
``ssl`` --- SSL wrapper for socket objects
******************************************

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, ``ssl.SSLSocket``, which is derived from
the ``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 ``read()`` and ``write()`` methods, along with
a method, ``getpeercert()``, to retrieve the certificate of the other
side of the connection, and a method, ``cipher()``, to retrieve the
cipher being used for the secure connection.


Functions, Constants, and Exceptions
====================================

exception exception ssl.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 ``socket.error``, which in
   turn is a subtype of ``IOError``.

ssl.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 ``socket.socket``, and returns an
   instance of ``ssl.SSLSocket``, a subtype of ``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 ``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
   ``accept()`` method.  ``wrap_socket()`` may raise ``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 *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
   ``CERT_NONE`` (certificates ignored), ``CERT_OPTIONAL`` (not
   required, but validated if provided), or ``CERT_REQUIRED``
   (required and validated).  If the value of this parameter is not
   ``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
   *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):

      +--------------------------+-----------+-----------+------------+-----------+
      | *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 ``socket.connect()``,
   or whether the application program will call it explicitly, by
   invoking the ``SSLSocket.do_handshake()`` method.  Calling
   ``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
   ``SSLSocket.read()`` method should signal unexpected EOF from the
   other end of the connection.  If specified as ``True`` (the
   default), it returns a normal EOF in response to unexpected EOF
   errors raised from the underlying socket; if ``False``, it will
   raise the exceptions back to the caller.

ssl.RAND_status()

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

ssl.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.

ssl.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 ``0.0``).
   See **RFC 1750** for more information on sources of entropy.

ssl.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'
      >>>

ssl.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 ``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.

ssl.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.

ssl.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.

ssl.CERT_NONE

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

ssl.CERT_OPTIONAL

   Value to pass to the ``cert_reqs`` parameter to ``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.

ssl.CERT_REQUIRED

   Value to pass to the ``cert_reqs`` parameter to ``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.

ssl.PROTOCOL_SSLv2

   Selects SSL version 2 as the channel encryption protocol.

ssl.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.

ssl.PROTOCOL_SSLv3

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

ssl.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
=================

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.

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.

SSLSocket.do_handshake()

   Performs the SSL setup handshake.  If the socket is non-blocking,
   this method may raise ``SSLError`` with the value of the exception
   instance's ``args[0]`` being either ``SSL_ERROR_WANT_READ`` or
   ``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

SSLSocket.unwrap()

   Performs the SSL shutdown handshake, which removes the TLS layer
   from the underlying socket, and returns the underlying socket
   object.  This can be used to go from encrypted operation over a
   connection to unencrypted.  The returned socket should always be
   used for further communication with the other side of the
   connection, rather than the original socket

SSLSocket.getpeercert(binary_form=False)

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

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

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``.

SSLSocket.unwrap()

   Performs the SSL shutdown handshake, which removes the TLS layer
   from the underlying socket, and returns the underlying socket
   object.  This can be used to go from encrypted operation over a
   connection to unencrypted.  The returned socket should always be
   used for further communication with the other side of the
   connection, rather than the original socket


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, Thawte,
Verisign, Positive SSL (used by python.org), Equifax and GeoTrust.

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 http.client 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 ``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 ``accept()`` on the socket to get the new
socket from the other end, and use ``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.

See also:

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

   Introducing SSL and Certificates using OpenSSL
      Frederick J. Hirsch

   RFC 1422: Privacy Enhancement for Internet Electronic Mail: Part
   II: Certificate-Based Key Management
      Steve Kent

   RFC 1750: Randomness Recommendations for Security
      D. Eastlake et. al.

   RFC 3280: Internet X.509 Public Key Infrastructure Certificate and
   CRL Profile
      Housley et. al.
