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Edit: /usr/lib/python3.6/site-packages/urllib3/util/ssl_.py

from __future__ import absolute_import
import errno
import warnings
import hmac
import socket

from binascii import hexlify, unhexlify
from hashlib import md5, sha1, sha256

from ..exceptions import SSLError, InsecurePlatformWarning, SNIMissingWarning
from ..packages import six

SSLContext = None
HAS_SNI = False
IS_PYOPENSSL = False
IS_SECURETRANSPORT = False

# Maps the length of a digest to a possible hash function producing this digest
HASHFUNC_MAP = {
    32: md5,
    40: sha1,
    64: sha256,
}

def _const_compare_digest_backport(a, b):
    """
    Compare two digests of equal length in constant time.

The digests must be of type str/bytes.
    Returns True if the digests match, and False otherwise.
    """
    result = abs(len(a) - len(b))
    for l, r in zip(bytearray(a), bytearray(b)):
        result |= l ^ r
    return result == 0

_const_compare_digest = getattr(hmac, 'compare_digest',
                                _const_compare_digest_backport)

try:  # Test for SSL features
    import ssl
    from ssl import wrap_socket, CERT_NONE, PROTOCOL_SSLv23
    from ssl import HAS_SNI  # Has SNI?
except ImportError:
    pass

try:
    from ssl import OP_NO_SSLv2, OP_NO_SSLv3, OP_NO_COMPRESSION
except ImportError:
    OP_NO_SSLv2, OP_NO_SSLv3 = 0x1000000, 0x2000000
    OP_NO_COMPRESSION = 0x20000

# Python 2.7 doesn't have inet_pton on non-Linux so we fallback on inet_aton in
# those cases. This means that we can only detect IPv4 addresses in this case.
if hasattr(socket, 'inet_pton'):
    inet_pton = socket.inet_pton
else:
    # Maybe we can use ipaddress if the user has urllib3[secure]?
    try:
        import ipaddress

def inet_pton(_, host):
            if isinstance(host, bytes):
                host = host.decode('ascii')
            return ipaddress.ip_address(host)

except ImportError:  # Platform-specific: Non-Linux
        def inet_pton(_, host):
            return socket.inet_aton(host)

# A secure default.
# Sources for more information on TLS ciphers:
#
# - https://wiki.mozilla.org/Security/Server_Side_TLS
# - https://www.ssllabs.com/projects/best-practices/index.html
# - https://hynek.me/articles/hardening-your-web-servers-ssl-ciphers/
#
# The general intent is:
# - Prefer TLS 1.3 cipher suites
# - prefer cipher suites that offer perfect forward secrecy (DHE/ECDHE),
# - prefer ECDHE over DHE for better performance,
# - prefer any AES-GCM and ChaCha20 over any AES-CBC for better performance and
#   security,
# - prefer AES-GCM over ChaCha20 because hardware-accelerated AES is common,
# - disable NULL authentication, MD5 MACs and DSS for security reasons.
DEFAULT_CIPHERS = ':'.join([
    'TLS13-AES-256-GCM-SHA384',
    'TLS13-CHACHA20-POLY1305-SHA256',
    'TLS13-AES-128-GCM-SHA256',
    'ECDH+AESGCM',
    'ECDH+CHACHA20',
    'DH+AESGCM',
    'DH+CHACHA20',
    'ECDH+AES256',
    'DH+AES256',
    'ECDH+AES128',
    'DH+AES',
    'RSA+AESGCM',
    'RSA+AES',
    '!aNULL',
    '!eNULL',
    '!MD5',
])

try:
    from ssl import SSLContext  # Modern SSL?
except ImportError:
    import sys

class SSLContext(object):  # Platform-specific: Python 2
        def __init__(self, protocol_version):
            self.protocol = protocol_version
            # Use default values from a real SSLContext
            self.check_hostname = False
            self.verify_mode = ssl.CERT_NONE
            self.ca_certs = None
            self.options = 0
            self.certfile = None
            self.keyfile = None
            self.ciphers = None

def load_cert_chain(self, certfile, keyfile):
            self.certfile = certfile
            self.keyfile = keyfile

def load_verify_locations(self, cafile=None, capath=None):
            self.ca_certs = cafile

if capath is not None:
                raise SSLError("CA directories not supported in older Pythons")

