ssl_.py

from __future__ import absolute_import
import errno
import logging
import warnings
import hmac
import sys

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

from .url import IPV4_RE, BRACELESS_IPV6_ADDRZ_RE
from ..exceptions import SSLError, InsecurePlatformWarning, SNIMissingWarning
from ..packages.ssl_match_hostname import (
    match_hostname as _match_hostname,
    CertificateError,
)
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}


log = logging.getLogger("hip.util.ssl_")


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 left, right in zip(bytearray(a), bytearray(b)):
        result |= left ^ right
    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_REQUIRED
    from ssl import HAS_SNI  # Has SNI?
    from ssl import SSLError as BaseSSLError
    from ssl import SSLWantReadError, SSLWantWriteError
except ImportError:

    class BaseSSLError(Exception):
        pass

    class SSLWantReadError(Exception):
        pass

    class SSLWantWriteError(Exception):
        pass


try:  # Platform-specific: Python 3.6
    from ssl import PROTOCOL_TLS

    PROTOCOL_SSLv23 = PROTOCOL_TLS
except ImportError:
    try:
        from ssl import PROTOCOL_SSLv23 as PROTOCOL_TLS

        PROTOCOL_SSLv23 = PROTOCOL_TLS
    except ImportError:
        PROTOCOL_SSLv23 = PROTOCOL_TLS = 2


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


# 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 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, DSS, and other
#   insecure ciphers for security reasons.
# - NOTE: TLS 1.3 cipher suites are managed through a different interface
#   not exposed by CPython (yet!) and are enabled by default if they're available.
DEFAULT_CIPHERS = ":".join(
    [
        "ECDHE+AESGCM",
        "ECDHE+CHACHA20",
        "DHE+AESGCM",
        "DHE+CHACHA20",
        "ECDH+AESGCM",
        "DH+AESGCM",
        "ECDH+AES",
        "DH+AES",
        "RSA+AESGCM",
        "RSA+AES",
        "!aNULL",
        "!eNULL",
        "!MD5",
        "!DSS",
    ]
)

try:
    from ssl import SSLContext  # Modern SSL?
except ImportError:
    # TODO: Can we remove this by choosing to support only platforms with
    # actual SSLContext objects?
    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 "
                "Hip 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://hip.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_REQUIRED

    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_TLS

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

    return candidate


def create_ssl_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 hip.util import ssl_
        context = ssl_.create_ssl_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 PROTOCOL_TLS)

    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.  We only enable on Python 3.7.4+ or if certificate
    # verification is enabled to work around Python issue #37428
    # See: https://bugs.python.org/issue37428
    if (cert_reqs == ssl.CERT_REQUIRED or sys.version_info >= (3, 7, 4)) and 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 merge_context_settings(
    context,
    keyfile=None,
    certfile=None,
    cert_reqs=None,
    key_password=None,
    ca_certs=None,
    ca_cert_dir=None,
):
    """
    Merges provided settings into an SSL Context.
    """
    if cert_reqs is not None:
        context.verify_mode = resolve_cert_reqs(cert_reqs)

    if ca_certs or ca_cert_dir:
        try:
            context.load_verify_locations(ca_certs, ca_cert_dir)
        except IOError as e:  # Platform-specific: Python 2.6, 2.7, 3.2
            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

    # Attempt to detect if we get the goofy behavior of the
    # keyfile being encrypted and OpenSSL asking for the
    # passphrase via the terminal and instead error out.
    if keyfile and key_password is None and _is_key_file_encrypted(keyfile):
        raise SSLError("Client private key is encrypted, password is required")

    if certfile:
        if key_password is None:
            context.load_cert_chain(certfile, keyfile)
        else:
            context.load_cert_chain(certfile, keyfile, key_password)

    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,
    key_password=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_ssl_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().
    :param key_password:
        Optional password if the keyfile is encrypted.
    """
    context = ssl_context
    if context is None:
        # Note: This branch of code and all the variables in it are no longer
        # used by Hip itself. We should consider deprecating and removing
        # this code.
        context = create_ssl_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

    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()

    # Attempt to detect if we get the goofy behavior of the
    # keyfile being encrypted and OpenSSL asking for the
    # passphrase via the terminal and instead error out.
    if keyfile and key_password is None and _is_key_file_encrypted(keyfile):
        raise SSLError("Client private key is encrypted, password is required")

    if certfile:
        if key_password is None:
            context.load_cert_chain(certfile, keyfile)
        else:
            context.load_cert_chain(certfile, keyfile, key_password)

    # 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://hip.readthedocs.io/en/latest/advanced-usage.html"
            "#ssl-warnings",
            SNIMissingWarning,
        )

    return context.wrap_socket(sock)


def match_hostname(cert, asserted_hostname):
    try:
        _match_hostname(cert, asserted_hostname)
    except CertificateError as e:
        log.warning(
            "Certificate did not match expected hostname: %s. Certificate: %s",
            asserted_hostname,
            cert,
        )
        # Add cert to exception and reraise so client code can inspect
        # the cert when catching the exception, if they want to
        e._peer_cert = cert
        raise


def is_ipaddress(hostname):
    """Detects whether the hostname given is an IPv4 or IPv6 address.
    Also detects IPv6 addresses with Zone IDs.

    :param str hostname: Hostname to examine.
    :return: True if the hostname is an IP address, False otherwise.
    """
    if not six.PY2 and isinstance(hostname, bytes):
        # IDN A-label bytes are ASCII compatible.
        hostname = hostname.decode("ascii")
    return bool(IPV4_RE.match(hostname) or BRACELESS_IPV6_ADDRZ_RE.match(hostname))


def _is_key_file_encrypted(key_file):
    """Detects if a key file is encrypted or not."""
    with open(key_file, "r") as f:
        for line in f:
            # Look for Proc-Type: 4,ENCRYPTED
            if "ENCRYPTED" in line:
                return True

    return False