ssl.py

# Wrapper module for _ssl, providing some additional facilities
# implemented in Python.  Written by Bill Janssen.

"""This module provides some more Pythonic support for SSL.

Object types:

  SSLSocket -- subtype of socket.socket which does SSL over the socket

Exceptions:

  SSLError -- exception raised for I/O errors

Functions:

  cert_time_to_seconds -- convert time string used for certificate
                          notBefore and notAfter functions to integer
                          seconds past the Epoch (the time values
                          returned from time.time())

  get_server_certificate (addr, ssl_version, ca_certs, timeout) -- Retrieve the
                          certificate from the server at the specified
                          address and return it as a PEM-encoded string


Integer constants:

SSL_ERROR_ZERO_RETURN
SSL_ERROR_WANT_READ
SSL_ERROR_WANT_WRITE
SSL_ERROR_WANT_X509_LOOKUP
SSL_ERROR_SYSCALL
SSL_ERROR_SSL
SSL_ERROR_WANT_CONNECT

SSL_ERROR_EOF
SSL_ERROR_INVALID_ERROR_CODE

The following group define certificate requirements that one side is
allowing/requiring from the other side:

CERT_NONE - no certificates from the other side are required (or will
            be looked at if provided)
CERT_OPTIONAL - certificates are not required, but if provided will be
                validated, and if validation fails, the connection will
                also fail
CERT_REQUIRED - certificates are required, and will be validated, and
                if validation fails, the connection will also fail

The following constants identify various SSL protocol variants:

PROTOCOL_SSLv2
PROTOCOL_SSLv3
PROTOCOL_SSLv23
PROTOCOL_TLS
PROTOCOL_TLS_CLIENT
PROTOCOL_TLS_SERVER
PROTOCOL_TLSv1
PROTOCOL_TLSv1_1
PROTOCOL_TLSv1_2

The following constants identify various SSL alert message descriptions as per
http://www.iana.org/assignments/tls-parameters/tls-parameters.xml#tls-parameters-6

ALERT_DESCRIPTION_CLOSE_NOTIFY
ALERT_DESCRIPTION_UNEXPECTED_MESSAGE
ALERT_DESCRIPTION_BAD_RECORD_MAC
ALERT_DESCRIPTION_RECORD_OVERFLOW
ALERT_DESCRIPTION_DECOMPRESSION_FAILURE
ALERT_DESCRIPTION_HANDSHAKE_FAILURE
ALERT_DESCRIPTION_BAD_CERTIFICATE
ALERT_DESCRIPTION_UNSUPPORTED_CERTIFICATE
ALERT_DESCRIPTION_CERTIFICATE_REVOKED
ALERT_DESCRIPTION_CERTIFICATE_EXPIRED
ALERT_DESCRIPTION_CERTIFICATE_UNKNOWN
ALERT_DESCRIPTION_ILLEGAL_PARAMETER
ALERT_DESCRIPTION_UNKNOWN_CA
ALERT_DESCRIPTION_ACCESS_DENIED
ALERT_DESCRIPTION_DECODE_ERROR
ALERT_DESCRIPTION_DECRYPT_ERROR
ALERT_DESCRIPTION_PROTOCOL_VERSION
ALERT_DESCRIPTION_INSUFFICIENT_SECURITY
ALERT_DESCRIPTION_INTERNAL_ERROR
ALERT_DESCRIPTION_USER_CANCELLED
ALERT_DESCRIPTION_NO_RENEGOTIATION
ALERT_DESCRIPTION_UNSUPPORTED_EXTENSION
ALERT_DESCRIPTION_CERTIFICATE_UNOBTAINABLE
ALERT_DESCRIPTION_UNRECOGNIZED_NAME
ALERT_DESCRIPTION_BAD_CERTIFICATE_STATUS_RESPONSE
ALERT_DESCRIPTION_BAD_CERTIFICATE_HASH_VALUE
ALERT_DESCRIPTION_UNKNOWN_PSK_IDENTITY
"""

import sys
import os
from collections import namedtuple
from enum import Enum as _Enum, IntEnum as _IntEnum, IntFlag as _IntFlag
from enum import _simple_enum

import _ssl             # if we can't import it, let the error propagate

from _ssl import OPENSSL_VERSION_NUMBER, OPENSSL_VERSION_INFO, OPENSSL_VERSION
from _ssl import _SSLContext, MemoryBIO, SSLSession
from _ssl import (
    SSLError, SSLZeroReturnError, SSLWantReadError, SSLWantWriteError,
    SSLSyscallError, SSLEOFError, SSLCertVerificationError
    )
from _ssl import txt2obj as _txt2obj, nid2obj as _nid2obj
from _ssl import RAND_status, RAND_add, RAND_bytes, RAND_pseudo_bytes
try:
    from _ssl import RAND_egd
except ImportError:
    # LibreSSL does not provide RAND_egd
    pass


from _ssl import (
    HAS_SNI, HAS_ECDH, HAS_NPN, HAS_ALPN, HAS_SSLv2, HAS_SSLv3, HAS_TLSv1,
    HAS_TLSv1_1, HAS_TLSv1_2, HAS_TLSv1_3
)
from _ssl import _DEFAULT_CIPHERS, _OPENSSL_API_VERSION

_IntEnum._convert_(
    '_SSLMethod', __name__,
    lambda name: name.startswith('PROTOCOL_') and name != 'PROTOCOL_SSLv23',
    source=_ssl)

_IntFlag._convert_(
    'Options', __name__,
    lambda name: name.startswith('OP_'),
    source=_ssl)

_IntEnum._convert_(
    'AlertDescription', __name__,
    lambda name: name.startswith('ALERT_DESCRIPTION_'),
    source=_ssl)

_IntEnum._convert_(
    'SSLErrorNumber', __name__,
    lambda name: name.startswith('SSL_ERROR_'),
    source=_ssl)

_IntFlag._convert_(
    'VerifyFlags', __name__,
    lambda name: name.startswith('VERIFY_'),
    source=_ssl)

_IntEnum._convert_(
    'VerifyMode', __name__,
    lambda name: name.startswith('CERT_'),
    source=_ssl)

PROTOCOL_SSLv23 = _SSLMethod.PROTOCOL_SSLv23 = _SSLMethod.PROTOCOL_TLS
_PROTOCOL_NAMES = {value: name for name, value in _SSLMethod.__members__.items()}

_SSLv2_IF_EXISTS = getattr(_SSLMethod, 'PROTOCOL_SSLv2', None)


