contain a certificate
   to be used to identify the local side of the connection.  See the discussion of :ref:`ssl-certificates`
   for more information on how the certificate is stored in the ``certfile``.

   Often the private key is stored
   in the same file as the certificate; in this case, only the ``certfile`` parameter need be
   passed.  If the private key is stored in a separate file, both parameters must be used.
   If the private key is stored in the ``certfile``, it should come before the first certificate
   in the certificate chain::

      -----BEGIN RSA PRIVATE KEY-----
      ... (private key in base64 encoding) ...
      -----END RSA PRIVATE KEY-----
      -----BEGIN CERTIFICATE-----
      ... (certificate in base64 PEM encoding) ...
      -----END CERTIFICATE-----

   The parameter ``server_side`` is a boolean which identifies whether server-side or client-side
   behavior is desired from this socket.

   The parameter ``cert_reqs`` specifies whether a certificate is
   required from the other side of the connection, and whether it will
   be validated if provided.  It must be one of the three values
   :const:`CERT_NONE` (certificates ignored), :const:`CERT_OPTIONAL` (not required,
   but validated if provided), or :const:`CERT_REQUIRED` (required and
   validated).  If the value of this parameter is not :const:`CERT_NONE`, then
   the ``ca_certs`` parameter must point to a file of CA certificates.

   The ``ca_certs`` file contains a set of concatenated "certification authority" certificates,
   which are used to validate certificates passed from the other end of the connection.
   See the discussion of :ref:`ssl-certificates` for more information about how to arrange
   the certificates in this file.

   The parameter ``ssl_version`` specifies which version of the SSL protocol to use.
   Typically, the server chooses a particular protocol version, and the client
   must adapt to the server's choice.  Most of the versions are not interoperable
   with the other versions.  If not specified, for client-side operation, the
   default SSL version is SSLv3; for server-side operation, SSLv23.  These
   version selections provide the most compatibility with other versions.

   Here's a table showing which versions in a client (down the side)
   can connect to which versions in a server (along the top):

     .. table::

       ========================  =========  =========  ==========  =========
        *client* / **server**    **SSLv2**  **SSLv3**  **SSLv23**  **TLSv1**
       ------------------------  ---------  ---------  ----------  ---------
        *SSLv2*                    yes        no         yes*        no
        *SSLv3*                    yes        yes        yes         no
        *SSLv23*                   yes        no         yes         no
        *TLSv1*                    no         no         yes         yes
       ========================  =========  =========  ==========  =========

   In some older versions of OpenSSL (for instance, 0.9.7l on OS X 10.4),
   an SSLv2 client could not connect to an SSLv23 server.

   The parameter ``do_handshake_on_connect`` specifies whether to do the SSL
   handshake automatically after doing a :meth:`socket.connect`, or whether the
   application program will call it explicitly, by invoking the :meth:`SSLSocket.do_handshake`
   method.  Calling :meth:`SSLSocket.do_handshake` explicitly gives the program control over
   the blocking behavior of the socket I/O involved in the handshake.

   The parameter ``suppress_ragged_eofs`` specifies how the :meth:`SSLSocket.read`
   method should signal unexpected EOF from the other end of the connection.  If specified
   as :const:`True` (the default), it returns a normal EOF in response to unexpected
   EOF errors raised from the underlying socket; if :const:`False`, it will raise
   the exceptions back the caller.

.. function:: RAND_status()

   Returns True if the SSL pseudo-random number generator has been
   seeded with 'enough' randomness, and False otherwise.  You can use
   :func:`ssl.RAND_egd` and :func:`ssl.RAND_add` to increase the randomness
   of the pseudo-random number generator.

.. function:: RAND_egd(path)

   If you are running an entropy-gathering daemon (EGD) somewhere, and ``path``
   is the pathname of a socket connection open to it, this will read
   256 bytes of randomness from the socket, and add it to the SSL pseudo-random number generator
   to increase the security of generated secret keys.  This is typically only
   necessary on systems without better sources of randomness.

   See http://egd.sourceforge.net/ or http://prngd.sourceforge.net/ for
   sources of entropy-gathering daemons.

