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

**Source code:** Lib/ssl.py

======================================================================

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 methods such as ``getpeercert()``, which
retrieves the certificate of the other side of the connection, and
``cipher()``,which retrieves the cipher being used for the secure
connection.

For more sophisticated applications, the ``ssl.SSLContext`` class
helps manage settings and certificates, which can then be inherited by
SSL sockets created through the ``SSLContext.wrap_socket()`` method.


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

exception exception ssl.SSLError

   Raised to signal an error from the underlying SSL implementation
   (currently provided by the OpenSSL library).  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 ``OSError``.  The error code and message of
   ``SSLError`` instances are provided by the OpenSSL library.

   Changed in version 3.3: ``SSLError`` used to be a subtype of
   ``socket.error``.

   library

      A string mnemonic designating the OpenSSL submodule in which the
      error occurred, such as ``SSL``, ``PEM`` or ``X509``.  The range
      of possible values depends on the OpenSSL version.

      New in version 3.3.

   reason

      A string mnemonic designating the reason this error occurred,
      for example ``CERTIFICATE_VERIFY_FAILED``.  The range of
      possible values depends on the OpenSSL version.

      New in version 3.3.

exception exception ssl.SSLZeroReturnError

   A subclass of ``SSLError`` raised when trying to read or write and
   the SSL connection has been closed cleanly.  Note that this doesn't
   mean that the underlying transport (read TCP) has been closed.

   New in version 3.3.

exception exception ssl.SSLWantReadError

   A subclass of ``SSLError`` raised by a *non-blocking SSL socket*
   when trying to read or write data, but more data needs to be
   received on the underlying TCP transport before the request can be
   fulfilled.

   New in version 3.3.

exception exception ssl.SSLWantWriteError

   A subclass of ``SSLError`` raised by a *non-blocking SSL socket*
   when trying to read or write data, but more data needs to be sent
   on the underlying TCP transport before the request can be
   fulfilled.

   New in version 3.3.

exception exception ssl.SSLSyscallError

   A subclass of ``SSLError`` raised when a system error was
   encountered while trying to fulfill an operation on a SSL socket.
   Unfortunately, there is no easy way to inspect the original errno
   number.

   New in version 3.3.

exception exception ssl.SSLEOFError

   A subclass of ``SSLError`` raised when the SSL connection has been
   terminated abruptly.  Generally, you shouldn't try to reuse the
   underlying transport when this error is encountered.

   New in version 3.3.

exception exception ssl.CertificateError

   Raised to signal an error with a certificate (such as mismatching
   hostname).  Certificate errors detected by OpenSSL, though, raise
   an ``SSLError``.


Socket creation
---------------

The following function allows for standalone socket creation.
Starting from Python 3.2, it can be more flexible to use
``SSLContext.wrap_socket()`` instead.

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, ciphers=None)

   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``.

   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, the default is ``PROTOCOL_SSLv23``; it provides
   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*                  | no        | yes       | yes        | no        |
      +--------------------------+-----------+-----------+------------+-----------+
      | *SSLv23*                 | yes       | no        | yes        | no        |
      +--------------------------+-----------+-----------+------------+-----------+
      | *TLSv1*                  | no        | no        | yes        | yes       |
      +--------------------------+-----------+-----------+------------+-----------+

   Note: Which connections succeed will vary depending on the version of
     OpenSSL.  For instance, in some older versions of OpenSSL (such
     as 0.9.7l on OS X 10.4), an SSLv2 client could not connect to an
     SSLv23 server.  Another example: beginning with OpenSSL 1.0.0, an
     SSLv23 client will not actually attempt SSLv2 connections unless
     you explicitly enable SSLv2 ciphers; for example, you might
     specify ``"ALL"`` or ``"SSLv2"`` as the *ciphers* parameter to
     enable them.

   The *ciphers* parameter sets the available ciphers for this SSL
   object. It should be a string in the OpenSSL cipher list format.