def set_ciphers(self, cipher_suite):
            self.ciphers = cipher_suite

def wrap_socket(self, socket, server_hostname=None, server_side=False):
            warnings.warn(
                'A true SSLContext object is not available. This prevents '
                'urllib3 from configuring SSL appropriately and may cause '
                'certain SSL connections to fail. You can upgrade to a newer '
                'version of Python to solve this. For more information, see '
                'https://urllib3.readthedocs.io/en/latest/advanced-usage.html'
                '#ssl-warnings',
                InsecurePlatformWarning
            )
            kwargs = {
                'keyfile': self.keyfile,
                'certfile': self.certfile,
                'ca_certs': self.ca_certs,
                'cert_reqs': self.verify_mode,
                'ssl_version': self.protocol,
                'server_side': server_side,
            }
            return wrap_socket(socket, ciphers=self.ciphers, **kwargs)

def assert_fingerprint(cert, fingerprint):
    """
    Checks if given fingerprint matches the supplied certificate.

:param cert:
        Certificate as bytes object.
    :param fingerprint:
        Fingerprint as string of hexdigits, can be interspersed by colons.
    """

fingerprint = fingerprint.replace(':', '').lower()
    digest_length = len(fingerprint)
    hashfunc = HASHFUNC_MAP.get(digest_length)
    if not hashfunc:
        raise SSLError(
            'Fingerprint of invalid length: {0}'.format(fingerprint))

# We need encode() here for py32; works on py2 and p33.
    fingerprint_bytes = unhexlify(fingerprint.encode())

cert_digest = hashfunc(cert).digest()

if not _const_compare_digest(cert_digest, fingerprint_bytes):
        raise SSLError('Fingerprints did not match. Expected "{0}", got "{1}".'
                       .format(fingerprint, hexlify(cert_digest)))

def resolve_cert_reqs(candidate):
    """
    Resolves the argument to a numeric constant, which can be passed to
    the wrap_socket function/method from the ssl module.
    Defaults to :data:`ssl.CERT_NONE`.
    If given a string it is assumed to be the name of the constant in the
    :mod:`ssl` module or its abbreviation.
    (So you can specify `REQUIRED` instead of `CERT_REQUIRED`.
    If it's neither `None` nor a string we assume it is already the numeric
    constant which can directly be passed to wrap_socket.
    """
    if candidate is None:
        return CERT_NONE

if isinstance(candidate, str):
        res = getattr(ssl, candidate, None)
        if res is None:
            res = getattr(ssl, 'CERT_' + candidate)
        return res

return candidate

def resolve_ssl_version(candidate):
    """
    like resolve_cert_reqs
    """
    if candidate is None:
        return PROTOCOL_SSLv23

if isinstance(candidate, str):
        res = getattr(ssl, candidate, None)
        if res is None:
            res = getattr(ssl, 'PROTOCOL_' + candidate)
        return res

return candidate

def create_urllib3_context(ssl_version=None, cert_reqs=None,
                           options=None, ciphers=None):
    """All arguments have the same meaning as ``ssl_wrap_socket``.

By default, this function does a lot of the same work that
    ``ssl.create_default_context`` does on Python 3.4+. It:

- Disables SSLv2, SSLv3, and compression
    - Sets a restricted set of server ciphers

If you wish to enable SSLv3, you can do::

from urllib3.util import ssl_
        context = ssl_.create_urllib3_context()
        context.options &= ~ssl_.OP_NO_SSLv3

You can do the same to enable compression (substituting ``COMPRESSION``
    for ``SSLv3`` in the last line above).

:param ssl_version:
        The desired protocol version to use. This will default to
        PROTOCOL_SSLv23 which will negotiate the highest protocol that both
        the server and your installation of OpenSSL support.
    :param cert_reqs:
        Whether to require the certificate verification. This defaults to
        ``ssl.CERT_REQUIRED``.
    :param options:
        Specific OpenSSL options. These default to ``ssl.OP_NO_SSLv2``,
        ``ssl.OP_NO_SSLv3``, ``ssl.OP_NO_COMPRESSION``.
    :param ciphers:
        Which cipher suites to allow the server to select.
    :returns:
        Constructed SSLContext object with specified options
    :rtype: SSLContext
    """
    context = SSLContext(ssl_version or ssl.PROTOCOL_SSLv23)

context.set_ciphers(ciphers or DEFAULT_CIPHERS)

