
"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. For example, TLSv1.1
  and TLSv1.2 come with openssl version 1.0.1.

Warning: Don't use this module without reading the Security
  considerations. Doing so may lead to a false sense of security, as
  the default settings of the ssl module are not necessarily
  appropriate for your application.

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.

Changed in version 3.6: OpenSSL 0.9.8, 1.0.0 and 1.0.1 are deprecated
and no longer supported. In the future the ssl module will require at
least OpenSSL 1.0.2 or 1.1.0.


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

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 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 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 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 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 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 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.  "sock" must be a
   "SOCK_STREAM" socket; other socket types are unsupported.

   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_TLS"; 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**    | **TLS**       | **TLSv1** | **TLSv1.1** | **TLSv1.2** |
      +--------------------------+--------------+--------------+---------------+-----------+-------------+-------------+
      | *SSLv2*                  | yes          | no           | no [1]        | no        | no          | no          |
      +--------------------------+--------------+--------------+---------------+-----------+-------------+-------------+
      | *SSLv3*                  | no           | yes          | no [2]        | no        | no          | no          |
      +--------------------------+--------------+--------------+---------------+-----------+-------------+-------------+
      | *TLS* (*SSLv23*)         | no [1]       | no [2]       | yes           | yes       | yes         | yes         |
      +--------------------------+--------------+--------------+---------------+-----------+-------------+-------------+
      | *TLSv1*                  | no           | no           | yes           | yes       | no          | no          |
      +--------------------------+--------------+--------------+---------------+-----------+-------------+-------------+
      | *TLSv1.1*                | no           | no           | yes           | no        | yes         | no          |
      +--------------------------+--------------+--------------+---------------+-----------+-------------+-------------+
      | *TLSv1.2*                | no           | no           | yes           | no        | no          | yes         |
      +--------------------------+--------------+--------------+---------------+-----------+-------------+-------------+

   -[ Footnotes ]-

   [1] "SSLContext" disables SSLv2 with "OP_NO_SSLv2" by default.

   [2] "SSLContext" disables SSLv3 with "OP_NO_SSLv3" by default.

   Note: Which connections succeed will vary depending on the
     version of OpenSSL.  For example, before OpenSSL 1.0.0, an SSLv23
     client would always attempt SSLv2 connections.

   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*.


Context creation
----------------

A convenience function helps create "SSLContext" objects for common
purposes.

ssl.create_default_context(purpose=Purpose.SERVER_AUTH, cafile=None, capath=None, cadata=None)

   Return a new "SSLContext" object with default settings for the
   given *purpose*.  The settings are chosen by the "ssl" module, and
   usually represent a higher security level than when calling the
   "SSLContext" constructor directly.

   *cafile*, *capath*, *cadata* represent optional CA certificates to
   trust for certificate verification, as in
   "SSLContext.load_verify_locations()".  If all three are "None",
   this function can choose to trust the system's default CA
   certificates instead.

   The settings are: "PROTOCOL_TLS", "OP_NO_SSLv2", and "OP_NO_SSLv3"
   with high encryption cipher suites without RC4 and without
   unauthenticated cipher suites. Passing "SERVER_AUTH" as *purpose*
   sets "verify_mode" to "CERT_REQUIRED" and either loads CA
   certificates (when at least one of *cafile*, *capath* or *cadata*
   is given) or uses "SSLContext.load_default_certs()" to load default
   CA certificates.

   Note: The protocol, options, cipher and other settings may change
     to more restrictive values anytime without prior deprecation.
     The values represent a fair balance between compatibility and
     security.If your application needs specific settings, you should
     create a "SSLContext" and apply the settings yourself.

   Note: If you find that when certain older clients or servers
     attempt to connect with a "SSLContext" created by this function
     that they get an error stating "Protocol or cipher suite
     mismatch", it may be that they only support SSL3.0 which this
     function excludes using the "OP_NO_SSLv3". SSL3.0 is widely
     considered to be completely broken. If you still wish to continue
     to use this function but still allow SSL 3.0 connections you can
     re-enable them using:

        ctx = ssl.create_default_context(Purpose.CLIENT_AUTH)
        ctx.options &= ~ssl.OP_NO_SSLv3

   New in version 3.4.

   Changed in version 3.4.4: RC4 was dropped from the default cipher
   string.

   Changed in version 3.6: ChaCha20/Poly1305 was added to the default
   cipher string.3DES was dropped from the default cipher string.


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

ssl.RAND_bytes(num)

   Return *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.

   For almost all applications "os.urandom()" is preferable.

   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)

   Return (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.

   For almost all applications "os.urandom()" is preferable.

   New in version 3.3.

   Deprecated since version 3.6: OpenSSL has deprecated
   "ssl.RAND_pseudo_bytes()", use "ssl.RAND_bytes()" instead.

ssl.RAND_status()

   Return "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.

   Availability: not available with LibreSSL and OpenSSL > 1.1.0

ssl.RAND_add(bytes, entropy)

   Mix 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.

   Changed in version 3.5: Writable *bytes-like object* is now
   accepted.


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**, **RFC 5280** and **RFC 6125**.  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.

   Changed in version 3.3.3: The function now follows **RFC 6125**,
   section 6.4.3 and does neither match multiple wildcards (e.g.
   "*.*.com" or "*a*.example.org") nor a wildcard inside an
   internationalized domain names (IDN) fragment. IDN A-labels such as
   "www*.xn--pthon-kva.org" are still supported, but "x*.python.org"
   no longer matches "xn--tda.python.org".