   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.recv()`` method should signal unexpected EOF from the
   other end of the connection.  If specified as ``True`` (the
   default), it returns a normal EOF (an empty bytes object) in
   response to unexpected EOF errors raised from the underlying
   socket; if ``False``, it will raise the exceptions back to the
   caller.

   Changed in version 3.2: New optional argument *ciphers*.


Random generation
-----------------

ssl.RAND_bytes(num)

   Returns *num* cryptographically strong pseudo-random bytes. Raises
   an ``SSLError`` if the PRNG has not been seeded with enough data or
   if the operation is not supported by the current RAND method.
   ``RAND_status()`` can be used to check the status of the PRNG and
   ``RAND_add()`` can be used to seed the PRNG.

   Read the Wikipedia article, Cryptographically secure pseudorandom
   number generator (CSPRNG), to get the requirements of a
   cryptographically generator.

   New in version 3.3.

ssl.RAND_pseudo_bytes(num)

   Returns (bytes, is_cryptographic): bytes are *num* pseudo-random
   bytes, is_cryptographic is True if the bytes generated are
   cryptographically strong. Raises an ``SSLError`` if the operation
   is not supported by the current RAND method.

   Generated pseudo-random byte sequences will be unique if they are
   of sufficient length, but are not necessarily unpredictable. They
   can be used for non-cryptographic purposes and for certain purposes
   in cryptographic protocols, but usually not for key generation etc.

   New in version 3.3.

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.


Certificate handling
--------------------

ssl.match_hostname(cert, hostname)

   Verify that *cert* (in decoded format as returned by
   ``SSLSocket.getpeercert()``) matches the given *hostname*.  The
   rules applied are those for checking the identity of HTTPS servers
   as outlined in **RFC 2818**, except that IP addresses are not
   currently supported. In addition to HTTPS, this function should be
   suitable for checking the identity of servers in various SSL-based
   protocols such as FTPS, IMAPS, POPS and others.

   ``CertificateError`` is raised on failure. On success, the function
   returns nothing:

      >>> cert = {'subject': ((('commonName', 'example.com'),),)}
      >>> ssl.match_hostname(cert, "example.com")
      >>> ssl.match_hostname(cert, "example.org")
      Traceback (most recent call last):
        File "<stdin>", line 1, in <module>
        File "/home/py3k/Lib/ssl.py", line 130, in match_hostname
      ssl.CertificateError: hostname 'example.org' doesn't match 'example.com'

   New in version 3.2.

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.

   Changed in version 3.3: This function is now IPv6-compatible.

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.


Constants
---------

ssl.CERT_NONE

   Possible value for ``SSLContext.verify_mode``, or the ``cert_reqs``
   parameter to ``wrap_socket()``.  In this mode (the default), no
   certificates will be required from the other side of the socket
   connection. If a certificate is received from the other end, no
   attempt to validate it is made.

   See the discussion of *Security considerations* below.

ssl.CERT_OPTIONAL

   Possible value for ``SSLContext.verify_mode``, or the ``cert_reqs``
   parameter to ``wrap_socket()``.  In this mode no certificates will
   be required from the other side of the socket connection; but if
   they are provided, validation will be attempted and an ``SSLError``
   will be raised on failure.

   Use of this setting requires a valid set of CA certificates to be
   passed, either to ``SSLContext.load_verify_locations()`` or as a
   value of the ``ca_certs`` parameter to ``wrap_socket()``.

ssl.CERT_REQUIRED

   Possible value for ``SSLContext.verify_mode``, or the ``cert_reqs``
   parameter to ``wrap_socket()``.  In this mode, certificates are
   required from the other side of the socket connection; an
   ``SSLError`` will be raised if no certificate is provided, or if
   its validation fails.

   Use of this setting requires a valid set of CA certificates to be
   passed, either to ``SSLContext.load_verify_locations()`` or as a
   value of the ``ca_certs`` parameter to ``wrap_socket()``.

ssl.PROTOCOL_SSLv2

   Selects SSL version 2 as the channel encryption protocol.

   This protocol is not available if OpenSSL is compiled with
   OPENSSL_NO_SSL2 flag.

   Warning: SSL version 2 is insecure.  Its use is highly discouraged.