@_simple_enum(_IntEnum)
class TLSVersion:
    MINIMUM_SUPPORTED = _ssl.PROTO_MINIMUM_SUPPORTED
    SSLv3 = _ssl.PROTO_SSLv3
    TLSv1 = _ssl.PROTO_TLSv1
    TLSv1_1 = _ssl.PROTO_TLSv1_1
    TLSv1_2 = _ssl.PROTO_TLSv1_2
    TLSv1_3 = _ssl.PROTO_TLSv1_3
    MAXIMUM_SUPPORTED = _ssl.PROTO_MAXIMUM_SUPPORTED


@_simple_enum(_IntEnum)
class _TLSContentType:
    """Content types (record layer)

    See RFC 8446, section B.1
    """
    CHANGE_CIPHER_SPEC = 20
    ALERT = 21
    HANDSHAKE = 22
    APPLICATION_DATA = 23
    # pseudo content types
    HEADER = 0x100
    INNER_CONTENT_TYPE = 0x101


@_simple_enum(_IntEnum)
class _TLSAlertType:
    """Alert types for TLSContentType.ALERT messages

    See RFC 8466, section B.2
    """
    CLOSE_NOTIFY = 0
    UNEXPECTED_MESSAGE = 10
    BAD_RECORD_MAC = 20
    DECRYPTION_FAILED = 21
    RECORD_OVERFLOW = 22
    DECOMPRESSION_FAILURE = 30
    HANDSHAKE_FAILURE = 40
    NO_CERTIFICATE = 41
    BAD_CERTIFICATE = 42
    UNSUPPORTED_CERTIFICATE = 43
    CERTIFICATE_REVOKED = 44
    CERTIFICATE_EXPIRED = 45
    CERTIFICATE_UNKNOWN = 46
    ILLEGAL_PARAMETER = 47
    UNKNOWN_CA = 48
    ACCESS_DENIED = 49
    DECODE_ERROR = 50
    DECRYPT_ERROR = 51
    EXPORT_RESTRICTION = 60
    PROTOCOL_VERSION = 70
    INSUFFICIENT_SECURITY = 71
    INTERNAL_ERROR = 80
    INAPPROPRIATE_FALLBACK = 86
    USER_CANCELED = 90
    NO_RENEGOTIATION = 100
    MISSING_EXTENSION = 109
    UNSUPPORTED_EXTENSION = 110
    CERTIFICATE_UNOBTAINABLE = 111
    UNRECOGNIZED_NAME = 112
    BAD_CERTIFICATE_STATUS_RESPONSE = 113
    BAD_CERTIFICATE_HASH_VALUE = 114
    UNKNOWN_PSK_IDENTITY = 115
    CERTIFICATE_REQUIRED = 116
    NO_APPLICATION_PROTOCOL = 120


@_simple_enum(_IntEnum)
class _TLSMessageType:
    """Message types (handshake protocol)

    See RFC 8446, section B.3
    """
    HELLO_REQUEST = 0
    CLIENT_HELLO = 1
    SERVER_HELLO = 2
    HELLO_VERIFY_REQUEST = 3
    NEWSESSION_TICKET = 4
    END_OF_EARLY_DATA = 5
    HELLO_RETRY_REQUEST = 6
    ENCRYPTED_EXTENSIONS = 8
    CERTIFICATE = 11
    SERVER_KEY_EXCHANGE = 12
    CERTIFICATE_REQUEST = 13
    SERVER_DONE = 14
    CERTIFICATE_VERIFY = 15
    CLIENT_KEY_EXCHANGE = 16
    FINISHED = 20
    CERTIFICATE_URL = 21
    CERTIFICATE_STATUS = 22
    SUPPLEMENTAL_DATA = 23
    KEY_UPDATE = 24
    NEXT_PROTO = 67
    MESSAGE_HASH = 254
    CHANGE_CIPHER_SPEC = 0x0101


if sys.platform == "win32":
    from _ssl import enum_certificates, enum_crls

from socket import socket, SOCK_STREAM, create_connection
from socket import SOL_SOCKET, SO_TYPE, _GLOBAL_DEFAULT_TIMEOUT
import socket as _socket
import base64        # for DER-to-PEM translation
import errno
import warnings


socket_error = OSError  # keep that public name in module namespace

CHANNEL_BINDING_TYPES = ['tls-unique']

HAS_NEVER_CHECK_COMMON_NAME = hasattr(_ssl, 'HOSTFLAG_NEVER_CHECK_SUBJECT')


_RESTRICTED_SERVER_CIPHERS = _DEFAULT_CIPHERS

CertificateError = SSLCertVerificationError


def _dnsname_match(dn, hostname):
    """Matching according to RFC 6125, section 6.4.3

    - Hostnames are compared lower-case.
    - For IDNA, both dn and hostname must be encoded as IDN A-label (ACE).
    - Partial wildcards like 'www*.example.org', multiple wildcards, sole
      wildcard or wildcards in labels other then the left-most label are not
      supported and a CertificateError is raised.
    - A wildcard must match at least one character.
    """
    if not dn:
        return False

    wildcards = dn.count('*')
    # speed up common case w/o wildcards
    if not wildcards:
        return dn.lower() == hostname.lower()

    if wildcards > 1:
        raise CertificateError(
            "too many wildcards in certificate DNS name: {!r}.".format(dn))

    dn_leftmost, sep, dn_remainder = dn.partition('.')

    if '*' in dn_remainder:
        # Only match wildcard in leftmost segment.
        raise CertificateError(
            "wildcard can only be present in the leftmost label: "
            "{!r}.".format(dn))

    if not sep:
        # no right side
        raise CertificateError(
            "sole wildcard without additional labels are not support: "
            "{!r}.".format(dn))

    if dn_leftmost != '*':
        # no partial wildcard matching
        raise CertificateError(
            "partial wildcards in leftmost label are not supported: "
            "{!r}.".format(dn))

    hostname_leftmost, sep, hostname_remainder = hostname.partition('.')
    if not hostname_leftmost or not sep:
        # wildcard must match at least one char
        return False
    return dn_remainder.lower() == hostname_remainder.lower()


def _inet_paton(ipname):
    """Try to convert an IP address to packed binary form

    Supports IPv4 addresses on all platforms and IPv6 on platforms with IPv6
    support.
    """
    # inet_aton() also accepts strings like '1', '127.1', some also trailing
    # data like '127.0.0.1 whatever'.
    try:
        addr = _socket.inet_aton(ipname)
    except OSError:
        # not an IPv4 address
        pass
    else:
        if _socket.inet_ntoa(addr) == ipname:
            # only accept injective ipnames
            return addr
        else:
            # refuse for short IPv4 notation and additional trailing data
            raise ValueError(
                "{!r} is not a quad-dotted IPv4 address.".format(ipname)
            )

    try:
        return _socket.inet_pton(_socket.AF_INET6, ipname)
    except OSError:
        raise ValueError("{!r} is neither an IPv4 nor an IP6 "
                         "address.".format(ipname))
    except AttributeError:
        # AF_INET6 not available
        pass

    raise ValueError("{!r} is not an IPv4 address.".format(ipname))


def _ipaddress_match(cert_ipaddress, host_ip):
    """Exact matching of IP addresses.