.. function:: RAND_add(bytes, entropy)

   Mixes the given ``bytes`` into the SSL pseudo-random number generator.
   The parameter ``entropy`` (a float) is a lower bound on the entropy
   contained in string (so you can always use :const:`0.0`).
   See :rfc:`1750` for more information on sources of entropy.

.. function:: cert_time_to_seconds(timestring)

   Returns a floating-point value containing a normal seconds-after-the-epoch time
   value, given the time-string representing the "notBefore" or "notAfter" date
   from a certificate.

   Here's an example::

     >>> import ssl
     >>> ssl.cert_time_to_seconds("May  9 00:00:00 2007 GMT")
     1178694000.0
     >>> import time
     >>> time.ctime(ssl.cert_time_to_seconds("May  9 00:00:00 2007 GMT"))
     'Wed May  9 00:00:00 2007'
     >>> 

.. function:: get_server_certificate (addr, ssl_version=PROTOCOL_SSLv3, ca_certs=None)

   Given the address ``addr`` of an SSL-protected server, as a
   (*hostname*, *port-number*) pair, fetches the server's certificate,
   and returns it as a PEM-encoded string.  If ``ssl_version`` is
   specified, uses that version of the SSL protocol to attempt to
   connect to the server.  If ``ca_certs`` is specified, it should be
   a file containing a list of root certificates, the same format as
   used for the same parameter in :func:`wrap_socket`.  The call will
   attempt to validate the server certificate against that set of root
   certificates, and will fail if the validation attempt fails.

.. function:: DER_cert_to_PEM_cert (DER_cert_bytes)

   Given a certificate as a DER-encoded blob of bytes, returns a PEM-encoded
   string version of the same certificate.

.. function:: PEM_cert_to_DER_cert (PEM_cert_string)

   Given a certificate as an ASCII PEM string, returns a DER-encoded
   sequence of bytes for that same certificate.

.. data:: CERT_NONE

   Value to pass to the ``cert_reqs`` parameter to :func:`sslobject`
   when no certificates will be required or validated from the other
   side of the socket connection.

.. data:: CERT_OPTIONAL

   Value to pass to the ``cert_reqs`` parameter to :func:`sslobject`
   when no certificates will be required from the other side of the
   socket connection, but if they are provided, will be validated.
   Note that use of this setting requires a valid certificate
   validation file also be passed as a value of the ``ca_certs``
   parameter.

.. data:: CERT_REQUIRED

   Value to pass to the ``cert_reqs`` parameter to :func:`sslobject`
   when certificates will be required from the other side of the
   socket connection.  Note that use of this setting requires a valid certificate
   validation file also be passed as a value of the ``ca_certs``
   parameter.

.. data:: PROTOCOL_SSLv2

   Selects SSL version 2 as the channel encryption protocol.

.. data:: PROTOCOL_SSLv23

   Selects SSL version 2 or 3 as the channel encryption protocol.
   This is a setting to use with servers for maximum compatibility
   with the other end of an SSL connection, but it may cause the
   specific ciphers chosen for the encryption to be of fairly low
   quality.

.. data:: PROTOCOL_SSLv3

   Selects SSL version 3 as the channel encryption protocol.
   For clients, this is the maximally compatible SSL variant.

.. data:: PROTOCOL_TLSv1

   Selects TLS version 1 as the channel encryption protocol.  This is
   the most modern version, and probably the best choice for maximum
   protection, if both sides can speak it.


SSLSocket Objects
-----------------

.. method:: SSLSocket.read(nbytes=1024, buffer=None)

   Reads up to ``nbytes`` bytes from the SSL-encrypted channel and returns them.
   If the ``buffer`` is specified, it will attempt to read into the buffer
   the minimum of the size of the buffer and ``nbytes``, if that is specified.
   If no buffer is specified, an immutable buffer is allocated and returned
   with the data read from the socket.

.. method:: SSLSocket.write(data)

   Writes the ``data`` to the other side of the connection, using the
   SSL channel to encrypt.  Returns the number of bytes written.

