urllib3/contrib/securetransport.py

"""
SecureTranport support for urllib3 via ctypes.

This makes platform-native TLS available to urllib3 users on macOS without the
use of a compiler. This is an important feature because the Python Package
Index is moving to become a TLSv1.2-or-higher server, and the default OpenSSL
that ships with macOS is not capable of doing TLSv1.2. The only way to resolve
this is to give macOS users an alternative solution to the problem, and that
solution is to use SecureTransport.

We use ctypes here because this solution must not require a compiler. That's
because pip is not allowed to require a compiler either.

This is not intended to be a seriously long-term solution to this problem.
The hope is that PEP 543 will eventually solve this issue for us, at which
point we can retire this contrib module. But in the short term, we need to
solve the impending tire fire that is Python on Mac without this kind of
contrib module. So...here we are.

To use this module, simply import and inject it::

    import urllib3.contrib.securetransport
    urllib3.contrib.securetransport.inject_into_urllib3()

Happy TLSing!

This code is a bastardised version of the code found in Will Bond's oscrypto
library. An enormous debt is owed to him for blazing this trail for us. For
that reason, this code should be considered to be covered both by urllib3's
license and by oscrypto's:

.. code-block::

    Copyright (c) 2015-2016 Will Bond <will@wbond.net>

    Permission is hereby granted, free of charge, to any person obtaining a
    copy of this software and associated documentation files (the "Software"),
    to deal in the Software without restriction, including without limitation
    the rights to use, copy, modify, merge, publish, distribute, sublicense,
    and/or sell copies of the Software, and to permit persons to whom the
    Software is furnished to do so, subject to the following conditions:

    The above copyright notice and this permission notice shall be included in
    all copies or substantial portions of the Software.

    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
    AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
    FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
    DEALINGS IN THE SOFTWARE.
"""
from __future__ import absolute_import

import contextlib
import ctypes
import errno
import os.path
import shutil
import socket
import ssl
import struct
import threading
import weakref

import six

from .. import util
from ..util.ssl_ import PROTOCOL_TLS_CLIENT
from ._securetransport.bindings import CoreFoundation, Security, SecurityConst
from ._securetransport.low_level import (
    _assert_no_error,
    _build_tls_unknown_ca_alert,
    _cert_array_from_pem,
    _create_cfstring_array,
    _load_client_cert_chain,
    _temporary_keychain,
)

try:  # Platform-specific: Python 2
    from socket import _fileobject
except ImportError:  # Platform-specific: Python 3
    _fileobject = None
    from ..packages.backports.makefile import backport_makefile

__all__ = ["inject_into_urllib3", "extract_from_urllib3"]

# SNI always works
HAS_SNI = True

orig_util_HAS_SNI = util.HAS_SNI
orig_util_SSLContext = util.ssl_.SSLContext

# This dictionary is used by the read callback to obtain a handle to the
# calling wrapped socket. This is a pretty silly approach, but for now it'll
# do. I feel like I should be able to smuggle a handle to the wrapped socket
# directly in the SSLConnectionRef, but for now this approach will work I
# guess.
#
# We need to lock around this structure for inserts, but we don't do it for
# reads/writes in the callbacks. The reasoning here goes as follows:
#
#    1. It is not possible to call into the callbacks before the dictionary is
#       populated, so once in the callback the id must be in the dictionary.
#    2. The callbacks don't mutate the dictionary, they only read from it, and
#       so cannot conflict with any of the insertions.
#
# This is good: if we had to lock in the callbacks we'd drastically slow down
# the performance of this code.
_connection_refs = weakref.WeakValueDictionary()
_connection_ref_lock = threading.Lock()

# Limit writes to 16kB. This is OpenSSL's limit, but we'll cargo-cult it over
# for no better reason than we need *a* limit, and this one is right there.
SSL_WRITE_BLOCKSIZE = 16384

