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 0.011
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
136 encode
137 my $string = $store->encode( $data, $expires );
138 The $data argument should be a reference to a data structure. By
140 default, it must not contain objects. (See "Objects not stored by
141 default" for rationale and alternatives.) If it is undefined, an empty
142 hash reference will be encoded instead.
143 The optional $expires argument should be the session expiration time
145 expressed as epoch seconds. If the $expires time is in the past, the
146 $data argument is cleared and an empty hash reference is encoded and
147 returned. If no $expires is given, then if the "default_duration"
148 attribute is set, it will be used to calculate an expiration time.
149 The method returns a string that securely encodes the session data.
151 All binary components are protected via the "transport_encoder"
152 attribute.
153 An exception is thrown on any errors.
155 decode
157 my $data = $store->decode( $string );
158 The $string argument must be the output of "encode".
160 If the message integrity check fails or if expiration exists and is in
162 the past, the method returns undef or an empty list (depending on
163 context).
164 An exception is thrown on any errors.
166
168 Secret key
169 You must protect the secret key, of course. Rekeying periodically
170 would improve security. Rekeying also invalidates all existing
171 sessions unless the "old_secrets" attribute contains old encryption
172 keys still used for decryption. In a multi-node application, all nodes
173 must share the same secret key.
174 Session size
176 If storing the encoded session in a cookie, keep in mind that cookies
177 must fit within 4k, so don't store too much data. This module uses
178 Sereal for serialization and enables the "snappy" compression option.
179 Sereal plus Snappy appears to be one of the fastest and most compact
180 serialization options for Perl, according to the Sereal benchmarks
181 <https://github.com/Sereal/Sereal/wiki/Sereal-Comparison-Graphs> page.
182 However, nothing prevents the encoded output from exceeding 4k.
184 Applications must check for this condition and handle it appropriately
185 with an error or by splitting the value across multiple cookies.
186 Objects not stored by default
188 The default Sereal options do not allow storing objects because object
189 deserialization can have undesirable side effects, including
190 potentially fatal errors if a class is not available at deserialization
191 time or if internal class structures changed from when the session data
192 was serialized to when it was deserialized. Applications should take
193 steps to deflate/inflate objects before storing them in session data.
194 Alternatively, applications can change "sereal_encoder_options" and
196 "sereal_decoder_options" to allow object serialization or other object
197 transformations and accept the risks of doing so.
198
200 Storing encrypted session data within a browser cookie avoids latency
201 and overhead of backend session storage, but has several additional
202 security considerations.
203 Transport security
205 If using cookies to store session data, an attacker could intercept
206 cookies and replay them to impersonate a valid user regardless of
207 encryption. SSL encryption of the transport channel is strongly
208 recommended.
209 Cookie replay
211 Because all session state is maintained in the session cookie, an
212 attacker or malicious user could replay an old cookie to return to a
213 previous state. Cookie-based sessions should not be used for recording
214 incremental steps in a transaction or to record "negative rights".
215 Because cookie expiration happens on the client-side, an attacker or
217 malicious user could replay a cookie after its scheduled expiration
218 date. It is strongly recommended to set "cookie_duration" or
219 "default_duration" to limit the window of opportunity for such replay
220 attacks.
221 Session authentication
223 A compromised secret key could be used to construct valid messages
224 appearing to be from any user. Applications should take extra steps in
225 their use of session data to ensure that sessions are authenticated to
226 the user.
227 One simple approach could be to store a hash of the user's hashed
229 password in the session on login and to verify it on each request.
230 # on login
232 my $hashed_pw = bcrypt( $password, $salt );
233 if ( $hashed_pw eq $hashed_pw_from_db ) {
234 session user => $user;
235 session auth => bcrypt( $hashed_pw, $salt ) );
236 }
237 # on each request
239 if ( bcrypt( $hashed_pw_from_db, $salt ) ne session("auth") ) {
240 context->destroy_session;
241 }
242 The downside of this is that if there is a read-only attack against the
244 database (SQL injection or leaked backup dump) and the secret key is
245 compromised, then an attacker can forge a cookie to impersonate any
246 user.
247 A more secure approach suggested by Stephen Murdoch in Hardened
249 Stateless Session Cookies
250 <http://www.cl.cam.ac.uk/~sjm217/papers/protocols08cookies.pdf> is to
251 store an iterated hash of the hashed password in the database and use
252 the hashed password itself within the session.
253 # on login
255 my $hashed_pw = bcrypt( $password, $salt );
256 if ( bcrypt( $hashed_pw, $salt ) eq $double_hashed_pw_from_db ) {
257 session user => $user;
258 session auth => $hashed_pw;
259 }
260 # on each request
262 if ( $double_hashed_pw_from_db ne bcrypt( session("auth"), $salt ) ) {
263 context->destroy_session;
264 }
265 This latter approach means that even a compromise of the secret key and
267 the database contents can't be used to impersonate a user because doing
268 so would requiring reversing a one-way hash to determine the correct
269 authenticator to put into the forged cookie.
270 Both methods require an additional database read per request. This
272 diminishes some of the scalability benefits of storing session data in
273 a cookie, but the read could be cached and there is still no database
274 write needed to store session data.
275
277 Papers on secure cookies and cookie session storage:
278 · Liu, Alex X., et al., A Secure Cookie Protocol
280 <http://www.cse.msu.edu/~alexliu/publications/Cookie/Cookie_COMNET.pdf>
281 · Murdoch, Stephen J., Hardened Stateless Session Cookies
283 <http://www.cl.cam.ac.uk/~sjm217/papers/protocols08cookies.pdf>
284 · Fu, Kevin, et al., Dos and Don'ts of Client Authentication on the
286 Web <http://pdos.csail.mit.edu/papers/webauth:sec10.pdf>
287 CPAN modules implementing cookie session storage:
289 · Catalyst::Plugin::CookiedSession -- encryption only
291 · Dancer::Session::Cookie -- Dancer 1, encryption only
293 · Dancer::SessionFactory::Cookie -- Dancer 2, forthcoming, based on
295 this module
296 · HTTP::CryptoCookie -- encryption only
298 · Mojolicious::Sessions -- MAC only
300 · Plack::Middleware::Session::Cookie -- MAC only
302 · Plack::Middleware::Session::SerializedCookie -- really just a
304 framework and you provide the guts with callbacks
305 Related CPAN modules that offer frameworks for serializing and
307 encrypting data, but without features relevant for sessions like
308 expiration and unique keying.
309 · Crypt::Util
311 · Data::Serializer
313
315 Bugs / Feature Requests
316 Please report any bugs or feature requests through the issue tracker at
317 <https://github.com/dagolden/Session-Storage-Secure/issues>. You will
318 be notified automatically of any progress on your issue.
319 Source Code
321 This is open source software. The code repository is available for
322 public review and contribution under the terms of the license.
323 <https://github.com/dagolden/Session-Storage-Secure>
325 git clone https://github.com/dagolden/Session-Storage-Secure.git
327
329 David Golden <dagolden@cpan.org>
330
332 Tom Hukins <tom@eborcom.com>
333
335 This software is Copyright (c) 2013 by David Golden.
336 This is free software, licensed under:
338 The Apache License, Version 2.0, January 2004
340perl v5.28.1 2018-04-22 Session::Storage::Secure(3)