mdw@git.distorted.org.uk Git - chopwood/blob - httpauth.py

   1 ### -*-python-*-
   2 ###
   3 ### HTTP authentication
   4 ###
   5 ### (c) 2013 Mark Wooding
   6 ###
   7
   8 ###----- Licensing notice ---------------------------------------------------
   9 ###
  10 ### This file is part of Chopwood: a password-changing service.
  11 ###
  12 ### Chopwood is free software; you can redistribute it and/or modify
  13 ### it under the terms of the GNU Affero General Public License as
  14 ### published by the Free Software Foundation; either version 3 of the
  15 ### License, or (at your option) any later version.
  16 ###
  17 ### Chopwood is distributed in the hope that it will be useful,
  18 ### but WITHOUT ANY WARRANTY; without even the implied warranty of
  19 ### MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  20 ### GNU Affero General Public License for more details.
  21 ###
  22 ### You should have received a copy of the GNU Affero General Public
  23 ### License along with Chopwood; if not, see
  24 ### <http://www.gnu.org/licenses/>.
  25
  26 from __future__ import with_statement
  27
  28 import base64 as BN
  29 import hashlib as H
  30 import hmac as HM
  31 import os as OS
  32
  33 import cgi as CGI
  34 import config as CONF; CFG = CONF.CFG
  35 import dbmaint as D
  36 import output as O; PRINT = O.PRINT
  37 import service as S
  38 import subcommand as SC
  39 import util as U
  40
  41 ###--------------------------------------------------------------------------
  42 ### About the authentication scheme.
  43 ###
  44 ### We mustn't allow a CGI user to make changes (or even learn about a user's
  45 ### accounts) without authenticating first.  Curently, that means a username
  46 ### and password, though I really dislike this; maybe I'll add a feature for
  47 ### handling TLS client certificates some time.
  48 ###
  49 ### We're particularly worried about cross-site request forgery: a forged
  50 ### request to change a password to some known value lets a bad guy straight
  51 ### into a restricted service -- and a change to the `master' account lets
  52 ### him into all of them.
  53 ###
  54 ### Once we've satisfied ourselves of the user's credentials, we issue a
  55 ### short-lived session token, stored in a cookie namde `chpwd-token'.  This
  56 ### token has the form `DATE.NONCE.TAG.USER': here, DATE is the POSIX time of
  57 ### issue, as a decimal number; NONCE is a randomly chosen string, encoded in
  58 ### base64, USER is the user's login name, and TAG is a cryptographic MAC tag
  59 ### on the string `DATE.NONCE.USER'.  (The USER name is on the end so that it
  60 ### can contain `.' characters without introducing parsing difficulties.)
  61 ###
  62 ### Secrets for these MAC tags are stored in the database: secrets expire
  63 ### after 30 minutes (invalidating all tokens issued with them); we only
  64 ### issue a token with a secret that's at most five minutes old.  A session's
  65 ### lifetime, then, is somewhere between 25 and 30 minutes.  We choose the
  66 ### lower bound as the cookie lifetime, just so that error messages end up
  67 ### consistent.
  68 ###
  69 ### A cookie with a valid token is sufficient to grant read-only access to a
  70 ### user's account details.  However, this authority is ambient: during the
  71 ### validity period of the token, a cross-site request forgery can easily
  72 ### succeed, since there's nothing about the rest of a request which is hard
  73 ### to forge, and the cookie will be supplied automatically by the user
  74 ### agent.  Showing the user some information we were quite happy to release
  75 ### anyway isn't an interesting attack, but we must certainly require
  76 ### something stronger for state-change requests.  Here, we also check that a
  77 ### special request parameter `%nonce' matches the token's NONCE field: forms
  78 ### setting up a `POST' action must include an appropriate hidden input
  79 ### element.