# This is our equivalent of util.ssl_.DEFAULT_CIPHERS, but expanded out to
# individual cipher suites. We need to do this because this is how
# SecureTransport wants them.
CIPHER_SUITES = [
    SecurityConst.TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384,
    SecurityConst.TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256,
    SecurityConst.TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384,
    SecurityConst.TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,
    SecurityConst.TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256,
    SecurityConst.TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256,
    SecurityConst.TLS_DHE_RSA_WITH_AES_256_GCM_SHA384,
    SecurityConst.TLS_DHE_RSA_WITH_AES_128_GCM_SHA256,
    SecurityConst.TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384,
    SecurityConst.TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA,
    SecurityConst.TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256,
    SecurityConst.TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA,
    SecurityConst.TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384,
    SecurityConst.TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA,
    SecurityConst.TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256,
    SecurityConst.TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA,
    SecurityConst.TLS_DHE_RSA_WITH_AES_256_CBC_SHA256,
    SecurityConst.TLS_DHE_RSA_WITH_AES_256_CBC_SHA,
    SecurityConst.TLS_DHE_RSA_WITH_AES_128_CBC_SHA256,
    SecurityConst.TLS_DHE_RSA_WITH_AES_128_CBC_SHA,
    SecurityConst.TLS_AES_256_GCM_SHA384,
    SecurityConst.TLS_AES_128_GCM_SHA256,
    SecurityConst.TLS_RSA_WITH_AES_256_GCM_SHA384,
    SecurityConst.TLS_RSA_WITH_AES_128_GCM_SHA256,
    SecurityConst.TLS_AES_128_CCM_8_SHA256,
    SecurityConst.TLS_AES_128_CCM_SHA256,
    SecurityConst.TLS_RSA_WITH_AES_256_CBC_SHA256,
    SecurityConst.TLS_RSA_WITH_AES_128_CBC_SHA256,
    SecurityConst.TLS_RSA_WITH_AES_256_CBC_SHA,
    SecurityConst.TLS_RSA_WITH_AES_128_CBC_SHA,
]

# Basically this is simple: for PROTOCOL_SSLv23 we turn it into a low of
# TLSv1 and a high of TLSv1.2. For everything else, we pin to that version.
# TLSv1 to 1.2 are supported on macOS 10.8+
_protocol_to_min_max = {
    util.PROTOCOL_TLS: (SecurityConst.kTLSProtocol1, SecurityConst.kTLSProtocol12),
    PROTOCOL_TLS_CLIENT: (SecurityConst.kTLSProtocol1, SecurityConst.kTLSProtocol12),
}

if hasattr(ssl, "PROTOCOL_SSLv2"):
    _protocol_to_min_max[ssl.PROTOCOL_SSLv2] = (
        SecurityConst.kSSLProtocol2,
        SecurityConst.kSSLProtocol2,
    )
if hasattr(ssl, "PROTOCOL_SSLv3"):
    _protocol_to_min_max[ssl.PROTOCOL_SSLv3] = (
        SecurityConst.kSSLProtocol3,
        SecurityConst.kSSLProtocol3,
    )
if hasattr(ssl, "PROTOCOL_TLSv1"):
    _protocol_to_min_max[ssl.PROTOCOL_TLSv1] = (
        SecurityConst.kTLSProtocol1,
        SecurityConst.kTLSProtocol1,
    )
if hasattr(ssl, "PROTOCOL_TLSv1_1"):
    _protocol_to_min_max[ssl.PROTOCOL_TLSv1_1] = (
        SecurityConst.kTLSProtocol11,
        SecurityConst.kTLSProtocol11,
    )
if hasattr(ssl, "PROTOCOL_TLSv1_2"):
    _protocol_to_min_max[ssl.PROTOCOL_TLSv1_2] = (
        SecurityConst.kTLSProtocol12,
        SecurityConst.kTLSProtocol12,
    )