   Changed in version 3.5: Matching of IP addresses, when present in
   the subjectAltName field of the certificate, is now supported.

ssl.cert_time_to_seconds(cert_time)

   Return the time in seconds since the Epoch, given the "cert_time"
   string representing the "notBefore" or "notAfter" date from a
   certificate in ""%b %d %H:%M:%S %Y %Z"" strptime format (C locale).

   Here's an example:

      >>> import ssl
      >>> timestamp = ssl.cert_time_to_seconds("Jan  5 09:34:43 2018 GMT")
      >>> timestamp
      1515144883
      >>> from datetime import datetime
      >>> print(datetime.utcfromtimestamp(timestamp))
      2018-01-05 09:34:43

   "notBefore" or "notAfter" dates must use GMT (**RFC 5280**).

   Changed in version 3.5: Interpret the input time as a time in UTC
   as specified by 'GMT' timezone in the input string. Local timezone
   was used previously. Return an integer (no fractions of a second in
   the input format)

ssl.get_server_certificate(addr, ssl_version=PROTOCOL_TLS, 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.

   Changed in version 3.5: The default *ssl_version* is changed from
   "PROTOCOL_SSLv3" to "PROTOCOL_TLS" for maximum compatibility with
   modern servers.

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.get_default_verify_paths()

   Returns a named tuple with paths to OpenSSL's default cafile and
   capath. The paths are the same as used by
   "SSLContext.set_default_verify_paths()". The return value is a
   *named tuple* "DefaultVerifyPaths":

   * "cafile" - resolved path to cafile or "None" if the file
     doesn't exist,

   * "capath" - resolved path to capath or "None" if the directory
     doesn't exist,

   * "openssl_cafile_env" - OpenSSL's environment key that points to
     a cafile,

   * "openssl_cafile" - hard coded path to a cafile,

   * "openssl_capath_env" - OpenSSL's environment key that points to
     a capath,

   * "openssl_capath" - hard coded path to a capath directory

   Availability: LibreSSL ignores the environment vars
   "openssl_cafile_env" and "openssl_capath_env"

   New in version 3.4.

ssl.enum_certificates(store_name)

   Retrieve certificates from Windows' system cert store. *store_name*
   may be one of "CA", "ROOT" or "MY". Windows may provide additional
   cert stores, too.

   The function returns a list of (cert_bytes, encoding_type, trust)
   tuples. The encoding_type specifies the encoding of cert_bytes. It
   is either "x509_asn" for X.509 ASN.1 data or "pkcs_7_asn" for
   PKCS#7 ASN.1 data. Trust specifies the purpose of the certificate
   as a set of OIDS or exactly "True" if the certificate is
   trustworthy for all purposes.

   Example:

      >>> ssl.enum_certificates("CA")
      [(b'data...', 'x509_asn', {'1.3.6.1.5.5.7.3.1', '1.3.6.1.5.5.7.3.2'}),
       (b'data...', 'x509_asn', True)]

   Availability: Windows.

   New in version 3.4.

ssl.enum_crls(store_name)

   Retrieve CRLs from Windows' system cert store. *store_name* may be
   one of "CA", "ROOT" or "MY". Windows may provide additional cert
   stores, too.

   The function returns a list of (cert_bytes, encoding_type, trust)
   tuples. The encoding_type specifies the encoding of cert_bytes. It
   is either "x509_asn" for X.509 ASN.1 data or "pkcs_7_asn" for
   PKCS#7 ASN.1 data.

   Availability: Windows.

   New in version 3.4.


Constants
---------

   All constants are now "enum.IntEnum" or "enum.IntFlag" collections.

   New in version 3.6.

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()".

class ssl.VerifyMode

   "enum.IntEnum" collection of CERT_* constants.

   New in version 3.6.

ssl.VERIFY_DEFAULT

   Possible value for "SSLContext.verify_flags". In this mode,
   certificate revocation lists (CRLs) are not checked. By default
   OpenSSL does neither require nor verify CRLs.

   New in version 3.4.

ssl.VERIFY_CRL_CHECK_LEAF

   Possible value for "SSLContext.verify_flags". In this mode, only
   the peer cert is check but non of the intermediate CA certificates.
   The mode requires a valid CRL that is signed by the peer cert's
   issuer (its direct ancestor CA). If no proper has been loaded
   "SSLContext.load_verify_locations", validation will fail.

   New in version 3.4.

ssl.VERIFY_CRL_CHECK_CHAIN

   Possible value for "SSLContext.verify_flags". In this mode, CRLs of
   all certificates in the peer cert chain are checked.

   New in version 3.4.

ssl.VERIFY_X509_STRICT

   Possible value for "SSLContext.verify_flags" to disable workarounds
   for broken X.509 certificates.

   New in version 3.4.

ssl.VERIFY_X509_TRUSTED_FIRST

   Possible value for "SSLContext.verify_flags". It instructs OpenSSL
   to prefer trusted certificates when building the trust chain to
   validate a certificate. This flag is enabled by default.

   New in version 3.4.4.

class ssl.VerifyFlags

   "enum.IntFlag" collection of VERIFY_* constants.

   New in version 3.6.

ssl.PROTOCOL_TLS

   Selects the highest protocol version that both the client and
   server support. Despite the name, this option can select both "SSL"
   and "TLS" protocols.

   New in version 3.6.

ssl.PROTOCOL_TLS_CLIENT

   Auto-negotiate the highest protocol version like "PROTOCOL_TLS",
   but only support client-side "SSLSocket" connections. The protocol
   enables "CERT_REQUIRED" and "check_hostname" by default.

   New in version 3.6.

ssl.PROTOCOL_TLS_SERVER

   Auto-negotiate the highest protocol version like "PROTOCOL_TLS",
   but only support server-side "SSLSocket" connections.

