PEP: 748
Title: A Unified TLS API for Python
Author: Joop van de Pol <joop.vandepol@trailofbits.com>,
        William Woodruff <william@yossarian.net>
Sponsor: Alyssa Coghlan <ncoghlan@gmail.com>
Discussions-To: https://discuss.python.org/t/pre-pep-discussion-revival-of-pep-543/51263
Status: Draft
Type: Standards Track
Created: 27-Jun-2024
Python-Version: 3.14
Post-History: `17-Apr-2024 <https://discuss.python.org/t/pre-pep-discussion-revival-of-pep-543/51263>`__
Replaces: 543

Abstract
========

This PEP defines a standard TLS interface in the form of a collection of
protocol classes. This interface will allow Python implementations and
third-party libraries to provide bindings to TLS libraries other than OpenSSL.

These bindings can be used by tools that expect the interface provided by the
Python standard library, with the goal of reducing the dependence of the Python
ecosystem on OpenSSL.

Rationale
=========

It has become increasingly clear that robust and user-friendly TLS support is an
extremely important part of the ecosystem of any popular programming language.
For most of its lifetime, this role in the Python ecosystem has primarily been
served by the :mod:`ssl` module, which provides a Python API to the `OpenSSL
library <https://www.openssl.org/>`_.

Because the :mod:`ssl` module is distributed with the Python standard library,
it has become the overwhelmingly most popular method for handling TLS in Python.
A majority of Python libraries, both in the standard library and
on the Python Package Index, rely on the :mod:`ssl` module for their TLS
connectivity.

Unfortunately, the preeminence of the :mod:`ssl` module has had a number of
tied the entire Python
ecosystem tightly to OpenSSL. This has forced Python users to use OpenSSL even
in situations where it may provide a worse user experience than alternative TLS
implementations, which imposes a cognitive burden and makes it hard to provide
“platform-native” experiences.

Problems
--------

The fact that the :mod:`ssl` module is built into the standard library has meant
that all standard-library Python networking libraries are entirely reliant on
the OpenSSL that the Python implementation has been linked against. This leads
to the following issues:

* It is difficult to take advantage of new, higher-security TLS without
  recompiling Python to get a new OpenSSL. While there are third-party bindings
  to OpenSSL (e.g. `pyOpenSSL <https://pypi.org/project/pyOpenSSL/>`_), these
  need to be shimmed into a format that the standard library understands,
  forcing projects that want to use them to maintain substantial compatibility
  layers.

* Windows distributions of Python need to be shipped with a copy of
  OpenSSL. This puts the CPython development team in the position of being
  OpenSSL redistributors, potentially needing to ship security updates to the
  Windows Python distributions when OpenSSL vulnerabilities are released.

* macOS distributions of Python need either to be shipped with a copy
  of OpenSSL or linked against the system OpenSSL library. Apple has formally
  deprecated linking against the system OpenSSL library, and even if they had
  not, that library version has been unsupported by upstream for nearly one year
  as of the time of writing. The CPython development team has started shipping
  newer OpenSSLs with the Python available from python.org, but this has the
  same problem as with Windows.

* Users may wish to integrate with TLS libraries other than OpenSSL for other
  reasons, such as maintenance burden versus a system-provided implementation,
  or because OpenSSL is simply too large and unwieldy for their platform (e.g.
  for embedded Python). Those users are left with the requirement to use
  third-party networking libraries that can interact with their preferred TLS
  library or to shim their preferred library into the OpenSSL-specific
  :mod:`ssl` module API.

Additionally, the :mod:`ssl` module as implemented today limits the ability of
CPython itself to add support for alternative TLS implementations, or remove
OpenSSL support entirely, should either of these become necessary or useful. The
:mod:`ssl` module exposes too many OpenSSL-specific function calls and features
to easily map to an alternative TLS implementation.

Proposal
========

This PEP proposes to introduce a few new Protocol Classes in Python 3.14 to
provide TLS functionality that is not so strongly tied to OpenSSL. It also
proposes to update standard library modules to use only the interface exposed by
these protocol classes wherever possible. There are three goals here:

#. To provide a common API surface for both core and third-party developers to
   target their TLS implementations to. This allows TLS developers to provide
   interfaces that can be used by most Python code, and allows network
   developers to have an interface that they can target that will work with a
   wide range of TLS implementations.

#. To provide an API that has few or no OpenSSL-specific concepts leak through.
   The :mod:`ssl` module today has a number of warts caused by leaking OpenSSL
   concepts through to the API: the new protocol classes would remove those
   specific concepts.

#. To provide a path for the core development team to make OpenSSL one of many
   possible TLS implementations, rather than requiring that it be present on a
   system in order for Python to have TLS support.

The proposed interface is laid out below.

Interfaces
----------

There are several interfaces that require standardization. Those interfaces are:

#. Configuring TLS, currently implemented by the :class:`~ssl.SSLContext` class
   in the :mod:`ssl` module.

#. Providing an in-memory buffer for doing in-memory encryption or decryption
   with no actual I/O (necessary for asynchronous I/O models), currently
   implemented by the :class:`~ssl.SSLObject` class in the :mod:`ssl` module.

#. Wrapping a socket object, currently implemented by the
   :class:`~ssl.SSLSocket` class in the :mod:`ssl` module.

#. Applying TLS configuration to the wrapping objects in (2) and (3). Currently
   this is also implemented by the SSLContext class in the :mod:`ssl` module.

#. Specifying TLS cipher suites. There is currently no code for doing this in
   the standard library: instead, the standard library uses OpenSSL cipher suite
   strings.

#. Specifying application-layer protocols that can be negotiated during the TLS
   handshake.

#. Specifying TLS versions.

#. Reporting errors to the caller, currently implemented by the
   :class:`~ssl.SSLError` class in the :mod:`ssl` module.

#. Specifying certificates to load, either as client or server certificates.

#. Specifying which trust database should be used to validate certificates
   presented by a remote peer.

#. Finding a way to get hold of these interfaces at run time.

For the sake of simplicity, this PEP proposes to remove interfaces (3) and (4),
and replace them by a simpler interface that returns a socket which ensures that
all communication through the socket is protected by TLS. In other words, this
interface treats concepts such as socket initialization, the TLS handshake,
Server Name Indication (SNI), etc., as an atomic part of creating a client or
server connection. However, in-memory buffers are still supported, as they are
useful for asynchronous communication.

Obviously, (5) doesn't require a protocol class: instead, it requires a richer
API for configuring supported cipher suites that can be easily updated with
supported cipher suites for different implementations.

