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:mod:`!ssl` --- TLS/SSL wrapper for socket objects
==================================================
.. module:: ssl
:synopsis: TLS/SSL wrapper for socket objects
.. moduleauthor:: Bill Janssen <bill.janssen@gmail.com>
.. sectionauthor:: Bill Janssen <bill.janssen@gmail.com>
**Source code:** :source:`Lib/ssl.py`
.. index:: single: OpenSSL; (use in module ssl)
.. index:: TLS, SSL, Transport Layer Security, Secure Sockets Layer
--------------
This module provides access to Transport Layer Security (often known as "Secure
Sockets Layer") encryption and peer authentication facilities for network
sockets, both client-side and server-side. This module uses the OpenSSL
library. It is available on all modern Unix systems, Windows, macOS, 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, TLSv1.3 comes with OpenSSL version
1.1.1.
.. warning::
Don't use this module without reading the :ref:`ssl-security`. 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.
.. include:: ../includes/wasm-notavail.rst
This section documents the objects and functions in the ``ssl`` module; for more
general information about TLS, SSL, and certificates, the reader is referred to
the documents in the "See Also" section at the bottom.
This module provides a class, :class:`ssl.SSLSocket`, which is derived from the
:class:`socket.socket` type, and provides a socket-like wrapper that also
encrypts and decrypts the data going over the socket with SSL. It supports
additional methods such as :meth:`getpeercert`, which retrieves the
certificate of the other side of the connection, :meth:`cipher`, which
retrieves the cipher being used for the secure connection or
:meth:`get_verified_chain`, :meth:`get_unverified_chain` which retrieves
certificate chain.
For more sophisticated applications, the :class:`ssl.SSLContext` class
helps manage settings and certificates, which can then be inherited
by SSL sockets created through the :meth:`SSLContext.wrap_socket` method.
.. versionchanged:: 3.5.3
Updated to support linking with OpenSSL 1.1.0
.. versionchanged:: 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.
.. versionchanged:: 3.10
:pep:`644` has been implemented. The ssl module requires OpenSSL 1.1.1
or newer.
Use of deprecated constants and functions result in deprecation warnings.
Functions, Constants, and Exceptions
------------------------------------
Socket creation
^^^^^^^^^^^^^^^
Instances of :class:`SSLSocket` must be created using the
:meth:`SSLContext.wrap_socket` method. The helper function
:func:`create_default_context` returns a new context with secure default
settings.
Client socket example with default context and IPv4/IPv6 dual stack::
import socket
import ssl
hostname = 'www.python.org'
context = ssl.create_default_context()
with socket.create_connection((hostname, 443)) as sock:
with context.wrap_socket(sock, server_hostname=hostname) as ssock:
print(ssock.version())
Client socket example with custom context and IPv4::
hostname = 'www.python.org'
# PROTOCOL_TLS_CLIENT requires valid cert chain and hostname
context = ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT)
context.load_verify_locations('path/to/cabundle.pem')
with socket.socket(socket.AF_INET, socket.SOCK_STREAM, 0) as sock:
with context.wrap_socket(sock, server_hostname=hostname) as ssock:
print(ssock.version())
Server socket example listening on localhost IPv4::
context = ssl.SSLContext(ssl.PROTOCOL_TLS_SERVER)
context.load_cert_chain('/path/to/certchain.pem', '/path/to/private.key')
with socket.socket(socket.AF_INET, socket.SOCK_STREAM, 0) as sock:
sock.bind(('127.0.0.1', 8443))
sock.listen(5)
with context.wrap_socket(sock, server_side=True) as ssock:
conn, addr = ssock.accept()
...
Context creation
^^^^^^^^^^^^^^^^
A convenience function helps create :class:`SSLContext` objects for common
purposes.
.. function:: create_default_context(purpose=Purpose.SERVER_AUTH, cafile=None, capath=None, cadata=None)
Return a new :class:`SSLContext` object with default settings for
the given *purpose*. The settings are chosen by the :mod:`ssl` module,
and usually represent a higher security level than when calling the
:class:`SSLContext` constructor directly.
*cafile*, *capath*, *cadata* represent optional CA certificates to
trust for certificate verification, as in
:meth:`SSLContext.load_verify_locations`. If all three are
:const:`None`, this function can choose to trust the system's default
CA certificates instead.
The settings are: :data:`PROTOCOL_TLS_CLIENT` or
:data:`PROTOCOL_TLS_SERVER`, :data:`OP_NO_SSLv2`, and :data:`OP_NO_SSLv3`
with high encryption cipher suites without RC4 and
without unauthenticated cipher suites. Passing :const:`~Purpose.SERVER_AUTH`
as *purpose* sets :data:`~SSLContext.verify_mode` to :data:`CERT_REQUIRED`
and either loads CA certificates (when at least one of *cafile*, *capath* or
*cadata* is given) or uses :meth:`SSLContext.load_default_certs` to load
default CA certificates.
When :attr:`~SSLContext.keylog_filename` is supported and the environment
variable :envvar:`SSLKEYLOGFILE` is set, :func:`create_default_context`
enables key logging.
The default settings for this context include
:data:`VERIFY_X509_PARTIAL_CHAIN` and :data:`VERIFY_X509_STRICT`.
These make the underlying OpenSSL implementation behave more like
a conforming implementation of :rfc:`5280`, in exchange for a small
amount of incompatibility with older X.509 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
:class:`SSLContext` and apply the settings yourself.
.. note::
If you find that when certain older clients or servers attempt to connect
with a :class:`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
:data:`OP_NO_SSLv3`. SSL3.0 is widely considered to be `completely broken
<https://en.wikipedia.org/wiki/POODLE>`_. 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
.. note::
This context enables :data:`VERIFY_X509_STRICT` by default, which
may reject pre-:rfc:`5280` or malformed certificates that the
underlying OpenSSL implementation otherwise would accept. While disabling
this is not recommended, you can do so using::
ctx = ssl.create_default_context()
ctx.verify_flags &= ~ssl.VERIFY_X509_STRICT
.. versionadded:: 3.4
.. versionchanged:: 3.4.4
RC4 was dropped from the default cipher string.
.. versionchanged:: 3.6
ChaCha20/Poly1305 was added to the default cipher string.
3DES was dropped from the default cipher string.
.. versionchanged:: 3.8
Support for key logging to :envvar:`SSLKEYLOGFILE` was added.
.. versionchanged:: 3.10
The context now uses :data:`PROTOCOL_TLS_CLIENT` or
:data:`PROTOCOL_TLS_SERVER` protocol instead of generic
:data:`PROTOCOL_TLS`.
.. versionchanged:: 3.13
The context now uses :data:`VERIFY_X509_PARTIAL_CHAIN` and
:data:`VERIFY_X509_STRICT` in its default verify flags.
Exceptions
^^^^^^^^^^
.. exception:: 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 :exc:`OSError`. The error code and message of
:exc:`SSLError` instances are provided by the OpenSSL library.
.. versionchanged:: 3.3
:exc:`SSLError` used to be a subtype of :exc:`socket.error`.
.. attribute:: 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.
.. versionadded:: 3.3
.. attribute:: 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.
.. versionadded:: 3.3
.. exception:: SSLZeroReturnError
A subclass of :exc:`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.
.. versionadded:: 3.3
.. exception:: SSLWantReadError
A subclass of :exc:`SSLError` raised by a :ref:`non-blocking SSL socket
<ssl-nonblocking>` 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.
.. versionadded:: 3.3
.. exception:: SSLWantWriteError
A subclass of :exc:`SSLError` raised by a :ref:`non-blocking SSL socket
<ssl-nonblocking>` 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.
.. versionadded:: 3.3
.. exception:: SSLSyscallError
A subclass of :exc:`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.
.. versionadded:: 3.3
.. exception:: SSLEOFError
A subclass of :exc:`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.
.. versionadded:: 3.3
.. exception:: SSLCertVerificationError
A subclass of :exc:`SSLError` raised when certificate validation has
failed.
.. versionadded:: 3.7
.. attribute:: verify_code
A numeric error number that denotes the verification error.
.. attribute:: verify_message
A human readable string of the verification error.
.. exception:: CertificateError
An alias for :exc:`SSLCertVerificationError`.
.. versionchanged:: 3.7
The exception is now an alias for :exc:`SSLCertVerificationError`.
Random generation
^^^^^^^^^^^^^^^^^
.. function:: RAND_bytes(num)
Return *num* cryptographically strong pseudo-random bytes. Raises an
:class:`SSLError` if the PRNG has not been seeded with enough data or if the
operation is not supported by the current RAND method. :func:`RAND_status`
can be used to check the status of the PRNG and :func:`RAND_add` can be used
to seed the PRNG.
For almost all applications :func:`os.urandom` is preferable.