   New in version 3.6.

ssl.PROTOCOL_SSLv23

   Alias for data:*PROTOCOL_TLS*.

   Deprecated since version 3.6: Use "PROTOCOL_TLS" instead.

ssl.PROTOCOL_SSLv2

   Selects SSL version 2 as the channel encryption protocol.

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

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

   Deprecated since version 3.6: OpenSSL has removed support for
   SSLv2.

ssl.PROTOCOL_SSLv3

   Selects SSL version 3 as the channel encryption protocol.

   This protocol is not be available if OpenSSL is compiled with the
   "OPENSSL_NO_SSLv3" flag.

   Warning: SSL version 3 is insecure.  Its use is highly
     discouraged.

   Deprecated since version 3.6: OpenSSL has deprecated all version
   specific protocols. Use the default protocol "PROTOCOL_TLS" with
   flags like "OP_NO_SSLv3" instead.

ssl.PROTOCOL_TLSv1

   Selects TLS version 1.0 as the channel encryption protocol.

   Deprecated since version 3.6: OpenSSL has deprecated all version
   specific protocols. Use the default protocol "PROTOCOL_TLS" with
   flags like "OP_NO_SSLv3" instead.

ssl.PROTOCOL_TLSv1_1

   Selects TLS version 1.1 as the channel encryption protocol.
   Available only with openssl version 1.0.1+.

   New in version 3.4.

   Deprecated since version 3.6: OpenSSL has deprecated all version
   specific protocols. Use the default protocol "PROTOCOL_TLS" with
   flags like "OP_NO_SSLv3" instead.

ssl.PROTOCOL_TLSv1_2

   Selects TLS version 1.2 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.  Available only with
   openssl version 1.0.1+.

   New in version 3.4.

   Deprecated since version 3.6: OpenSSL has deprecated all version
   specific protocols. Use the default protocol "PROTOCOL_TLS" with
   flags like "OP_NO_SSLv3" instead.

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_TLS".  It prevents the peers from
   choosing SSLv2 as the protocol version.

   New in version 3.2.

   Deprecated since version 3.6: SSLv2 is deprecated

ssl.OP_NO_SSLv3

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

   New in version 3.2.

   Deprecated since version 3.6: SSLv3 is deprecated

ssl.OP_NO_TLSv1

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

   New in version 3.2.

ssl.OP_NO_TLSv1_1

   Prevents a TLSv1.1 connection. This option is only applicable in
   conjunction with "PROTOCOL_TLS". It prevents the peers from
   choosing TLSv1.1 as the protocol version. Available only with
   openssl version 1.0.1+.

   New in version 3.4.

ssl.OP_NO_TLSv1_2

   Prevents a TLSv1.2 connection. This option is only applicable in
   conjunction with "PROTOCOL_TLS". It prevents the peers from
   choosing TLSv1.2 as the protocol version. Available only with
   openssl version 1.0.1+.

   New in version 3.4.

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.

class ssl.Options

   "enum.IntFlag" collection of OP_* constants.

ssl.OP_NO_TICKET

   Prevent client side from requesting a session ticket.

   New in version 3.6.

ssl.HAS_ALPN

   Whether the OpenSSL library has built-in support for the
   *Application-Layer Protocol Negotiation* TLS extension as described
   in **RFC 7301**.

   New in version 3.5.

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 (as defined in **RFC 6066**).

   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 1.0.2k  26 Jan 2017'

   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
      (1, 0, 2, 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
      268443839
      >>> hex(ssl.OPENSSL_VERSION_NUMBER)
      '0x100020bf'

   New in version 3.2.

ssl.ALERT_DESCRIPTION_HANDSHAKE_FAILURE
ssl.ALERT_DESCRIPTION_INTERNAL_ERROR
ALERT_DESCRIPTION_*

   Alert Descriptions from **RFC 5246** and others. The IANA TLS Alert
   Registry contains this list and references to the RFCs where their
   meaning is defined.

   Used as the return value of the callback function in
   "SSLContext.set_servername_callback()".

   New in version 3.4.

class ssl.AlertDescription

   "enum.IntEnum" collection of ALERT_DESCRIPTION_* constants.

   New in version 3.6.

Purpose.SERVER_AUTH

   Option for "create_default_context()" and
   "SSLContext.load_default_certs()".  This value indicates that the
   context may be used to authenticate Web servers (therefore, it will
   be used to create client-side sockets).

   New in version 3.4.

Purpose.CLIENT_AUTH

   Option for "create_default_context()" and
   "SSLContext.load_default_certs()".  This value indicates that the
   context may be used to authenticate Web clients (therefore, it will
   be used to create server-side sockets).

   New in version 3.4.

class ssl.SSLErrorNumber

   "enum.IntEnum" collection of SSL_ERROR_* constants.

   New in version 3.6.


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

class ssl.SSLSocket(socket.socket)

   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)

   * "sendfile()" (but "os.sendfile" will be used for plain-text
     sockets only, else "send()" will be used)

   * "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.

   Usually, "SSLSocket" are not created directly, but using the
   "SSLContext.wrap_socket()" method.

   Changed in version 3.5: The "sendfile()" method was added.

   Changed in version 3.5: The "shutdown()" does not reset the socket
   timeout each time bytes are received or sent. The socket timeout is
   now to maximum total duration of the shutdown.

   Deprecated since version 3.6: It is deprecated to create a
   "SSLSocket" instance directly, use "SSLContext.wrap_socket()" to
   wrap a socket.

SSL sockets also have the following additional methods and attributes:

SSLSocket.read(len=1024, buffer=None)

   Read up to *len* bytes of data from the SSL socket and return the
   result as a "bytes" instance. If *buffer* is specified, then read
   into the buffer instead, and return the number of bytes read.