(9) is a thorny problem, because in an ideal world the private keys associated
with these certificates would never end up in-memory in the Python process
(that is, the TLS library would collaborate with a Hardware Security Module
(HSM) to provide the private key in such a way that it cannot be extracted
from process memory). Thus, we need to provide an extensible model of
providing certificates that allows concrete implementations the ability to
provide this higher level of security, while also allowing a lower bar for
those implementations that cannot. This lower bar would be the same as the
status quo: that is, the certificate may be loaded from an in-memory buffer,
from a file on disk, or additionally referenced by some arbitrary ID
corresponding to a system certificate store.

(10) also represents an issue because different TLS implementations vary wildly
in how they allow users to select trust stores. Some implementations have
specific trust store formats that only they can use (such as the OpenSSL CA
directory format that is created by c_rehash), and others may not allow you
to specify a trust store that does not include their default trust store.
On the other hand, most implementations will support some form of loading custom
DER- or PEM-encoded certificates.

For this reason, we need to provide a model that assumes very little about the
form that trust stores take, while maintaining type-compatibility with other
implementations. The sections “Certificate”, “Private Keys”, and “Trust Store”
below go into more detail about how this is achieved.

Finally, this API will split the responsibilities currently assumed by the
:class:`~ssl.SSLContext` object: specifically, the responsibility for holding
and managing configuration and the responsibility for using that configuration
to build buffers or sockets.

This is necessary primarily for supporting functionality like Server Name
Indication (SNI). In OpenSSL (and thus in the :mod:`ssl` module), the server has
the ability to modify the TLS configuration in response to the client telling
the server what hostname it is trying to reach. This is mostly used to change
the certificate chain so as to present the correct TLS certificate chain for the
given hostname. The specific mechanism by which this is done is by returning a
new :class:`~ssl.SSLContext` object with the appropriate configuration as part
of a user-provided SNI callback function.

This is not a model that maps well to other TLS implementations, and puts a
burden on users to write callback functions. Instead, we propose that the
concrete implementations handle SNI transparently for every user after receiving
the relevant certificates.

For this reason, we split the responsibility of :class:`~ssl.SSLContext` into
two separate objects, which are each split into server and client versions. The
``TLSServerConfiguration`` and ``TLSClientConfiguration`` objects act as
containers for a TLS configuration: the ClientContext and ServerContext objects
are instantiated with a ``TLSClientConfiguration`` and
``TLSServerConfiguration`` object, respectively, and are used to create buffers
or sockets. All four objects would be immutable.

.. note::

    The following API declarations uniformly use type hints to aid reading.

Configuration
~~~~~~~~~~~~~

The ``TLSServerConfiguration`` and ``TLSClientConfiguration`` concrete classes
define objects that can hold and manage TLS configuration. The goals of these
classes are as follows:

#. To provide a method of specifying TLS configuration that avoids the risk of
   errors in typing (this excludes the use of a simple dictionary).

#. To provide an object that can be safely compared to other configuration
   objects to detect changes in TLS configuration, for use with the SNI
   callback.

These classes are not protocol classes, primarily because they are not expected to
have implementation-specific behavior. The responsibility for transforming a
``TLSServerConfiguration``  or ``TLSClientConfiguration`` object into a useful
set of configurations for a given TLS implementation belongs to the Context
objects discussed below.

These classes have one other notable property: they are immutable. This is a
desirable trait for a few reasons. The most important one is that immutability
by default is a good engineering practice. As a side benefit, it allows these
objects to be used as dictionary keys, which is potentially useful for specific
TLS implementations and their SNI configuration. On top of this, it frees
implementations from needing to worry about their configuration objects being
changed under their feet, which allows them to avoid needing to carefully
synchronize changes between their concrete data structures and the configuration
object.

These objects are extendable: that is, future releases of Python may add
configuration fields to these objects as they become useful. For
backwards-compatibility purposes, new fields are only appended to these objects.
Existing fields will never be removed, renamed, or reordered. They are split
between client and server to minimize API confusion.

The ``TLSClientConfiguration`` class would be defined by the following code:

.. code-block:: python

    class TLSClientConfiguration:
        __slots__ = (
            "_certificate_chain",
            "_ciphers",
            "_inner_protocols",
            "_lowest_supported_version",
            "_highest_supported_version",
            "_trust_store",
        )

        def __init__(
            self,
            certificate_chain: SigningChain | None = None,
            ciphers: Sequence[CipherSuite] | None = None,
            inner_protocols: Sequence[NextProtocol | bytes] | None = None,
            lowest_supported_version: TLSVersion | None = None,
            highest_supported_version: TLSVersion | None = None,
            trust_store: TrustStore | None = None,
        ) -> None:
            if inner_protocols is None:
                inner_protocols = []

            self._certificate_chain = certificate_chain
            self._ciphers = ciphers
            self._inner_protocols = inner_protocols
            self._lowest_supported_version = lowest_supported_version
            self._highest_supported_version = highest_supported_version
            self._trust_store = trust_store

        @property
        def certificate_chain(self) -> SigningChain | None:
            return self._certificate_chain

        @property
        def ciphers(self) -> Sequence[CipherSuite | int] | None:
            return self._ciphers

        @property
        def inner_protocols(self) -> Sequence[NextProtocol | bytes]:
            return self._inner_protocols

        @property
        def lowest_supported_version(self) -> TLSVersion | None:
            return self._lowest_supported_version

        @property
        def highest_supported_version(self) -> TLSVersion | None:
            return self._highest_supported_version

        @property
        def trust_store(self) -> TrustStore | None:
            return self._trust_store

The ``TLSServerConfiguration`` object is similar to the client one, except that
it takes a ``Sequence[SigningChain]`` as the ``certificate_chain`` parameter.

Context
~~~~~~~

We define two Context protocol classes. These protocol classes define objects
that allow configuration of TLS to be applied to specific connections. They can
be thought of as factories for ``TLSSocket`` and ``TLSBuffer`` objects.

Unlike the current :mod:`ssl` module, we provide two context classes instead of
one. Specifically, we provide the ``ClientContext`` and ``ServerContext``
classes. This simplifies the APIs (for example, there is no sense in the server
providing the ``server_hostname`` parameter to
:meth:`~ssl.SSLContext.wrap_socket`, but because there is only one context class
that parameter is still available), and ensures that implementations know as
early as possible which side of a TLS connection they will serve. Additionally,
it allows implementations to opt-out of one or either side of the connection.

As much as possible implementers should aim to make these classes immutable:
that is, they should prefer not to allow users to mutate their internal state
directly, instead preferring to create new contexts from new TLSConfiguration
objects. Obviously, the protocol classes cannot enforce this constraint, and so
they do not attempt to.

The ``ClientContext`` protocol class has the following class definition:

.. code-block:: python

    class ClientContext(Protocol):
        @abstractmethod
        def __init__(self, configuration: TLSClientConfiguration) -> None:
            """Create a new client context object from a given TLS client configuration."""
            ...