Read the Wikipedia article, `Cryptographically secure pseudorandom number
generator (CSPRNG)
<https://en.wikipedia.org/wiki/Cryptographically_secure_pseudorandom_number_generator>`_,
to get the requirements of a cryptographically strong generator.
.. versionadded:: 3.3
.. function:: RAND_status()
Return ``True`` if the SSL pseudo-random number generator has been seeded
with 'enough' randomness, and ``False`` otherwise. You can use
:func:`ssl.RAND_egd` and :func:`ssl.RAND_add` to increase the randomness of
the pseudo-random number generator.
.. function:: RAND_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.
.. versionchanged:: 3.5
Writable :term:`bytes-like object` is now accepted.
Certificate handling
^^^^^^^^^^^^^^^^^^^^
.. testsetup::
import ssl
.. function:: 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:
.. doctest:: newcontext
>>> import ssl
>>> timestamp = ssl.cert_time_to_seconds("Jan 5 09:34:43 2018 GMT")
>>> timestamp # doctest: +SKIP
1515144883
>>> from datetime import datetime
>>> print(datetime.utcfromtimestamp(timestamp)) # doctest: +SKIP
2018-01-05 09:34:43
"notBefore" or "notAfter" dates must use GMT (:rfc:`5280`).
.. versionchanged:: 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)
.. function:: get_server_certificate(addr, ssl_version=PROTOCOL_TLS_CLIENT, \
ca_certs=None[, timeout])
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 *cafile* parameter in
:meth:`SSLContext.load_verify_locations`. The call will attempt to validate the
server certificate against that set of root certificates, and will fail
if the validation attempt fails. A timeout can be specified with the
``timeout`` parameter.
.. versionchanged:: 3.3
This function is now IPv6-compatible.
.. versionchanged:: 3.5
The default *ssl_version* is changed from :data:`PROTOCOL_SSLv3` to
:data:`PROTOCOL_TLS` for maximum compatibility with modern servers.
.. versionchanged:: 3.10
The *timeout* parameter was added.
.. function:: DER_cert_to_PEM_cert(DER_cert_bytes)
Given a certificate as a DER-encoded blob of bytes, returns a PEM-encoded
string version of the same certificate.
.. function:: PEM_cert_to_DER_cert(PEM_cert_string)
Given a certificate as an ASCII PEM string, returns a DER-encoded sequence of
bytes for that same certificate.
.. function:: 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
:meth:`SSLContext.set_default_verify_paths`. The return value is a
:term:`named tuple` ``DefaultVerifyPaths``:
* :attr:`cafile` - resolved path to cafile or ``None`` if the file doesn't exist,
* :attr:`capath` - resolved path to capath or ``None`` if the directory doesn't exist,
* :attr:`openssl_cafile_env` - OpenSSL's environment key that points to a cafile,
* :attr:`openssl_cafile` - hard coded path to a cafile,
* :attr:`openssl_capath_env` - OpenSSL's environment key that points to a capath,
* :attr:`openssl_capath` - hard coded path to a capath directory
.. versionadded:: 3.4
.. function:: 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
:const:`x509_asn` for X.509 ASN.1 data or :const:`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.
.. versionadded:: 3.4
.. function:: 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
:const:`x509_asn` for X.509 ASN.1 data or :const:`pkcs_7_asn` for
PKCS#7 ASN.1 data.
.. availability:: Windows.
.. versionadded:: 3.4
Constants
^^^^^^^^^
All constants are now :class:`enum.IntEnum` or :class:`enum.IntFlag` collections.
.. versionadded:: 3.6
.. data:: CERT_NONE
Possible value for :attr:`SSLContext.verify_mode`.
Except for :const:`PROTOCOL_TLS_CLIENT`,
it is the default mode. With client-side sockets, just about any
cert is accepted. Validation errors, such as untrusted or expired cert,
are ignored and do not abort the TLS/SSL handshake.
In server mode, no certificate is requested from the client, so the client
does not send any for client cert authentication.
See the discussion of :ref:`ssl-security` below.
.. data:: CERT_OPTIONAL
Possible value for :attr:`SSLContext.verify_mode`.
In client mode, :const:`CERT_OPTIONAL`
has the same meaning as :const:`CERT_REQUIRED`. It is recommended to
use :const:`CERT_REQUIRED` for client-side sockets instead.
In server mode, a client certificate request is sent to the client. The
client may either ignore the request or send a certificate in order
perform TLS client cert authentication. If the client chooses to send
a certificate, it is verified. Any verification error immediately aborts
the TLS handshake.
Use of this setting requires a valid set of CA certificates to
be passed to :meth:`SSLContext.load_verify_locations`.
.. data:: CERT_REQUIRED
Possible value for :attr:`SSLContext.verify_mode`.
In this mode, certificates are
required from the other side of the socket connection; an :class:`SSLError`
will be raised if no certificate is provided, or if its validation fails.
This mode is **not** sufficient to verify a certificate in client mode as
it does not match hostnames. :attr:`~SSLContext.check_hostname` must be
enabled as well to verify the authenticity of a cert.
:const:`PROTOCOL_TLS_CLIENT` uses :const:`CERT_REQUIRED` and
enables :attr:`~SSLContext.check_hostname` by default.
With server socket, this mode provides mandatory TLS client cert
authentication. A client certificate request is sent to the client and
the client must provide a valid and trusted certificate.
Use of this setting requires a valid set of CA certificates to
be passed to :meth:`SSLContext.load_verify_locations`.
.. class:: VerifyMode
:class:`enum.IntEnum` collection of CERT_* constants.
.. versionadded:: 3.6
.. data:: VERIFY_DEFAULT
Possible value for :attr:`SSLContext.verify_flags`. In this mode, certificate
revocation lists (CRLs) are not checked. By default OpenSSL does neither
require nor verify CRLs.
.. versionadded:: 3.4
.. data:: VERIFY_CRL_CHECK_LEAF
Possible value for :attr:`SSLContext.verify_flags`. In this mode, only the
peer cert is checked but none 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 CRL has been loaded with
:attr:`SSLContext.load_verify_locations`, validation will fail.
.. versionadded:: 3.4
.. data:: VERIFY_CRL_CHECK_CHAIN
Possible value for :attr:`SSLContext.verify_flags`. In this mode, CRLs of
all certificates in the peer cert chain are checked.
.. versionadded:: 3.4
.. data:: VERIFY_X509_STRICT
Possible value for :attr:`SSLContext.verify_flags` to disable workarounds
for broken X.509 certificates.
.. versionadded:: 3.4
.. data:: VERIFY_ALLOW_PROXY_CERTS
Possible value for :attr:`SSLContext.verify_flags` to enables proxy
certificate verification.
.. versionadded:: 3.10
.. data:: VERIFY_X509_TRUSTED_FIRST
Possible value for :attr:`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.
.. versionadded:: 3.4.4
.. data:: VERIFY_X509_PARTIAL_CHAIN
Possible value for :attr:`SSLContext.verify_flags`. It instructs OpenSSL to
accept intermediate CAs in the trust store to be treated as trust-anchors,
in the same way as the self-signed root CA certificates. This makes it
possible to trust certificates issued by an intermediate CA without having
to trust its ancestor root CA.
.. versionadded:: 3.10
.. class:: VerifyFlags
:class:`enum.IntFlag` collection of VERIFY_* constants.
.. versionadded:: 3.6
.. data:: 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.
.. versionadded:: 3.6
.. deprecated:: 3.10
TLS clients and servers require different default settings for secure
communication. The generic TLS protocol constant is deprecated in
favor of :data:`PROTOCOL_TLS_CLIENT` and :data:`PROTOCOL_TLS_SERVER`.
.. data:: PROTOCOL_TLS_CLIENT
Auto-negotiate the highest protocol version that both the client and
server support, and configure the context client-side connections. The
protocol enables :data:`CERT_REQUIRED` and
:attr:`~SSLContext.check_hostname` by default.
.. versionadded:: 3.6
.. data:: PROTOCOL_TLS_SERVER
Auto-negotiate the highest protocol version that both the client and
server support, and configure the context server-side connections.
.. versionadded:: 3.6
.. data:: PROTOCOL_SSLv23
Alias for :data:`PROTOCOL_TLS`.
.. deprecated:: 3.6
Use :data:`PROTOCOL_TLS` instead.
.. data:: PROTOCOL_SSLv3
Selects SSL version 3 as the channel encryption protocol.
This protocol is not available if OpenSSL is compiled with the
``no-ssl3`` option.
.. warning::
SSL version 3 is insecure. Its use is highly discouraged.
.. deprecated:: 3.6
OpenSSL has deprecated all version specific protocols. Use the default
protocol :data:`PROTOCOL_TLS_SERVER` or :data:`PROTOCOL_TLS_CLIENT`
with :attr:`SSLContext.minimum_version` and
:attr:`SSLContext.maximum_version` instead.