.. method:: SSLSocket.do_handshake()

   Performs the SSL setup handshake.  If the socket is non-blocking,
   this method may raise :exc:`SSLError` with the value of the exception
   instance's ``args[0]``
   being either :const:`SSL_ERROR_WANT_READ` or
   :const:`SSL_ERROR_WANT_WRITE`, and should be called again until
   it stops raising those exceptions.  Here's an example of how to do
   that::

        while True:
            try:
                sock.do_handshake()
                break
            except ssl.SSLError as err:
                if err.args[0] == ssl.SSL_ERROR_WANT_READ:
                    select.select([sock], [], [])
                elif err.args[0] == ssl.SSL_ERROR_WANT_WRITE:
                    select.select([], [sock], [])
                else:
                    raise

.. method:: SSLSocket.getpeercert(binary_form=False)

   If there is no certificate for the peer on the other end of the
   connection, returns ``None``.

   If the the parameter ``binary_form`` is :const:`False`, and a
   certificate was received from the peer, this method returns a
   :class:`dict` instance.  If the certificate was not validated, the
   dict is empty.  If the certificate was validated, it returns a dict
   with the keys ``subject`` (the principal for which the certificate
   was issued), and ``notAfter`` (the time after which the certificate
   should not be trusted).  The certificate was already validated, so
   the ``notBefore`` and ``issuer`` fields are not returned.  If a
   certificate contains an instance of the *Subject Alternative Name*
   extension (see :rfc:`3280`), there will also be a
   ``subjectAltName`` key in the dictionary.

   The "subject" field is a tuple containing the sequence of relative
   distinguished names (RDNs) given in the certificate's data
   structure for the principal, and each RDN is a sequence of
   name-value pairs::

      {'notAfter': 'Feb 16 16:54:50 2013 GMT',
       'subject': ((('countryName', u'US'),),
                   (('stateOrProvinceName', u'Delaware'),),
                   (('localityName', u'Wilmington'),),
                   (('organizationName', u'Python Software Foundation'),),
                   (('organizationalUnitName', u'SSL'),),
                   (('commonName', u'somemachine.python.org'),))}

   If the ``binary_form`` parameter is :const:`True`, and a
   certificate was provided, this method returns the DER-encoded form
   of the entire certificate as a sequence of bytes, or :const:`None` if the
   peer did not provide a certificate.  This return
   value is independent of validation; if validation was required
   (:const:`CERT_OPTIONAL` or :const:`CERT_REQUIRED`), it will have
   been validated, but if :const:`CERT_NONE` was used to establish the
   connection, the certificate, if present, will not have been validated.

.. method:: SSLSocket.cipher()

   Returns a three-value tuple containing the name of the cipher being
   used, the version of the SSL protocol that defines its use, and the
   number of secret bits being used.  If no connection has been
   established, returns ``None``.


.. index:: single: certificates

.. index:: single: X509 certificate

.. _ssl-certificates:

Certificates
------------

Certificates in general are part of a public-key / private-key system.  In this system, each *principal*,
(which may be a machine, or a person, or an organization) is assigned a unique two-part encryption key.
One part of the key is public, and is called the *public key*; the other part is kept secret, and is called
the *private key*.  The two parts are related, in that if you encrypt a message with one of the parts, you can
decrypt it with the other part, and **only** with the other part.

A certificate contains information about two principals.  It contains
the name of a *subject*, and the subject's public key.  It also
contains a statement by a second principal, the *issuer*, that the
subject is who he claims to be, and that this is indeed the subject's
public key.  The issuer's statement is signed with the issuer's
private key, which only the issuer knows.  However, anyone can verify
the issuer's statement by finding the issuer's public key, decrypting
the statement with it, and comparing it to the other information in
the certificate.  The certificate also contains information about the
time period over which it is valid.  This is expressed as two fields,
called "notBefore" and "notAfter".

In the Python use of certificates, a client or server
can use a certificate to prove who they are.  The other
side of a network connection can also be required to produce a certificate,
and that certificate can be validated to the satisfaction
of the client or server that requires such validation.
The connection attempt can be set to raise an exception if
the validation fails.  Validation is done
automatically, by the underlying OpenSSL framework; the
application need not concern itself with its mechanics.
But the application does usually need to provide
sets of certificates to allow this process to take place.