def inject_into_urllib3():
    """
    Monkey-patch urllib3 with SecureTransport-backed SSL-support.
    """
    util.SSLContext = SecureTransportContext
    util.ssl_.SSLContext = SecureTransportContext
    util.HAS_SNI = HAS_SNI
    util.ssl_.HAS_SNI = HAS_SNI
    util.IS_SECURETRANSPORT = True
    util.ssl_.IS_SECURETRANSPORT = True


def extract_from_urllib3():
    """
    Undo monkey-patching by :func:`inject_into_urllib3`.
    """
    util.SSLContext = orig_util_SSLContext
    util.ssl_.SSLContext = orig_util_SSLContext
    util.HAS_SNI = orig_util_HAS_SNI
    util.ssl_.HAS_SNI = orig_util_HAS_SNI
    util.IS_SECURETRANSPORT = False
    util.ssl_.IS_SECURETRANSPORT = False


def _read_callback(connection_id, data_buffer, data_length_pointer):
    """
    SecureTransport read callback. This is called by ST to request that data
    be returned from the socket.
    """
    wrapped_socket = None
    try:
        wrapped_socket = _connection_refs.get(connection_id)
        if wrapped_socket is None:
            return SecurityConst.errSSLInternal
        base_socket = wrapped_socket.socket

        requested_length = data_length_pointer[0]

        timeout = wrapped_socket.gettimeout()
        error = None
        read_count = 0

        try:
            while read_count < requested_length:
                if timeout is None or timeout >= 0:
                    if not util.wait_for_read(base_socket, timeout):
                        raise socket.error(errno.EAGAIN, "timed out")

                remaining = requested_length - read_count
                buffer = (ctypes.c_char * remaining).from_address(
                    data_buffer + read_count
                )
                chunk_size = base_socket.recv_into(buffer, remaining)
                read_count += chunk_size
                if not chunk_size:
                    if not read_count:
                        return SecurityConst.errSSLClosedGraceful
                    break
        except (socket.error) as e:
            error = e.errno

            if error is not None and error != errno.EAGAIN:
                data_length_pointer[0] = read_count
                if error == errno.ECONNRESET or error == errno.EPIPE:
                    return SecurityConst.errSSLClosedAbort
                raise

        data_length_pointer[0] = read_count

        if read_count != requested_length:
            return SecurityConst.errSSLWouldBlock

        return 0
    except Exception as e:
        if wrapped_socket is not None:
            wrapped_socket._exception = e
        return SecurityConst.errSSLInternal


def _write_callback(connection_id, data_buffer, data_length_pointer):
    """
    SecureTransport write callback. This is called by ST to request that data
    actually be sent on the network.
    """
    wrapped_socket = None
    try:
        wrapped_socket = _connection_refs.get(connection_id)
        if wrapped_socket is None:
            return SecurityConst.errSSLInternal
        base_socket = wrapped_socket.socket

        bytes_to_write = data_length_pointer[0]
        data = ctypes.string_at(data_buffer, bytes_to_write)

        timeout = wrapped_socket.gettimeout()
        error = None
        sent = 0

        try:
            while sent < bytes_to_write:
                if timeout is None or timeout >= 0:
                    if not util.wait_for_write(base_socket, timeout):
                        raise socket.error(errno.EAGAIN, "timed out")
                chunk_sent = base_socket.send(data)
                sent += chunk_sent

                # This has some needless copying here, but I'm not sure there's
                # much value in optimising this data path.
                data = data[chunk_sent:]
        except (socket.error) as e:
            error = e.errno

            if error is not None and error != errno.EAGAIN:
                data_length_pointer[0] = sent
                if error == errno.ECONNRESET or error == errno.EPIPE:
                    return SecurityConst.errSSLClosedAbort
                raise

        data_length_pointer[0] = sent

        if sent != bytes_to_write:
            return SecurityConst.errSSLWouldBlock

        return 0
    except Exception as e:
        if wrapped_socket is not None:
            wrapped_socket._exception = e
        return SecurityConst.errSSLInternal


# We need to keep these two objects references alive: if they get GC'd while
# in use then SecureTransport could attempt to call a function that is in freed
# memory. That would be...uh...bad. Yeah, that's the word. Bad.
_read_callback_pointer = Security.SSLReadFunc(_read_callback)
_write_callback_pointer = Security.SSLWriteFunc(_write_callback)


class WrappedSocket(object):
    """
    API-compatibility wrapper for Python's OpenSSL wrapped socket object.