.. data:: PROTOCOL_TLSv1
Selects TLS version 1.0 as the channel encryption protocol.
.. deprecated:: 3.6
OpenSSL has deprecated all version specific protocols.
.. data:: PROTOCOL_TLSv1_1
Selects TLS version 1.1 as the channel encryption protocol.
Available only with openssl version 1.0.1+.
.. versionadded:: 3.4
.. deprecated:: 3.6
OpenSSL has deprecated all version specific protocols.
.. data:: PROTOCOL_TLSv1_2
Selects TLS version 1.2 as the channel encryption protocol.
Available only with openssl version 1.0.1+.
.. versionadded:: 3.4
.. deprecated:: 3.6
OpenSSL has deprecated all version specific protocols.
.. data:: 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.
.. versionadded:: 3.2
.. data:: OP_NO_SSLv2
Prevents an SSLv2 connection. This option is only applicable in
conjunction with :const:`PROTOCOL_TLS`. It prevents the peers from
choosing SSLv2 as the protocol version.
.. versionadded:: 3.2
.. deprecated:: 3.6
SSLv2 is deprecated
.. data:: OP_NO_SSLv3
Prevents an SSLv3 connection. This option is only applicable in
conjunction with :const:`PROTOCOL_TLS`. It prevents the peers from
choosing SSLv3 as the protocol version.
.. versionadded:: 3.2
.. deprecated:: 3.6
SSLv3 is deprecated
.. data:: OP_NO_TLSv1
Prevents a TLSv1 connection. This option is only applicable in
conjunction with :const:`PROTOCOL_TLS`. It prevents the peers from
choosing TLSv1 as the protocol version.
.. versionadded:: 3.2
.. deprecated:: 3.7
The option is deprecated since OpenSSL 1.1.0, use the new
:attr:`SSLContext.minimum_version` and
:attr:`SSLContext.maximum_version` instead.
.. data:: OP_NO_TLSv1_1
Prevents a TLSv1.1 connection. This option is only applicable in conjunction
with :const:`PROTOCOL_TLS`. It prevents the peers from choosing TLSv1.1 as
the protocol version. Available only with openssl version 1.0.1+.
.. versionadded:: 3.4
.. deprecated:: 3.7
The option is deprecated since OpenSSL 1.1.0.
.. data:: OP_NO_TLSv1_2
Prevents a TLSv1.2 connection. This option is only applicable in conjunction
with :const:`PROTOCOL_TLS`. It prevents the peers from choosing TLSv1.2 as
the protocol version. Available only with openssl version 1.0.1+.
.. versionadded:: 3.4
.. deprecated:: 3.7
The option is deprecated since OpenSSL 1.1.0.
.. data:: OP_NO_TLSv1_3
Prevents a TLSv1.3 connection. This option is only applicable in conjunction
with :const:`PROTOCOL_TLS`. It prevents the peers from choosing TLSv1.3 as
the protocol version. TLS 1.3 is available with OpenSSL 1.1.1 or later.
When Python has been compiled against an older version of OpenSSL, the
flag defaults to *0*.
.. versionadded:: 3.6.3
.. deprecated:: 3.7
The option is deprecated since OpenSSL 1.1.0. It was added to 2.7.15 and
3.6.3 for backwards compatibility with OpenSSL 1.0.2.
.. data:: OP_NO_RENEGOTIATION
Disable all renegotiation in TLSv1.2 and earlier. Do not send
HelloRequest messages, and ignore renegotiation requests via ClientHello.
This option is only available with OpenSSL 1.1.0h and later.
.. versionadded:: 3.7
.. data:: 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.
.. versionadded:: 3.3
.. data:: OP_SINGLE_DH_USE
Prevents reuse 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.
.. versionadded:: 3.3
.. data:: OP_SINGLE_ECDH_USE
Prevents reuse 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.
.. versionadded:: 3.3
.. data:: OP_ENABLE_MIDDLEBOX_COMPAT
Send dummy Change Cipher Spec (CCS) messages in TLS 1.3 handshake to make
a TLS 1.3 connection look more like a TLS 1.2 connection.
This option is only available with OpenSSL 1.1.1 and later.
.. versionadded:: 3.8
.. data:: OP_NO_COMPRESSION
Disable compression on the SSL channel. This is useful if the application
protocol supports its own compression scheme.
.. versionadded:: 3.3
.. class:: Options
:class:`enum.IntFlag` collection of OP_* constants.
.. data:: OP_NO_TICKET
Prevent client side from requesting a session ticket.
.. versionadded:: 3.6
.. data:: OP_IGNORE_UNEXPECTED_EOF
Ignore unexpected shutdown of TLS connections.
This option is only available with OpenSSL 3.0.0 and later.
.. versionadded:: 3.10
.. data:: OP_ENABLE_KTLS
Enable the use of the kernel TLS. To benefit from the feature, OpenSSL must
have been compiled with support for it, and the negotiated cipher suites and
extensions must be supported by it (a list of supported ones may vary by
platform and kernel version).
Note that with enabled kernel TLS some cryptographic operations are
performed by the kernel directly and not via any available OpenSSL
Providers. This might be undesirable if, for example, the application
requires all cryptographic operations to be performed by the FIPS provider.
This option is only available with OpenSSL 3.0.0 and later.
.. versionadded:: 3.12
.. data:: OP_LEGACY_SERVER_CONNECT
Allow legacy insecure renegotiation between OpenSSL and unpatched servers
only.
.. versionadded:: 3.12
.. data:: HAS_ALPN
Whether the OpenSSL library has built-in support for the *Application-Layer
Protocol Negotiation* TLS extension as described in :rfc:`7301`.
.. versionadded:: 3.5
.. data:: HAS_NEVER_CHECK_COMMON_NAME
Whether the OpenSSL library has built-in support not checking subject
common name and :attr:`SSLContext.hostname_checks_common_name` is
writeable.
.. versionadded:: 3.7
.. data:: HAS_ECDH
Whether the OpenSSL library has built-in support for the Elliptic Curve-based
Diffie-Hellman key exchange. This should be true unless the feature was
explicitly disabled by the distributor.
.. versionadded:: 3.3
.. data:: HAS_SNI
Whether the OpenSSL library has built-in support for the *Server Name
Indication* extension (as defined in :rfc:`6066`).
.. versionadded:: 3.2
.. data:: HAS_NPN
Whether the OpenSSL library has built-in support for the *Next Protocol
Negotiation* as described in the `Application Layer Protocol
Negotiation <https://en.wikipedia.org/wiki/Application-Layer_Protocol_Negotiation>`_.
When true, you can use the :meth:`SSLContext.set_npn_protocols` method to advertise
which protocols you want to support.
.. versionadded:: 3.3
.. data:: HAS_SSLv2
Whether the OpenSSL library has built-in support for the SSL 2.0 protocol.
.. versionadded:: 3.7
.. data:: HAS_SSLv3
Whether the OpenSSL library has built-in support for the SSL 3.0 protocol.
.. versionadded:: 3.7
.. data:: HAS_TLSv1
Whether the OpenSSL library has built-in support for the TLS 1.0 protocol.
.. versionadded:: 3.7
.. data:: HAS_TLSv1_1
Whether the OpenSSL library has built-in support for the TLS 1.1 protocol.
.. versionadded:: 3.7
.. data:: HAS_TLSv1_2
Whether the OpenSSL library has built-in support for the TLS 1.2 protocol.
.. versionadded:: 3.7
.. data:: HAS_TLSv1_3
Whether the OpenSSL library has built-in support for the TLS 1.3 protocol.
.. versionadded:: 3.7
.. data:: HAS_PSK
Whether the OpenSSL library has built-in support for TLS-PSK.
.. versionadded:: 3.13
.. data:: CHANNEL_BINDING_TYPES
List of supported TLS channel binding types. Strings in this list
can be used as arguments to :meth:`SSLSocket.get_channel_binding`.
.. versionadded:: 3.3
.. data:: OPENSSL_VERSION
The version string of the OpenSSL library loaded by the interpreter::
>>> ssl.OPENSSL_VERSION
'OpenSSL 1.0.2k 26 Jan 2017'
.. versionadded:: 3.2
.. data:: OPENSSL_VERSION_INFO
A tuple of five integers representing version information about the
OpenSSL library::
>>> ssl.OPENSSL_VERSION_INFO
(1, 0, 2, 11, 15)
.. versionadded:: 3.2
.. data:: 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'
.. versionadded:: 3.2
.. data:: ALERT_DESCRIPTION_HANDSHAKE_FAILURE
ALERT_DESCRIPTION_INTERNAL_ERROR
ALERT_DESCRIPTION_*
Alert Descriptions from :rfc:`5246` and others. The `IANA TLS Alert Registry
<https://www.iana.org/assignments/tls-parameters/tls-parameters.xml#tls-parameters-6>`_
contains this list and references to the RFCs where their meaning is defined.