   Raise "SSLWantReadError" or "SSLWantWriteError" if the socket is
   non-blocking and the read would block.

   As at any time a re-negotiation is possible, a call to "read()" can
   also cause write operations.

   Changed in version 3.5: The socket timeout is no more reset each
   time bytes are received or sent. The socket timeout is now to
   maximum total duration to read up to *len* bytes.

   Deprecated since version 3.6: Use "recv()" instead of "read()".

SSLSocket.write(buf)

   Write *buf* to the SSL socket and return the number of bytes
   written. The *buf* argument must be an object supporting the buffer
   interface.

   Raise "SSLWantReadError" or "SSLWantWriteError" if the socket is
   non-blocking and the write would block.

   As at any time a re-negotiation is possible, a call to "write()"
   can also cause read operations.

   Changed in version 3.5: The socket timeout is no more reset each
   time bytes are received or sent. The socket timeout is now to
   maximum total duration to write *buf*.

   Deprecated since version 3.6: Use "send()" instead of "write()".

Note: The "read()" and "write()" methods are the low-level methods
  that read and write unencrypted, application-level data and
  decrypt/encrypt it to encrypted, wire-level data. These methods
  require an active SSL connection, i.e. the handshake was completed
  and "SSLSocket.unwrap()" was not called.Normally you should use the
  socket API methods like "recv()" and "send()" instead of these
  methods.

SSLSocket.do_handshake()

   Perform the SSL setup handshake.

   Changed in version 3.4: The handshake method also performs
   "match_hostname()" when the "check_hostname" attribute of the
   socket's "context" is true.

   Changed in version 3.5: The socket timeout is no more reset each
   time bytes are received or sent. The socket timeout is now to
   maximum total duration of the handshake.

SSLSocket.getpeercert(binary_form=False)

   If there is no certificate for the peer on the other end of the
   connection, return "None".  If the SSL handshake hasn't been done
   yet, raise "ValueError".

   If the "binary_form" parameter 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.  Whether the peer provides a certificate
   depends on the SSL socket's role:

   * for a client SSL socket, the server will always provide a
     certificate, regardless of whether validation was required;

   * for a server SSL socket, the client will only provide a
     certificate when requested by the server; therefore
     "getpeercert()" will return "None" if you used "CERT_NONE"
     (rather than "CERT_OPTIONAL" or "CERT_REQUIRED").

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

   Changed in version 3.4: "ValueError" is raised when the handshake
   isn't done. The returned dictionary includes additional X509v3
   extension items   such as "crlDistributionPoints", "caIssuers" and
   "OCSP" URIs.

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.shared_ciphers()

   Return the list of ciphers shared by the client during the
   handshake.  Each entry of the returned list is a three-value tuple
   containing the name of the cipher, the version of the SSL protocol
   that defines its use, and the number of secret bits the cipher
   uses.  "shared_ciphers()" returns "None" if no connection has been
   established or the socket is a client socket.

   New in version 3.5.

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_alpn_protocol()

   Return the protocol that was selected during the TLS handshake.  If
   "SSLContext.set_alpn_protocols()" was not called, if the other
   party does not support ALPN, if this socket does not support any of
   the client's proposed protocols, or if the handshake has not
   happened yet, "None" is returned.

   New in version 3.5.

SSLSocket.selected_npn_protocol()

   Return the higher-level 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.version()

   Return the actual SSL protocol version negotiated by the connection
   as a string, or "None" is no secure connection is established. As
   of this writing, possible return values include ""SSLv2"",
   ""SSLv3"", ""TLSv1"", ""TLSv1.1"" and ""TLSv1.2"". Recent OpenSSL
   versions may define more return values.

   New in version 3.5.

SSLSocket.pending()

   Returns the number of already decrypted bytes available for read,
   pending on the connection.

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.

SSLSocket.server_side

   A boolean which is "True" for server-side sockets and "False" for
   client-side sockets.

   New in version 3.2.

SSLSocket.server_hostname

   Hostname of the server: "str" type, or "None" for server-side
   socket or if the hostname was not specified in the constructor.

   New in version 3.2.

SSLSocket.session

   The "SSLSession" for this SSL connection. The session is available
   for client and server side sockets after the TLS handshake has been
   performed. For client sockets the session can be set before
   "do_handshake()" has been called to reuse a session.

   New in version 3.6.

SSLSocket.session_reused

   New in version 3.6.


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 ssl.SSLContext(protocol=PROTOCOL_TLS)

   Create a new SSL context.  You may pass *protocol* which must be
   one of the "PROTOCOL_*" constants defined in this module.
   "PROTOCOL_TLS" is currently recommended for maximum
   interoperability and default value.

   See also: "create_default_context()" lets the "ssl" module choose
     security settings for a given purpose.

   Changed in version 3.6: The context is created with secure default
   values. The options "OP_NO_COMPRESSION",
   "OP_CIPHER_SERVER_PREFERENCE", "OP_SINGLE_DH_USE",
   "OP_SINGLE_ECDH_USE", "OP_NO_SSLv2" (except for "PROTOCOL_SSLv2"),
   and "OP_NO_SSLv3" (except for "PROTOCOL_SSLv3") are set by default.
   The initial cipher suite list contains only "HIGH" ciphers, no
   "NULL" ciphers and no "MD5" ciphers (except for "PROTOCOL_SSLv2").

"SSLContext" objects have the following methods and attributes:

SSLContext.cert_store_stats()

   Get statistics about quantities of loaded X.509 certificates, count
   of X.509 certificates flagged as CA certificates and certificate
   revocation lists as dictionary.

