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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 | None: None | |
179 | } | |
180 | ||
181 | class AuthenticationFailed (U.ExpectedError): | |
182 | """ | |
183 | An authentication error. The most interesting extra feature is an | |
184 | attribute `why' carrying a reason code, which can be looked up in | |
185 | `LOGIN_REASONS'. | |
186 | """ | |
187 | def __init__(me, why): | |
188 | msg = LOGIN_REASONS[why] | |
189 | U.ExpectedError.__init__(me, 403, msg) | |
190 | me.why = why | |
191 | ||
192 | def check_auth(token, nonce = None): | |
193 | """ | |
194 | Check that the TOKEN is valid, comparing it against the NONCE if this is | |
195 | not `None'. | |
196 | ||
197 | If the token is OK, then return the correct user name, and set `NONCE' set | |
198 | to the appropriate portion of the token. Otherwise raise an | |
199 | `AuthenticationFailed' exception with an appropriate `why'. | |
200 | """ | |
201 | ||
202 | global NONCE | |
203 | ||
204 | ## Parse the token. | |
205 | bits = token.split('.', 3) | |
206 | if len(bits) != 4: raise AuthenticationFailed, 'BADTOKEN' | |
207 | stamp, NONCE, tag, user = bits | |
208 | ||
209 | ## Check that the nonce matches, if one was supplied. | |
210 | if nonce is not None and nonce != NONCE: | |
211 | raise AuthenticationFailed, 'BADNONCE' | |
212 | ||
213 | ## Check the stamp, and find the right secret. | |
214 | if not stamp.isdigit(): raise AuthenticationFailed, 'BADTIME' | |
215 | when = int(stamp) | |
216 | sec = getsecret(when) | |
217 | if sec is None: raise AuthenticationFailed, 'EXPIRED' | |
218 | ||
219 | ## Check the tag. | |
220 | t = auth_tag(sec, when, NONCE, user) | |
221 | if t != tag: raise AuthenticationFailed, 'BADTAG' | |
222 | ||
223 | ## Make sure the user still exists. | |
224 | try: acct = S.SERVICES['master'].find(user) | |
225 | except S.UnknownUser: raise AuthenticationFailed, 'NOUSER' | |
226 | ||
227 | ## Done. | |
228 | return user | |
229 | ||
230 | ###-------------------------------------------------------------------------- | |
231 | ### Authentication commands. | |
232 | ||
233 | ## A dummy string, for when we're invoked from the command-line. | |
234 | NONCE = '@DUMMY-NONCE' | |
235 | ||
236 | @CGI.subcommand( | |
237 | 'login', ['cgi-noauth'], | |
238 | 'Authenticate to the CGI machinery', | |
239 | opts = [SC.Opt('why', '-w', '--why', | |
240 | 'Reason for redirection back to the login page.', | |
241 | argname = 'WHY')]) | |
242 | def cmd_login(why = None): | |
243 | CGI.page('login.fhtml', | |
244 | title = 'Chopwood: login', | |
245 | why =LOGIN_REASONS.get(why, '<unknown error %s>' % why)) | |
246 | ||
247 | @CGI.subcommand( | |
248 | 'auth', ['cgi-noauth'], | |
249 | 'Verify a user name and password', | |
250 | params = [SC.Arg('u'), SC.Arg('pw')]) | |
251 | def cmd_auth(u, pw): | |
252 | svc = S.SERVICES['master'] | |
253 | try: | |
254 | acct = svc.find(u) | |
255 | acct.check(pw) | |
256 | except (S.UnknownUser, S.IncorrectPassword): | |
257 | CGI.redirect(CGI.action('login', why = 'AUTHFAIL')) | |
258 | else: | |
259 | t = mint_token(u) | |
260 | CGI.redirect(CGI.action('list'), | |
261 | set_cookie = CGI.cookie('chpwd-token', t, | |
262 | httponly = True, | |
263 | path = CFG.SCRIPT_NAME, | |
264 | max_age = (CFG.SECRETLIFE - | |
265 | CFG.SECRETFRESH))) | |
266 | ||
267 | ###----- That's all, folks -------------------------------------------------- |