Used as the return value of the callback function in
:meth:`SSLContext.set_servername_callback`.
.. versionadded:: 3.4
.. class:: AlertDescription
:class:`enum.IntEnum` collection of ALERT_DESCRIPTION_* constants.
.. versionadded:: 3.6
.. data:: Purpose.SERVER_AUTH
Option for :func:`create_default_context` and
:meth:`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).
.. versionadded:: 3.4
.. data:: Purpose.CLIENT_AUTH
Option for :func:`create_default_context` and
:meth:`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).
.. versionadded:: 3.4
.. class:: SSLErrorNumber
:class:`enum.IntEnum` collection of SSL_ERROR_* constants.
.. versionadded:: 3.6
.. class:: TLSVersion
:class:`enum.IntEnum` collection of SSL and TLS versions for
:attr:`SSLContext.maximum_version` and :attr:`SSLContext.minimum_version`.
.. versionadded:: 3.7
.. attribute:: TLSVersion.MINIMUM_SUPPORTED
.. attribute:: TLSVersion.MAXIMUM_SUPPORTED
The minimum or maximum supported SSL or TLS version. These are magic
constants. Their values don't reflect the lowest and highest available
TLS/SSL versions.
.. attribute:: TLSVersion.SSLv3
.. attribute:: TLSVersion.TLSv1
.. attribute:: TLSVersion.TLSv1_1
.. attribute:: TLSVersion.TLSv1_2
.. attribute:: TLSVersion.TLSv1_3
SSL 3.0 to TLS 1.3.
.. deprecated:: 3.10
All :class:`TLSVersion` members except :attr:`TLSVersion.TLSv1_2` and
:attr:`TLSVersion.TLSv1_3` are deprecated.
SSL Sockets
-----------
.. class:: SSLSocket(socket.socket)
SSL sockets provide the following methods of :ref:`socket-objects`:
- :meth:`~socket.socket.accept`
- :meth:`~socket.socket.bind`
- :meth:`~socket.socket.close`
- :meth:`~socket.socket.connect`
- :meth:`~socket.socket.detach`
- :meth:`~socket.socket.fileno`
- :meth:`~socket.socket.getpeername`, :meth:`~socket.socket.getsockname`
- :meth:`~socket.socket.getsockopt`, :meth:`~socket.socket.setsockopt`
- :meth:`~socket.socket.gettimeout`, :meth:`~socket.socket.settimeout`,
:meth:`~socket.socket.setblocking`
- :meth:`~socket.socket.listen`
- :meth:`~socket.socket.makefile`
- :meth:`~socket.socket.recv`, :meth:`~socket.socket.recv_into`
(but passing a non-zero ``flags`` argument is not allowed)
- :meth:`~socket.socket.send`, :meth:`~socket.socket.sendall` (with
the same limitation)
- :meth:`~socket.socket.sendfile` (but :mod:`os.sendfile` will be used
for plain-text sockets only, else :meth:`~socket.socket.send` will be used)
- :meth:`~socket.socket.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
:ref:`notes on non-blocking sockets <ssl-nonblocking>`.
Instances of :class:`SSLSocket` must be created using the
:meth:`SSLContext.wrap_socket` method.
.. versionchanged:: 3.5
The :meth:`sendfile` method was added.
.. versionchanged:: 3.5
The :meth:`shutdown` does not reset the socket timeout each time bytes
are received or sent. The socket timeout is now the maximum total duration
of the shutdown.
.. deprecated:: 3.6
It is deprecated to create a :class:`SSLSocket` instance directly, use
:meth:`SSLContext.wrap_socket` to wrap a socket.
.. versionchanged:: 3.7
:class:`SSLSocket` instances must to created with
:meth:`~SSLContext.wrap_socket`. In earlier versions, it was possible
to create instances directly. This was never documented or officially
supported.
.. versionchanged:: 3.10
Python now uses ``SSL_read_ex`` and ``SSL_write_ex`` internally. The
functions support reading and writing of data larger than 2 GB. Writing
zero-length data no longer fails with a protocol violation error.
SSL sockets also have the following additional methods and attributes:
.. method:: 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 :exc:`SSLWantReadError` or :exc:`SSLWantWriteError` if the socket is
:ref:`non-blocking <ssl-nonblocking>` and the read would block.
As at any time a re-negotiation is possible, a call to :meth:`read` can also
cause write operations.
.. versionchanged:: 3.5
The socket timeout is no longer reset each time bytes are received or sent.
The socket timeout is now the maximum total duration to read up to *len*
bytes.
.. deprecated:: 3.6
Use :meth:`~SSLSocket.recv` instead of :meth:`~SSLSocket.read`.
.. method:: 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 :exc:`SSLWantReadError` or :exc:`SSLWantWriteError` if the socket is
:ref:`non-blocking <ssl-nonblocking>` and the write would block.
As at any time a re-negotiation is possible, a call to :meth:`write` can
also cause read operations.
.. versionchanged:: 3.5
The socket timeout is no longer reset each time bytes are received or sent.
The socket timeout is now the maximum total duration to write *buf*.
.. deprecated:: 3.6
Use :meth:`~SSLSocket.send` instead of :meth:`~SSLSocket.write`.
.. note::
The :meth:`~SSLSocket.read` and :meth:`~SSLSocket.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
:meth:`SSLSocket.unwrap` was not called.
Normally you should use the socket API methods like
:meth:`~socket.socket.recv` and :meth:`~socket.socket.send` instead of these
methods.
.. method:: SSLSocket.do_handshake()
Perform the SSL setup handshake.
.. versionchanged:: 3.4
The handshake method also performs :func:`match_hostname` when the
:attr:`~SSLContext.check_hostname` attribute of the socket's
:attr:`~SSLSocket.context` is true.
.. versionchanged:: 3.5
The socket timeout is no longer reset each time bytes are received or sent.
The socket timeout is now the maximum total duration of the handshake.
.. versionchanged:: 3.7
Hostname or IP address is matched by OpenSSL during handshake. The
function :func:`match_hostname` is no longer used. In case OpenSSL
refuses a hostname or IP address, the handshake is aborted early and
a TLS alert message is sent to the peer.
.. method:: 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
:exc:`ValueError`.
If the ``binary_form`` parameter is :const:`False`, and a certificate was
received from the peer, this method returns a :class:`dict` instance. If the
certificate was not validated, the dict is empty. If the certificate was
validated, it returns a dict with 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}
If the ``binary_form`` parameter is :const:`True`, and a certificate was
provided, this method returns the DER-encoded form of the entire certificate
as a sequence of bytes, or :const:`None` if the peer did not provide a
certificate. 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 :meth:`getpeercert` will return
:const:`None` if you used :const:`CERT_NONE` (rather than
:const:`CERT_OPTIONAL` or :const:`CERT_REQUIRED`).
See also :attr:`SSLContext.check_hostname`.
.. versionchanged:: 3.2
The returned dictionary includes additional items such as ``issuer``
and ``notBefore``.
.. versionchanged:: 3.4
:exc:`ValueError` is raised when the handshake isn't done.
The returned dictionary includes additional X509v3 extension items
such as ``crlDistributionPoints``, ``caIssuers`` and ``OCSP`` URIs.
.. versionchanged:: 3.9
IPv6 address strings no longer have a trailing new line.
.. method:: SSLSocket.get_verified_chain()
Returns verified certificate chain provided by the other
end of the SSL channel as a list of DER-encoded bytes.
If certificate verification was disabled method acts the same as
:meth:`~SSLSocket.get_unverified_chain`.
.. versionadded:: 3.13
.. method:: SSLSocket.get_unverified_chain()
Returns raw certificate chain provided by the other
end of the SSL channel as a list of DER-encoded bytes.
.. versionadded:: 3.13
.. method:: SSLSocket.cipher()
Returns a three-value tuple containing the name of the cipher being used, the
version of the SSL protocol that defines its use, and the number of secret
bits being used. If no connection has been established, returns ``None``.
.. method:: SSLSocket.shared_ciphers()
Return the list of ciphers available in both the client and server. 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. :meth:`~SSLSocket.shared_ciphers` returns
``None`` if no connection has been established or the socket is a client
socket.
.. versionadded:: 3.5
.. method:: 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 :data:`OP_NO_COMPRESSION` to disable SSL-level compression.
.. versionadded:: 3.3
.. method:: 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
:data:`CHANNEL_BINDING_TYPES` list. Currently only the 'tls-unique' channel
binding, defined by :rfc:`5929`, is supported. :exc:`ValueError` will be
raised if an unsupported channel binding type is requested.
.. versionadded:: 3.3
.. method:: SSLSocket.selected_alpn_protocol()
Return the protocol that was selected during the TLS handshake. If
:meth:`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.