   Example for a context with one CA cert and one other cert:

      >>> context.cert_store_stats()
      {'crl': 0, 'x509_ca': 1, 'x509': 2}

   New in version 3.4.

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_default_certs(purpose=Purpose.SERVER_AUTH)

   Load a set of default "certification authority" (CA) certificates
   from default locations. On Windows it loads CA certs from the "CA"
   and "ROOT" system stores. On other systems it calls
   "SSLContext.set_default_verify_paths()". In the future the method
   may load CA certificates from other locations, too.

   The *purpose* flag specifies what kind of CA certificates are
   loaded. The default settings "Purpose.SERVER_AUTH" loads
   certificates, that are flagged and trusted for TLS web server
   authentication (client side sockets). "Purpose.CLIENT_AUTH" loads
   CA certificates for client certificate verification on the server
   side.

   New in version 3.4.

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

   This method can also load certification revocation lists (CRLs) in
   PEM or DER format. In order to make use of CRLs,
   "SSLContext.verify_flags" must be configured properly.

   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.

   The *cadata* object, if present, is either an ASCII string of one
   or more PEM-encoded certificates or a *bytes-like object* of DER-
   encoded certificates. Like with *capath* extra lines around PEM-
   encoded certificates are ignored but at least one certificate must
   be present.

   Changed in version 3.4: New optional argument *cadata*

SSLContext.get_ca_certs(binary_form=False)

   Get a list of loaded "certification authority" (CA) certificates.
   If the "binary_form" parameter is "False" each list entry is a dict
   like the output of "SSLSocket.getpeercert()". Otherwise the method
   returns a list of DER-encoded certificates. The returned list does
   not contain certificates from *capath* unless a certificate was
   requested and loaded by a SSL connection.

   Note: Certificates in a capath directory aren't loaded unless
     they have been used at least once.

   New in version 3.4.

SSLContext.get_ciphers()

   Get a list of enabled ciphers. The list is in order of cipher
   priority. See "SSLContext.set_ciphers()".

   Example:

      >>> ctx = ssl.SSLContext(ssl.PROTOCOL_SSLv23)
      >>> ctx.set_ciphers('ECDHE+AESGCM:!ECDSA')
      >>> ctx.get_ciphers()  # OpenSSL 1.0.x
      [{'alg_bits': 256,
        'description': 'ECDHE-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH     Au=RSA  '
                       'Enc=AESGCM(256) Mac=AEAD',
        'id': 50380848,
        'name': 'ECDHE-RSA-AES256-GCM-SHA384',
        'protocol': 'TLSv1/SSLv3',
        'strength_bits': 256},
       {'alg_bits': 128,
        'description': 'ECDHE-RSA-AES128-GCM-SHA256 TLSv1.2 Kx=ECDH     Au=RSA  '
                       'Enc=AESGCM(128) Mac=AEAD',
        'id': 50380847,
        'name': 'ECDHE-RSA-AES128-GCM-SHA256',
        'protocol': 'TLSv1/SSLv3',
        'strength_bits': 128}]

   On OpenSSL 1.1 and newer the cipher dict contains additional
   fields::
      >>> ctx.get_ciphers()  # OpenSSL 1.1+
      [{'aead': True,
        'alg_bits': 256,
        'auth': 'auth-rsa',
        'description': 'ECDHE-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH     Au=RSA  '
                       'Enc=AESGCM(256) Mac=AEAD',
        'digest': None,
        'id': 50380848,
        'kea': 'kx-ecdhe',
        'name': 'ECDHE-RSA-AES256-GCM-SHA384',
        'protocol': 'TLSv1.2',
        'strength_bits': 256,
        'symmetric': 'aes-256-gcm'},
       {'aead': True,
        'alg_bits': 128,
        'auth': 'auth-rsa',
        'description': 'ECDHE-RSA-AES128-GCM-SHA256 TLSv1.2 Kx=ECDH     Au=RSA  '
                       'Enc=AESGCM(128) Mac=AEAD',
        'digest': None,
        'id': 50380847,
        'kea': 'kx-ecdhe',
        'name': 'ECDHE-RSA-AES128-GCM-SHA256',
        'protocol': 'TLSv1.2',
        'strength_bits': 128,
        'symmetric': 'aes-128-gcm'}]

   Availability: OpenSSL 1.0.2+

   New in version 3.6.

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_alpn_protocols(protocols)

   Specify which protocols the socket should advertise during the
   SSL/TLS handshake. It should be a list of ASCII 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 **RFC 7301**. After a successful handshake, the
   "SSLSocket.selected_alpn_protocol()" method will return the agreed-
   upon protocol.

   This method will raise "NotImplementedError" if "HAS_ALPN" is
   False.

   OpenSSL 1.1.0+ will abort the handshake and raise "SSLError" when
   both sides support ALPN but cannot agree on a protocol.

   New in version 3.5.

SSLContext.set_npn_protocols(protocols)

   Specify which protocols the socket should advertise 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.set_servername_callback(server_name_callback)

   Register a callback function that will be called after the TLS
   Client Hello handshake message has been received by the SSL/TLS
   server when the TLS client specifies a server name indication. The
   server name indication mechanism is specified in **RFC 6066**
   section 3 - Server Name Indication.

   Only one callback can be set per "SSLContext".  If
   *server_name_callback* is "None" then the callback is disabled.
   Calling this function a subsequent time will disable the previously
   registered callback.

   The callback function, *server_name_callback*, will be called with
   three arguments; the first being the "ssl.SSLSocket", the second is
   a string that represents the server name that the client is
   intending to communicate (or "None" if the TLS Client Hello does
   not contain a server name) and the third argument is the original
   "SSLContext". The server name argument is the IDNA decoded server
   name.