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.

ssl.OP_ALL

   Enables workarounds for various bugs present in other SSL
   implementations. This option is set by default.  It does not
   necessarily set the same flags as OpenSSL's ``SSL_OP_ALL``
   constant.

   New in version 3.2.

ssl.OP_NO_SSLv2

   Prevents an SSLv2 connection.  This option is only applicable in
   conjunction with ``PROTOCOL_SSLv23``.  It prevents the peers from
   choosing SSLv2 as the protocol version.

   New in version 3.2.

ssl.OP_NO_SSLv3

   Prevents an SSLv3 connection.  This option is only applicable in
   conjunction with ``PROTOCOL_SSLv23``.  It prevents the peers from
   choosing SSLv3 as the protocol version.

   New in version 3.2.

ssl.OP_NO_TLSv1

   Prevents a TLSv1 connection.  This option is only applicable in
   conjunction with ``PROTOCOL_SSLv23``.  It prevents the peers from
   choosing TLSv1 as the protocol version.

   New in version 3.2.

ssl.OP_CIPHER_SERVER_PREFERENCE

   Use the server's cipher ordering preference, rather than the
   client's. This option has no effect on client sockets and SSLv2
   server sockets.

   New in version 3.3.

ssl.OP_SINGLE_DH_USE

   Prevents re-use of the same DH key for distinct SSL sessions.  This
   improves forward secrecy but requires more computational resources.
   This option only applies to server sockets.

   New in version 3.3.

ssl.OP_SINGLE_ECDH_USE

   Prevents re-use of the same ECDH key for distinct SSL sessions.
   This improves forward secrecy but requires more computational
   resources. This option only applies to server sockets.

   New in version 3.3.

ssl.OP_NO_COMPRESSION

   Disable compression on the SSL channel.  This is useful if the
   application protocol supports its own compression scheme.

   This option is only available with OpenSSL 1.0.0 and later.

   New in version 3.3.

ssl.HAS_ECDH

   Whether the OpenSSL library has built-in support for Elliptic
   Curve-based Diffie-Hellman key exchange.  This should be true
   unless the feature was explicitly disabled by the distributor.

   New in version 3.3.

ssl.HAS_SNI

   Whether the OpenSSL library has built-in support for the *Server
   Name Indication* extension to the SSLv3 and TLSv1 protocols (as
   defined in **RFC 4366**).  When true, you can use the
   *server_hostname* argument to ``SSLContext.wrap_socket()``.

   New in version 3.2.

ssl.HAS_NPN

   Whether the OpenSSL library has built-in support for *Next Protocol
   Negotiation* as described in the NPN draft specification. When
   true, you can use the ``SSLContext.set_npn_protocols()`` method to
   advertise which protocols you want to support.

   New in version 3.3.

ssl.CHANNEL_BINDING_TYPES

   List of supported TLS channel binding types.  Strings in this list
   can be used as arguments to ``SSLSocket.get_channel_binding()``.

   New in version 3.3.

ssl.OPENSSL_VERSION

   The version string of the OpenSSL library loaded by the
   interpreter:

      >>> ssl.OPENSSL_VERSION
      'OpenSSL 0.9.8k 25 Mar 2009'

   New in version 3.2.

ssl.OPENSSL_VERSION_INFO

   A tuple of five integers representing version information about the
   OpenSSL library:

      >>> ssl.OPENSSL_VERSION_INFO
      (0, 9, 8, 11, 15)

   New in version 3.2.

ssl.OPENSSL_VERSION_NUMBER

   The raw version number of the OpenSSL library, as a single integer:

      >>> ssl.OPENSSL_VERSION_NUMBER
      9470143
      >>> hex(ssl.OPENSSL_VERSION_NUMBER)
      '0x9080bf'

   New in version 3.2.