    RFC 6125 explicitly doesn't define an algorithm for this
    (section 1.7.2 - "Out of Scope").
    """
    # OpenSSL may add a trailing newline to a subjectAltName's IP address,
    # commonly with IPv6 addresses. Strip off trailing \n.
    ip = _inet_paton(cert_ipaddress.rstrip())
    return ip == host_ip


def match_hostname(cert, hostname):
    """Verify that *cert* (in decoded format as returned by
    SSLSocket.getpeercert()) matches the *hostname*.  RFC 2818 and RFC 6125
    rules are followed.

    The function matches IP addresses rather than dNSNames if hostname is a
    valid ipaddress string. IPv4 addresses are supported on all platforms.
    IPv6 addresses are supported on platforms with IPv6 support (AF_INET6
    and inet_pton).

    CertificateError is raised on failure. On success, the function
    returns nothing.
    """
    warnings.warn(
        "ssl.match_hostname() is deprecated",
        category=DeprecationWarning,
        stacklevel=2
    )
    if not cert:
        raise ValueError("empty or no certificate, match_hostname needs a "
                         "SSL socket or SSL context with either "
                         "CERT_OPTIONAL or CERT_REQUIRED")
    try:
        host_ip = _inet_paton(hostname)
    except ValueError:
        # Not an IP address (common case)
        host_ip = None
    dnsnames = []
    san = cert.get('subjectAltName', ())
    for key, value in san:
        if key == 'DNS':
            if host_ip is None and _dnsname_match(value, hostname):
                return
            dnsnames.append(value)
        elif key == 'IP Address':
            if host_ip is not None and _ipaddress_match(value, host_ip):
                return
            dnsnames.append(value)
    if not dnsnames:
        # The subject is only checked when there is no dNSName entry
        # in subjectAltName
        for sub in cert.get('subject', ()):
            for key, value in sub:
                # XXX according to RFC 2818, the most specific Common Name
                # must be used.
                if key == 'commonName':
                    if _dnsname_match(value, hostname):
                        return
                    dnsnames.append(value)
    if len(dnsnames) > 1:
        raise CertificateError("hostname %r "
            "doesn't match either of %s"
            % (hostname, ', '.join(map(repr, dnsnames))))
    elif len(dnsnames) == 1:
        raise CertificateError("hostname %r "
            "doesn't match %r"
            % (hostname, dnsnames[0]))
    else:
        raise CertificateError("no appropriate commonName or "
            "subjectAltName fields were found")


DefaultVerifyPaths = namedtuple("DefaultVerifyPaths",
    "cafile capath openssl_cafile_env openssl_cafile openssl_capath_env "
    "openssl_capath")

def get_default_verify_paths():
    """Return paths to default cafile and capath.
    """
    parts = _ssl.get_default_verify_paths()

    # environment vars shadow paths
    cafile = os.environ.get(parts[0], parts[1])
    capath = os.environ.get(parts[2], parts[3])

    return DefaultVerifyPaths(cafile if os.path.isfile(cafile) else None,
                              capath if os.path.isdir(capath) else None,
                              *parts)


class _ASN1Object(namedtuple("_ASN1Object", "nid shortname longname oid")):
    """ASN.1 object identifier lookup
    """
    __slots__ = ()

    def __new__(cls, oid):
        return super().__new__(cls, *_txt2obj(oid, name=False))

    @classmethod
    def fromnid(cls, nid):
        """Create _ASN1Object from OpenSSL numeric ID
        """
        return super().__new__(cls, *_nid2obj(nid))

    @classmethod
    def fromname(cls, name):
        """Create _ASN1Object from short name, long name or OID
        """
        return super().__new__(cls, *_txt2obj(name, name=True))


class Purpose(_ASN1Object, _Enum):
    """SSLContext purpose flags with X509v3 Extended Key Usage objects
    """
    SERVER_AUTH = '1.3.6.1.5.5.7.3.1'
    CLIENT_AUTH = '1.3.6.1.5.5.7.3.2'


class SSLContext(_SSLContext):
    """An SSLContext holds various SSL-related configuration options and
    data, such as certificates and possibly a private key."""
    _windows_cert_stores = ("CA", "ROOT")

    sslsocket_class = None  # SSLSocket is assigned later.
    sslobject_class = None  # SSLObject is assigned later.

    def __new__(cls, protocol=None, *args, **kwargs):
        if protocol is None:
            warnings.warn(
                "ssl.SSLContext() without protocol argument is deprecated.",
                category=DeprecationWarning,
                stacklevel=2
            )
            protocol = PROTOCOL_TLS
        self = _SSLContext.__new__(cls, protocol)
        return self

    def _encode_hostname(self, hostname):
        if hostname is None:
            return None
        elif isinstance(hostname, str):
            return hostname.encode('idna').decode('ascii')
        else:
            return hostname.decode('ascii')

    def wrap_socket(self, sock, server_side=False,
                    do_handshake_on_connect=True,
                    suppress_ragged_eofs=True,
                    server_hostname=None, session=None):
        # SSLSocket class handles server_hostname encoding before it calls
        # ctx._wrap_socket()
        return self.sslsocket_class._create(
            sock=sock,
            server_side=server_side,
            do_handshake_on_connect=do_handshake_on_connect,
            suppress_ragged_eofs=suppress_ragged_eofs,
            server_hostname=server_hostname,
            context=self,
            session=session
        )

    def wrap_bio(self, incoming, outgoing, server_side=False,
                 server_hostname=None, session=None):
        # Need to encode server_hostname here because _wrap_bio() can only
        # handle ASCII str.
        return self.sslobject_class._create(
            incoming, outgoing, server_side=server_side,
            server_hostname=self._encode_hostname(server_hostname),
            session=session, context=self,
        )