# Setting the default here, as we may have no ssl module on import
    cert_reqs = ssl.CERT_REQUIRED if cert_reqs is None else cert_reqs

if options is None:
        options = 0
        # SSLv2 is easily broken and is considered harmful and dangerous
        options |= OP_NO_SSLv2
        # SSLv3 has several problems and is now dangerous
        options |= OP_NO_SSLv3
        # Disable compression to prevent CRIME attacks for OpenSSL 1.0+
        # (issue #309)
        options |= OP_NO_COMPRESSION

context.options |= options

# Enable post-handshake authentication for TLS 1.3, see GH #1634. PHA is
    # necessary for conditional client cert authentication with TLS 1.3.
    # The attribute is None for OpenSSL <= 1.1.0 or does not exist in older
    # versions of Python.
    if getattr(context, "post_handshake_auth", None) is not None:
        context.post_handshake_auth = True

context.verify_mode = cert_reqs
    if getattr(context, 'check_hostname', None) is not None:  # Platform-specific: Python 3.2
        # We do our own verification, including fingerprints and alternative
        # hostnames. So disable it here
        context.check_hostname = False
    return context

def ssl_wrap_socket(sock, keyfile=None, certfile=None, cert_reqs=None,
                    ca_certs=None, server_hostname=None,
                    ssl_version=None, ciphers=None, ssl_context=None,
                    ca_cert_dir=None):
    """
    All arguments except for server_hostname, ssl_context, and ca_cert_dir have
    the same meaning as they do when using :func:`ssl.wrap_socket`.

:param server_hostname:
        When SNI is supported, the expected hostname of the certificate
    :param ssl_context:
        A pre-made :class:`SSLContext` object. If none is provided, one will
        be created using :func:`create_urllib3_context`.
    :param ciphers:
        A string of ciphers we wish the client to support.
    :param ca_cert_dir:
        A directory containing CA certificates in multiple separate files, as
        supported by OpenSSL's -CApath flag or the capath argument to
        SSLContext.load_verify_locations().
    """
    context = ssl_context
    if context is None:
        # Note: This branch of code and all the variables in it are no longer
        # used by urllib3 itself. We should consider deprecating and removing
        # this code.
        context = create_urllib3_context(ssl_version, cert_reqs,
                                         ciphers=ciphers)

if ca_certs or ca_cert_dir:
        try:
            context.load_verify_locations(ca_certs, ca_cert_dir)
        except IOError as e:  # Platform-specific: Python 2.7
            raise SSLError(e)
        # Py33 raises FileNotFoundError which subclasses OSError
        # These are not equivalent unless we check the errno attribute
        except OSError as e:  # Platform-specific: Python 3.3 and beyond
            if e.errno == errno.ENOENT:
                raise SSLError(e)
            raise

# Don't load system certs unless there were no CA certs or
    # SSLContext object specified manually.
    elif ssl_context is None and hasattr(context, 'load_default_certs'):
        # try to load OS default certs; works well on Windows (require Python3.4+)
        context.load_default_certs()

if certfile:
        context.load_cert_chain(certfile, keyfile)

# If we detect server_hostname is an IP address then the SNI
    # extension should not be used according to RFC3546 Section 3.1
    # We shouldn't warn the user if SNI isn't available but we would
    # not be using SNI anyways due to IP address for server_hostname.
    if ((server_hostname is not None and not is_ipaddress(server_hostname))
            or IS_SECURETRANSPORT):
        if HAS_SNI and server_hostname is not None:
            return context.wrap_socket(sock, server_hostname=server_hostname)

warnings.warn(
            'An HTTPS request has been made, but the SNI (Server Name '
            'Indication) extension to TLS is not available on this platform. '
            'This may cause the server to present an incorrect TLS '
            'certificate, which can cause validation failures. You can upgrade to '
            'a newer version of Python to solve this. For more information, see '
            'https://urllib3.readthedocs.io/en/latest/advanced-usage.html'
            '#ssl-warnings',
            SNIMissingWarning
        )

return context.wrap_socket(sock)

def is_ipaddress(hostname):
    """Detects whether the hostname given is an IP address.

:param str hostname: Hostname to examine.
    :return: True if the hostname is an IP address, False otherwise.
    """
    if six.PY3 and isinstance(hostname, bytes):
        # IDN A-label bytes are ASCII compatible.
        hostname = hostname.decode('ascii')

families = [socket.AF_INET]
    if hasattr(socket, 'AF_INET6'):
        families.append(socket.AF_INET6)

for af in families:
        try:
            inet_pton(af, hostname)
        except (socket.error, ValueError, OSError):
            pass
        else:
            return True
    return False

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