SSL Sockets
===========

SSL sockets provide the following methods of *Socket Objects*:

* ``accept()``

* ``bind()``

* ``close()``

* ``connect()``

* ``detach()``

* ``fileno()``

* ``getpeername()``, ``getsockname()``

* ``getsockopt()``, ``setsockopt()``

* ``gettimeout()``, ``settimeout()``, ``setblocking()``

* ``listen()``

* ``makefile()``

* ``recv()``, ``recv_into()`` (but passing a non-zero ``flags``
  argument is not allowed)

* ``send()``, ``sendall()`` (with the same limitation)

* ``shutdown()``

However, since the SSL (and TLS) protocol has its own framing atop of
TCP, the SSL sockets abstraction can, in certain respects, diverge
from the specification of normal, OS-level sockets.  See especially
the *notes on non-blocking sockets*.

SSL sockets also have the following additional methods and attributes:

SSLSocket.do_handshake()

   Perform the SSL setup handshake.

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 several
   keys, amongst them ``subject`` (the principal for which the
   certificate was issued) and ``issuer`` (the principal issuing the
   certificate).  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`` and ``issuer`` fields are tuples containing the
   sequence of relative distinguished names (RDNs) given in the
   certificate's data structure for the respective fields, and each
   RDN is a sequence of name-value pairs.  Here is a real-world
   example:

      {'issuer': ((('countryName', 'IL'),),
                  (('organizationName', 'StartCom Ltd.'),),
                  (('organizationalUnitName',
                    'Secure Digital Certificate Signing'),),
                  (('commonName',
                    'StartCom Class 2 Primary Intermediate Server CA'),)),
       'notAfter': 'Nov 22 08:15:19 2013 GMT',
       'notBefore': 'Nov 21 03:09:52 2011 GMT',
       'serialNumber': '95F0',
       'subject': ((('description', '571208-SLe257oHY9fVQ07Z'),),
                   (('countryName', 'US'),),
                   (('stateOrProvinceName', 'California'),),
                   (('localityName', 'San Francisco'),),
                   (('organizationName', 'Electronic Frontier Foundation, Inc.'),),
                   (('commonName', '*.eff.org'),),
                   (('emailAddress', 'hostmaster@eff.org'),)),
       'subjectAltName': (('DNS', '*.eff.org'), ('DNS', 'eff.org')),
       'version': 3}

   Note: To validate a certificate for a particular service, you can use
     the ``match_hostname()`` function.

   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.

   Changed in version 3.2: The returned dictionary includes additional
   items such as ``issuer`` and ``notBefore``.

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.compression()

   Return the compression algorithm being used as a string, or
   ``None`` if the connection isn't compressed.

   If the higher-level protocol supports its own compression
   mechanism, you can use ``OP_NO_COMPRESSION`` to disable SSL-level
   compression.

   New in version 3.3.

SSLSocket.get_channel_binding(cb_type="tls-unique")

   Get channel binding data for current connection, as a bytes object.
   Returns ``None`` if not connected or the handshake has not been
   completed.

   The *cb_type* parameter allow selection of the desired channel
   binding type. Valid channel binding types are listed in the
   ``CHANNEL_BINDING_TYPES`` list.  Currently only the 'tls-unique'
   channel binding, defined by **RFC 5929**, is supported.
   ``ValueError`` will be raised if an unsupported channel binding
   type is requested.

   New in version 3.3.

SSLSocket.selected_npn_protocol()

   Returns the protocol that was selected during the TLS/SSL
   handshake. If ``SSLContext.set_npn_protocols()`` was not called, or
   if the other party does not support NPN, or if the handshake has
   not yet happened, this will return ``None``.

   New in version 3.3.

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.context

   The ``SSLContext`` object this SSL socket is tied to.  If the SSL
   socket was created using the top-level ``wrap_socket()`` function
   (rather than ``SSLContext.wrap_socket()``), this is a custom
   context object created for this SSL socket.

   New in version 3.2.


SSL Contexts
============

New in version 3.2.

An SSL context holds various data longer-lived than single SSL
connections, such as SSL configuration options, certificate(s) and
private key(s). It also manages a cache of SSL sessions for server-
side sockets, in order to speed up repeated connections from the same
clients.

class class ssl.SSLContext(protocol)

   Create a new SSL context.  You must pass *protocol* which must be
   one of the ``PROTOCOL_*`` constants defined in this module.
   ``PROTOCOL_SSLv23`` is recommended for maximum interoperability.