        @property
        @abstractmethod
        def configuration(self) -> TLSClientConfiguration:
            """Returns the TLS client configuration that was used to create the client context."""
            ...

        @abstractmethod
        def connect(self, address: tuple[str | None, int]) -> TLSSocket:
            """Creates a TLSSocket that behaves like a socket.socket, and
            contains information about the TLS exchange
            (cipher, negotiated_protocol, negotiated_tls_version, etc.).
            """
            ...

        @abstractmethod
        def create_buffer(self, server_hostname: str) -> TLSBuffer:
            """Creates a TLSBuffer that acts as an in-memory channel,
            and contains information about the TLS exchange
            (cipher, negotiated_protocol, negotiated_tls_version, etc.)."""
            ...

The ``ServerContext`` is similar, taking a ``TLSServerConfiguration`` instead.

Socket
~~~~~~

The context can be used to create sockets, which have to follow the
specification of the ``TLSSocket`` protocol class. Specifically, implementations
need to implement the following:

* ``recv`` and ``send``
* ``listen`` and ``accept``
* ``close``
* ``getsockname``
* ``getpeername``

They also need to implement some interfaces that give information about the TLS
connection, such as:

* The underlying context object that was used to create this socket
* The negotiated cipher
* The negotiated "next" protocol
* The negotiated TLS version

The following code describes these functions in more detail:

.. code-block:: python

    class TLSSocket(Protocol):
        """This class implements a socket.socket-like object that creates an OS
        socket, wraps it in an SSL context, and provides read and write methods
        over that channel."""

        @abstractmethod
        def __init__(self, *args: tuple, **kwargs: tuple) -> None:
            """TLSSockets should not be constructed by the user.
            The implementation should implement a method to construct a TLSSocket
            object and call it in ClientContext.connect() and
            ServerContext.connect()."""
            ...

        @abstractmethod
        def recv(self, bufsize: int) -> bytes:
            """Receive data from the socket. The return value is a bytes object
            representing the data received. Should not work before the handshake
            is completed."""
            ...

        @abstractmethod
        def send(self, bytes: bytes) -> int:
            """Send data to the socket. The socket must be connected to a remote socket."""
            ...

        @abstractmethod
        def close(self, force: bool = False) -> None:
            """Shuts down the connection and mark the socket closed.
            If force is True, this method should send the close_notify alert and shut down
            the socket without waiting for the other side.
            If force is False, this method should send the close_notify alert and raise
            the WantReadError exception until a corresponding close_notify alert has been
            received from the other side.
            In either case, this method should return WantWriteError if sending the
            close_notify alert currently fails."""
            ...

        @abstractmethod
        def listen(self, backlog: int) -> None:
            """Enable a server to accept connections. If backlog is specified, it
            specifies the number of unaccepted connections that the system will allow
            before refusing new connections."""
            ...

        @abstractmethod
        def accept(self) -> tuple[TLSSocket, tuple[str | None, int]]:
            """Accept a connection. The socket must be bound to an address and listening
            for connections. The return value is a pair (conn, address) where conn is a
            new TLSSocket object usable to send and receive data on the connection, and
            address is the address bound to the socket on the other end of the connection."""
            ...

        @abstractmethod
        def getsockname(self) -> tuple[str | None, int]:
            """Return the local address to which the socket is connected."""
            ...

        @abstractmethod
        def getpeercert(self) -> bytes | None:
            """
            Return the raw DER bytes of the certificate provided by the peer
            during the handshake, if applicable.
            """
            ...

        @abstractmethod
        def getpeername(self) -> tuple[str | None, int]:
            """Return the remote address to which the socket is connected."""
            ...

        @property
        @abstractmethod
        def context(self) -> ClientContext | ServerContext:
            """The ``Context`` object this socket is tied to."""
            ...

        @abstractmethod
        def cipher(self) -> CipherSuite | int | None:
            """
            Returns the CipherSuite entry for the cipher that has been negotiated on the connection.

            If no connection has been negotiated, returns ``None``. If the cipher negotiated is not
            defined in CipherSuite, returns the 16-bit integer representing that cipher directly.
            """
            ...

        @abstractmethod
        def negotiated_protocol(self) -> NextProtocol | bytes | None:
            """
            Returns the protocol that was selected during the TLS handshake.

            This selection may have been made using ALPN or some future
            negotiation mechanism.

            If the negotiated protocol is one of the protocols defined in the
            ``NextProtocol`` enum, the value from that enum will be returned.
            Otherwise, the raw bytestring of the negotiated protocol will be
            returned.

            If ``Context.set_inner_protocols()`` was not called, if the other
            party does not support protocol negotiation, if this socket does
            not support any of the peer's proposed protocols, or if the
            handshake has not happened yet, ``None`` is returned.
            """
            ...

        @property
        @abstractmethod
        def negotiated_tls_version(self) -> TLSVersion | None:
            """The version of TLS that has been negotiated on this connection."""
            ...

Buffer
~~~~~~

The context can also be used to create buffers, which have to follow the
specification of the ``TLSBuffer`` protocol class. Specifically, implementations
need to implement the following:

* ``read`` and ``write``
* ``do_handshake``
* ``shutdown``
* ``process_incoming`` and ``process_outgoing``
* ``incoming_bytes_buffered`` and ``outgoing_bytes_buffered``
* ``getpeercert``

Similarly to the socket case, they also need to implement some interfaces that
give information about the TLS connection, such as:

* The underlying context object that was used to create this buffer
* The negotiated cipher
* The negotiated "next" protocol
* The negotiated TLS version

The following code describes these functions in more detail:

.. code-block:: python

    class TLSBuffer(Protocol):
        """This class implements an in memory-channel that creates two buffers,
        wraps them in an SSL context, and provides read and write methods over
        that channel."""

        @abstractmethod
        def read(self, amt: int, buffer: Buffer | None) -> bytes | int:
            """
            Read up to ``amt`` bytes of data from the input buffer and return
            the result as a ``bytes`` instance. If an optional buffer is
            provided, the result is written into the buffer and the number of
            bytes is returned instead.

            Once EOF is reached, all further calls to this method return the
            empty byte string ``b''``.

            May read "short": that is, fewer bytes may be returned than were
            requested.

            Raise ``WantReadError`` or ``WantWriteError`` if there is
            insufficient data in either the input or output buffer and the
            operation would have caused data to be written or read.

            May raise ``RaggedEOF`` if the connection has been closed without a
            graceful TLS shutdown. Whether this is an exception that should be
            ignored or not is up to the specific application.

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

        @abstractmethod
        def write(self, buf: Buffer) -> int:
            """
            Write ``buf`` in encrypted form to the output buffer and return the
            number of bytes written. The ``buf`` argument must be an object
            supporting the buffer interface.