.. versionadded:: 3.5
.. method:: SSLSocket.selected_npn_protocol()
Return the higher-level protocol that was selected during the TLS/SSL
handshake. If :meth:`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``.
.. versionadded:: 3.3
.. deprecated:: 3.10
NPN has been superseded by ALPN
.. method:: 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.
.. method:: SSLSocket.verify_client_post_handshake()
Requests post-handshake authentication (PHA) from a TLS 1.3 client. PHA
can only be initiated for a TLS 1.3 connection from a server-side socket,
after the initial TLS handshake and with PHA enabled on both sides, see
:attr:`SSLContext.post_handshake_auth`.
The method does not perform a cert exchange immediately. The server-side
sends a CertificateRequest during the next write event and expects the
client to respond with a certificate on the next read event.
If any precondition isn't met (e.g. not TLS 1.3, PHA not enabled), an
:exc:`SSLError` is raised.
.. note::
Only available with OpenSSL 1.1.1 and TLS 1.3 enabled. Without TLS 1.3
support, the method raises :exc:`NotImplementedError`.
.. versionadded:: 3.8
.. method:: SSLSocket.version()
Return the actual SSL protocol version negotiated by the connection
as a string, or ``None`` if 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.
.. versionadded:: 3.5
.. method:: SSLSocket.pending()
Returns the number of already decrypted bytes available for read, pending on
the connection.
.. attribute:: SSLSocket.context
The :class:`SSLContext` object this SSL socket is tied to.
.. versionadded:: 3.2
.. attribute:: SSLSocket.server_side
A boolean which is ``True`` for server-side sockets and ``False`` for
client-side sockets.
.. versionadded:: 3.2
.. attribute:: SSLSocket.server_hostname
Hostname of the server: :class:`str` type, or ``None`` for server-side
socket or if the hostname was not specified in the constructor.
.. versionadded:: 3.2
.. versionchanged:: 3.7
The attribute is now always ASCII text. When ``server_hostname`` is
an internationalized domain name (IDN), this attribute now stores the
A-label form (``"xn--pythn-mua.org"``), rather than the U-label form
(``"pythön.org"``).
.. attribute:: SSLSocket.session
The :class:`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
:meth:`~SSLSocket.do_handshake` has been called to reuse a session.
.. versionadded:: 3.6
.. attribute:: SSLSocket.session_reused
.. versionadded:: 3.6
SSL Contexts
------------
.. versionadded:: 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:: SSLContext(protocol=None)
Create a new SSL context. You may pass *protocol* which must be one
of the ``PROTOCOL_*`` constants defined in this module. The parameter
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
:data:`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):
.. table::
======================== ============ ============ ============= ========= =========== ===========
*client* / **server** **SSLv2** **SSLv3** **TLS** [3]_ **TLSv1** **TLSv1.1** **TLSv1.2**
------------------------ ------------ ------------ ------------- --------- ----------- -----------
*SSLv2* yes no no [1]_ no no no
*SSLv3* no yes no [2]_ no no no
*TLS* (*SSLv23*) [3]_ 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
======================== ============ ============ ============= ========= =========== ===========
.. rubric:: Footnotes
.. [1] :class:`SSLContext` disables SSLv2 with :data:`OP_NO_SSLv2` by default.
.. [2] :class:`SSLContext` disables SSLv3 with :data:`OP_NO_SSLv3` by default.
.. [3] TLS 1.3 protocol will be available with :data:`PROTOCOL_TLS` in
OpenSSL >= 1.1.1. There is no dedicated PROTOCOL constant for just
TLS 1.3.
.. seealso::
:func:`create_default_context` lets the :mod:`ssl` module choose
security settings for a given purpose.
.. versionchanged:: 3.6
The context is created with secure default values. The options
:data:`OP_NO_COMPRESSION`, :data:`OP_CIPHER_SERVER_PREFERENCE`,
:data:`OP_SINGLE_DH_USE`, :data:`OP_SINGLE_ECDH_USE`,
:data:`OP_NO_SSLv2`,
and :data:`OP_NO_SSLv3` (except for :data:`PROTOCOL_SSLv3`) are
set by default. The initial cipher suite list contains only ``HIGH``
ciphers, no ``NULL`` ciphers and no ``MD5`` ciphers.
.. deprecated:: 3.10
:class:`SSLContext` without protocol argument is deprecated. The
context class will either require :data:`PROTOCOL_TLS_CLIENT` or
:data:`PROTOCOL_TLS_SERVER` protocol in the future.
.. versionchanged:: 3.10
The default cipher suites now include only secure AES and ChaCha20
ciphers with forward secrecy and security level 2. RSA and DH keys with
less than 2048 bits and ECC keys with less than 224 bits are prohibited.
:data:`PROTOCOL_TLS`, :data:`PROTOCOL_TLS_CLIENT`, and
:data:`PROTOCOL_TLS_SERVER` use TLS 1.2 as minimum TLS version.
.. note::
:class:`SSLContext` only supports limited mutation once it has been used
by a connection. Adding new certificates to the internal trust store is
allowed, but changing ciphers, verification settings, or mTLS
certificates may result in surprising behavior.
.. note::
:class:`SSLContext` is designed to be shared and used by multiple
connections.
Thus, it is thread-safe as long as it is not reconfigured after being
used by a connection.
:class:`SSLContext` objects have the following methods and attributes:
.. method:: 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}
.. versionadded:: 3.4
.. method:: 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. Otherwise the private
key will be taken from *certfile* as well. See the discussion of
:ref:`ssl-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 :class:`SSLError` is raised if the private key doesn't
match with the certificate.
.. versionchanged:: 3.3
New optional argument *password*.
.. method:: 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 all systems it calls
:meth:`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 :const:`Purpose.SERVER_AUTH` loads certificates, that are
flagged and trusted for TLS web server authentication (client side
sockets). :const:`Purpose.CLIENT_AUTH` loads CA certificates for client
certificate verification on the server side.
.. versionadded:: 3.4
.. method:: 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 :data:`verify_mode` is other than
:data:`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, :attr:`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
:ref:`ssl-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
<https://docs.openssl.org/master/man3/SSL_CTX_load_verify_locations/>`_.
The *cadata* object, if present, is either an ASCII string of one or more
PEM-encoded certificates or a :term:`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.
.. versionchanged:: 3.4
New optional argument *cadata*
.. method:: SSLContext.get_ca_certs(binary_form=False)
Get a list of loaded "certification authority" (CA) certificates. If the
``binary_form`` parameter is :const:`False` each list
entry is a dict like the output of :meth:`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.
.. versionadded:: 3.4
.. method:: SSLContext.get_ciphers()
Get a list of enabled ciphers. The list is in order of cipher priority.
See :meth:`SSLContext.set_ciphers`.
Example::
>>> ctx = ssl.SSLContext(ssl.PROTOCOL_SSLv23)
>>> ctx.set_ciphers('ECDHE+AESGCM:!ECDSA')
>>> ctx.get_ciphers()
[{'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'}]
.. versionadded:: 3.6
.. method:: 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.
.. method:: 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
<https://docs.openssl.org/master/man1/ciphers/>`_.
If no cipher can be selected (because compile-time options or other
configuration forbids use of all the specified ciphers), an
:class:`SSLError` will be raised.
.. note::
when connected, the :meth:`SSLSocket.cipher` method of SSL sockets will
give the currently selected cipher.
TLS 1.3 cipher suites cannot be disabled with
:meth:`~SSLContext.set_ciphers`.
.. method:: 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 :meth:`SSLSocket.selected_alpn_protocol` method will
return the agreed-upon protocol.
This method will raise :exc:`NotImplementedError` if :data:`HAS_ALPN` is
``False``.
.. versionadded:: 3.5
.. method:: 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 `Application Layer Protocol Negotiation
<https://en.wikipedia.org/wiki/Application-Layer_Protocol_Negotiation>`_. After a
successful handshake, the :meth:`SSLSocket.selected_npn_protocol` method will
return the agreed-upon protocol.
This method will raise :exc:`NotImplementedError` if :data:`HAS_NPN` is
``False``.
.. versionadded:: 3.3
.. deprecated:: 3.10
NPN has been superseded by ALPN
.. attribute:: SSLContext.sni_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 *sni_callback*
is set to ``None`` then the callback is disabled. Calling this function a
subsequent time will disable the previously registered callback.
The callback function will be called with three
arguments; the first being the :class:`ssl.SSLSocket`, the second is a string
that represents the server name that the client is intending to communicate
(or :const:`None` if the TLS Client Hello does not contain a server name)
and the third argument is the original :class:`SSLContext`. The server name
argument is text. For internationalized domain name, the server
name is an IDN A-label (``"xn--pythn-mua.org"``).