    Note: _makefile_refs, _drop(), and _reuse() are needed for the garbage
    collector of PyPy.
    """

    def __init__(self, socket):
        self.socket = socket
        self.context = None
        self._makefile_refs = 0
        self._closed = False
        self._exception = None
        self._keychain = None
        self._keychain_dir = None
        self._client_cert_chain = None

        # We save off the previously-configured timeout and then set it to
        # zero. This is done because we use select and friends to handle the
        # timeouts, but if we leave the timeout set on the lower socket then
        # Python will "kindly" call select on that socket again for us. Avoid
        # that by forcing the timeout to zero.
        self._timeout = self.socket.gettimeout()
        self.socket.settimeout(0)

    @contextlib.contextmanager
    def _raise_on_error(self):
        """
        A context manager that can be used to wrap calls that do I/O from
        SecureTransport. If any of the I/O callbacks hit an exception, this
        context manager will correctly propagate the exception after the fact.
        This avoids silently swallowing those exceptions.

        It also correctly forces the socket closed.
        """
        self._exception = None

        # We explicitly don't catch around this yield because in the unlikely
        # event that an exception was hit in the block we don't want to swallow
        # it.
        yield
        if self._exception is not None:
            exception, self._exception = self._exception, None
            self.close()
            raise exception

    def _set_ciphers(self):
        """
        Sets up the allowed ciphers. By default this matches the set in
        util.ssl_.DEFAULT_CIPHERS, at least as supported by macOS. This is done
        custom and doesn't allow changing at this time, mostly because parsing
        OpenSSL cipher strings is going to be a freaking nightmare.
        """
        ciphers = (Security.SSLCipherSuite * len(CIPHER_SUITES))(*CIPHER_SUITES)
        result = Security.SSLSetEnabledCiphers(
            self.context, ciphers, len(CIPHER_SUITES)
        )
        _assert_no_error(result)

    def _set_alpn_protocols(self, protocols):
        """
        Sets up the ALPN protocols on the context.
        """
        if not protocols:
            return
        protocols_arr = _create_cfstring_array(protocols)
        try:
            result = Security.SSLSetALPNProtocols(self.context, protocols_arr)
            _assert_no_error(result)
        finally:
            CoreFoundation.CFRelease(protocols_arr)

    def _custom_validate(self, verify, trust_bundle):
        """
        Called when we have set custom validation. We do this in two cases:
        first, when cert validation is entirely disabled; and second, when
        using a custom trust DB.
        Raises an SSLError if the connection is not trusted.
        """
        # If we disabled cert validation, just say: cool.
        if not verify:
            return

        successes = (
            SecurityConst.kSecTrustResultUnspecified,
            SecurityConst.kSecTrustResultProceed,
        )
        try:
            trust_result = self._evaluate_trust(trust_bundle)
            if trust_result in successes:
                return
            reason = "error code: %d" % (trust_result,)
        except Exception as e:
            # Do not trust on error
            reason = "exception: %r" % (e,)

        # SecureTransport does not send an alert nor shuts down the connection.
        rec = _build_tls_unknown_ca_alert(self.version())
        self.socket.sendall(rec)
        # close the connection immediately
        # l_onoff = 1, activate linger
        # l_linger = 0, linger for 0 seoncds
        opts = struct.pack("ii", 1, 0)
        self.socket.setsockopt(socket.SOL_SOCKET, socket.SO_LINGER, opts)
        self.close()
        raise ssl.SSLError("certificate verify failed, %s" % reason)

    def _evaluate_trust(self, trust_bundle):
        # We want data in memory, so load it up.
        if os.path.isfile(trust_bundle):
            with open(trust_bundle, "rb") as f:
                trust_bundle = f.read()

        cert_array = None
        trust = Security.SecTrustRef()

        try:
            # Get a CFArray that contains the certs we want.
            cert_array = _cert_array_from_pem(trust_bundle)

