"""Certificate verification. Stolen from AV-98, whose license is reproduced below (and should be interpreted as pertaining to this code only):

Copyright (c) 2020, Solderpunk <solderpunk@sdf.org> and contributors.
All rights reserved.

Redistribution and use in source and binary forms, with or without modification,
are permitted provided that the following conditions are met:

1. Redistributions of source code must retain the above copyright notice,
this list of conditions and the following disclaimer.

2. Redistributions in binary form must reproduce the above copyright notice,
this list of conditions and the following disclaimer in the documentation
and/or other materials provided with the distribution.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE
USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
"""
import ssl
from ssl import CertificateError
try:
	from cryptography import x509
	from cryptography.hazmat.backends import default_backend
	_HAS_CRYPTOGRAPHY = True
	_BACKEND = default_backend()
	_WARN_NO_CRYPTOGRAPHY = False
except ModuleNotFoundError:
	_HAS_CRYPTOGRAPHY = False
	_WARN_NO_CRYPTOGRAPHY = True
from sys import stderr
import datetime

def validate_cert(host, cert):
	"""Validate a TLS certificate in TOFU mode.
	If the cryptography module is installed:
	 - Check the certificate Common Name or SAN matches `host`
	 - Check the certificate's not valid before date is in the past
	 - Check the certificate's not valid after date is in the future
	"""
	now = datetime.datetime.utcnow()
	if _HAS_CRYPTOGRAPHY:
		# Using the cryptography module we can get detailed access
		# to the properties of even self-signed certs, unlike in
		# the standard ssl library...
		c = x509.load_der_x509_certificate(cert, _BACKEND)
		# Check certificate validity dates
		if c.not_valid_before >= now:
			raise CertificateError("Certificate not valid until: {}!".format(c.not_valid_before))
		elif c.not_valid_after <= now:
			raise CertificateError("Certificate expired as of: {}!".format(c.not_valid_after))
		# Check certificate hostnames
		names = []
		common_name = c.subject.get_attributes_for_oid(x509.oid.NameOID.COMMON_NAME)
		if common_name:
			names.append(common_name[0].value)
		try:
			names.extend([alt.value for alt in c.extensions.get_extension_for_oid(x509.oid.ExtensionOID.SUBJECT_ALTERNATIVE_NAME).value if type(alt.value)==str])
		except x509.ExtensionNotFound:
			pass
		names = set(names)
		for name in names:
			try:
				ssl._dnsname_match(name, host)
				break
			except CertificateError:
				continue
		else:
			# If we didn't break out, none of the names were valid
			raise CertificateError("Hostname does not match certificate common name or any alternative names.")
	else:
		if _WARN_NO_CRYPTOGRAPHY:
			print("WARNING: cryptography library not installed!",file=stderr)
			print("All certificates will be treated as valid!",file=stderr)
			_WARN_NO_CRYPTOGRAPHY = False # only warn once per run