``SSLContext`` objects have the following methods and attributes:

SSLContext.load_cert_chain(certfile, keyfile=None, password=None)

   Load a private key and the corresponding certificate.  The
   *certfile* string must be the path to a single file in PEM format
   containing the certificate as well as any number of CA certificates
   needed to establish the certificate's authenticity.  The *keyfile*
   string, if present, must point to a file containing the private key
   in.  Otherwise the private key will be taken from *certfile* as
   well.  See the discussion of *Certificates* for more information on
   how the certificate is stored in the *certfile*.

   The *password* argument may be a function to call to get the
   password for decrypting the private key.  It will only be called if
   the private key is encrypted and a password is necessary.  It will
   be called with no arguments, and it should return a string, bytes,
   or bytearray.  If the return value is a string it will be encoded
   as UTF-8 before using it to decrypt the key. Alternatively a
   string, bytes, or bytearray value may be supplied directly as the
   *password* argument.  It will be ignored if the private key is not
   encrypted and no password is needed.

   If the *password* argument is not specified and a password is
   required, OpenSSL's built-in password prompting mechanism will be
   used to interactively prompt the user for a password.

   An ``SSLError`` is raised if the private key doesn't match with the
   certificate.

   Changed in version 3.3: New optional argument *password*.

SSLContext.load_verify_locations(cafile=None, capath=None)

   Load a set of "certification authority" (CA) certificates used to
   validate other peers' certificates when ``verify_mode`` is other
   than ``CERT_NONE``.  At least one of *cafile* or *capath* must be
   specified.

   The *cafile* string, if present, is the path to a file of
   concatenated CA certificates in PEM format. See the discussion of
   *Certificates* for more information about how to arrange the
   certificates in this file.

   The *capath* string, if present, is the path to a directory
   containing several CA certificates in PEM format, following an
   OpenSSL specific layout.

SSLContext.set_default_verify_paths()

   Load a set of default "certification authority" (CA) certificates
   from a filesystem path defined when building the OpenSSL library.
   Unfortunately, there's no easy way to know whether this method
   succeeds: no error is returned if no certificates are to be found.
   When the OpenSSL library is provided as part of the operating
   system, though, it is likely to be configured properly.

SSLContext.set_ciphers(ciphers)

   Set the available ciphers for sockets created with this context. It
   should be a string in the OpenSSL cipher list format. If no cipher
   can be selected (because compile-time options or other
   configuration forbids use of all the specified ciphers), an
   ``SSLError`` will be raised.

   Note: when connected, the ``SSLSocket.cipher()`` method of SSL sockets
     will give the currently selected cipher.

SSLContext.set_npn_protocols(protocols)

   Specify which protocols the socket should avertise during the
   SSL/TLS handshake. It should be a list of strings, like
   ``['http/1.1', 'spdy/2']``, ordered by preference. The selection of
   a protocol will happen during the handshake, and will play out
   according to the NPN draft specification. After a successful
   handshake, the ``SSLSocket.selected_npn_protocol()`` method will
   return the agreed-upon protocol.

   This method will raise ``NotImplementedError`` if ``HAS_NPN`` is
   False.

   New in version 3.3.

SSLContext.load_dh_params(dhfile)

   Load the key generation parameters for Diffie-Helman (DH) key
   exchange. Using DH key exchange improves forward secrecy at the
   expense of computational resources (both on the server and on the
   client). The *dhfile* parameter should be the path to a file
   containing DH parameters in PEM format.

   This setting doesn't apply to client sockets.  You can also use the
   ``OP_SINGLE_DH_USE`` option to further improve security.

   New in version 3.3.

SSLContext.set_ecdh_curve(curve_name)

   Set the curve name for Elliptic Curve-based Diffie-Hellman (ECDH)
   key exchange.  ECDH is significantly faster than regular DH while
   arguably as secure.  The *curve_name* parameter should be a string
   describing a well-known elliptic curve, for example ``prime256v1``
   for a widely supported curve.