A typical use of this callback is to change the :class:`ssl.SSLSocket`'s
:attr:`SSLSocket.context` attribute to a new object of type
:class:`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
:meth:`SSLSocket.selected_alpn_protocol` and :attr:`SSLSocket.context`.
The :meth:`SSLSocket.getpeercert`, :meth:`SSLSocket.get_verified_chain`,
:meth:`SSLSocket.get_unverified_chain` :meth:`SSLSocket.cipher`
and :meth:`SSLSocket.compression` methods require that
the TLS connection has progressed beyond the TLS Client Hello and therefore
will not return meaningful values nor can they be called safely.
The *sni_callback* function must return ``None`` to allow the
TLS negotiation to continue. If a TLS failure is required, a constant
:const:`ALERT_DESCRIPTION_* <ALERT_DESCRIPTION_INTERNAL_ERROR>` can be
returned. Other return values will result in a TLS fatal error with
:const:`ALERT_DESCRIPTION_INTERNAL_ERROR`.
If an exception is raised from the *sni_callback* function the TLS
connection will terminate with a fatal TLS alert message
:const:`ALERT_DESCRIPTION_HANDSHAKE_FAILURE`.
This method will raise :exc:`NotImplementedError` if the OpenSSL library
had OPENSSL_NO_TLSEXT defined when it was built.
.. versionadded:: 3.7
.. attribute:: SSLContext.set_servername_callback(server_name_callback)
This is a legacy API retained for backwards compatibility. When possible,
you should use :attr:`sni_callback` instead. The given *server_name_callback*
is similar to *sni_callback*, except that when the server hostname is an
IDN-encoded internationalized domain name, the *server_name_callback*
receives a decoded U-label (``"pythön.org"``).
If there is a decoding error on the server name, the TLS connection will
terminate with an :const:`ALERT_DESCRIPTION_INTERNAL_ERROR` fatal TLS
alert message to the client.
.. versionadded:: 3.4
.. method:: SSLContext.load_dh_params(dhfile)
Load the key generation parameters for Diffie-Hellman (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
:data:`OP_SINGLE_DH_USE` option to further improve security.
.. versionadded:: 3.3
.. method:: 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
:data:`OP_SINGLE_ECDH_USE` option to further improve security.
This method is not available if :data:`HAS_ECDH` is ``False``.
.. versionadded:: 3.3
.. seealso::
`SSL/TLS & Perfect Forward Secrecy <https://vincent.bernat.ch/en/blog/2011-ssl-perfect-forward-secrecy>`_
Vincent Bernat.
.. method:: 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 instance of
:attr:`SSLContext.sslsocket_class` (default :class:`SSLSocket`). The
returned SSL socket is tied to the context, its settings and certificates.
*sock* must be a :const:`~socket.SOCK_STREAM` socket; other
socket types are unsupported.
The parameter ``server_side`` is a boolean which identifies whether
server-side or client-side behavior is desired from this socket.
For client-side sockets, the context construction is lazy; if the
underlying socket isn't connected yet, the context construction will be
performed after :meth:`connect` is called on the socket. For
server-side sockets, if the socket has no remote peer, it is assumed
to be a listening socket, and the server-side SSL wrapping is
automatically performed on client connections accepted via the
:meth:`accept` method. The method may raise :exc:`SSLError`.
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 :exc:`ValueError` if *server_side* is true.
The parameter ``do_handshake_on_connect`` specifies whether to do the SSL
handshake automatically after doing a :meth:`socket.connect`, or whether the
application program will call it explicitly, by invoking the
:meth:`SSLSocket.do_handshake` method. Calling
:meth:`SSLSocket.do_handshake` explicitly gives the program control over the
blocking behavior of the socket I/O involved in the handshake.
The parameter ``suppress_ragged_eofs`` specifies how the
:meth:`SSLSocket.recv` method should signal unexpected EOF from the other end
of the connection. If specified as :const:`True` (the default), it returns a
normal EOF (an empty bytes object) in response to unexpected EOF errors
raised from the underlying socket; if :const:`False`, it will raise the
exceptions back to the caller.
*session*, see :attr:`~SSLSocket.session`.
To wrap an :class:`SSLSocket` in another :class:`SSLSocket`, use
:meth:`SSLContext.wrap_bio`.
.. versionchanged:: 3.5
Always allow a server_hostname to be passed, even if OpenSSL does not
have SNI.
.. versionchanged:: 3.6
*session* argument was added.
.. versionchanged:: 3.7
The method returns an instance of :attr:`SSLContext.sslsocket_class`
instead of hard-coded :class:`SSLSocket`.
.. attribute:: SSLContext.sslsocket_class
The return type of :meth:`SSLContext.wrap_socket`, defaults to
:class:`SSLSocket`. The attribute can be overridden on instance of class
in order to return a custom subclass of :class:`SSLSocket`.
.. versionadded:: 3.7
.. method:: SSLContext.wrap_bio(incoming, outgoing, server_side=False, \
server_hostname=None, session=None)
Wrap the BIO objects *incoming* and *outgoing* and return an instance of
:attr:`SSLContext.sslobject_class` (default :class:`SSLObject`). 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 :meth:`SSLContext.wrap_socket`.
.. versionchanged:: 3.6
*session* argument was added.
.. versionchanged:: 3.7
The method returns an instance of :attr:`SSLContext.sslobject_class`
instead of hard-coded :class:`SSLObject`.
.. attribute:: SSLContext.sslobject_class
The return type of :meth:`SSLContext.wrap_bio`, defaults to
:class:`SSLObject`. The attribute can be overridden on instance of class
in order to return a custom subclass of :class:`SSLObject`.
.. versionadded:: 3.7
.. method:: 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 <https://docs.openssl.org/1.1.1/man3/SSL_CTX_sess_number/>`_ 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)
.. attribute:: SSLContext.check_hostname
Whether to match the peer cert's hostname in
:meth:`SSLSocket.do_handshake`. The context's
:attr:`~SSLContext.verify_mode` must be set to :data:`CERT_OPTIONAL` or
:data:`CERT_REQUIRED`, and you must pass *server_hostname* to
:meth:`~SSLContext.wrap_socket` in order to match the hostname. Enabling
hostname checking automatically sets :attr:`~SSLContext.verify_mode` from
:data:`CERT_NONE` to :data:`CERT_REQUIRED`. It cannot be set back to
:data:`CERT_NONE` as long as hostname checking is enabled. The
:data:`PROTOCOL_TLS_CLIENT` protocol enables hostname checking by default.
With other protocols, hostname checking must be enabled explicitly.
Example::
import socket, ssl
context = ssl.SSLContext(ssl.PROTOCOL_TLSv1_2)
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))
.. versionadded:: 3.4
.. versionchanged:: 3.7
:attr:`~SSLContext.verify_mode` is now automatically changed
to :data:`CERT_REQUIRED` when hostname checking is enabled and
:attr:`~SSLContext.verify_mode` is :data:`CERT_NONE`. Previously
the same operation would have failed with a :exc:`ValueError`.
.. attribute:: SSLContext.keylog_filename
Write TLS keys to a keylog file, whenever key material is generated or
received. The keylog file is designed for debugging purposes only. The
file format is specified by NSS and used by many traffic analyzers such
as Wireshark. The log file is opened in append-only mode. Writes are
synchronized between threads, but not between processes.
.. versionadded:: 3.8
.. attribute:: SSLContext.maximum_version
A :class:`TLSVersion` enum member representing the highest supported
TLS version. The value defaults to :attr:`TLSVersion.MAXIMUM_SUPPORTED`.
The attribute is read-only for protocols other than :attr:`PROTOCOL_TLS`,
:attr:`PROTOCOL_TLS_CLIENT`, and :attr:`PROTOCOL_TLS_SERVER`.
The attributes :attr:`~SSLContext.maximum_version`,
:attr:`~SSLContext.minimum_version` and
:attr:`SSLContext.options` all affect the supported SSL
and TLS versions of the context. The implementation does not prevent
invalid combination. For example a context with
:attr:`OP_NO_TLSv1_2` in :attr:`~SSLContext.options` and
:attr:`~SSLContext.maximum_version` set to :attr:`TLSVersion.TLSv1_2`
will not be able to establish a TLS 1.2 connection.
.. versionadded:: 3.7
.. attribute:: SSLContext.minimum_version
Like :attr:`SSLContext.maximum_version` except it is the lowest
supported version or :attr:`TLSVersion.MINIMUM_SUPPORTED`.
.. versionadded:: 3.7
.. attribute:: SSLContext.num_tickets
Control the number of TLS 1.3 session tickets of a
:attr:`PROTOCOL_TLS_SERVER` context. The setting has no impact on TLS
1.0 to 1.2 connections.
.. versionadded:: 3.8
.. attribute:: SSLContext.options
An integer representing the set of SSL options enabled on this context.
The default value is :data:`OP_ALL`, but you can specify other options
such as :data:`OP_NO_SSLv2` by ORing them together.