    def set_npn_protocols(self, npn_protocols):
        warnings.warn(
            "ssl NPN is deprecated, use ALPN instead",
            DeprecationWarning,
            stacklevel=2
        )
        protos = bytearray()
        for protocol in npn_protocols:
            b = bytes(protocol, 'ascii')
            if len(b) == 0 or len(b) > 255:
                raise SSLError('NPN protocols must be 1 to 255 in length')
            protos.append(len(b))
            protos.extend(b)

        self._set_npn_protocols(protos)

    def set_servername_callback(self, server_name_callback):
        if server_name_callback is None:
            self.sni_callback = None
        else:
            if not callable(server_name_callback):
                raise TypeError("not a callable object")

            def shim_cb(sslobj, servername, sslctx):
                servername = self._encode_hostname(servername)
                return server_name_callback(sslobj, servername, sslctx)

            self.sni_callback = shim_cb

    def set_alpn_protocols(self, alpn_protocols):
        protos = bytearray()
        for protocol in alpn_protocols:
            b = bytes(protocol, 'ascii')
            if len(b) == 0 or len(b) > 255:
                raise SSLError('ALPN protocols must be 1 to 255 in length')
            protos.append(len(b))
            protos.extend(b)

        self._set_alpn_protocols(protos)

    def _load_windows_store_certs(self, storename, purpose):
        certs = bytearray()
        try:
            for cert, encoding, trust in enum_certificates(storename):
                # CA certs are never PKCS#7 encoded
                if encoding == "x509_asn":
                    if trust is True or purpose.oid in trust:
                        certs.extend(cert)
        except PermissionError:
            warnings.warn("unable to enumerate Windows certificate store")
        if certs:
            self.load_verify_locations(cadata=certs)
        return certs

    def load_default_certs(self, purpose=Purpose.SERVER_AUTH):
        if not isinstance(purpose, _ASN1Object):
            raise TypeError(purpose)
        if sys.platform == "win32":
            for storename in self._windows_cert_stores:
                self._load_windows_store_certs(storename, purpose)
        self.set_default_verify_paths()

    if hasattr(_SSLContext, 'minimum_version'):
        @property
        def minimum_version(self):
            return TLSVersion(super().minimum_version)

        @minimum_version.setter
        def minimum_version(self, value):
            if value == TLSVersion.SSLv3:
                self.options &= ~Options.OP_NO_SSLv3
            super(SSLContext, SSLContext).minimum_version.__set__(self, value)

        @property
        def maximum_version(self):
            return TLSVersion(super().maximum_version)

        @maximum_version.setter
        def maximum_version(self, value):
            super(SSLContext, SSLContext).maximum_version.__set__(self, value)

    @property
    def options(self):
        return Options(super().options)

    @options.setter
    def options(self, value):
        super(SSLContext, SSLContext).options.__set__(self, value)

    if hasattr(_ssl, 'HOSTFLAG_NEVER_CHECK_SUBJECT'):
        @property
        def hostname_checks_common_name(self):
            ncs = self._host_flags & _ssl.HOSTFLAG_NEVER_CHECK_SUBJECT
            return ncs != _ssl.HOSTFLAG_NEVER_CHECK_SUBJECT

        @hostname_checks_common_name.setter
        def hostname_checks_common_name(self, value):
            if value:
                self._host_flags &= ~_ssl.HOSTFLAG_NEVER_CHECK_SUBJECT
            else:
                self._host_flags |= _ssl.HOSTFLAG_NEVER_CHECK_SUBJECT
    else:
        @property
        def hostname_checks_common_name(self):
            return True

    @property
    def _msg_callback(self):
        """TLS message callback

        The message callback provides a debugging hook to analyze TLS
        connections. The callback is called for any TLS protocol message
        (header, handshake, alert, and more), but not for application data.
        Due to technical  limitations, the callback can't be used to filter
        traffic or to abort a connection. Any exception raised in the
        callback is delayed until the handshake, read, or write operation
        has been performed.

        def msg_cb(conn, direction, version, content_type, msg_type, data):
            pass

        conn
            :class:`SSLSocket` or :class:`SSLObject` instance
        direction
            ``read`` or ``write``
        version
            :class:`TLSVersion` enum member or int for unknown version. For a
            frame header, it's the header version.
        content_type
            :class:`_TLSContentType` enum member or int for unsupported
            content type.
        msg_type
            Either a :class:`_TLSContentType` enum number for a header
            message, a :class:`_TLSAlertType` enum member for an alert
            message, a :class:`_TLSMessageType` enum member for other
            messages, or int for unsupported message types.
        data
            Raw, decrypted message content as bytes
        """
        inner = super()._msg_callback
        if inner is not None:
            return inner.user_function
        else:
            return None

    @_msg_callback.setter
    def _msg_callback(self, callback):
        if callback is None:
            super(SSLContext, SSLContext)._msg_callback.__set__(self, None)
            return

        if not hasattr(callback, '__call__'):
            raise TypeError(f"{callback} is not callable.")

        def inner(conn, direction, version, content_type, msg_type, data):
            try:
                version = TLSVersion(version)
            except ValueError:
                pass

            try:
                content_type = _TLSContentType(content_type)
            except ValueError:
                pass

            if content_type == _TLSContentType.HEADER:
                msg_enum = _TLSContentType
            elif content_type == _TLSContentType.ALERT:
                msg_enum = _TLSAlertType
            else:
                msg_enum = _TLSMessageType
            try:
                msg_type = msg_enum(msg_type)
            except ValueError:
                pass

            return callback(conn, direction, version,
                            content_type, msg_type, data)

        inner.user_function = callback

        super(SSLContext, SSLContext)._msg_callback.__set__(self, inner)

    @property
    def protocol(self):
        return _SSLMethod(super().protocol)

    @property
    def verify_flags(self):
        return VerifyFlags(super().verify_flags)

    @verify_flags.setter
    def verify_flags(self, value):
        super(SSLContext, SSLContext).verify_flags.__set__(self, value)

    @property
    def verify_mode(self):
        value = super().verify_mode
        try:
            return VerifyMode(value)
        except ValueError:
            return value

    @verify_mode.setter
    def verify_mode(self, value):
        super(SSLContext, SSLContext).verify_mode.__set__(self, value)


def create_default_context(purpose=Purpose.SERVER_AUTH, *, cafile=None,
                           capath=None, cadata=None):
    """Create a SSLContext object with default settings.