            Raise ``WantReadError`` or ``WantWriteError`` if there is
            insufficient data in either the input or output buffer and the
            operation would have caused data to be written or read. In either
            case, users should endeavour to resolve that situation and then
            re-call this method. When re-calling this method users *should*
            re-use the exact same ``buf`` object, as some implementations require that
            the exact same buffer be used.

            This operation may write "short": that is, fewer bytes may be
            written than were in the buffer.

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

        @abstractmethod
        def do_handshake(self) -> None:
            """
            Performs the TLS handshake. Also performs certificate validation
            and hostname verification.
            """
            ...

        @abstractmethod
        def cipher(self) -> CipherSuite | int | None:
            """
            Returns the CipherSuite entry for the cipher that has been
            negotiated on the connection. If no connection has been negotiated,
            returns ``None``. If the cipher negotiated is not defined in
            CipherSuite, returns the 16-bit integer representing that cipher
            directly.
            """
            ...

        @abstractmethod
        def negotiated_protocol(self) -> NextProtocol | bytes | None:
            """
            Returns the protocol that was selected during the TLS handshake.
            This selection may have been made using ALPN, NPN, or some future
            negotiation mechanism.

            If the negotiated protocol is one of the protocols defined in the
            ``NextProtocol`` enum, the value from that enum will be returned.
            Otherwise, the raw bytestring of the negotiated protocol will be
            returned.

            If ``Context.set_inner_protocols()`` was not called, if the other
            party does not support protocol negotiation, if this socket does
            not support any of the peer's proposed protocols, or if the
            handshake has not happened yet, ``None`` is returned.
            """
            ...

        @property
        @abstractmethod
        def context(self) -> ClientContext | ServerContext:
            """
            The ``Context`` object this buffer is tied to.
            """
            ...

        @property
        @abstractmethod
        def negotiated_tls_version(self) -> TLSVersion | None:
            """
            The version of TLS that has been negotiated on this connection.
            """
            ...

        @abstractmethod
        def shutdown(self) -> None:
            """
            Performs a clean TLS shut down. This should generally be used
            whenever possible to signal to the remote peer that the content is
            finished.
            """
            ...

        @abstractmethod
        def process_incoming(self, data_from_network: bytes) -> None:
            """
            Receives some TLS data from the network and stores it in an
            internal buffer.

            If the internal buffer is overfull, this method will raise
            ``WantReadError`` and store no data. At this point, the user must
            call ``read`` to remove some data from the internal buffer
            before repeating this call.
            """
            ...

        @abstractmethod
        def incoming_bytes_buffered(self) -> int:
            """
            Returns how many bytes are in the incoming buffer waiting to be processed.
            """
            ...

        @abstractmethod
        def process_outgoing(self, amount_bytes_for_network: int) -> bytes:
            """
            Returns the next ``amt`` bytes of data that should be written to
            the network from the outgoing data buffer, removing it from the
            internal buffer.
            """
            ...

        @abstractmethod
        def outgoing_bytes_buffered(self) -> int:
            """
            Returns how many bytes are in the outgoing buffer waiting to be sent.
            """
            ...

        @abstractmethod
        def getpeercert(self) -> bytes | None:
            """
            Return the raw DER bytes of the certificate provided by the peer
            during the handshake, if applicable.
            """
            ...