.. versionchanged:: 3.6
:attr:`SSLContext.options` returns :class:`Options` flags:
>>> ssl.create_default_context().options # doctest: +SKIP
<Options.OP_ALL|OP_NO_SSLv3|OP_NO_SSLv2|OP_NO_COMPRESSION: 2197947391>
.. deprecated:: 3.7
All ``OP_NO_SSL*`` and ``OP_NO_TLS*`` options have been deprecated since
Python 3.7. Use :attr:`SSLContext.minimum_version` and
:attr:`SSLContext.maximum_version` instead.
.. attribute:: SSLContext.post_handshake_auth
Enable TLS 1.3 post-handshake client authentication. Post-handshake auth
is disabled by default and a server can only request a TLS client
certificate during the initial handshake. When enabled, a server may
request a TLS client certificate at any time after the handshake.
When enabled on client-side sockets, the client signals the server that
it supports post-handshake authentication.
When enabled on server-side sockets, :attr:`SSLContext.verify_mode` must
be set to :data:`CERT_OPTIONAL` or :data:`CERT_REQUIRED`, too. The
actual client cert exchange is delayed until
:meth:`SSLSocket.verify_client_post_handshake` is called and some I/O is
performed.
.. versionadded:: 3.8
.. attribute:: SSLContext.protocol
The protocol version chosen when constructing the context. This attribute
is read-only.
.. attribute:: SSLContext.hostname_checks_common_name
Whether :attr:`~SSLContext.check_hostname` falls back to verify the cert's
subject common name in the absence of a subject alternative name
extension (default: true).
.. versionadded:: 3.7
.. versionchanged:: 3.10
The flag had no effect with OpenSSL before version 1.1.1l. Python 3.8.9,
3.9.3, and 3.10 include workarounds for previous versions.
.. attribute:: SSLContext.security_level
An integer representing the `security level
<https://docs.openssl.org/master/man3/SSL_CTX_get_security_level/>`_
for the context. This attribute is read-only.
.. versionadded:: 3.10
.. attribute:: SSLContext.verify_flags
The flags for certificate verification operations. You can set flags like
:data:`VERIFY_CRL_CHECK_LEAF` by ORing them together. By default OpenSSL
does neither require nor verify certificate revocation lists (CRLs).
.. versionadded:: 3.4
.. versionchanged:: 3.6
:attr:`SSLContext.verify_flags` returns :class:`VerifyFlags` flags:
>>> ssl.create_default_context().verify_flags # doctest: +SKIP
<VerifyFlags.VERIFY_X509_TRUSTED_FIRST: 32768>
.. attribute:: SSLContext.verify_mode
Whether to try to verify other peers' certificates and how to behave
if verification fails. This attribute must be one of
:data:`CERT_NONE`, :data:`CERT_OPTIONAL` or :data:`CERT_REQUIRED`.
.. versionchanged:: 3.6
:attr:`SSLContext.verify_mode` returns :class:`VerifyMode` enum:
>>> ssl.create_default_context().verify_mode # doctest: +SKIP
<VerifyMode.CERT_REQUIRED: 2>
.. method:: SSLContext.set_psk_client_callback(callback)
Enables TLS-PSK (pre-shared key) authentication on a client-side connection.
In general, certificate based authentication should be preferred over this method.
The parameter ``callback`` is a callable object with the signature:
``def callback(hint: str | None) -> tuple[str | None, bytes]``.
The ``hint`` parameter is an optional identity hint sent by the server.
The return value is a tuple in the form (client-identity, psk).
Client-identity is an optional string which may be used by the server to
select a corresponding PSK for the client. The string must be less than or
equal to ``256`` octets when UTF-8 encoded. PSK is a
:term:`bytes-like object` representing the pre-shared key. Return a zero
length PSK to reject the connection.
Setting ``callback`` to :const:`None` removes any existing callback.
.. note::
When using TLS 1.3:
- the ``hint`` parameter is always :const:`None`.
- client-identity must be a non-empty string.
Example usage::
context = ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT)
context.check_hostname = False
context.verify_mode = ssl.CERT_NONE
context.maximum_version = ssl.TLSVersion.TLSv1_2
context.set_ciphers('PSK')
# A simple lambda:
psk = bytes.fromhex('c0ffee')
context.set_psk_client_callback(lambda hint: (None, psk))
# A table using the hint from the server:
psk_table = { 'ServerId_1': bytes.fromhex('c0ffee'),
'ServerId_2': bytes.fromhex('facade')
}
def callback(hint):
return 'ClientId_1', psk_table.get(hint, b'')
context.set_psk_client_callback(callback)
This method will raise :exc:`NotImplementedError` if :data:`HAS_PSK` is
``False``.
.. versionadded:: 3.13
.. method:: SSLContext.set_psk_server_callback(callback, identity_hint=None)
Enables TLS-PSK (pre-shared key) authentication on a server-side connection.
In general, certificate based authentication should be preferred over this method.
The parameter ``callback`` is a callable object with the signature:
``def callback(identity: str | None) -> bytes``.
The ``identity`` parameter is an optional identity sent by the client which can
be used to select a corresponding PSK.
The return value is a :term:`bytes-like object` representing the pre-shared key.
Return a zero length PSK to reject the connection.
Setting ``callback`` to :const:`None` removes any existing callback.
The parameter ``identity_hint`` is an optional identity hint string sent to
the client. The string must be less than or equal to ``256`` octets when
UTF-8 encoded.
.. note::
When using TLS 1.3 the ``identity_hint`` parameter is not sent to the client.
Example usage::
context = ssl.SSLContext(ssl.PROTOCOL_TLS_SERVER)
context.maximum_version = ssl.TLSVersion.TLSv1_2
context.set_ciphers('PSK')
# A simple lambda:
psk = bytes.fromhex('c0ffee')
context.set_psk_server_callback(lambda identity: psk)
# A table using the identity of the client:
psk_table = { 'ClientId_1': bytes.fromhex('c0ffee'),
'ClientId_2': bytes.fromhex('facade')
}
def callback(identity):
return psk_table.get(identity, b'')
context.set_psk_server_callback(callback, 'ServerId_1')
This method will raise :exc:`NotImplementedError` if :data:`HAS_PSK` is
``False``.
.. versionadded:: 3.13
.. index:: single: certificates
.. index:: single: X509 certificate
.. _ssl-certificates:
Certificates
------------
Certificates in general are part of a public-key / private-key system. In this
system, each *principal*, (which may be a machine, or a person, or an
organization) is assigned a unique two-part encryption key. One part of the key
is public, and is called the *public key*; the other part is kept secret, and is
called the *private key*. The two parts are related, in that if you encrypt a
message with one of the parts, you can decrypt it with the other part, and
**only** with the other part.
A certificate contains information about two principals. It contains the name
of a *subject*, and the subject's public key. It also contains a statement by a
second principal, the *issuer*, that the subject is who they claim 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 :meth:`SSLContext.load_default_certs`, this is done
automatically with :func:`.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 :meth:`SSLContext.load_cert_chain`
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_CLIENT)
>>> 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)
The :data:`PROTOCOL_TLS_CLIENT` protocol configures the context for cert
validation and hostname verification. :attr:`~SSLContext.verify_mode` is
set to :data:`CERT_REQUIRED` and :attr:`~SSLContext.check_hostname` is set
to ``True``. All other protocols create SSL contexts with insecure defaults.
When you use the context to connect to a server, :const:`CERT_REQUIRED`
and :attr:`~SSLContext.check_hostname` validate the server certificate: it
ensures that the server certificate was signed with one of the CA
certificates, checks the signature for correctness, and verifies other
properties like validity and identity of the hostname::
>>> 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.org'),
('DNS', 'docs.python.org'),
('DNS', 'testpypi.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 :ref:`ssl-security` 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 :meth:`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.example.com', 10023))
bindsocket.listen(5)
When a client connects, you'll call :meth:`accept` on the socket to get the
new socket from the other end, and use the context's :meth:`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 :ref:`non-blocking mode <ssl-nonblocking>` and use an event loop).
.. _ssl-nonblocking:
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 :class:`SSLSocket` methods will raise either
:exc:`SSLWantWriteError` or :exc:`SSLWantReadError` instead of
:exc:`BlockingIOError` if an I/O operation would
block. :exc:`SSLWantReadError` will be raised if a read operation on
the underlying socket is necessary, and :exc:`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.
.. versionchanged:: 3.5
In earlier Python versions, the :meth:`!SSLSocket.send` method
returned zero instead of raising :exc:`SSLWantWriteError` or
:exc:`SSLWantReadError`.
- Calling :func:`~select.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 :meth:`SSLSocket.recv`
and :meth:`SSLSocket.send` failures, and retry after another call to
:func:`~select.select`.