    NOTE: The protocol and settings may change anytime without prior
          deprecation. The values represent a fair balance between maximum
          compatibility and security.
    """
    if not isinstance(purpose, _ASN1Object):
        raise TypeError(purpose)

    # SSLContext sets OP_NO_SSLv2, OP_NO_SSLv3, OP_NO_COMPRESSION,
    # OP_CIPHER_SERVER_PREFERENCE, OP_SINGLE_DH_USE and OP_SINGLE_ECDH_USE
    # by default.
    if purpose == Purpose.SERVER_AUTH:
        # verify certs and host name in client mode
        context = SSLContext(PROTOCOL_TLS_CLIENT)
        context.verify_mode = CERT_REQUIRED
        context.check_hostname = True
    elif purpose == Purpose.CLIENT_AUTH:
        context = SSLContext(PROTOCOL_TLS_SERVER)
    else:
        raise ValueError(purpose)

    if cafile or capath or cadata:
        context.load_verify_locations(cafile, capath, cadata)
    elif context.verify_mode != CERT_NONE:
        # no explicit cafile, capath or cadata but the verify mode is
        # CERT_OPTIONAL or CERT_REQUIRED. Let's try to load default system
        # root CA certificates for the given purpose. This may fail silently.
        context.load_default_certs(purpose)
    # OpenSSL 1.1.1 keylog file
    if hasattr(context, 'keylog_filename'):
        keylogfile = os.environ.get('SSLKEYLOGFILE')
        if keylogfile and not sys.flags.ignore_environment:
            context.keylog_filename = keylogfile
    return context

def _create_unverified_context(protocol=None, *, cert_reqs=CERT_NONE,
                           check_hostname=False, purpose=Purpose.SERVER_AUTH,
                           certfile=None, keyfile=None,
                           cafile=None, capath=None, cadata=None):
    """Create a SSLContext object for Python stdlib modules

    All Python stdlib modules shall use this function to create SSLContext
    objects in order to keep common settings in one place. The configuration
    is less restrict than create_default_context()'s to increase backward
    compatibility.
    """
    if not isinstance(purpose, _ASN1Object):
        raise TypeError(purpose)

    # SSLContext sets OP_NO_SSLv2, OP_NO_SSLv3, OP_NO_COMPRESSION,
    # OP_CIPHER_SERVER_PREFERENCE, OP_SINGLE_DH_USE and OP_SINGLE_ECDH_USE
    # by default.
    if purpose == Purpose.SERVER_AUTH:
        # verify certs and host name in client mode
        if protocol is None:
            protocol = PROTOCOL_TLS_CLIENT
    elif purpose == Purpose.CLIENT_AUTH:
        if protocol is None:
            protocol = PROTOCOL_TLS_SERVER
    else:
        raise ValueError(purpose)

    context = SSLContext(protocol)
    context.check_hostname = check_hostname
    if cert_reqs is not None:
        context.verify_mode = cert_reqs
    if check_hostname:
        context.check_hostname = True

    if keyfile and not certfile:
        raise ValueError("certfile must be specified")
    if certfile or keyfile:
        context.load_cert_chain(certfile, keyfile)

    # load CA root certs
    if cafile or capath or cadata:
        context.load_verify_locations(cafile, capath, cadata)
    elif context.verify_mode != CERT_NONE:
        # no explicit cafile, capath or cadata but the verify mode is
        # CERT_OPTIONAL or CERT_REQUIRED. Let's try to load default system
        # root CA certificates for the given purpose. This may fail silently.
        context.load_default_certs(purpose)
    # OpenSSL 1.1.1 keylog file
    if hasattr(context, 'keylog_filename'):
        keylogfile = os.environ.get('SSLKEYLOGFILE')
        if keylogfile and not sys.flags.ignore_environment:
            context.keylog_filename = keylogfile
    return context

# Used by http.client if no context is explicitly passed.
_create_default_https_context = create_default_context


# Backwards compatibility alias, even though it's not a public name.
_create_stdlib_context = _create_unverified_context


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

    This class does not have a public constructor. Instances are returned by
    ``SSLContext.wrap_bio``. This class is typically used by framework authors
    that want to implement asynchronous IO for SSL through memory buffers.

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

     * Any form of network IO, including methods such as ``recv`` and ``send``.
     * The ``do_handshake_on_connect`` and ``suppress_ragged_eofs`` machinery.
    """
    def __init__(self, *args, **kwargs):
        raise TypeError(
            f"{self.__class__.__name__} does not have a public "
            f"constructor. Instances are returned by SSLContext.wrap_bio()."
        )

    @classmethod
    def _create(cls, incoming, outgoing, server_side=False,
                 server_hostname=None, session=None, context=None):
        self = cls.__new__(cls)
        sslobj = context._wrap_bio(
            incoming, outgoing, server_side=server_side,
            server_hostname=server_hostname,
            owner=self, session=session
        )
        self._sslobj = sslobj
        return self

    @property
    def context(self):
        """The SSLContext that is currently in use."""
        return self._sslobj.context

    @context.setter
    def context(self, ctx):
        self._sslobj.context = ctx

    @property
    def session(self):
        """The SSLSession for client socket."""
        return self._sslobj.session

    @session.setter
    def session(self, session):
        self._sslobj.session = session

    @property
    def session_reused(self):
        """Was the client session reused during handshake"""
        return self._sslobj.session_reused

    @property
    def server_side(self):
        """Whether this is a server-side socket."""
        return self._sslobj.server_side

    @property
    def server_hostname(self):
        """The currently set server hostname (for SNI), or ``None`` if no
        server hostname is set."""
        return self._sslobj.server_hostname

    def read(self, len=1024, buffer=None):
        """Read up to 'len' bytes from the SSL object and return them.

        If 'buffer' is provided, read into this buffer and return the number of
        bytes read.
        """
        if buffer is not None:
            v = self._sslobj.read(len, buffer)
        else:
            v = self._sslobj.read(len)
        return v

    def write(self, data):
        """Write 'data' to the SSL object and return the number of bytes
        written.

        The 'data' argument must support the buffer interface.
        """
        return self._sslobj.write(data)

    def getpeercert(self, binary_form=False):
        """Returns a formatted version of the data in the certificate provided
        by the other end of the SSL channel.