Cipher Suites
~~~~~~~~~~~~~

Supporting cipher suites in a truly library-agnostic fashion is a remarkably
difficult undertaking. Different TLS implementations often have radically
different APIs for specifying cipher suites, but more problematically these APIs
frequently differ in capability as well as in style.

Below are examples of different cipher suite selection APIs. These examples are
not intended to obligate implementation against each API, only to illuminate the
constraints imposed by each.

OpenSSL
^^^^^^^

OpenSSL uses a well-known cipher string format. This format has been adopted as
a configuration language by most products that use OpenSSL, including Python.
This format is relatively easy to read, but has a number of downsides: it is a
string, which makes it easy to provide bad inputs; it lacks much
detailed validation, meaning that it is possible to configure OpenSSL in a way
that doesn't allow it to negotiate any cipher at all; and it allows specifying
cipher suites in a number of different ways that make it tricky to parse. The
biggest problem with this format is that there is no formal specification for
it, meaning that the only way to parse a given string the way OpenSSL would is
to get OpenSSL to parse it.

OpenSSL's cipher strings can look like this:

.. code-block:: python

    "ECDH+AESGCM:ECDH+CHACHA20:DH+AESGCM:DH+CHACHA20:ECDH+AES256:DH+AES256:ECDH+AES128:DH+AES:RSA+AESGCM:RSA+AES:!aNULL:!eNULL:!MD5"


This string demonstrates some of the complexity of the OpenSSL format. For
example, it is possible for one entry to specify multiple cipher suites: the
entry ``ECDH+AESGCM`` means “all ciphers suites that include both elliptic-curve
Diffie-Hellman key exchange and AES in Galois Counter Mode”. More explicitly,
that will expand to four cipher suites:


.. code-block:: python

    "ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256"


That makes parsing a complete OpenSSL cipher string extremely tricky. Add to the
fact that there are other meta-characters, such as “!” (exclude all cipher
suites that match this criterion, even if they would otherwise be included:
“!MD5” means that no cipher suites using the MD5 hash algorithm should be
included), “-” (exclude matching ciphers if they were already included, but
allow them to be re-added later if they get included again), and “+” (include
the matching ciphers, but place them at the end of the list), and you get an
extremely complex format to parse. On top of this complexity it should be noted
that the actual result depends on the OpenSSL version, as an OpenSSL cipher
string is valid so long as it contains at least one cipher that OpenSSL
recognizes.

OpenSSL also uses different names for its ciphers than the names used in the
relevant specifications. See the manual page for ``ciphers(1)`` for more
details.

The actual API inside OpenSSL for the cipher string is simple:

.. code-block:: c

    char *cipher_list = <some cipher list>;
    int rc = SSL_CTX_set_cipher_list(context, cipher_list);


This means that any format that is used by this module must be able to be
converted to an OpenSSL cipher string for use with OpenSSL.

Network Framework
^^^^^^^^^^^^^^^^^

Network Framework is the macOS (10.15+) system TLS library. This library is
substantially more restricted than OpenSSL in many ways, as it has a much more
restricted class of users. One of these substantial restrictions is in
controlling supported cipher suites.

Ciphers in Network Framework are represented by a Objective-C ``uint16_t`` enum.
This enum has one entry per cipher suite, with no aggregate entries, meaning
that it is not possible to reproduce the meaning of an OpenSSL cipher string
like ``“ECDH+AESGCM”`` without hand-coding which categories each enum member
falls into.

However, the names of most of the enum members are in line with the formal names
of the cipher suites: that is, the cipher suite that OpenSSL calls
``“ECDHE-ECDSA-AES256-GCM-SHA384”`` is called
``“tls_ciphersuite_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384”`` in Network Framework.

The API for configuring cipher suites inside Network Framework is simple:

.. code-block:: c

    void sec_protocol_options_append_tls_ciphersuite(sec_protocol_options_t options, tls_ciphersuite_t ciphersuite);

SChannel
^^^^^^^^

SChannel is the Windows system TLS library.

SChannel has extremely restrictive support for controlling available TLS cipher
suites, and additionally adopts a third method of expressing what TLS cipher
suites are supported.

Specifically, SChannel defines a set of ``ALG_ID`` constants (C unsigned ints).
Each of these constants does not refer to an entire cipher suite, but instead an
individual algorithm. Some examples are ``CALG_3DES`` and ``CALG_AES_256``,
which refer to the bulk encryption algorithm used in a cipher suite,
``CALG_ECDH_EPHEM`` and ``CALG_RSA_KEYX`` which refer to part of the key
exchange algorithm used in a cipher suite, ``CALG_SHA_256`` and ``CALG_SHA_384``
which refer to the message authentication code used in a cipher suite, and
``CALG_ECDSA`` and ``CALG_RSA_SIGN`` which refer to the signing portions of the
key exchange algorithm.

In earlier versions of the SChannel API, these constants were used to define the
algorithms that could be used. The latest version, however, uses these constants
to prohibit which algorithms can be used.

This can be thought of as the half of OpenSSL's functionality that Network
Framework doesn't have: Network Framework only allows specifying exact cipher
suites (and a limited number of pre-defined cipher suite groups), whereas
SChannel only allows specifying parts of the cipher suite, while OpenSSL allows
both.

Determining which cipher suites are allowed on a given connection is done by
providing a pointer to an array of these ``ALG_ID`` constants. This means that
any suitable API must allow the Python code to determine which ``ALG_ID``
constants must be provided.

Network Security Services (NSS)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

NSS is Mozilla's crypto and TLS library. It's used in Firefox, Thunderbird, and
as an alternative to OpenSSL in multiple libraries, e.g. curl.

By default, NSS comes with secure configuration of allowed ciphers. On some
platforms such as Fedora, the list of enabled ciphers is globally configured in
a system policy. Generally, applications should not modify cipher suites unless
they have specific reasons to do so.

NSS has both process global and per-connection settings for cipher suites. It
does not have a concept of :class:`~ssl.SSLContext` like OpenSSL. A
:class:`~ssl.SSLContext`-like behavior can be easily emulated. Specifically,
ciphers can be enabled or disabled globally with
``SSL_CipherPrefSetDefault(PRInt32 cipher, PRBool enabled)``, and
``SSL_CipherPrefSet(PRFileDesc *fd, PRInt32 cipher, PRBool enabled)`` for a
connection. The cipher ``PRInt32`` number is a signed 32-bit integer that
directly corresponds to an registered IANA id, e.g. ``0x1301`` is
``TLS_AES_128_GCM_SHA256``. Contrary to OpenSSL, the preference order of ciphers
is fixed and cannot be modified at runtime.

Like Network Framework, NSS has no API for aggregated entries. Some consumers of
NSS have implemented custom mappings from OpenSSL cipher names and rules to NSS
ciphers, e.g. ``mod_nss``.

Proposed Interface
^^^^^^^^^^^^^^^^^^

The proposed interface for the new module is influenced by the combined set of
limitations of the above implementations. Specifically, as every implementation
except OpenSSL requires that each individual cipher be provided, there is no
option but to provide that lowest common denominator approach.

The simplest approach is to provide an enumerated type that includes a large
subset of the cipher suites defined for TLS. The values of the enum members will
be their two-octet cipher identifier as used in the TLS handshake, stored as a
16 bit integer. The names of the enum members will be their IANA-registered
cipher suite names.

As of now, the `IANA cipher suite registry
<https://www.iana.org/assignments/tls-parameters/tls-parameters.xhtml#tls-parameters-4>`_
contains over 320 cipher suites. A large portion of the cipher suites are
irrelevant for TLS connections to network services. Other suites specify
deprecated and insecure algorithms that are no longer provided by recent
versions of implementations. The enum contains the five fixed cipher suites
defined for TLS v1.3. For TLS v1.2, it only contains the cipher suites that
correspond to the TLS v1.3 cipher suites, with ECDHE key exchange (for perfect
forward secrecy) and ECDSA or RSA signatures, which are an additional ten cipher
suites.

In addition to this enum, the interface defines a default cipher suite list for
TLS v1.2, which includes only those defined cipher suites based on AES-GCM or
ChaCha20-Poly1305. The default cipher suite list for TLS v1.3 will
comprise the five cipher suites defined in the specification.

The current enum is quite restricted, including only cipher suites that provide
forward secrecy. Because the enum doesn't contain every defined cipher, and also
to allow for forward-looking applications, all parts of this API that accept
``CipherSuite`` objects will also accept raw 16-bit integers directly.

.. code-block:: python

    class CipherSuite(IntEnum):
        """
        Known cipher suites.

        See: <https://www.iana.org/assignments/tls-parameters/tls-parameters.xhtml>
        """

        TLS_AES_128_GCM_SHA256 = 0x1301
        TLS_AES_256_GCM_SHA384 = 0x1302
        TLS_CHACHA20_POLY1305_SHA256 = 0x1303
        TLS_AES_128_CCM_SHA256 = 0x1304
        TLS_AES_128_CCM_8_SHA256 = 0x1305
        TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 = 0xC02B
        TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 = 0xC02C
        TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 = 0xC02F
        TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 = 0xC030
        TLS_ECDHE_ECDSA_WITH_AES_128_CCM = 0xC0AC
        TLS_ECDHE_ECDSA_WITH_AES_256_CCM = 0xC0AD
        TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 = 0xC0AE
        TLS_ECDHE_ECDSA_WITH_AES_256_CCM_8 = 0xC0AF
        TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256 = 0xCCA8
        TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256 = 0xCCA9


For Network Framework, these enum members directly refer to the values of the
cipher suite constants. For example, Network Framework defines the cipher suite
enum member ``tls_ciphersuite_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384`` as having
the value ``0xC02C``. Not coincidentally, that is identical to its value in the
above enum. This makes mapping between Network Framework and the above enum very
easy indeed.

For SChannel there is no easy direct mapping, due to the fact that SChannel
configures ciphers, instead of cipher suites. This represents an ongoing concern
with SChannel, which is that it is very difficult to configure in a specific
manner compared to other TLS implementations.

For the purposes of this PEP, any SChannel implementation will need to determine
which ciphers to choose based on the enum members. This may be more open than
the actual cipher suite list actually wants to allow, or it may be more
restrictive, depending on the choices of the implementation. This PEP recommends
that it be more restrictive, but of course this cannot be enforced.

Finally, we expect that for most users, secure defaults will be enough. When
specifying no list of ciphers, the implementations should use secure defaults
(possibly derived from system recommended settings).

Protocol Negotiation
~~~~~~~~~~~~~~~~~~~~

ALPN allows for protocol negotiation as part of the HTTP/2 handshake. While ALPN
is at a fundamental level built on top of bytestrings, string-based APIs are
frequently problematic as they allow for errors in typing that can be hard to
detect.