- Conversely, since the SSL layer has its own framing, a SSL socket may
still have data available for reading without :func:`~select.select`
being aware of it. Therefore, you should first call
:meth:`SSLSocket.recv` to drain any potentially available data, and then
only block on a :func:`~select.select` call if still necessary.
(of course, similar provisions apply when using other primitives such as
:func:`~select.poll`, or those in the :mod:`selectors` module)
- The SSL handshake itself will be non-blocking: the
:meth:`SSLSocket.do_handshake` method has to be retried until it returns
successfully. Here is a synopsis using :func:`~select.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], [])
.. seealso::
The :mod:`asyncio` module supports :ref:`non-blocking SSL sockets
<ssl-nonblocking>` and provides a
higher level API. It polls for events using the :mod:`selectors` module and
handles :exc:`SSLWantWriteError`, :exc:`SSLWantReadError` and
:exc:`BlockingIOError` exceptions. It runs the SSL handshake asynchronously
as well.
Memory BIO Support
------------------
.. versionadded:: 3.5
Ever since the SSL module was introduced in Python 2.6, the :class:`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 :class:`socket.socket`,
from which :class:`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 :class:`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 :class:`SSLSocket` called :class:`SSLObject` is
provided.
.. class:: SSLObject
A reduced-scope variant of :class:`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.
This class has no public constructor. An :class:`SSLObject` instance
must be created using the :meth:`~SSLContext.wrap_bio` method. This
method will create the :class:`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:
- :attr:`~SSLSocket.context`
- :attr:`~SSLSocket.server_side`
- :attr:`~SSLSocket.server_hostname`
- :attr:`~SSLSocket.session`
- :attr:`~SSLSocket.session_reused`
- :meth:`~SSLSocket.read`
- :meth:`~SSLSocket.write`
- :meth:`~SSLSocket.getpeercert`
- :meth:`~SSLSocket.get_verified_chain`
- :meth:`~SSLSocket.get_unverified_chain`
- :meth:`~SSLSocket.selected_alpn_protocol`
- :meth:`~SSLSocket.selected_npn_protocol`
- :meth:`~SSLSocket.cipher`
- :meth:`~SSLSocket.shared_ciphers`
- :meth:`~SSLSocket.compression`
- :meth:`~SSLSocket.pending`
- :meth:`~SSLSocket.do_handshake`
- :meth:`~SSLSocket.verify_client_post_handshake`
- :meth:`~SSLSocket.unwrap`
- :meth:`~SSLSocket.get_channel_binding`
- :meth:`~SSLSocket.version`
When compared to :class:`SSLSocket`, this object lacks the following
features:
- Any form of network IO; ``recv()`` and ``send()`` read and write only to
the underlying :class:`MemoryBIO` buffers.
- There is no *do_handshake_on_connect* machinery. You must always manually
call :meth:`~SSLSocket.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
:exc:`SSLEOFError` exception.
- The method :meth:`~SSLSocket.unwrap` call does not return anything,
unlike for an SSL socket where it returns the underlying socket.
- The *server_name_callback* callback passed to
:meth:`SSLContext.set_servername_callback` will get an :class:`SSLObject`
instance instead of a :class:`SSLSocket` instance as its first parameter.
Some notes related to the use of :class:`SSLObject`:
- All IO on an :class:`SSLObject` is :ref:`non-blocking <ssl-nonblocking>`.
This means that for example :meth:`~SSLSocket.read` will raise an
:exc:`SSLWantReadError` if it needs more data than the incoming BIO has
available.
.. versionchanged:: 3.7
:class:`SSLObject` instances must be created with
:meth:`~SSLContext.wrap_bio`. In earlier versions, it was possible to
create instances directly. This was never documented or officially
supported.
An SSLObject communicates with the outside world using memory buffers. The
class :class:`MemoryBIO` provides a memory buffer that can be used for this
purpose. It wraps an OpenSSL memory BIO (Basic IO) object:
.. class:: MemoryBIO
A memory buffer that can be used to pass data between Python and an SSL
protocol instance.
.. attribute:: MemoryBIO.pending
Return the number of bytes currently in the memory buffer.
.. attribute:: MemoryBIO.eof
A boolean indicating whether the memory BIO is current at the end-of-file
position.
.. method:: MemoryBIO.read(n=-1)
Read up to *n* bytes from the memory buffer. If *n* is not specified or
negative, all bytes are returned.
.. method:: MemoryBIO.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*.
.. method:: MemoryBIO.write_eof()
Write an EOF marker to the memory BIO. After this method has been called, it
is illegal to call :meth:`~MemoryBIO.write`. The attribute :attr:`eof` will
become true after all data currently in the buffer has been read.
SSL session
-----------
.. versionadded:: 3.6
.. class:: SSLSession
Session object used by :attr:`~SSLSocket.session`.
.. attribute:: id
.. attribute:: time
.. attribute:: timeout
.. attribute:: ticket_lifetime_hint
.. attribute:: has_ticket
.. _ssl-security:
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
:func:`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 :class:`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
:meth:`SSLContext.load_cert_chain`.
By contrast, if you create the SSL context by calling the :class:`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 :class:`SSLContext` constructor directly,
:const:`CERT_NONE` is the default. Since it does not authenticate the other
peer, it can be insecure, especially in client mode where most of the 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
:const:`CERT_REQUIRED`. However, it is in itself not sufficient; you also
have to check that the server certificate, which can be obtained by calling
:meth:`SSLSocket.getpeercert`, matches the desired service. For many
protocols and applications, the service can be identified by the hostname.
This common check is automatically performed when
:attr:`SSLContext.check_hostname` is enabled.
.. versionchanged:: 3.7
Hostname matchings is now performed by OpenSSL. Python no longer uses
:func:`match_hostname`.
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 :const:`CERT_REQUIRED` and similarly check the client certificate.
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 :const:`PROTOCOL_TLS_CLIENT` or
:const:`PROTOCOL_TLS_SERVER` as the protocol version. SSLv2 and SSLv3 are
disabled by default.
::
>>> client_context = ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT)
>>> client_context.minimum_version = ssl.TLSVersion.TLSv1_3
>>> client_context.maximum_version = ssl.TLSVersion.TLSv1_3
The SSL context created above will only allow TLSv1.3 and later (if
supported by your system) connections to a server. :const:`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
:meth:`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 <https://docs.openssl.org/1.1.1/man1/ciphers/#cipher-list-format>`_.
If you want to check which ciphers are enabled by a given cipher list, use
:meth:`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 :mod:`multiprocessing` or :mod:`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 :func:`os.fork`. Any
successful call of :func:`~ssl.RAND_add` or :func:`~ssl.RAND_bytes` is
sufficient.
.. _ssl-tlsv1_3:
TLS 1.3
-------
.. versionadded:: 3.7
The TLS 1.3 protocol behaves slightly differently than previous version
of TLS/SSL. Some new TLS 1.3 features are not yet available.
- TLS 1.3 uses a disjunct set of cipher suites. All AES-GCM and
ChaCha20 cipher suites are enabled by default. The method
:meth:`SSLContext.set_ciphers` cannot enable or disable any TLS 1.3
ciphers yet, but :meth:`SSLContext.get_ciphers` returns them.
- Session tickets are no longer sent as part of the initial handshake and
are handled differently. :attr:`SSLSocket.session` and :class:`SSLSession`
are not compatible with TLS 1.3.
- Client-side certificates are also no longer verified during the initial
handshake. A server can request a certificate at any time. Clients
process certificate requests while they send or receive application data
from the server.
- TLS 1.3 features like early data, deferred TLS client cert request,
signature algorithm configuration, and rekeying are not supported yet.
.. seealso::
Class :class:`socket.socket`
Documentation of underlying :mod:`socket` class
`SSL/TLS Strong Encryption: An Introduction <https://httpd.apache.org/docs/trunk/en/ssl/ssl_intro.html>`_
Intro from the Apache HTTP Server documentation
:rfc:`RFC 1422: Privacy Enhancement for Internet Electronic Mail: Part II: Certificate-Based Key Management <1422>`
Steve Kent
:rfc:`RFC 4086: Randomness Requirements for Security <4086>`
Donald E., Jeffrey I. Schiller
:rfc:`RFC 5280: Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile <5280>`
D. Cooper
:rfc:`RFC 5246: The Transport Layer Security (TLS) Protocol Version 1.2 <5246>`
T. Dierks et. al.
:rfc:`RFC 6066: Transport Layer Security (TLS) Extensions <6066>`
D. Eastlake
`IANA TLS: Transport Layer Security (TLS) Parameters <https://www.iana.org/assignments/tls-parameters/tls-parameters.xml>`_
IANA
:rfc:`RFC 7525: Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) <7525>`
IETF
`Mozilla's Server Side TLS recommendations <https://wiki.mozilla.org/Security/Server_Side_TLS>`_
Mozilla
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