1Session::Storage::SecurUes(e3r)Contributed Perl DocumentSaetsisoinon::Storage::Secure(3)
2
6 Session::Storage::Secure - Encrypted, expiring, compressed, serialized
7 session data with integrity
8
10 version 1.000
11
13 my $store = Session::Storage::Secure->new(
14 secret_key => "your pass phrase here",
15 default_duration => 86400 * 7,
16 );
17 my $encoded = $store->encode( $data, $expires );
19 my $decoded = $store->decode( $encoded );
21
23 This module implements a secure way to encode session data. It is
24 primarily intended for storing session data in browser cookies, but
25 could be used with other backend storage where security of stored
26 session data is important.
27 Features include:
29 • Data serialization and compression using Sereal
31 • Data encryption using AES with a unique derived key per encoded
33 session
34 • Enforced expiration timestamp (optional)
36 • Integrity protected with a message authentication code (MAC)
38 The storage protocol used in this module is based heavily on A Secure
40 Cookie Protocol
41 <http://www.cse.msu.edu/~alexliu/publications/Cookie/Cookie_COMNET.pdf>
42 by Alex Liu and others. Liu proposes a session cookie value as
43 follows:
44 user|expiration|E(data,k)|HMAC(user|expiration|data|ssl-key,k)
46 where
48 | denotes concatenation with a separator character
50 E(p,q) is a symmetric encryption of p with key q
51 HMAC(p,q) is a keyed message hash of p with key q
52 k is HMAC(user|expiration, sk)
53 sk is a secret key shared by all servers
54 ssl-key is an SSL session key
55 Because SSL session keys are not readily available (and SSL termination
57 may happen prior to the application server), we omit "ssl-key". This
58 weakens protection against replay attacks if an attacker can break the
59 SSL session key and intercept messages.
60 Using "user" and "expiration" to generate the encryption and MAC keys
62 was a method proposed to ensure unique keys to defeat volume attacks
63 against the secret key. Rather than rely on those for uniqueness (with
64 the unfortunate side effect of revealing user names and prohibiting
65 anonymous sessions), we replace "user" with a cryptographically-strong
66 random salt value.
67 The original proposal also calculates a MAC based on unencrypted data.
69 We instead calculate the MAC based on the encrypted data. This avoids
70 an extra step decrypting invalid messages. Because the salt is already
71 encoded into the key, we omit it from the MAC input.
72 Therefore, the session storage protocol used by this module is as
74 follows:
75 salt|expiration|E(data,k)|HMAC(expiration|E(data,k),k)
77 where
79 | denotes concatenation with a separator character
81 E(p,q) is a symmetric encryption of p with key q
82 HMAC(p,q) is a keyed message hash of p with key q
83 k is HMAC(salt, sk)
84 sk is a secret key shared by all servers
85 The salt value is generated using Math::Random::ISAAC::XS, seeded from
87 Crypt::URandom.
88 The HMAC algorithm is "hmac_sha256" from Digest::SHA. Encryption is
90 done by Crypt::CBC using Crypt::Rijndael (AES). The ciphertext and
91 MAC's in the cookie are Base64 encoded by MIME::Base64 by default.
92 During session retrieval, if the MAC does not authenticate or if the
94 expiration is set and in the past, the session will be discarded.
95
97 secret_key (required)
98 This is used to secure the session data. The encryption and message
99 authentication key is derived from this using a one-way function.
100 Changing it will invalidate all sessions.
101 default_duration
103 Number of seconds for which the session may be considered valid. If an
104 expiration is not provided to "encode", this is used instead to expire
105 the session after a period of time. It is unset by default, meaning
106 that session expiration is not capped.
107 old_secrets
109 An optional array reference of strings containing old secret keys no
110 longer used for encryption but still supported for decrypting session
111 data.