        Return None if no certificate was provided, {} if a certificate was
        provided, but not validated.
        """
        return self._sslobj.getpeercert(binary_form)

    def selected_npn_protocol(self):
        """Return the currently selected NPN protocol as a string, or ``None``
        if a next protocol was not negotiated or if NPN is not supported by one
        of the peers."""
        warnings.warn(
            "ssl NPN is deprecated, use ALPN instead",
            DeprecationWarning,
            stacklevel=2
        )

    def selected_alpn_protocol(self):
        """Return the currently selected ALPN protocol as a string, or ``None``
        if a next protocol was not negotiated or if ALPN is not supported by one
        of the peers."""
        return self._sslobj.selected_alpn_protocol()

    def cipher(self):
        """Return the currently selected cipher as a 3-tuple ``(name,
        ssl_version, secret_bits)``."""
        return self._sslobj.cipher()

    def shared_ciphers(self):
        """Return a list of ciphers shared by the client during the handshake or
        None if this is not a valid server connection.
        """
        return self._sslobj.shared_ciphers()

    def compression(self):
        """Return the current compression algorithm in use, or ``None`` if
        compression was not negotiated or not supported by one of the peers."""
        return self._sslobj.compression()

    def pending(self):
        """Return the number of bytes that can be read immediately."""
        return self._sslobj.pending()

    def do_handshake(self):
        """Start the SSL/TLS handshake."""
        self._sslobj.do_handshake()

    def unwrap(self):
        """Start the SSL shutdown handshake."""
        return self._sslobj.shutdown()

    def get_channel_binding(self, cb_type="tls-unique"):
        """Get channel binding data for current connection.  Raise ValueError
        if the requested `cb_type` is not supported.  Return bytes of the data
        or None if the data is not available (e.g. before the handshake)."""
        return self._sslobj.get_channel_binding(cb_type)

    def version(self):
        """Return a string identifying the protocol version used by the
        current SSL channel. """
        return self._sslobj.version()

    def verify_client_post_handshake(self):
        return self._sslobj.verify_client_post_handshake()


def _sslcopydoc(func):
    """Copy docstring from SSLObject to SSLSocket"""
    func.__doc__ = getattr(SSLObject, func.__name__).__doc__
    return func


class SSLSocket(socket):
    """This class implements a subtype of socket.socket that wraps
    the underlying OS socket in an SSL context when necessary, and
    provides read and write methods over that channel. """

    def __init__(self, *args, **kwargs):
        raise TypeError(
            f"{self.__class__.__name__} does not have a public "
            f"constructor. Instances are returned by "
            f"SSLContext.wrap_socket()."
        )

    @classmethod
    def _create(cls, sock, server_side=False, do_handshake_on_connect=True,
                suppress_ragged_eofs=True, server_hostname=None,
                context=None, session=None):
        if sock.getsockopt(SOL_SOCKET, SO_TYPE) != SOCK_STREAM:
            raise NotImplementedError("only stream sockets are supported")
        if server_side:
            if server_hostname:
                raise ValueError("server_hostname can only be specified "
                                 "in client mode")
            if session is not None:
                raise ValueError("session can only be specified in "
                                 "client mode")
        if context.check_hostname and not server_hostname:
            raise ValueError("check_hostname requires server_hostname")

        sock_timeout = sock.gettimeout()
        kwargs = dict(
            family=sock.family, type=sock.type, proto=sock.proto,
            fileno=sock.fileno()
        )
        self = cls.__new__(cls, **kwargs)
        super(SSLSocket, self).__init__(**kwargs)
        sock.detach()
        # Now SSLSocket is responsible for closing the file descriptor.
        try:
            self._context = context
            self._session = session
            self._closed = False
            self._sslobj = None
            self.server_side = server_side
            self.server_hostname = context._encode_hostname(server_hostname)
            self.do_handshake_on_connect = do_handshake_on_connect
            self.suppress_ragged_eofs = suppress_ragged_eofs

            # See if we are connected
            try:
                self.getpeername()
            except OSError as e:
                if e.errno != errno.ENOTCONN:
                    raise
                connected = False
                blocking = self.getblocking()
                self.setblocking(False)
                try:
                    # We are not connected so this is not supposed to block, but
                    # testing revealed otherwise on macOS and Windows so we do
                    # the non-blocking dance regardless. Our raise when any data
                    # is found means consuming the data is harmless.
                    notconn_pre_handshake_data = self.recv(1)
                except OSError as e:
                    # EINVAL occurs for recv(1) on non-connected on unix sockets.
                    if e.errno not in (errno.ENOTCONN, errno.EINVAL):
                        raise
                    notconn_pre_handshake_data = b''
                self.setblocking(blocking)
                if notconn_pre_handshake_data:
                    # This prevents pending data sent to the socket before it was
                    # closed from escaping to the caller who could otherwise
                    # presume it came through a successful TLS connection.
                    reason = "Closed before TLS handshake with data in recv buffer."
                    notconn_pre_handshake_data_error = SSLError(e.errno, reason)
                    # Add the SSLError attributes that _ssl.c always adds.
                    notconn_pre_handshake_data_error.reason = reason
                    notconn_pre_handshake_data_error.library = None
                    try:
                        raise notconn_pre_handshake_data_error
                    finally:
                        # Explicitly break the reference cycle.
                        notconn_pre_handshake_data_error = None
            else:
                connected = True

            self.settimeout(sock_timeout)  # Must come after setblocking() calls.
            self._connected = connected
            if connected:
                # create the SSL object
                self._sslobj = self._context._wrap_socket(
                    self, server_side, self.server_hostname,
                    owner=self, session=self._session,
                )
                if do_handshake_on_connect:
                    timeout = self.gettimeout()
                    if timeout == 0.0:
                        # non-blocking
                        raise ValueError("do_handshake_on_connect should not be specified for non-blocking sockets")
                    self.do_handshake()
        except:
            try:
                self.close()
            except OSError:
                pass
            raise
        return self

    @property
    @_sslcopydoc
    def context(self):
        return self._context

    @context.setter
    def context(self, ctx):
        self._context = ctx
        self._sslobj.context = ctx

    @property
    @_sslcopydoc
    def session(self):
        if self._sslobj is not None:
            return self._sslobj.session

    @session.setter
    def session(self, session):
        self._session = session
        if self._sslobj is not None:
            self._sslobj.session = session

    @property
    @_sslcopydoc
    def session_reused(self):
        if self._sslobj is not None:
            return self._sslobj.session_reused

    def dup(self):
        raise NotImplementedError("Can't dup() %s instances" %
                                  self.__class__.__name__)

    def _checkClosed(self, msg=None):
        # raise an exception here if you wish to check for spurious closes
        pass

    def _check_connected(self):
        if not self._connected:
            # getpeername() will raise ENOTCONN if the socket is really
            # not connected; note that we can be connected even without
            # _connected being set, e.g. if connect() first returned
            # EAGAIN.
            self.getpeername()

    def read(self, len=1024, buffer=None):
        """Read up to LEN bytes and return them.
        Return zero-length string on EOF."""