For this reason, this module will define a type that protocol negotiation
implementations can pass and be passed. This type would wrap a bytestring to
allow for aliases for well-known protocols. This allows us to avoid the problems
inherent in typos for well-known protocols, while allowing the full
extensibility of the protocol negotiation layer if needed by letting users pass
byte strings directly.

.. code-block:: python

    class NextProtocol(Enum):
        """The underlying negotiated ("next") protocol."""

        H2 = b"h2"
        H2C = b"h2c"
        HTTP1 = b"http/1.1"
        WEBRTC = b"webrtc"
        C_WEBRTC = b"c-webrtc"
        FTP = b"ftp"
        STUN = b"stun.nat-discovery"
        TURN = b"stun.turn"

TLS Versions
~~~~~~~~~~~~

It is often useful to be able to restrict the versions of TLS you're willing to
support. There are many security advantages in refusing to use old versions of
TLS, and some misbehaving servers will mishandle TLS clients advertising support
for newer versions.

The following enumerated type can be used to gate TLS versions. Forward-looking
applications should almost never set a maximum TLS version unless they
absolutely must, as a TLS implementation that is newer than the Python that uses
it may support TLS versions that are not in this enumerated type.

Additionally, this enumerated type defines two additional flags that can always
be used to request either the lowest or highest TLS version supported by an
implementation. As for cipher suites, we expect that for most users, secure
defaults will be enough. When specifying no list of TLS versions, the
implementations should use secure defaults (possibly derived from system
recommended settings).

.. code-block:: python

    class TLSVersion(Enum):
        """
        TLS versions.

        The `MINIMUM_SUPPORTED` and `MAXIMUM_SUPPORTED` variants are "open ended",
        and refer to the "lowest mutually supported" and "highest mutually supported"
        TLS versions, respectively.
        """

        MINIMUM_SUPPORTED = "MINIMUM_SUPPORTED"
        TLSv1_2 = "TLSv1.2"
        TLSv1_3 = "TLSv1.3"
        MAXIMUM_SUPPORTED = "MAXIMUM_SUPPORTED"

Errors
~~~~~~

This module would define four base classes for use with error handling. Unlike
many of the other classes defined here, these classes are not abstract, as they
have no behavior. They exist simply to signal certain common behaviors. TLS
implementations should subclass these exceptions in their own packages, but
needn't define any behavior for them.

In general, concrete implementations should subclass these exceptions rather
than throw them directly. This makes it moderately easier to determine which
concrete TLS implementation is in use during debugging of unexpected errors.
However, this is not mandatory.

The definitions of the errors are below:

.. code-block:: python

    class TLSError(Exception):
        """
        The base exception for all TLS related errors from any implementation.

        Catching this error should be sufficient to catch *all* TLS errors,
        regardless of what implementation is used.
        """


    class WantWriteError(TLSError):
        """
        A special signaling exception used only when non-blocking or buffer-only I/O is used.

        This error signals that the requested
        operation cannot complete until more data is written to the network,
        or until the output buffer is drained.

        This error is should only be raised when it is completely impossible
        to write any data. If a partial write is achievable then this should
        not be raised.
        """


    class WantReadError(TLSError):
        """
        A special signaling exception used only when non-blocking or buffer-only I/O is used.

        This error signals that the requested
        operation cannot complete until more data is read from the network, or
        until more data is available in the input buffer.

        This error should only be raised when it is completely impossible to
        write any data. If a partial write is achievable then this should not
        be raised.
        """


    class RaggedEOF(TLSError):
        """A special signaling exception used when a TLS connection has been
        closed gracelessly: that is, when a TLS CloseNotify was not received
        from the peer before the underlying TCP socket reached EOF. This is a
        so-called "ragged EOF".

        This exception is not guaranteed to be raised in the face of a ragged
        EOF: some implementations may not be able to detect or report the
        ragged EOF.

        This exception is not always a problem. Ragged EOFs are a concern only
        when protocols are vulnerable to length truncation attacks. Any
        protocol that can detect length truncation attacks at the application
        layer (e.g. HTTP/1.1 and HTTP/2) is not vulnerable to this kind of
        attack and so can ignore this exception.
        """


    class ConfigurationError(TLSError):
        """An special exception that implementations can use when the provided
        configuration uses features not supported by that implementation."""


Certificates
~~~~~~~~~~~~

This module would define a concrete certificate class. This class would have
almost no behavior, as the goal of this module is not to provide all possible
relevant cryptographic functionality that could be provided by X.509
certificates. Instead, all we need is the ability to signal the source of a
certificate to a concrete implementation.

For that reason, this certificate class defines three attributes, corresponding
to the three envisioned constructors: certificates from files, certificates from
memory, or certificates from arbitrary identifiers. It is possible that
implementations do not support all of these constructors, and they can
communicate this to users as described in the “Runtime” section below.
Certificates from arbitrary identifiers, in particular, are expected to be
useful primarily to users seeking to build integrations on top of HSMs, TPMs,
SSMs, and similar.

Specifically, this class does not parse any provided input to validate that it
is a correct certificate, and also does not provide any form of introspection
into a particular certificate. TLS implementations are not required to provide
such introspection either. Peer certificates that are received during the
handshake are provided as raw DER bytes.

.. code-block:: python

    class Certificate:
        """Object representing a certificate used in TLS."""

        __slots__ = (
            "_buffer",
            "_path",
            "_id",
        )

        def __init__(
            self, buffer: bytes | None = None, path: os.PathLike[str] | None = None, id: bytes | None = None
        ):
            """
            Creates a Certificate object from a path, buffer, or ID.

            If none of these is given, an exception is raised.
            """

            if buffer is None and path is None and id is None:
                raise ValueError("Certificate cannot be empty.")

            self._buffer = buffer
            self._path = path
            self._id = id

        @classmethod
        def from_buffer(cls, buffer: bytes) -> Certificate:
            """
            Creates a Certificate object from a byte buffer. This byte buffer
            may be either PEM-encoded or DER-encoded. If the buffer is PEM
            encoded it *must* begin with the standard PEM preamble (a series of
            dashes followed by the ASCII bytes "BEGIN CERTIFICATE" and another
            series of dashes). In the absence of that preamble, the
            implementation may assume that the certificate is DER-encoded
            instead.
            """
            return cls(buffer=buffer)

        @classmethod
        def from_file(cls, path: os.PathLike[str]) -> Certificate:
            """
            Creates a Certificate object from a file on disk. The file on disk
            should contain a series of bytes corresponding to a certificate that
            may be either PEM-encoded or DER-encoded. If the bytes are PEM encoded
            it *must* begin with the standard PEM preamble (a series of dashes
            followed by the ASCII bytes "BEGIN CERTIFICATE" and another series of
            dashes). In the absence of that preamble, the implementation may
            assume that the certificate is DER-encoded instead.
            """
            return cls(path=path)

        @classmethod
        def from_id(cls, id: bytes) -> Certificate:
            """
            Creates a Certificate object from an arbitrary identifier. This may
            be useful for implementations that rely on system certificate stores.
            """
            return cls(id=id)

Private Keys
~~~~~~~~~~~~