112 separator
114 A character used to separate fields. It defaults to "~".
115 sereal_encoder_options
117 A hash reference with constructor arguments for Sereal::Encoder.
118 Defaults to "{ snappy => 1, croak_on_bless => 1 }".
119 sereal_decoder_options
121 A hash reference with constructor arguments for Sereal::Decoder.
122 Defaults to "{ refuse_objects => 1, validate_utf8 => 1 }".
123 transport_encoder
125 A code reference to convert binary data elements (the encrypted data
126 and the MAC) into a transport-safe form. Defaults to
127 MIME::Base64::encode_base64url. The output must not include the
128 "separator" attribute used to delimit fields.
129 transport_decoder
131 A code reference to extract binary data (the encrypted data and the
132 MAC) from a transport-safe form. It must be the complement to
133 "encode". Defaults to MIME::Base64::decode_base64url.
134 protocol_version
136 An integer representing the protocol used by
137 "Session::Storage::Secure". Protocol 1 was the initial version, which
138 used a now-deprecated mode of Crypt::CBC. Protocol 2 is the current
139 default.
140
142 encode
143 my $string = $store->encode( $data, $expires );
144 The $data argument should be a reference to a data structure. By
146 default, it must not contain objects. (See "Objects not stored by
147 default" for rationale and alternatives.) If it is undefined, an empty
148 hash reference will be encoded instead.
149 The optional $expires argument should be the session expiration time
151 expressed as epoch seconds. If the $expires time is in the past, the
152 $data argument is cleared and an empty hash reference is encoded and
153 returned. If no $expires is given, then if the "default_duration"
154 attribute is set, it will be used to calculate an expiration time.
155 The method returns a string that securely encodes the session data.
157 All binary components are protected via the "transport_encoder"
158 attribute.
159 An exception is thrown on any errors.
161 decode
163 my $data = $store->decode( $string );
164 The $string argument must be the output of "encode".
166 If the message integrity check fails or if expiration exists and is in
168 the past, the method returns undef or an empty list (depending on
169 context).
170 An exception is thrown on any errors.
172
174 Secret key
175 You must protect the secret key, of course. Rekeying periodically
176 would improve security. Rekeying also invalidates all existing
177 sessions unless the "old_secrets" attribute contains old encryption
178 keys still used for decryption. In a multi-node application, all nodes
179 must share the same secret key.
180 Session size
182 If storing the encoded session in a cookie, keep in mind that cookies
183 must fit within 4k, so don't store too much data. This module uses
184 Sereal for serialization and enables the "snappy" compression option.
185 Sereal plus Snappy appears to be one of the fastest and most compact
186 serialization options for Perl, according to the Sereal benchmarks
187 <https://github.com/Sereal/Sereal/wiki/Sereal-Comparison-Graphs> page.
188 However, nothing prevents the encoded output from exceeding 4k.
190 Applications must check for this condition and handle it appropriately
191 with an error or by splitting the value across multiple cookies.
192 Objects not stored by default
194 The default Sereal options do not allow storing objects because object
195 deserialization can have undesirable side effects, including
196 potentially fatal errors if a class is not available at deserialization
197 time or if internal class structures changed from when the session data
198 was serialized to when it was deserialized. Applications should take
199 steps to deflate/inflate objects before storing them in session data.
200 Alternatively, applications can change "sereal_encoder_options" and
202 "sereal_decoder_options" to allow object serialization or other object
203 transformations and accept the risks of doing so.
204
206 Storing encrypted session data within a browser cookie avoids latency
207 and overhead of backend session storage, but has several additional
208 security considerations.
209 Transport security
211 If using cookies to store session data, an attacker could intercept
212 cookies and replay them to impersonate a valid user regardless of
213 encryption. SSL encryption of the transport channel is strongly
214 recommended.
215 Cookie replay
217 Because all session state is maintained in the session cookie, an
218 attacker or malicious user could replay an old cookie to return to a
219 previous state. Cookie-based sessions should not be used for recording
220 incremental steps in a transaction or to record "negative rights".