            # Ok, now the hard part. We want to get the SecTrustRef that ST has
            # created for this connection, shove our CAs into it, tell ST to
            # ignore everything else it knows, and then ask if it can build a
            # chain. This is a buuuunch of code.
            result = Security.SSLCopyPeerTrust(self.context, ctypes.byref(trust))
            _assert_no_error(result)
            if not trust:
                raise ssl.SSLError("Failed to copy trust reference")

            result = Security.SecTrustSetAnchorCertificates(trust, cert_array)
            _assert_no_error(result)

            result = Security.SecTrustSetAnchorCertificatesOnly(trust, True)
            _assert_no_error(result)

            trust_result = Security.SecTrustResultType()
            result = Security.SecTrustEvaluate(trust, ctypes.byref(trust_result))
            _assert_no_error(result)
        finally:
            if trust:
                CoreFoundation.CFRelease(trust)

            if cert_array is not None:
                CoreFoundation.CFRelease(cert_array)

        return trust_result.value

    def handshake(
        self,
        server_hostname,
        verify,
        trust_bundle,
        min_version,
        max_version,
        client_cert,
        client_key,
        client_key_passphrase,
        alpn_protocols,
    ):
        """
        Actually performs the TLS handshake. This is run automatically by
        wrapped socket, and shouldn't be needed in user code.
        """
        # First, we do the initial bits of connection setup. We need to create
        # a context, set its I/O funcs, and set the connection reference.
        self.context = Security.SSLCreateContext(
            None, SecurityConst.kSSLClientSide, SecurityConst.kSSLStreamType
        )
        result = Security.SSLSetIOFuncs(
            self.context, _read_callback_pointer, _write_callback_pointer
        )
        _assert_no_error(result)

        # Here we need to compute the handle to use. We do this by taking the
        # id of self modulo 2**31 - 1. If this is already in the dictionary, we
        # just keep incrementing by one until we find a free space.
        with _connection_ref_lock:
            handle = id(self) % 2147483647
            while handle in _connection_refs:
                handle = (handle + 1) % 2147483647
            _connection_refs[handle] = self

        result = Security.SSLSetConnection(self.context, handle)
        _assert_no_error(result)

        # If we have a server hostname, we should set that too.
        if server_hostname:
            if not isinstance(server_hostname, bytes):
                server_hostname = server_hostname.encode("utf-8")

            result = Security.SSLSetPeerDomainName(
                self.context, server_hostname, len(server_hostname)
            )
            _assert_no_error(result)

        # Setup the ciphers.
        self._set_ciphers()

        # Setup the ALPN protocols.
        self._set_alpn_protocols(alpn_protocols)

        # Set the minimum and maximum TLS versions.
        result = Security.SSLSetProtocolVersionMin(self.context, min_version)
        _assert_no_error(result)

        result = Security.SSLSetProtocolVersionMax(self.context, max_version)
        _assert_no_error(result)

        # If there's a trust DB, we need to use it. We do that by telling
        # SecureTransport to break on server auth. We also do that if we don't
        # want to validate the certs at all: we just won't actually do any
        # authing in that case.
        if not verify or trust_bundle is not None:
            result = Security.SSLSetSessionOption(
                self.context, SecurityConst.kSSLSessionOptionBreakOnServerAuth, True
            )
            _assert_no_error(result)