   A typical use of this callback is to change the "ssl.SSLSocket"'s
   "SSLSocket.context" attribute to a new object of type "SSLContext"
   representing a certificate chain that matches the server name.

   Due to the early negotiation phase of the TLS connection, only
   limited methods and attributes are usable like
   "SSLSocket.selected_alpn_protocol()" and "SSLSocket.context".
   "SSLSocket.getpeercert()", "SSLSocket.getpeercert()",
   "SSLSocket.cipher()" and "SSLSocket.compress()" methods require
   that the TLS connection has progressed beyond the TLS Client Hello
   and therefore will not contain return meaningful values nor can
   they be called safely.

   The *server_name_callback* function must return "None" to allow the
   TLS negotiation to continue.  If a TLS failure is required, a
   constant "ALERT_DESCRIPTION_*" can be returned.  Other return
   values will result in a TLS fatal error with
   "ALERT_DESCRIPTION_INTERNAL_ERROR".

   If there is an IDNA decoding error on the server name, the TLS
   connection will terminate with an
   "ALERT_DESCRIPTION_INTERNAL_ERROR" fatal TLS alert message to the
   client.

   If an exception is raised from the *server_name_callback* function
   the TLS connection will terminate with a fatal TLS alert message
   "ALERT_DESCRIPTION_HANDSHAKE_FAILURE".

   This method will raise "NotImplementedError" if the OpenSSL library
   had OPENSSL_NO_TLSEXT defined when it was built.

   New in version 3.4.

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, session=None)

   Wrap an existing Python socket *sock* and return an "SSLSocket"
   object.  *sock* must be a "SOCK_STREAM" socket; other socket types
   are unsupported.

   The returned 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
   *server_side* is true.

   *session*, see "session".

   Changed in version 3.5: Always allow a server_hostname to be
   passed, even if OpenSSL does not have SNI.

   Changed in version 3.6: *session* argument was added.

SSLContext.wrap_bio(incoming, outgoing, server_side=False, server_hostname=None, session=None)

   Create a new "SSLObject" instance by wrapping the BIO objects
   *incoming* and *outgoing*. The SSL routines will read input data
   from the incoming BIO and write data to the outgoing BIO.

   The *server_side*, *server_hostname* and *session* parameters have
   the same meaning as in "SSLContext.wrap_socket()".

   Changed in version 3.6: *session* argument was added.

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.check_hostname

   Whether to match the peer cert's hostname with "match_hostname()"
   in "SSLSocket.do_handshake()". The context's "verify_mode" must be
   set to "CERT_OPTIONAL" or "CERT_REQUIRED", and you must pass
   *server_hostname* to "wrap_socket()" in order to match the
   hostname.

   Example:

      import socket, ssl

      context = ssl.SSLContext(ssl.PROTOCOL_TLSv1)
      context.verify_mode = ssl.CERT_REQUIRED
      context.check_hostname = True
      context.load_default_certs()

      s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
      ssl_sock = context.wrap_socket(s, server_hostname='www.verisign.com')
      ssl_sock.connect(('www.verisign.com', 443))

   New in version 3.4.

   Note: This features requires OpenSSL 0.9.8f or newer.

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".

   Changed in version 3.6: "SSLContext.options" returns "Options"
   flags:

   >>> ssl.create_default_context().options
   <Options.OP_ALL|OP_NO_SSLv3|OP_NO_SSLv2|OP_NO_COMPRESSION: 2197947391>

SSLContext.protocol

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

SSLContext.verify_flags

   The flags for certificate verification operations. You can set
   flags like "VERIFY_CRL_CHECK_LEAF" by ORing them together. By
   default OpenSSL does neither require nor verify certificate
   revocation lists (CRLs). Available only with openssl version
   0.9.8+.

   New in version 3.4.

   Changed in version 3.6: "SSLContext.verify_flags" returns
   "VerifyFlags" flags:

   >>> ssl.create_default_context().verify_flags
   <VerifyFlags.VERIFY_X509_TRUSTED_FIRST: 32768>

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".

   Changed in version 3.6: "SSLContext.verify_mode" returns
   "VerifyMode" enum:

   >>> ssl.create_default_context().verify_mode
   <VerifyMode.CERT_REQUIRED: 2>


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.  The platform's certificates file can be used
by calling "SSLContext.load_default_certs()", this is done
automatically with "create_default_context()".


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 creates a SSL context with the recommended security
settings for client sockets, including automatic certificate
verification:

   >>> context = ssl.create_default_context()

If you prefer to tune security settings yourself, you might create a
context from scratch (but beware that you might not get the settings
right):

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

(this snippet assumes 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),
   ...                            server_hostname="www.python.org")
   >>> conn.connect(("www.python.org", 443))

You may then fetch the certificate:

   >>> cert = conn.getpeercert()

Visual inspection shows that the certificate does identify the desired
service (that is, the HTTPS host "www.python.org"):