        self._checkClosed()
        if self._sslobj is None:
            raise ValueError("Read on closed or unwrapped SSL socket.")
        try:
            if buffer is not None:
                return self._sslobj.read(len, buffer)
            else:
                return self._sslobj.read(len)
        except SSLError as x:
            if x.args[0] == SSL_ERROR_EOF and self.suppress_ragged_eofs:
                if buffer is not None:
                    return 0
                else:
                    return b''
            else:
                raise

    def write(self, data):
        """Write DATA to the underlying SSL channel.  Returns
        number of bytes of DATA actually transmitted."""

        self._checkClosed()
        if self._sslobj is None:
            raise ValueError("Write on closed or unwrapped SSL socket.")
        return self._sslobj.write(data)

    @_sslcopydoc
    def getpeercert(self, binary_form=False):
        self._checkClosed()
        self._check_connected()
        return self._sslobj.getpeercert(binary_form)

    @_sslcopydoc
    def selected_npn_protocol(self):
        self._checkClosed()
        warnings.warn(
            "ssl NPN is deprecated, use ALPN instead",
            DeprecationWarning,
            stacklevel=2
        )
        return None

    @_sslcopydoc
    def selected_alpn_protocol(self):
        self._checkClosed()
        if self._sslobj is None or not _ssl.HAS_ALPN:
            return None
        else:
            return self._sslobj.selected_alpn_protocol()

    @_sslcopydoc
    def cipher(self):
        self._checkClosed()
        if self._sslobj is None:
            return None
        else:
            return self._sslobj.cipher()

    @_sslcopydoc
    def shared_ciphers(self):
        self._checkClosed()
        if self._sslobj is None:
            return None
        else:
            return self._sslobj.shared_ciphers()

    @_sslcopydoc
    def compression(self):
        self._checkClosed()
        if self._sslobj is None:
            return None
        else:
            return self._sslobj.compression()

    def send(self, data, flags=0):
        self._checkClosed()
        if self._sslobj is not None:
            if flags != 0:
                raise ValueError(
                    "non-zero flags not allowed in calls to send() on %s" %
                    self.__class__)
            return self._sslobj.write(data)
        else:
            return super().send(data, flags)

    def sendto(self, data, flags_or_addr, addr=None):
        self._checkClosed()
        if self._sslobj is not None:
            raise ValueError("sendto not allowed on instances of %s" %
                             self.__class__)
        elif addr is None:
            return super().sendto(data, flags_or_addr)
        else:
            return super().sendto(data, flags_or_addr, addr)

    def sendmsg(self, *args, **kwargs):
        # Ensure programs don't send data unencrypted if they try to
        # use this method.
        raise NotImplementedError("sendmsg not allowed on instances of %s" %
                                  self.__class__)

    def sendall(self, data, flags=0):
        self._checkClosed()
        if self._sslobj is not None:
            if flags != 0:
                raise ValueError(
                    "non-zero flags not allowed in calls to sendall() on %s" %
                    self.__class__)
            count = 0
            with memoryview(data) as view, view.cast("B") as byte_view:
                amount = len(byte_view)
                while count < amount:
                    v = self.send(byte_view[count:])
                    count += v
        else:
            return super().sendall(data, flags)

    def sendfile(self, file, offset=0, count=None):
        """Send a file, possibly by using os.sendfile() if this is a
        clear-text socket.  Return the total number of bytes sent.
        """
        if self._sslobj is not None:
            return self._sendfile_use_send(file, offset, count)
        else:
            # os.sendfile() works with plain sockets only
            return super().sendfile(file, offset, count)

    def recv(self, buflen=1024, flags=0):
        self._checkClosed()
        if self._sslobj is not None:
            if flags != 0:
                raise ValueError(
                    "non-zero flags not allowed in calls to recv() on %s" %
                    self.__class__)
            return self.read(buflen)
        else:
            return super().recv(buflen, flags)

    def recv_into(self, buffer, nbytes=None, flags=0):
        self._checkClosed()
        if nbytes is None:
            if buffer is not None:
                with memoryview(buffer) as view:
                    nbytes = view.nbytes
                if not nbytes:
                    nbytes = 1024
            else:
                nbytes = 1024
        if self._sslobj is not None:
            if flags != 0:
                raise ValueError(
                  "non-zero flags not allowed in calls to recv_into() on %s" %
                  self.__class__)
            return self.read(nbytes, buffer)
        else:
            return super().recv_into(buffer, nbytes, flags)

    def recvfrom(self, buflen=1024, flags=0):
        self._checkClosed()
        if self._sslobj is not None:
            raise ValueError("recvfrom not allowed on instances of %s" %
                             self.__class__)
        else:
            return super().recvfrom(buflen, flags)

    def recvfrom_into(self, buffer, nbytes=None, flags=0):
        self._checkClosed()
        if self._sslobj is not None:
            raise ValueError("recvfrom_into not allowed on instances of %s" %
                             self.__class__)
        else:
            return super().recvfrom_into(buffer, nbytes, flags)

    def recvmsg(self, *args, **kwargs):
        raise NotImplementedError("recvmsg not allowed on instances of %s" %
                                  self.__class__)

    def recvmsg_into(self, *args, **kwargs):
        raise NotImplementedError("recvmsg_into not allowed on instances of "
                                  "%s" % self.__class__)

    @_sslcopydoc
    def pending(self):
        self._checkClosed()
        if self._sslobj is not None:
            return self._sslobj.pending()
        else:
            return 0

    def shutdown(self, how):
        self._checkClosed()
        self._sslobj = None
        super().shutdown(how)

    @_sslcopydoc
    def unwrap(self):
        if self._sslobj:
            s = self._sslobj.shutdown()
            self._sslobj = None
            return s
        else:
            raise ValueError("No SSL wrapper around " + str(self))

    @_sslcopydoc
    def verify_client_post_handshake(self):
        if self._sslobj:
            return self._sslobj.verify_client_post_handshake()
        else:
            raise ValueError("No SSL wrapper around " + str(self))

    def _real_close(self):
        self._sslobj = None
        super()._real_close()

