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)
context = SSLContext(PROTOCOL_SSLv23)
# SSLv2 considered harmful.
context.options |= OP_NO_SSLv2
# SSLv3 has problematic security and is only required for really old
# clients such as IE6 on Windows XP
context.options |= OP_NO_SSLv3
# disable compression to prevent CRIME attacks (OpenSSL 1.0+)
context.options |= getattr(_ssl, "OP_NO_COMPRESSION", 0)
if purpose == Purpose.SERVER_AUTH:
# verify certs and host name in client mode
context.verify_mode = CERT_REQUIRED
context.check_hostname = True
elif purpose == Purpose.CLIENT_AUTH:
# Prefer the server's ciphers by default so that we get stronger
# encryption
context.options |= getattr(_ssl, "OP_CIPHER_SERVER_PREFERENCE", 0)
# Use single use keys in order to improve forward secrecy
context.options |= getattr(_ssl, "OP_SINGLE_DH_USE", 0)
context.options |= getattr(_ssl, "OP_SINGLE_ECDH_USE", 0)
# disallow ciphers with known vulnerabilities
context.set_ciphers(_RESTRICTED_SERVER_CIPHERS)
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)
return context
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