   >>> pprint.pprint(cert)
   {'OCSP': ('http://ocsp.digicert.com',),
    'caIssuers': ('http://cacerts.digicert.com/DigiCertSHA2ExtendedValidationServerCA.crt',),
    'crlDistributionPoints': ('http://crl3.digicert.com/sha2-ev-server-g1.crl',
                              'http://crl4.digicert.com/sha2-ev-server-g1.crl'),
    'issuer': ((('countryName', 'US'),),
               (('organizationName', 'DigiCert Inc'),),
               (('organizationalUnitName', 'www.digicert.com'),),
               (('commonName', 'DigiCert SHA2 Extended Validation Server CA'),)),
    'notAfter': 'Sep  9 12:00:00 2016 GMT',
    'notBefore': 'Sep  5 00:00:00 2014 GMT',
    'serialNumber': '01BB6F00122B177F36CAB49CEA8B6B26',
    'subject': ((('businessCategory', 'Private Organization'),),
                (('1.3.6.1.4.1.311.60.2.1.3', 'US'),),
                (('1.3.6.1.4.1.311.60.2.1.2', 'Delaware'),),
                (('serialNumber', '3359300'),),
                (('streetAddress', '16 Allen Rd'),),
                (('postalCode', '03894-4801'),),
                (('countryName', 'US'),),
                (('stateOrProvinceName', 'NH'),),
                (('localityName', 'Wolfeboro,'),),
                (('organizationName', 'Python Software Foundation'),),
                (('commonName', 'www.python.org'),)),
    'subjectAltName': (('DNS', 'www.python.org'),
                       ('DNS', 'python.org'),
                       ('DNS', 'pypi.python.org'),
                       ('DNS', 'docs.python.org'),
                       ('DNS', 'testpypi.python.org'),
                       ('DNS', 'bugs.python.org'),
                       ('DNS', 'wiki.python.org'),
                       ('DNS', 'hg.python.org'),
                       ('DNS', 'mail.python.org'),
                       ('DNS', 'packaging.python.org'),
                       ('DNS', 'pythonhosted.org'),
                       ('DNS', 'www.pythonhosted.org'),
                       ('DNS', 'test.pythonhosted.org'),
                       ('DNS', 'us.pycon.org'),
                       ('DNS', 'id.python.org')),
    'version': 3}

Now the SSL channel is established and the certificate verified, 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/1.1 200 OK',
    b'Date: Sat, 18 Oct 2014 18:27:20 GMT',
    b'Server: nginx',
    b'Content-Type: text/html; charset=utf-8',
    b'X-Frame-Options: SAMEORIGIN',
    b'Content-Length: 45679',
    b'Accept-Ranges: bytes',
    b'Via: 1.1 varnish',
    b'Age: 2188',
    b'X-Served-By: cache-lcy1134-LCY',
    b'X-Cache: HIT',
    b'X-Cache-Hits: 11',
    b'Vary: Cookie',
    b'Strict-Transport-Security: max-age=63072000; includeSubDomains',
    b'Connection: close',
    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.create_default_context(ssl.Purpose.CLIENT_AUTH)
   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
=============================

SSL sockets behave slightly different than regular sockets in non-
blocking mode. When working with non-blocking sockets, there are thus
several things you need to be aware of:

* Most "SSLSocket" methods will raise either "SSLWantWriteError" or
  "SSLWantReadError" instead of "BlockingIOError" if an I/O operation
  would block. "SSLWantReadError" will be raised if a read operation
  on the underlying socket is necessary, and "SSLWantWriteError" for a
  write operation on the underlying socket. Note that attempts to
  *write* to an SSL socket may require *reading* from the underlying
  socket first, and attempts to *read* from the SSL socket may require
  a prior *write* to the underlying socket.

  Changed in version 3.5: In earlier Python versions, the
  "SSLSocket.send()" method returned zero instead of raising
  "SSLWantWriteError" or "SSLWantReadError".

* 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()".

* Conversely, since the SSL layer has its own framing, a SSL socket
  may still have data available for reading without "select()" being
  aware of it.  Therefore, you should first call "SSLSocket.recv()" to
  drain any potentially available data, and then only block on a
  "select()" call if still necessary.

  (of course, similar provisions apply when using other primitives
  such as "poll()", or those in the "selectors" module)

* 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], [])

See also: The "asyncio" module supports non-blocking SSL sockets and
  provides a higher level API. It polls for events using the
  "selectors" module and handles "SSLWantWriteError",
  "SSLWantReadError" and "BlockingIOError" exceptions. It runs the SSL
  handshake asynchronously as well.


Memory BIO Support
==================

New in version 3.5.

Ever since the SSL module was introduced in Python 2.6, the
"SSLSocket" class has provided two related but distinct areas of
functionality:

* SSL protocol handling

* Network IO

The network IO API is identical to that provided by "socket.socket",
from which "SSLSocket" also inherits. This allows an SSL socket to be
used as a drop-in replacement for a regular socket, making it very
easy to add SSL support to an existing application.

Combining SSL protocol handling and network IO usually works well, but
there are some cases where it doesn't. An example is async IO
frameworks that want to use a different IO multiplexing model than the
"select/poll on a file descriptor" (readiness based) model that is
assumed by "socket.socket" and by the internal OpenSSL socket IO
routines. This is mostly relevant for platforms like Windows where
this model is not efficient. For this purpose, a reduced scope variant
of "SSLSocket" called "SSLObject" is provided.

class ssl.SSLObject

   A reduced-scope variant of "SSLSocket" representing an SSL protocol
   instance that does not contain any network IO methods. This class
   is typically used by framework authors that want to implement
   asynchronous IO for SSL through memory buffers.

   This class implements an interface on top of a low-level SSL object
   as implemented by OpenSSL. This object captures the state of an SSL
   connection but does not provide any network IO itself. IO needs to
   be performed through separate "BIO" objects which are OpenSSL's IO
   abstraction layer.

   An "SSLObject" instance can be created using the "wrap_bio()"
   method. This method will create the "SSLObject" instance and bind
   it to a pair of BIOs. The *incoming* BIO is used to pass data from
   Python to the SSL protocol instance, while the *outgoing* BIO is
   used to pass data the other way around.