    @_sslcopydoc
    def do_handshake(self, block=False):
        self._check_connected()
        timeout = self.gettimeout()
        try:
            if timeout == 0.0 and block:
                self.settimeout(None)
            self._sslobj.do_handshake()
        finally:
            self.settimeout(timeout)

    def _real_connect(self, addr, connect_ex):
        if self.server_side:
            raise ValueError("can't connect in server-side mode")
        # Here we assume that the socket is client-side, and not
        # connected at the time of the call.  We connect it, then wrap it.
        if self._connected or self._sslobj is not None:
            raise ValueError("attempt to connect already-connected SSLSocket!")
        self._sslobj = self.context._wrap_socket(
            self, False, self.server_hostname,
            owner=self, session=self._session
        )
        try:
            if connect_ex:
                rc = super().connect_ex(addr)
            else:
                rc = None
                super().connect(addr)
            if not rc:
                self._connected = True
                if self.do_handshake_on_connect:
                    self.do_handshake()
            return rc
        except (OSError, ValueError):
            self._sslobj = None
            raise

    def connect(self, addr):
        """Connects to remote ADDR, and then wraps the connection in
        an SSL channel."""
        self._real_connect(addr, False)

    def connect_ex(self, addr):
        """Connects to remote ADDR, and then wraps the connection in
        an SSL channel."""
        return self._real_connect(addr, True)

    def accept(self):
        """Accepts a new connection from a remote client, and returns
        a tuple containing that new connection wrapped with a server-side
        SSL channel, and the address of the remote client."""

        newsock, addr = super().accept()
        newsock = self.context.wrap_socket(newsock,
                    do_handshake_on_connect=self.do_handshake_on_connect,
                    suppress_ragged_eofs=self.suppress_ragged_eofs,
                    server_side=True)
        return newsock, addr

    @_sslcopydoc
    def get_channel_binding(self, cb_type="tls-unique"):
        if self._sslobj is not None:
            return self._sslobj.get_channel_binding(cb_type)
        else:
            if cb_type not in CHANNEL_BINDING_TYPES:
                raise ValueError(
                    "{0} channel binding type not implemented".format(cb_type)
                )
            return None

    @_sslcopydoc
    def version(self):
        if self._sslobj is not None:
            return self._sslobj.version()
        else:
            return None


# Python does not support forward declaration of types.
SSLContext.sslsocket_class = SSLSocket
SSLContext.sslobject_class = SSLObject


def wrap_socket(sock, keyfile=None, certfile=None,
                server_side=False, cert_reqs=CERT_NONE,
                ssl_version=PROTOCOL_TLS, ca_certs=None,
                do_handshake_on_connect=True,
                suppress_ragged_eofs=True,
                ciphers=None):
    warnings.warn(
        "ssl.wrap_socket() is deprecated, use SSLContext.wrap_socket()",
        category=DeprecationWarning,
        stacklevel=2
    )
    if server_side and not certfile:
        raise ValueError("certfile must be specified for server-side "
                         "operations")
    if keyfile and not certfile:
        raise ValueError("certfile must be specified")
    context = SSLContext(ssl_version)
    context.verify_mode = cert_reqs
    if ca_certs:
        context.load_verify_locations(ca_certs)
    if certfile:
        context.load_cert_chain(certfile, keyfile)
    if ciphers:
        context.set_ciphers(ciphers)
    return context.wrap_socket(
        sock=sock, server_side=server_side,
        do_handshake_on_connect=do_handshake_on_connect,
        suppress_ragged_eofs=suppress_ragged_eofs
    )

# some utility functions

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

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

    Month is one of: Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
    UTC should be specified as GMT (see ASN1_TIME_print())
    """
    from time import strptime
    from calendar import timegm

    months = (
        "Jan","Feb","Mar","Apr","May","Jun",
        "Jul","Aug","Sep","Oct","Nov","Dec"
    )
    time_format = ' %d %H:%M:%S %Y GMT' # NOTE: no month, fixed GMT
    try:
        month_number = months.index(cert_time[:3].title()) + 1
    except ValueError:
        raise ValueError('time data %r does not match '
                         'format "%%b%s"' % (cert_time, time_format))
    else:
        # found valid month
        tt = strptime(cert_time[3:], time_format)
        # return an integer, the previous mktime()-based implementation
        # returned a float (fractional seconds are always zero here).
        return timegm((tt[0], month_number) + tt[2:6])

PEM_HEADER = "-----BEGIN CERTIFICATE-----"
PEM_FOOTER = "-----END CERTIFICATE-----"

def DER_cert_to_PEM_cert(der_cert_bytes):
    """Takes a certificate in binary DER format and returns the
    PEM version of it as a string."""

    f = str(base64.standard_b64encode(der_cert_bytes), 'ASCII', 'strict')
    ss = [PEM_HEADER]
    ss += [f[i:i+64] for i in range(0, len(f), 64)]
    ss.append(PEM_FOOTER + '\n')
    return '\n'.join(ss)

def PEM_cert_to_DER_cert(pem_cert_string):
    """Takes a certificate in ASCII PEM format and returns the
    DER-encoded version of it as a byte sequence"""

    if not pem_cert_string.startswith(PEM_HEADER):
        raise ValueError("Invalid PEM encoding; must start with %s"
                         % PEM_HEADER)
    if not pem_cert_string.strip().endswith(PEM_FOOTER):
        raise ValueError("Invalid PEM encoding; must end with %s"
                         % PEM_FOOTER)
    d = pem_cert_string.strip()[len(PEM_HEADER):-len(PEM_FOOTER)]
    return base64.decodebytes(d.encode('ASCII', 'strict'))

def get_server_certificate(addr, ssl_version=PROTOCOL_TLS_CLIENT,
                           ca_certs=None, timeout=_GLOBAL_DEFAULT_TIMEOUT):
    """Retrieve the certificate from the server at the specified address,
    and return it as a PEM-encoded string.
    If 'ca_certs' is specified, validate the server cert against it.
    If 'ssl_version' is specified, use it in the connection attempt.
    If 'timeout' is specified, use it in the connection attempt.
    """

    host, port = addr
    if ca_certs is not None:
        cert_reqs = CERT_REQUIRED
    else:
        cert_reqs = CERT_NONE
    context = _create_stdlib_context(ssl_version,
                                     cert_reqs=cert_reqs,
                                     cafile=ca_certs)
    with create_connection(addr, timeout=timeout) as sock:
        with context.wrap_socket(sock, server_hostname=host) as sslsock:
            dercert = sslsock.getpeercert(True)
    return DER_cert_to_PEM_cert(dercert)

def get_protocol_name(protocol_code):
    return _PROTOCOL_NAMES.get(protocol_code, '<unknown>')