        # If there's a client cert, we need to use it.
        if client_cert:
            self._keychain, self._keychain_dir = _temporary_keychain()
            self._client_cert_chain = _load_client_cert_chain(
                self._keychain, client_cert, client_key
            )
            result = Security.SSLSetCertificate(self.context, self._client_cert_chain)
            _assert_no_error(result)

        while True:
            with self._raise_on_error():
                result = Security.SSLHandshake(self.context)

                if result == SecurityConst.errSSLWouldBlock:
                    raise socket.timeout("handshake timed out")
                elif result == SecurityConst.errSSLServerAuthCompleted:
                    self._custom_validate(verify, trust_bundle)
                    continue
                else:
                    _assert_no_error(result)
                    break

    def fileno(self):
        return self.socket.fileno()

    # Copy-pasted from Python 3.5 source code
    def _decref_socketios(self):
        if self._makefile_refs > 0:
            self._makefile_refs -= 1
        if self._closed:
            self.close()

    def recv(self, bufsiz):
        buffer = ctypes.create_string_buffer(bufsiz)
        bytes_read = self.recv_into(buffer, bufsiz)
        data = buffer[:bytes_read]
        return data

    def recv_into(self, buffer, nbytes=None):
        # Read short on EOF.
        if self._closed:
            return 0

        if nbytes is None:
            nbytes = len(buffer)

        buffer = (ctypes.c_char * nbytes).from_buffer(buffer)
        processed_bytes = ctypes.c_size_t(0)

        with self._raise_on_error():
            result = Security.SSLRead(
                self.context, buffer, nbytes, ctypes.byref(processed_bytes)
            )

        # There are some result codes that we want to treat as "not always
        # errors". Specifically, those are errSSLWouldBlock,
        # errSSLClosedGraceful, and errSSLClosedNoNotify.
        if result == SecurityConst.errSSLWouldBlock:
            # If we didn't process any bytes, then this was just a time out.
            # However, we can get errSSLWouldBlock in situations when we *did*
            # read some data, and in those cases we should just read "short"
            # and return.
            if processed_bytes.value == 0:
                # Timed out, no data read.
                raise socket.timeout("recv timed out")
        elif result in (
            SecurityConst.errSSLClosedGraceful,
            SecurityConst.errSSLClosedNoNotify,
        ):
            # The remote peer has closed this connection. We should do so as
            # well. Note that we don't actually return here because in
            # principle this could actually be fired along with return data.
            # It's unlikely though.
            self.close()
        else:
            _assert_no_error(result)

        # Ok, we read and probably succeeded. We should return whatever data
        # was actually read.
        return processed_bytes.value

    def settimeout(self, timeout):
        self._timeout = timeout

    def gettimeout(self):
        return self._timeout

    def send(self, data):
        processed_bytes = ctypes.c_size_t(0)

        with self._raise_on_error():
            result = Security.SSLWrite(
                self.context, data, len(data), ctypes.byref(processed_bytes)
            )

        if result == SecurityConst.errSSLWouldBlock and processed_bytes.value == 0:
            # Timed out
            raise socket.timeout("send timed out")
        else:
            _assert_no_error(result)

        # We sent, and probably succeeded. Tell them how much we sent.
        return processed_bytes.value

    def sendall(self, data):
        total_sent = 0
        while total_sent < len(data):
            sent = self.send(data[total_sent : total_sent + SSL_WRITE_BLOCKSIZE])
            total_sent += sent

    def shutdown(self):
        with self._raise_on_error():
            Security.SSLClose(self.context)

    def close(self):
        # TODO: should I do clean shutdown here? Do I have to?
        if self._makefile_refs < 1:
            self._closed = True
            if self.context:
                CoreFoundation.CFRelease(self.context)
                self.context = None
            if self._client_cert_chain:
                CoreFoundation.CFRelease(self._client_cert_chain)
                self._client_cert_chain = None
            if self._keychain:
                Security.SecKeychainDelete(self._keychain)
                CoreFoundation.CFRelease(self._keychain)
                shutil.rmtree(self._keychain_dir)
                self._keychain = self._keychain_dir = None
            return self.socket.close()
        else:
            self._makefile_refs -= 1