   The following methods are available:

   * "context"

   * "server_side"

   * "server_hostname"

   * "session"

   * "session_reused"

   * "read()"

   * "write()"

   * "getpeercert()"

   * "selected_npn_protocol()"

   * "cipher()"

   * "shared_ciphers()"

   * "compression()"

   * "pending()"

   * "do_handshake()"

   * "unwrap()"

   * "get_channel_binding()"

   When compared to "SSLSocket", this object lacks the following
   features:

   * Any form of network IO; "recv()" and "send()" read and write
     only to the underlying "MemoryBIO" buffers.

   * There is no *do_handshake_on_connect* machinery. You must
     always manually call "do_handshake()" to start the handshake.

   * There is no handling of *suppress_ragged_eofs*. All end-of-file
     conditions that are in violation of the protocol are reported via
     the "SSLEOFError" exception.

   * The method "unwrap()" call does not return anything, unlike for
     an SSL socket where it returns the underlying socket.

   * The *server_name_callback* callback passed to
     "SSLContext.set_servername_callback()" will get an "SSLObject"
     instance instead of a "SSLSocket" instance as its first
     parameter.

   Some notes related to the use of "SSLObject":

   * All IO on an "SSLObject" is non-blocking. This means that for
     example "read()" will raise an "SSLWantReadError" if it needs
     more data than the incoming BIO has available.

   * There is no module-level "wrap_bio()" call like there is for
     "wrap_socket()". An "SSLObject" is always created via an
     "SSLContext".

An SSLObject communicates with the outside world using memory buffers.
The class "MemoryBIO" provides a memory buffer that can be used for
this purpose.  It wraps an OpenSSL memory BIO (Basic IO) object:

class ssl.MemoryBIO

   A memory buffer that can be used to pass data between Python and an
   SSL protocol instance.

   pending

      Return the number of bytes currently in the memory buffer.

   eof

      A boolean indicating whether the memory BIO is current at the
      end-of-file position.

   read(n=-1)

      Read up to *n* bytes from the memory buffer. If *n* is not
      specified or negative, all bytes are returned.

   write(buf)

      Write the bytes from *buf* to the memory BIO. The *buf* argument
      must be an object supporting the buffer protocol.

      The return value is the number of bytes written, which is always
      equal to the length of *buf*.

   write_eof()

      Write an EOF marker to the memory BIO. After this method has
      been called, it is illegal to call "write()". The attribute
      "eof" will become true after all data currently in the buffer
      has been read.


SSL session
===========

New in version 3.6.

class ssl.SSLSession

   Session object used by "session".

   id

   time

   timeout

   ticket_lifetime_hint

   has_ticket


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


Best defaults
-------------

For **client use**, if you don't have any special requirements for
your security policy, it is highly recommended that you use the
"create_default_context()" function to create your SSL context. It
will load the system's trusted CA certificates, enable certificate
validation and hostname checking, and try to choose reasonably secure
protocol and cipher settings.

For example, here is how you would use the "smtplib.SMTP" class to
create a trusted, secure connection to a SMTP server:

   >>> import ssl, smtplib
   >>> smtp = smtplib.SMTP("mail.python.org", port=587)
   >>> context = ssl.create_default_context()
   >>> smtp.starttls(context=context)
   (220, b'2.0.0 Ready to start TLS')

If a client certificate is needed for the connection, it can be added
with "SSLContext.load_cert_chain()".

By contrast, if you create the SSL context by calling the "SSLContext"
constructor yourself, it will not have certificate validation nor
hostname checking enabled by default.  If you do so, please read the
paragraphs below to achieve a good security level.


Manual settings
---------------


Verifying certificates
~~~~~~~~~~~~~~~~~~~~~~

When calling the "SSLContext" constructor directly, "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.
This common check is automatically performed when
"SSLContext.check_hostname" is enabled.

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 versions 2 and 3 are considered insecure and are therefore
dangerous to use.  If you want maximum compatibility between clients
and servers, it is recommended to use "PROTOCOL_TLS_CLIENT" or
"PROTOCOL_TLS_SERVER" as the protocol version. SSLv2 and SSLv3 are
disabled by default.

>>> client_context = ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT)
>>> client_context.options |= ssl.OP_NO_TLSv1
>>> client_context.options |= ssl.OP_NO_TLSv1_1

The SSL context created above will only allow TLSv1.2 and later (if
supported by your system) connections to a server.
"PROTOCOL_TLS_CLIENT" implies certificate validation and hostname
checks by default. You have to load certificates into the context.


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. 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
"SSLContext.get_ciphers()" or the "openssl ciphers" command on your
system.


Multi-processing
----------------

If using this module as part of a multi-processed application (using,
for example the "multiprocessing" or "concurrent.futures" modules), be
aware that OpenSSL's internal random number generator does not
properly handle forked processes.  Applications must change the PRNG
state of the parent process if they use any SSL feature with
"os.fork()".  Any successful call of "RAND_add()", "RAND_bytes()" or
"RAND_pseudo_bytes()" is sufficient.

See also:

  Class "socket.socket"
     Documentation of underlying "socket" class

  SSL/TLS Strong Encryption: An Introduction
     Intro from the Apache webserver documentation

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

  RFC 4086: Randomness Requirements for Security
     Donald E., Jeffrey I. Schiller

  RFC 5280: Internet X.509 Public Key Infrastructure Certificate and
  Certificate Revocation List (CRL) Profile
     D. Cooper

  RFC 5246: The Transport Layer Security (TLS) Protocol Version 1.2
     T. Dierks et. al.

  RFC 6066: Transport Layer Security (TLS) Extensions
     D. Eastlake

  IANA TLS: Transport Layer Security (TLS) Parameters
     IANA