   This setting doesn't apply to client sockets.  You can also use the
   ``OP_SINGLE_ECDH_USE`` option to further improve security.

   This method is not available if ``HAS_ECDH`` is False.

   New in version 3.3.

   See also:

      SSL/TLS & Perfect Forward Secrecy    Vincent Bernat.

SSLContext.wrap_socket(sock, server_side=False, do_handshake_on_connect=True, suppress_ragged_eofs=True, server_hostname=None)

   Wrap an existing Python socket *sock* and return an ``SSLSocket``
   object.  The SSL socket is tied to the context, its settings and
   certificates.  The parameters *server_side*,
   *do_handshake_on_connect* and *suppress_ragged_eofs* have the same
   meaning as in the top-level ``wrap_socket()`` function.

   On client connections, the optional parameter *server_hostname*
   specifies the hostname of the service which we are connecting to.
   This allows a single server to host multiple SSL-based services
   with distinct certificates, quite similarly to HTTP virtual hosts.
   Specifying *server_hostname* will raise a ``ValueError`` if the
   OpenSSL library doesn't have support for it (that is, if
   ``HAS_SNI`` is ``False``).  Specifying *server_hostname* will also
   raise a ``ValueError`` if *server_side* is true.

SSLContext.session_stats()

   Get statistics about the SSL sessions created or managed by this
   context. A dictionary is returned which maps the names of each
   piece of information to their numeric values.  For example, here is
   the total number of hits and misses in the session cache since the
   context was created:

      >>> stats = context.session_stats()
      >>> stats['hits'], stats['misses']
      (0, 0)

SSLContext.options

   An integer representing the set of SSL options enabled on this
   context. The default value is ``OP_ALL``, but you can specify other
   options such as ``OP_NO_SSLv2`` by ORing them together.

   Note: With versions of OpenSSL older than 0.9.8m, it is only possible
     to set options, not to clear them.  Attempting to clear an option
     (by resetting the corresponding bits) will raise a
     ``ValueError``.

SSLContext.protocol

   The protocol version chosen when constructing the context.  This
   attribute is read-only.

SSLContext.verify_mode

   Whether to try to verify other peers' certificates and how to
   behave if verification fails.  This attribute must be one of
   ``CERT_NONE``, ``CERT_OPTIONAL`` or ``CERT_REQUIRED``.


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-----


Certificate chains
------------------

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-----


CA certificates
---------------

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.


Combined key and certificate
----------------------------

Often the private key is stored in the same file as the certificate;
in this case, only the ``certfile`` parameter to
``SSLContext.load_cert_chain()`` and ``wrap_socket()`` needs to be
passed.  If the private key is stored with the certificate, 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-----


Self-signed certificates
------------------------

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 and prints the server's
certificate:

   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))

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

As of January 6, 2012, the certificate printed by this program looks
like this:

   {'issuer': ((('countryName', 'US'),),
               (('organizationName', 'VeriSign, Inc.'),),
               (('organizationalUnitName', 'VeriSign Trust Network'),),
               (('organizationalUnitName',
                 'Terms of use at https://www.verisign.com/rpa (c)06'),),
               (('commonName',
                 'VeriSign Class 3 Extended Validation SSL SGC CA'),)),
    'notAfter': 'May 25 23:59:59 2012 GMT',
    'notBefore': 'May 26 00:00:00 2010 GMT',
    'serialNumber': '53D2BEF924A7245E83CA01E46CAA2477',
    'subject': ((('1.3.6.1.4.1.311.60.2.1.3', 'US'),),
                (('1.3.6.1.4.1.311.60.2.1.2', 'Delaware'),),
                (('businessCategory', 'V1.0, Clause 5.(b)'),),
                (('serialNumber', '2497886'),),
                (('countryName', 'US'),),
                (('postalCode', '94043'),),
                (('stateOrProvinceName', 'California'),),
                (('localityName', 'Mountain View'),),
                (('streetAddress', '487 East Middlefield Road'),),
                (('organizationName', 'VeriSign, Inc.'),),
                (('organizationalUnitName', ' Production Security Services'),),
                (('commonName', 'www.verisign.com'),)),
    'subjectAltName': (('DNS', 'www.verisign.com'),
                       ('DNS', 'verisign.com'),
                       ('DNS', 'www.verisign.net'),
                       ('DNS', 'verisign.net'),
                       ('DNS', 'www.verisign.mobi'),
                       ('DNS', 'verisign.mobi'),
                       ('DNS', 'www.verisign.eu'),
                       ('DNS', 'verisign.eu')),
    'version': 3}