Python uses files to contain certificates.  They should be formatted
as "PEM" (see :rfc:`1422`), which is a base-64 encoded form wrapped
with a header line and a footer line::

      -----BEGIN CERTIFICATE-----
      ... (certificate in base64 PEM encoding) ...
      -----END CERTIFICATE-----

The Python files which contain certificates can contain a sequence
of certificates, sometimes called a *certificate chain*.  This chain
should start with the specific certificate for the principal who "is"
the client or server, and then the certificate for the issuer of that
certificate, and then the certificate for the issuer of *that* certificate,
and so on up the chain till you get to a certificate which is *self-signed*,
that is, a certificate which has the same subject and issuer, 
sometimes called a *root certificate*.  The certificates should just
be concatenated together in the certificate file.  For example, suppose
we had a three certificate chain, from our server certificate to the
certificate of the certification authority that signed our server certificate,
to the root certificate of the agency which issued the certification authority's
certificate::

      -----BEGIN CERTIFICATE-----
      ... (certificate for your server)...
      -----END CERTIFICATE-----
      -----BEGIN CERTIFICATE-----
      ... (the certificate for the CA)...
      -----END CERTIFICATE-----
      -----BEGIN CERTIFICATE-----
      ... (the root certificate for the CA's issuer)...
      -----END CERTIFICATE-----

If you are going to require validation of the other side of the connection's
certificate, you need to provide a "CA certs" file, filled with the certificate
chains for each issuer you are willing to trust.  Again, this file just
contains these chains concatenated together.  For validation, Python will
use the first chain it finds in the file which matches.
Some "standard" root certificates are available from various certification
authorities:
`CACert.org <http://www.cacert.org/index.php?id=3>`_,
`Thawte <http://www.thawte.com/roots/>`_,
`Verisign <http://www.verisign.com/support/roots.html>`_,
`Positive SSL <http://www.PositiveSSL.com/ssl-certificate-support/cert_installation/UTN-USERFirst-Hardware.crt>`_ (used by python.org),
`Equifax and GeoTrust <http://www.geotrust.com/resources/root_certificates/index.asp>`_.

In general, if you are using
SSL3 or TLS1, you don't need to put the full chain in your "CA certs" file;
you only need the root certificates, and the remote peer is supposed to
furnish the other certificates necessary to chain from its certificate to
a root certificate.
See :rfc:`4158` for more discussion of the way in which 
certification chains can be built.

If you are going to create a server that provides SSL-encrypted
connection services, you will need to acquire a certificate for that
service.  There are many ways of acquiring appropriate certificates,
such as buying one from a certification authority.  Another common 
practice is to generate a self-signed certificate.  The simplest
way to do this is with the OpenSSL package, using something like
the following::

  % openssl req -new -x509 -days 365 -nodes -out cert.pem -keyout cert.pem
  Generating a 1024 bit RSA private key
  .......++++++
  .............................++++++
  writing new private key to 'cert.pem'
  -----
  You are about to be asked to enter information that will be incorporated
  into your certificate request.
  What you are about to enter is what is called a Distinguished Name or a DN.
  There are quite a few fields but you can leave some blank
  For some fields there will be a default value,
  If you enter '.', the field will be left blank.
  -----
  Country Name (2 letter code) [AU]:US
  State or Province Name (full name) [Some-State]:MyState
  Locality Name (eg, city) []:Some City
  Organization Name (eg, company) [Internet Widgits Pty Ltd]:My Organization, Inc.
  Organizational Unit Name (eg, section) []:My Group
  Common Name (eg, YOUR name) []:myserver.mygroup.myorganization.com
  Email Address []:ops@myserver.mygroup.myorganization.com
  %

The disadvantage of a self-signed certificate is that it is its
own root certificate, and no one else will have it in their cache
of known (and trusted) root certificates.