This module would define a concrete private key class. Much like the
``Certificate`` class, this class has three attributes to correspond to the
three constructors, and further has all the caveats of the ``Certificate``
class.

.. code-block:: python

    class PrivateKey:
        """Object representing a private key corresponding to a public key
        for a certificate used in TLS."""

        __slots__ = (
            "_buffer",
            "_path",
            "_id",
        )

        def __init__(
            self, buffer: bytes | None = None, path: os.PathLike | None = None, id: bytes | None = None
        ):
            """
            Creates a PrivateKey object from a path, buffer, or ID.

            If none of these is given, an exception is raised.
            """

            if buffer is None and path is None and id is None:
                raise ValueError("PrivateKey cannot be empty.")

            self._buffer = buffer
            self._path = path
            self._id = id

        @classmethod
        def from_buffer(cls, buffer: bytes) -> PrivateKey:
            """
            Creates a PrivateKey object from a byte buffer. This byte buffer
            may be either PEM-encoded or DER-encoded. If the buffer is PEM
            encoded it *must* begin with the standard PEM preamble (a series of
            dashes followed by the ASCII bytes "BEGIN", the key type, and
            another series of dashes). In the absence of that preamble, the
            implementation may assume that the private key is DER-encoded
            instead.
            """
            return cls(buffer=buffer)

        @classmethod
        def from_file(cls, path: os.PathLike) -> PrivateKey:
            """
            Creates a PrivateKey object from a file on disk. The file on disk
            should contain a series of bytes corresponding to a certificate that
            may be either PEM-encoded or DER-encoded. If the bytes are PEM encoded
            it *must* begin with the standard PEM preamble (a series of dashes
            followed by the ASCII bytes "BEGIN", the key type, and another series
            of dashes). In the absence of that preamble, the implementation may
            assume that the certificate is DER-encoded instead.
            """
            return cls(path=path)

        @classmethod
        def from_id(cls, id: bytes) -> PrivateKey:
            """
            Creates a PrivateKey object from an arbitrary identifier. This may
            be useful for implementations that rely on system private key stores.
            """
            return cls(id=id)

Signing Chain
~~~~~~~~~~~~~

In order to authenticate themselves, TLS participants need to provide a leaf
certificate with a chain leading up to some root certificate that is trusted by
the other side. Servers always need to authenticate themselves to clients, but
clients can also authenticate themselves to servers during client
authentication. Additionally, the leaf certificate must be accompanied by a
private key, which can either be stored in a separate object, or together with
the leaf certificate itself. This module defines the collection of these objects
as a ``SigningChain`` as detailed below:

.. code-block:: python

    class SigningChain:
        """Object representing a certificate chain used in TLS."""

        leaf: tuple[Certificate, PrivateKey | None]
        chain: list[Certificate]

        def __init__(
            self,
            leaf: tuple[Certificate, PrivateKey | None],
            chain: Sequence[Certificate] | None = None,
        ):
            """Initializes a SigningChain object."""
            self.leaf = leaf
            if chain is None:
                chain = []
            self.chain = list(chain)

As shown in the configuration classes above, a client can have one signing chain
in the case of client authentication or none otherwise. A server can have a
sequence of signing chains, which is useful when it is responsible for multiple
domains.

Trust Store
~~~~~~~~~~~

As discussed above, loading a trust store represents an issue because different
TLS implementations vary wildly in how they allow users to select trust stores.
For this reason, we need to provide a model that assumes very little about the
form that trust stores take.

This problem is the same as the one that the ``Certificate`` and ``PrivateKey``
types need to solve. For this reason, we use the exact same model, by creating a
concrete class that captures the various means of how users could define a trust
store.

A given TLS implementation is not required to handle all possible trust stores.
However, it is strongly recommended that a given TLS implementation handles the
``system`` constructor if at all possible, as this is the most common validation
trust store that is used. TLS implementations can communicate unsupported
options as described in the “Runtime” section below.

.. code-block:: python

    class TrustStore:
        """
        The trust store that is used to verify certificate validity.
        """

        __slots__ = (
            "_buffer",
            "_path",
            "_id",
        )

        def __init__(
            self, buffer: bytes | None = None, path: os.PathLike | None = None, id: bytes | None = None
        ):
            """
            Creates a TrustStore object from a path, buffer, or ID.

            If none of these is given, the default system trust store is used.
            """

            self._buffer = buffer
            self._path = path
            self._id = id

        @classmethod
        def system(cls) -> TrustStore:
            """
            Returns a TrustStore object that represents the system trust
            database.
            """
            return cls()

        @classmethod
        def from_buffer(cls, buffer: bytes) -> TrustStore:
            """
            Initializes a trust store from a buffer of PEM-encoded certificates.
            """
            return cls(buffer=buffer)

        @classmethod
        def from_file(cls, path: os.PathLike) -> TrustStore:
            """
            Initializes a trust store from a single file containing PEMs.
            """
            return cls(path=path)

        @classmethod
        def from_id(cls, id: bytes) -> TrustStore:
            """
            Initializes a trust store from an arbitrary identifier.
            """
            return cls(id=id)