This other example first creates an SSL context, instructs it to
verify certificates sent by peers, and feeds it a set of recognized
certificate authorities (CA):

   >>> context = ssl.SSLContext(ssl.PROTOCOL_SSLv23)
   >>> context.verify_mode = ssl.CERT_REQUIRED
   >>> context.load_verify_locations("/etc/ssl/certs/ca-bundle.crt")

(it is assumed your operating system places a bundle of all CA
certificates in ``/etc/ssl/certs/ca-bundle.crt``; if not, you'll get
an error and have to adjust the location)

When you use the context to connect to a server, ``CERT_REQUIRED``
validates the server certificate: it ensures that the server
certificate was signed with one of the CA certificates, and checks the
signature for correctness:

   >>> conn = context.wrap_socket(socket.socket(socket.AF_INET))
   >>> conn.connect(("linuxfr.org", 443))

You should then fetch the certificate and check its fields for
conformity:

   >>> cert = conn.getpeercert()
   >>> ssl.match_hostname(cert, "linuxfr.org")

Visual inspection shows that the certificate does identify the desired
service (that is, the HTTPS host ``linuxfr.org``):

   >>> pprint.pprint(cert)
   {'issuer': ((('organizationName', 'CAcert Inc.'),),
               (('organizationalUnitName', 'http://www.CAcert.org'),),
               (('commonName', 'CAcert Class 3 Root'),)),
    'notAfter': 'Jun  7 21:02:24 2013 GMT',
    'notBefore': 'Jun  8 21:02:24 2011 GMT',
    'serialNumber': 'D3E9',
    'subject': ((('commonName', 'linuxfr.org'),),),
    'subjectAltName': (('DNS', 'linuxfr.org'),
                       ('othername', '<unsupported>'),
                       ('DNS', 'linuxfr.org'),
                       ('othername', '<unsupported>'),
                       ('DNS', 'dev.linuxfr.org'),
                       ('othername', '<unsupported>'),
                       ('DNS', 'prod.linuxfr.org'),
                       ('othername', '<unsupported>'),
                       ('DNS', 'alpha.linuxfr.org'),
                       ('othername', '<unsupported>'),
                       ('DNS', '*.linuxfr.org'),
                       ('othername', '<unsupported>')),
    'version': 3}

Now that you are assured of its authenticity, you can proceed to talk
with the server:

   >>> conn.sendall(b"HEAD / HTTP/1.0\r\nHost: linuxfr.org\r\n\r\n")
   >>> pprint.pprint(conn.recv(1024).split(b"\r\n"))
   [b'HTTP/1.1 302 Found',
    b'Date: Sun, 16 May 2010 13:43:28 GMT',
    b'Server: Apache/2.2',
    b'Location: https://linuxfr.org/pub/',
    b'Vary: Accept-Encoding',
    b'Connection: close',
    b'Content-Type: text/html; charset=iso-8859-1',
    b'',
    b'']

See the discussion of *Security considerations* below.