    def getpeercert(self, binary_form=False):
        # Urgh, annoying.
        #
        # Here's how we do this:
        #
        # 1. Call SSLCopyPeerTrust to get hold of the trust object for this
        #    connection.
        # 2. Call SecTrustGetCertificateAtIndex for index 0 to get the leaf.
        # 3. To get the CN, call SecCertificateCopyCommonName and process that
        #    string so that it's of the appropriate type.
        # 4. To get the SAN, we need to do something a bit more complex:
        #    a. Call SecCertificateCopyValues to get the data, requesting
        #       kSecOIDSubjectAltName.
        #    b. Mess about with this dictionary to try to get the SANs out.
        #
        # This is gross. Really gross. It's going to be a few hundred LoC extra
        # just to repeat something that SecureTransport can *already do*. So my
        # operating assumption at this time is that what we want to do is
        # instead to just flag to urllib3 that it shouldn't do its own hostname
        # validation when using SecureTransport.
        if not binary_form:
            raise ValueError("SecureTransport only supports dumping binary certs")
        trust = Security.SecTrustRef()
        certdata = None
        der_bytes = None

        try:
            # Grab the trust store.
            result = Security.SSLCopyPeerTrust(self.context, ctypes.byref(trust))
            _assert_no_error(result)
            if not trust:
                # Probably we haven't done the handshake yet. No biggie.
                return None

            cert_count = Security.SecTrustGetCertificateCount(trust)
            if not cert_count:
                # Also a case that might happen if we haven't handshaked.
                # Handshook? Handshaken?
                return None

            leaf = Security.SecTrustGetCertificateAtIndex(trust, 0)
            assert leaf

            # Ok, now we want the DER bytes.
            certdata = Security.SecCertificateCopyData(leaf)
            assert certdata

            data_length = CoreFoundation.CFDataGetLength(certdata)
            data_buffer = CoreFoundation.CFDataGetBytePtr(certdata)
            der_bytes = ctypes.string_at(data_buffer, data_length)
        finally:
            if certdata:
                CoreFoundation.CFRelease(certdata)
            if trust:
                CoreFoundation.CFRelease(trust)

        return der_bytes

    def version(self):
        protocol = Security.SSLProtocol()
        result = Security.SSLGetNegotiatedProtocolVersion(
            self.context, ctypes.byref(protocol)
        )
        _assert_no_error(result)
        if protocol.value == SecurityConst.kTLSProtocol13:
            raise ssl.SSLError("SecureTransport does not support TLS 1.3")
        elif protocol.value == SecurityConst.kTLSProtocol12:
            return "TLSv1.2"
        elif protocol.value == SecurityConst.kTLSProtocol11:
            return "TLSv1.1"
        elif protocol.value == SecurityConst.kTLSProtocol1:
            return "TLSv1"
        elif protocol.value == SecurityConst.kSSLProtocol3:
            return "SSLv3"
        elif protocol.value == SecurityConst.kSSLProtocol2:
            return "SSLv2"
        else:
            raise ssl.SSLError("Unknown TLS version: %r" % protocol)

    def _reuse(self):
        self._makefile_refs += 1

    def _drop(self):
        if self._makefile_refs < 1:
            self.close()
        else:
            self._makefile_refs -= 1


if _fileobject:  # Platform-specific: Python 2

    def makefile(self, mode, bufsize=-1):
        self._makefile_refs += 1
        return _fileobject(self, mode, bufsize, close=True)


else:  # Platform-specific: Python 3

    def makefile(self, mode="r", buffering=None, *args, **kwargs):
        # We disable buffering with SecureTransport because it conflicts with
        # the buffering that ST does internally (see issue #1153 for more).
        buffering = 0
        return backport_makefile(self, mode, buffering, *args, **kwargs)