221 Because cookie expiration happens on the client-side, an attacker or
223 malicious user could replay a cookie after its scheduled expiration
224 date. It is strongly recommended to set "cookie_duration" or
225 "default_duration" to limit the window of opportunity for such replay
226 attacks.
227 Session authentication
229 A compromised secret key could be used to construct valid messages
230 appearing to be from any user. Applications should take extra steps in
231 their use of session data to ensure that sessions are authenticated to
232 the user.
233 One simple approach could be to store a hash of the user's hashed
235 password in the session on login and to verify it on each request.
236 # on login
238 my $hashed_pw = bcrypt( $password, $salt );
239 if ( $hashed_pw eq $hashed_pw_from_db ) {
240 session user => $user;
241 session auth => bcrypt( $hashed_pw, $salt ) );
242 }
243 # on each request
245 if ( bcrypt( $hashed_pw_from_db, $salt ) ne session("auth") ) {
246 context->destroy_session;
247 }
248 The downside of this is that if there is a read-only attack against the
250 database (SQL injection or leaked backup dump) and the secret key is
251 compromised, then an attacker can forge a cookie to impersonate any
252 user.
253 A more secure approach suggested by Stephen Murdoch in Hardened
255 Stateless Session Cookies
256 <http://www.cl.cam.ac.uk/~sjm217/papers/protocols08cookies.pdf> is to
257 store an iterated hash of the hashed password in the database and use
258 the hashed password itself within the session.
259 # on login
261 my $hashed_pw = bcrypt( $password, $salt );
262 if ( bcrypt( $hashed_pw, $salt ) eq $double_hashed_pw_from_db ) {
263 session user => $user;
264 session auth => $hashed_pw;
265 }
266 # on each request
268 if ( $double_hashed_pw_from_db ne bcrypt( session("auth"), $salt ) ) {
269 context->destroy_session;
270 }
271 This latter approach means that even a compromise of the secret key and
273 the database contents can't be used to impersonate a user because doing
274 so would requiring reversing a one-way hash to determine the correct
275 authenticator to put into the forged cookie.
276 Both methods require an additional database read per request. This
278 diminishes some of the scalability benefits of storing session data in
279 a cookie, but the read could be cached and there is still no database
280 write needed to store session data.
281
283 Papers on secure cookies and cookie session storage:
284 • Liu, Alex X., et al., A Secure Cookie Protocol
286 <http://www.cse.msu.edu/~alexliu/publications/Cookie/Cookie_COMNET.pdf>
287 • Murdoch, Stephen J., Hardened Stateless Session Cookies
289 <http://www.cl.cam.ac.uk/~sjm217/papers/protocols08cookies.pdf>
290 • Fu, Kevin, et al., Dos and Don'ts of Client Authentication on the
292 Web <http://pdos.csail.mit.edu/papers/webauth:sec10.pdf>
293 CPAN modules implementing cookie session storage:
295 • Catalyst::Plugin::CookiedSession -- encryption only
297 • Dancer::Session::Cookie -- Dancer 1, encryption only
299 • Dancer::SessionFactory::Cookie -- Dancer 2, forthcoming, based on
301 this module
302 • HTTP::CryptoCookie -- encryption only
304 • Mojolicious::Sessions -- MAC only
306 • Plack::Middleware::Session::Cookie -- MAC only
308 • Plack::Middleware::Session::SerializedCookie -- really just a
310 framework and you provide the guts with callbacks
311 Related CPAN modules that offer frameworks for serializing and
313 encrypting data, but without features relevant for sessions like
314 expiration and unique keying.
315 • Crypt::Util
317 • Data::Serializer
319
321 David Golden <dagolden@cpan.org>
322
324 • Petr Písař <ppisar@redhat.com>
325 • Tom Hukins <tom@eborcom.com>
327
329 This software is Copyright (c) 2013 by David Golden.
330 This is free software, licensed under:
332 The Apache License, Version 2.0, January 2004
334perl v5.32.1 2021-03-24 Session::Storage::Secure(3)