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

For server operation, typically you'll need to have a server
certificate, and private key, each in a file.  You'll first create a
context holding the key and the certificate, so that clients can check
your authenticity.  Then you'll open a socket, bind it to a port, call
``listen()`` on it, and start waiting for clients to connect:

   import socket, ssl

   context = ssl.SSLContext(ssl.PROTOCOL_TLSv1)
   context.load_cert_chain(certfile="mycertfile", keyfile="mykeyfile")

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

When a client connects, you'll call ``accept()`` on the socket to get
the new socket from the other end, and use the context's
``SSLContext.wrap_socket()`` method to create a server-side SSL socket
for the connection:

   while True:
       newsocket, fromaddr = bindsocket.accept()
       connstream = context.wrap_socket(newsocket, server_side=True)
       try:
           deal_with_client(connstream)
       finally:
           connstream.shutdown(socket.SHUT_RDWR)
           connstream.close()

Then you'll 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.recv(1024)
       # empty 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.recv(1024)
       # finished with client

And go back to listening for new client connections (of course, a real
server would probably handle each client connection in a separate
thread, or put the sockets in non-blocking mode and use an event
loop).


Notes on non-blocking sockets
=============================

When working with non-blocking sockets, there are several things you
need to be aware of:

* Calling ``select()`` tells you that the OS-level socket can be read
  from (or written to), but it does not imply that there is sufficient
  data at the upper SSL layer.  For example, only part of an SSL frame
  might have arrived.  Therefore, you must be ready to handle
  ``SSLSocket.recv()`` and ``SSLSocket.send()`` failures, and retry
  after another call to ``select()``.

  (of course, similar provisions apply when using other primitives
  such as ``poll()``)

* The SSL handshake itself will be non-blocking: the
  ``SSLSocket.do_handshake()`` method has to be retried until it
  returns successfully.  Here is a synopsis using ``select()`` to wait
  for the socket's readiness:

     while True:
         try:
             sock.do_handshake()
             break
         except ssl.SSLWantReadError:
             select.select([sock], [], [])
         except ssl.SSLWantWriteError:
             select.select([], [sock], [])


Security considerations
=======================


Verifying certificates
----------------------

``CERT_NONE`` is the default.  Since it does not authenticate the
other peer, it can be insecure, especially in client mode where most
of time you would like to ensure the authenticity of the server you're
talking to. Therefore, when in client mode, it is highly recommended
to use ``CERT_REQUIRED``.  However, it is in itself not sufficient;
you also have to check that the server certificate, which can be
obtained by calling ``SSLSocket.getpeercert()``, matches the desired
service.  For many protocols and applications, the service can be
identified by the hostname; in this case, the ``match_hostname()``
function can be used.

In server mode, if you want to authenticate your clients using the SSL
layer (rather than using a higher-level authentication mechanism),
you'll also have to specify ``CERT_REQUIRED`` and similarly check the
client certificate.

   Note: In client mode, ``CERT_OPTIONAL`` and ``CERT_REQUIRED`` are
     equivalent unless anonymous ciphers are enabled (they are
     disabled by default).


Protocol versions
-----------------

SSL version 2 is considered insecure and is therefore dangerous to
use.  If you want maximum compatibility between clients and servers,
it is recommended to use ``PROTOCOL_SSLv23`` as the protocol version
and then disable SSLv2 explicitly using the ``SSLContext.options``
attribute:

   context = ssl.SSLContext(ssl.PROTOCOL_SSLv23)
   context.options |= ssl.OP_NO_SSLv2

The SSL context created above will allow SSLv3 and TLSv1 connections,
but not SSLv2.


Cipher selection
----------------

If you have advanced security requirements, fine-tuning of the ciphers
enabled when negotiating a SSL session is possible through the
``SSLContext.set_ciphers()`` method.  Starting from Python 3.2.3, the
ssl module disables certain weak ciphers by default, but you may want
to further restrict the cipher choice.  For example:

   context = ssl.SSLContext(ssl.PROTOCOL_TLSv1)
   context.set_ciphers('HIGH:!aNULL:!eNULL')

The ``!aNULL:!eNULL`` part of the cipher spec is necessary to disable
ciphers which don't provide both encryption and authentication.  Be
sure to read OpenSSL's documentation about the cipher list format. If
you want to check which ciphers are enabled by a given cipher list,
use the ``openssl ciphers`` command on your system.

See also:

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

   TLS (Transport Layer Security) and SSL (Secure Socket Layer)
      Debby Koren

   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.

   RFC 4366: Transport Layer Security (TLS) Extensions
      Blake-Wilson et. al.
