#----------------------------------------------------------------------------- # Copyright (c) 2012 - 2019, Anaconda, Inc., and Bokeh Contributors. # All rights reserved. # # The full license is in the file LICENSE.txt, distributed with this software. #----------------------------------------------------------------------------- ''' Utilities for generating and manipulating session IDs. A session ID would typically be associated with each browser tab viewing an application or plot. Each session has its own state separate from any other sessions hosted by the server. ''' #----------------------------------------------------------------------------- # Boilerplate #----------------------------------------------------------------------------- from __future__ import absolute_import, division, print_function, unicode_literals import logging log = logging.getLogger(__name__) #----------------------------------------------------------------------------- # Imports #----------------------------------------------------------------------------- # Standard library imports import base64 import codecs import hashlib import hmac import time # External imports from six import binary_type # Bokeh imports from bokeh.settings import settings from bokeh.util.string import encode_utf8 #----------------------------------------------------------------------------- # Globals and constants #----------------------------------------------------------------------------- __all__ = ( 'check_session_id_signature', 'generate_secret_key', 'generate_session_id', ) #----------------------------------------------------------------------------- # General API #----------------------------------------------------------------------------- [docs]def generate_secret_key(): """ Generate a new securely-generated secret key appropriate for SHA-256 HMAC signatures. This key could be used to sign Bokeh server session IDs for example. """ return _get_random_string() [docs]def generate_session_id(secret_key=settings.secret_key_bytes(), signed=settings.sign_sessions()): """Generate a random session ID. Typically, each browser tab connected to a Bokeh application has its own session ID. In production deployments of a Bokeh app, session IDs should be random and unguessable - otherwise users of the app could interfere with one another. If session IDs are signed with a secret key, the server can verify that the generator of the session ID was "authorized" (the generator had to know the secret key). This can be used to have a separate process, such as another web application, which generates new sessions on a Bokeh server. This other process may require users to log in before redirecting them to the Bokeh server with a valid session ID, for example. Args: secret_key (str, optional) : Secret key (default: value of 'BOKEH_SECRET_KEY' env var) signed (bool, optional) : Whether to sign the session ID (default: value of 'BOKEH_SIGN_SESSIONS' env var) """ secret_key = _ensure_bytes(secret_key) if signed: # note: '-' can also be in the base64 encoded signature base_id = _get_random_string(secret_key=secret_key) return base_id + '-' + _signature(base_id, secret_key) else: return _get_random_string(secret_key=secret_key) [docs]def check_session_id_signature(session_id, secret_key=settings.secret_key_bytes(), signed=settings.sign_sessions()): """Check the signature of a session ID, returning True if it's valid. The server uses this function to check whether a session ID was generated with the correct secret key. If signed sessions are disabled, this function always returns True. Args: session_id (str) : The session ID to check secret_key (str, optional) : Secret key (default: value of 'BOKEH_SECRET_KEY' env var) signed (bool, optional) : Whether to check anything (default: value of 'BOKEH_SIGN_SESSIONS' env var) """ secret_key = _ensure_bytes(secret_key) if signed: pieces = session_id.split('-', 1) if len(pieces) != 2: return False base_id = pieces[0] provided_signature = pieces[1] expected_signature = _signature(base_id, secret_key) # hmac.compare_digest() uses a string compare algorithm that doesn't # short-circuit so we don't allow timing analysis # encode_utf8 is used to ensure that strings have same encoding return hmac.compare_digest(encode_utf8(expected_signature), encode_utf8(provided_signature)) else: return True #----------------------------------------------------------------------------- # Dev API #----------------------------------------------------------------------------- #----------------------------------------------------------------------------- # Private API #----------------------------------------------------------------------------- def _get_sysrandom(): # Use the system PRNG for session id generation (if possible) # NOTE: secure random string generation implementation is adapted # from the Django project. Reference: # https://github.com/django/django/blob/0ed7d155635da9f79d4dd67e4889087d3673c6da/django/utils/crypto.py import random try: random = random.SystemRandom() using_sysrandom = True except NotImplementedError: import warnings warnings.warn('A secure pseudo-random number generator is not available ' 'on your system. Falling back to Mersenne Twister.') if settings.secret_key() is None: warnings.warn('A secure pseudo-random number generator is not available ' 'and no BOKEH_SECRET_KEY has been set. ' 'Setting a secret key will mitigate the lack of a secure ' 'generator.') using_sysrandom = False return random, using_sysrandom def _ensure_bytes(secret_key): if secret_key is None: return None elif isinstance(secret_key, binary_type): return secret_key else: return codecs.encode(secret_key, 'utf-8') # this is broken out for unit testability def _reseed_if_needed(using_sysrandom, secret_key): secret_key = _ensure_bytes(secret_key) if not using_sysrandom: # This is ugly, and a hack, but it makes things better than # the alternative of predictability. This re-seeds the PRNG # using a value that is hard for an attacker to predict, every # time a random string is required. This may change the # properties of the chosen random sequence slightly, but this # is better than absolute predictability. random.seed( hashlib.sha256( ("%s%s%s" % ( random.getstate(), time.time(), secret_key)).encode('utf-8') ).digest()) def _base64_encode(decoded): # base64 encode both takes and returns bytes, we want to work with strings. # If 'decoded' isn't bytes already, assume it's utf-8 decoded_as_bytes = _ensure_bytes(decoded) encoded = codecs.decode(base64.urlsafe_b64encode(decoded_as_bytes), 'ascii') # remove padding '=' chars that cause trouble return str(encoded.rstrip('=')) def _signature(base_id, secret_key): secret_key = _ensure_bytes(secret_key) base_id = codecs.encode(base_id, "utf-8") signer = hmac.new(secret_key, base_id, hashlib.sha256) return _base64_encode(signer.digest()) def _get_random_string(length=44, allowed_chars='abcdefghijklmnopqrstuvwxyz' 'ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789', secret_key=settings.secret_key_bytes()): """ Return a securely generated random string. With the a-z, A-Z, 0-9 character set: Length 12 is a 71-bit value. log_2((26+26+10)^12) =~ 71 Length 44 is a 261-bit value. log_2((26+26+10)^44) = 261 """ secret_key = _ensure_bytes(secret_key) _reseed_if_needed(using_sysrandom, secret_key) return ''.join(random.choice(allowed_chars) for i in range(length)) #----------------------------------------------------------------------------- # Code #----------------------------------------------------------------------------- random, using_sysrandom = _get_sysrandom()