  80 ###
  81 ### Messing about with cookies is a bit annoying, but it's hard to come up
  82 ### with alternatives.  I'm trying to keep the URLs fairly pretty, and anyway
  83 ### putting secrets into them is asking for trouble, since user agents have
  84 ### an awful tendecy to store URLs in a history database, send them to
  85 ### motherships, leak them in `Referer' headers, and other awful things.  Our
  86 ### cookie is marked `HttpOnly' so, in particular, user agents must keep them
  87 ### out of the grubby mitts of Javascript programs.
  88 ###
  89 ### I promise that I'm only using these cookies for the purposes of
  90 ### maintaining security: I don't log them or do anything else at all with
  91 ### them.
  92
  93 ###--------------------------------------------------------------------------
  94 ### Generating and checking authentication tokens.
  95
  96 ## Secret lifetime parameters.
  97 CONF.DEFAULTS.update(
  98
  99   ## The lifetime of a session cookie, in seconds.
 100   SECRETLIFE = 30*60,
 101
 102   ## Maximum age of an authentication key, in seconds.
 103   SECRETFRESH = 5*60)
 104
 105 def cleansecrets():
 106   """Remove dead secrets from the database."""
 107   with D.DB:
 108     D.DB.execute("DELETE FROM secrets WHERE stamp < $stale",
 109                  stale = U.NOW - CFG.SECRETLIFE)
 110
 111 def getsecret(when):
 112   """
 113   Return the newest and most shiny secret no older than WHEN.
 114
 115   If there is no such secret, or the only one available would have been stale
 116   at WHEN, then return `None'.
 117   """
 118   cleansecrets()
 119   with D.DB:
 120     D.DB.execute("""SELECT stamp, secret FROM secrets
 121                     WHERE stamp <= $when
 122                     ORDER BY stamp DESC""",
 123                when = when)
 124     row = D.DB.fetchone()
 125     if row is None: return None
 126     if row[0] < when - CFG.SECRETFRESH: return None
 127     return row[1].decode('base64')
 128
 129 def freshsecret():
 130   """Return a fresh secret."""
 131   cleansecrets()
 132   with D.DB:
 133     D.DB.execute("""SELECT secret FROM secrets
 134                     WHERE stamp >= $fresh
 135                     ORDER BY stamp DESC""",
 136                  fresh = U.NOW - CFG.SECRETFRESH)
 137     row = D.DB.fetchone()
 138     if row is not None:
 139       sec = row[0].decode('base64')
 140     else:
 141       sec = OS.urandom(16)
 142       D.DB.execute("""INSERT INTO secrets(stamp, secret)
 143                       VALUES ($stamp, $secret)""",
 144                    stamp = U.NOW, secret = sec.encode('base64'))
 145     return sec
 146
 147 def hack_octets(s):
 148   """Return the octet string S, in a vaguely pretty form."""
 149   return BN.b64encode(s) \
 150            .rstrip('=') \
 151            .replace('/', '$')
 152
 153 def auth_tag(sec, stamp, nonce, user):
 154   """Compute a tag using secret SEC on `STAMP.NONCE.USER'."""
 155   hmac = HM.HMAC(sec, digestmod = H.sha256)
 156   hmac.update('%d.%s.%s' % (stamp, nonce, user))
 157   return hack_octets(hmac.digest())
 158
 159 def mint_token(user):
 160   """Make and return a fresh token for USER."""
 161   sec = freshsecret()
 162   nonce = hack_octets(OS.urandom(16))
 163   tag = auth_tag(sec, U.NOW, nonce, user)
 164   return '%d.%s.%s.%s' % (U.NOW, nonce, tag, user)
 165
 166 ## Long messages for reasons why one might have been redirected back to the
 167 ## login page.
 168 LOGIN_REASONS = {
 169   'AUTHFAIL': 'incorrect user name or password',
 170   'NOAUTH': 'not authenticated',
 171   'NONONCE': 'missing nonce',
 172   'BADTOKEN': 'malformed token',
 173   'BADTIME': 'invalid timestamp',
 174   'BADNONCE': 'nonce mismatch',
 175   'EXPIRED': 'session timed out',
 176   'BADTAG': 'incorrect tag',
 177   'NOUSER': 'unknown user name',
 178   'LOGOUT': 'explicitly logged out',
 179   None: None
 180 }
 181
 182 class AuthenticationFailed (U.ExpectedError):
 183   """
 184   An authentication error.  The most interesting extra feature is an
 185   attribute `why' carrying a reason code, which can be looked up in
 186   `LOGIN_REASONS'.