WrappedSocket.makefile = makefile


class SecureTransportContext(object):
    """
    I am a wrapper class for the SecureTransport library, to translate the
    interface of the standard library ``SSLContext`` object to calls into
    SecureTransport.
    """

    def __init__(self, protocol):
        self._min_version, self._max_version = _protocol_to_min_max[protocol]
        self._options = 0
        self._verify = False
        self._trust_bundle = None
        self._client_cert = None
        self._client_key = None
        self._client_key_passphrase = None
        self._alpn_protocols = None

    @property
    def check_hostname(self):
        """
        SecureTransport cannot have its hostname checking disabled. For more,
        see the comment on getpeercert() in this file.
        """
        return True

    @check_hostname.setter
    def check_hostname(self, value):
        """
        SecureTransport cannot have its hostname checking disabled. For more,
        see the comment on getpeercert() in this file.
        """
        pass

    @property
    def options(self):
        # TODO: Well, crap.
        #
        # So this is the bit of the code that is the most likely to cause us
        # trouble. Essentially we need to enumerate all of the SSL options that
        # users might want to use and try to see if we can sensibly translate
        # them, or whether we should just ignore them.
        return self._options

    @options.setter
    def options(self, value):
        # TODO: Update in line with above.
        self._options = value

    @property
    def verify_mode(self):
        return ssl.CERT_REQUIRED if self._verify else ssl.CERT_NONE

    @verify_mode.setter
    def verify_mode(self, value):
        self._verify = True if value == ssl.CERT_REQUIRED else False

    def set_default_verify_paths(self):
        # So, this has to do something a bit weird. Specifically, what it does
        # is nothing.
        #
        # This means that, if we had previously had load_verify_locations
        # called, this does not undo that. We need to do that because it turns
        # out that the rest of the urllib3 code will attempt to load the
        # default verify paths if it hasn't been told about any paths, even if
        # the context itself was sometime earlier. We resolve that by just
        # ignoring it.
        pass

    def load_default_certs(self):
        return self.set_default_verify_paths()

    def set_ciphers(self, ciphers):
        # For now, we just require the default cipher string.
        if ciphers != util.ssl_.DEFAULT_CIPHERS:
            raise ValueError("SecureTransport doesn't support custom cipher strings")

    def load_verify_locations(self, cafile=None, capath=None, cadata=None):
        # OK, we only really support cadata and cafile.
        if capath is not None:
            raise ValueError("SecureTransport does not support cert directories")

        # Raise if cafile does not exist.
        if cafile is not None:
            with open(cafile):
                pass

        self._trust_bundle = cafile or cadata

    def load_cert_chain(self, certfile, keyfile=None, password=None):
        self._client_cert = certfile
        self._client_key = keyfile
        self._client_cert_passphrase = password

    def set_alpn_protocols(self, protocols):
        """
        Sets the ALPN protocols that will later be set on the context.

        Raises a NotImplementedError if ALPN is not supported.
        """
        if not hasattr(Security, "SSLSetALPNProtocols"):
            raise NotImplementedError(
                "SecureTransport supports ALPN only in macOS 10.12+"
            )
        self._alpn_protocols = [six.ensure_binary(p) for p in protocols]

    def wrap_socket(
        self,
        sock,
        server_side=False,
        do_handshake_on_connect=True,
        suppress_ragged_eofs=True,
        server_hostname=None,
    ):
        # So, what do we do here? Firstly, we assert some properties. This is a
        # stripped down shim, so there is some functionality we don't support.
        # See PEP 543 for the real deal.
        assert not server_side
        assert do_handshake_on_connect
        assert suppress_ragged_eofs

        # Ok, we're good to go. Now we want to create the wrapped socket object
        # and store it in the appropriate place.
        wrapped_socket = WrappedSocket(sock)

        # Now we can handshake
        wrapped_socket.handshake(
            server_hostname,
            self._verify,
            self._trust_bundle,
            self._min_version,
            self._max_version,
            self._client_cert,
            self._client_key,
            self._client_key_passphrase,
            self._alpn_protocols,
        )
        return wrapped_socket