Examples
--------

Testing for SSL support
^^^^^^^^^^^^^^^^^^^^^^^

To test for the presence of SSL support in a Python installation, user code should use the following idiom::

   try:
      import ssl
   except ImportError:
      pass
   else:
      [ do something that requires SSL support ]

Client-side operation
^^^^^^^^^^^^^^^^^^^^^

This example connects to an SSL server, prints the server's address and certificate,
sends some bytes, and reads part of the response::

   import socket, ssl, pprint

   s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

   # require a certificate from the server
   ssl_sock = ssl.wrap_socket(s,
                              ca_certs="/etc/ca_certs_file",
                              cert_reqs=ssl.CERT_REQUIRED)

   ssl_sock.connect(('www.verisign.com', 443))

   print(repr(ssl_sock.getpeername()))
   pprint.pprint(ssl_sock.getpeercert())
   print(pprint.pformat(ssl_sock.getpeercert()))

   # Set a simple HTTP request -- use httplib in actual code.
   ssl_sock.write("""GET / HTTP/1.0\r
   Host: www.verisign.com\r\n\r\n""")

   # Read a chunk of data.  Will not necessarily
   # read all the data returned by the server.
   data = ssl_sock.read()

   # note that closing the SSLSocket will also close the underlying socket
   ssl_sock.close()

As of September 6, 2007, the certificate printed by this program
looked like this::

      {'notAfter': 'May  8 23:59:59 2009 GMT',
       'subject': ((('serialNumber', u'2497886'),),
                   (('1.3.6.1.4.1.311.60.2.1.3', u'US'),),
                   (('1.3.6.1.4.1.311.60.2.1.2', u'Delaware'),),
                   (('countryName', u'US'),),
                   (('postalCode', u'94043'),),
                   (('stateOrProvinceName', u'California'),),
                   (('localityName', u'Mountain View'),),
                   (('streetAddress', u'487 East Middlefield Road'),),
                   (('organizationName', u'VeriSign, Inc.'),),
                   (('organizationalUnitName',
                     u'Production Security Services'),),
                   (('organizationalUnitName',
                     u'Terms of use at www.verisign.com/rpa (c)06'),),
                   (('commonName', u'www.verisign.com'),))}

which is a fairly poorly-formed ``subject`` field.

Server-side operation
^^^^^^^^^^^^^^^^^^^^^

For server operation, typically you'd need to have a server certificate, and private key, each in a file.
You'd open a socket, bind it to a port, call :meth:`listen` on it, then start waiting for clients
to connect::

   import socket, ssl

   bindsocket = socket.socket()
   bindsocket.bind(('myaddr.mydomain.com', 10023))
   bindsocket.listen(5)

When one did, you'd call :meth:`accept` on the socket to get the new socket from the other
end, and use :func:`wrap_socket` to create a server-side SSL context for it::

   while True:
      newsocket, fromaddr = bindsocket.accept()
      connstream = ssl.wrap_socket(newsocket,
                                   server_side=True,
                                   certfile="mycertfile",
                                   keyfile="mykeyfile",
                                   ssl_version=ssl.PROTOCOL_TLSv1)
      deal_with_client(connstream)

Then you'd read data from the ``connstream`` and do something with it till you are finished with the client (or the client is finished with you)::

   def deal_with_client(connstream):

      data = connstream.read()
      # null data means the client is finished with us
      while data:
         if not do_something(connstream, data):
            # we'll assume do_something returns False
            # when we're finished with client
            break
         data = connstream.read()
      # finished with client
      connstream.close()

And go back to listening for new client connections.

           
.. seealso::

   Class :class:`socket.socket`
            Documentation of underlying :mod:`socket` class

   `Introducing SSL and Certificates using OpenSSL <http://old.pseudonym.org/ssl/wwwj-index.html>`_
       Frederick J. Hirsch

   `RFC 1422: Privacy Enhancement for Internet Electronic Mail: Part II: Certificate-Based Key Management <http://www.ietf.org/rfc/rfc1422>`_
       Steve Kent

   `RFC 1750: Randomness Recommendations for Security <http://www.ietf.org/rfc/rfc1750>`_
       D. Eastlake et. al.

   `RFC 3280: Internet X.509 Public Key Infrastructure Certificate and CRL Profile <http://www.ietf.org/rfc/rfc3280>`_
       Housley et. al.