 187   """
 188   def __init__(me, why):
 189     msg = LOGIN_REASONS[why]
 190     U.ExpectedError.__init__(me, 403, msg)
 191     me.why = why
 192
 193 def check_auth(token, nonce = None):
 194   """
 195   Check that the TOKEN is valid, comparing it against the NONCE if this is
 196   not `None'.
 197
 198   If the token is OK, then return the correct user name, and set `NONCE' set
 199   to the appropriate portion of the token.  Otherwise raise an
 200   `AuthenticationFailed' exception with an appropriate `why'.
 201   """
 202
 203   global NONCE
 204
 205   ## If the token has been explicitly clobbered, then we're logged out.
 206   if token == 'logged-out': raise AuthenticationFailed, 'LOGOUT'
 207
 208   ## Parse the token.
 209   bits = token.split('.', 3)
 210   if len(bits) != 4: raise AuthenticationFailed, 'BADTOKEN'
 211   stamp, NONCE, tag, user = bits
 212
 213   ## Check that the nonce matches, if one was supplied.
 214   if nonce is not None and nonce != NONCE:
 215     raise AuthenticationFailed, 'BADNONCE'
 216
 217   ## Check the stamp, and find the right secret.
 218   if not stamp.isdigit(): raise AuthenticationFailed, 'BADTIME'
 219   when = int(stamp)
 220   sec = getsecret(when)
 221   if sec is None: raise AuthenticationFailed, 'EXPIRED'
 222
 223   ## Check the tag.
 224   t = auth_tag(sec, when, NONCE, user)
 225   if t != tag: raise AuthenticationFailed, 'BADTAG'
 226
 227   ## Make sure the user still exists.
 228   try: acct = S.SERVICES['master'].find(user)
 229   except S.UnknownUser: raise AuthenticationFailed, 'NOUSER'
 230
 231   ## Done.
 232   return user
 233
 234 def bake_cookie(value):
 235   """
 236   Return a properly baked authentication-token cookie with the given VALUE.
 237   """
 238   return CGI.cookie('chpwd-token', value,
 239                     httponly = True,
 240                     secure = CGI.SSLP,
 241                     path = CFG.SCRIPT_NAME,
 242                     max_age = (CFG.SECRETLIFE - CFG.SECRETFRESH))
 243
 244 ###--------------------------------------------------------------------------
 245 ### Authentication commands.
 246
 247 ## A dummy string, for when we're invoked from the command-line.
 248 NONCE = '@DUMMY-NONCE'
 249
 250 @CGI.subcommand(
 251   'login', ['cgi-noauth'],
 252   'Authenticate to the CGI machinery',
 253   opts = [SC.Opt('why', '-w', '--why',
 254                  'Reason for redirection back to the login page.',
 255                  argname = 'WHY')])
 256 def cmd_login(why = None):
 257   CGI.page('login.fhtml',
 258            title = 'Chopwood: login',
 259            why =LOGIN_REASONS.get(why, '<unknown error %s>' % why))
 260
 261 @CGI.subcommand(
 262   'auth', ['cgi-noauth'],
 263   'Verify a user name and password',
 264   params = [SC.Arg('u'), SC.Arg('pw')])
 265 def cmd_auth(u, pw):
 266   svc = S.SERVICES['master']
 267   try:
 268     acct = svc.find(u)
 269     acct.check(pw)
 270   except (S.UnknownUser, S.IncorrectPassword):
 271     CGI.redirect(CGI.action('login', why = 'AUTHFAIL'))
 272   else:
 273     t = mint_token(u)
 274     CGI.redirect(CGI.action('list', u),
 275                  set_cookie = bake_cookie(t))
 276
 277 ###----- That's all, folks --------------------------------------------------