Runtime Access
~~~~~~~~~~~~~~

A not-uncommon use case is for library users to want to specify the TLS
implementation to use while allowing the library to configure the details of the
actual TLS connection. For example, users of :pypi:`requests` may want to be
able to select between OpenSSL or a platform-native solution on Windows and
macOS, or between OpenSSL and NSS on some Linux platforms. These users, however,
may not care about exactly how their TLS configuration is done.

This poses two problems: given an arbitrary concrete implementation, how can a
library:

* Work out whether the implementation supports particular constructors for certificates
  or trust stores (e.g. from arbitrary identifiers)?

* Get the correct types for the two context classes?

Constructing certificate and trust store objects should be possible outside of
the implementation. Therefore, the implementations need to provide a way for
users to verify whether the implementation is compatible with user-constructed
certificates and trust stores. Therefore, each implementation should implement a
``validate_config`` method that takes a ``TLSClientConfiguration`` or
``TLSServerConfiguration`` object and raises an exception if unsupported
constructors were used.

For the types, there are two options: either all concrete implementations can be
required to fit into a specific naming scheme, or we can provide an API that
makes it possible to grab these objects.

This PEP proposes that we use the second approach. This grants the greatest
freedom to concrete implementations to structure their code as they see fit,
requiring only that they provide a single object that has the appropriate
properties in place. Users can then pass this implementation object to libraries
that support it, and those libraries can take care of configuring and using the
concrete implementation.

All concrete implementations must provide a method of obtaining a
``TLSImplementation`` object. The ``TLSImplementation`` object can be a global
singleton or can be created by a callable if there is an advantage in doing
that.

The ``TLSImplementation`` object has the following definition:

.. code-block:: python

    class TLSImplementation(Generic[_ClientContext, _ServerContext]):
        __slots__ = (
            "_client_context",
            "_server_context",
            "_validate_config",
        )

        def __init__(
            self,
            client_context: type[_ClientContext],
            server_context: type[_ServerContext],
            validate_config: Callable[[TLSClientConfiguration | TLSServerConfiguration], None],
        ) -> None:
            self._client_context = client_context
            self._server_context = server_context
            self._validate_config = validate_config

The first two properties must provide the concrete implementation of the
relevant Protocol class. For example, for the client context:

.. code-block:: python

    @property
    def client_context(self) -> type[_ClientContext]:
        """The concrete implementation of the PEP 543 Client Context object,
        if this TLS implementation supports being the client on a TLS connection.
        """
        return self._client_context

This ensures that code like this will work for any implementation:

.. code-block:: python

    client_config = TLSClientConfiguration()
    client_context = implementation.client_context(client_config)

The third property must provide a function that verifies whether a given TLS
configuration contains implementation-compatible certificates, private keys, and
a trust store:

.. code-block:: python

    @property
    def validate_config(self) -> Callable[[TLSClientConfiguration | TLSServerConfiguration], None]:
        """A function that reveals whether this TLS implementation supports a
        particular TLS configuration.
        """
        return self._validate_config

Note that this function only needs to verify that supported constructors were
used for the certificates, private keys, and trust store. It does not need to
parse or retrieve the objects to validate them further.

Insecure Usage
--------------

All of the above assumes that users want to use the module in a secure way.
Sometimes, users want to do imprudent things like disable certificate validation
for testing purposes. To this end, we propose a separate ``insecure`` module
that allows users to do this. This module contains insecure variants of the
configuration, context, and implementation objects, which allow to disable
certificate validation as well as the server hostname check.

This functionality is placed in a separate module to make it as hard as possible
for legitimate users to accidentally use the insecure functionality.
Additionally, it defines a new warning called ``SecurityWarning``, and loudly
warns at every step of the way when trying to create an insecure connection.

This module is only intended for testing purposes. In real-world situations
where a user wants to connect to some IoT device which only has a self-signed
certificate, it is strongly recommended to add this certificate into a custom
trust store, rather than using the insecure module to disable certificate
validation.

Changes to the Standard Library
===============================

The portions of the standard library that interact with TLS should be revised to
use these Protocol classes. This will allow them to function with other TLS
implementations. This includes the following modules:

* :mod:`asyncio`
* :mod:`ftplib`
* :mod:`http`
* :mod:`imaplib`
* :mod:`nntplib`
* :mod:`poplib`
* :mod:`smtplib`
* :mod:`urllib`

Migration of the ssl module
---------------------------

Naturally, we will need to extend the :mod:`ssl` module itself to conform to
these Protocol classes. This extension will take the form of new classes,
potentially in an entirely new module. This will allow applications that take
advantage of the current :mod:`ssl` module to continue to do so, while enabling
the new APIs for applications and libraries that want to use them.

In general, migrating from the :mod:`ssl` module to the new Protocol classes is
not expected to be one-to-one. This is normally acceptable: most tools that use
the :mod:`ssl` module hide it from the user, and so refactoring to use the new
module should be invisible.

However, a specific problem comes from libraries or applications that leak
exceptions from the :mod:`ssl` module, either as part of their defined API or by
accident (which is easily done). Users of those tools may have written code that
tolerates and handles exceptions from the :mod:`ssl` module being raised:
migrating to the protocol classes presented here would potentially cause the
exceptions defined above to be thrown instead, and existing ``except`` blocks
will not catch them.

For this reason, part of the migration of the :mod:`ssl` module would require
that the exceptions in the :mod:`ssl` module alias those defined above. That is,
they would require the following statements to all succeed:

.. code-block:: python

    assert ssl.SSLError is tls.TLSError
    assert ssl.SSLWantReadError is tls.WantReadError
    assert ssl.SSLWantWriteError is tls.WantWriteError


The exact mechanics of how this will be done are beyond the scope of this PEP,
as they are made more complex due to the fact that the current ssl exceptions
are defined in C code, but more details can be found in `an email sent to the
Security-SIG by Christian Heimes
<https://mail.python.org/pipermail/security-sig/2017-January/000213.html>`_.

Future
======

Major future TLS features may require revisions of these protocol classes. These
revisions should be made cautiously: many implementations may not be able to
move forward swiftly, and will be invalidated by changes in these protocol
classes. This is acceptable, but wherever possible features that are specific to
individual implementations should not be added to the protocol classes. The
protocol classes should restrict themselves to high-level descriptions of
IETF-specified features.

However, well-justified extensions to this API absolutely should be made. The
focus of this API is to provide a unifying lowest-common-denominator
configuration option for the Python community. TLS is not a static target, and
as TLS evolves so must this API.

Credits
=======

This PEP is adapted substantially from :pep:`543`, which was withdrawn in 2020.
:pep:`543` was authored by Cory Benfield and Christian Heimes, and received
extensive review from a number of individuals in the community who have
substantially helped shape it. Detailed review for both :pep:`543` and this
PEP was provided by:

* Alex Chan
* Alex Gaynor
* Antoine Pitrou
* Ashwini Oruganti
* Donald Stufft
* Ethan Furman
* Glyph
* Hynek Schlawack
* Jim J Jewett
* Nathaniel J. Smith
* Alyssa Coghlan
* Paul Kehrer
* Steve Dower
* Steven Fackler
* Wes Turner
* Will Bond
* Cory Benfield
* Marc-André Lemburg
* Seth M. Larson
* Victor Stinner
* Ronald Oussoren

Further review of :pep:`543` was provided by the Security-SIG and python-ideas
mailing lists.


Copyright
=========

This document is placed in the public domain or under the CC0-1.0-Universal
license, whichever is more permissive.
