Provided by: libsession-storage-secure-perl_0.011-1_all bug


       Session::Storage::Secure - Encrypted, expiring, compressed, serialized session data with


       version 0.011


         my $store = Session::Storage::Secure->new(
           secret_key   => "your pass phrase here",
           default_duration => 86400 * 7,

         my $encoded = $store->encode( $data, $expires );

         my $decoded = $store->decode( $encoded );


       This module implements a secure way to encode session data.  It is primarily intended for
       storing session data in browser cookies, but could be used with other backend storage
       where security of stored session data is important.

       Features include:

       ·   Data serialization and compression using Sereal

       ·   Data encryption using AES with a unique derived key per encoded session

       ·   Enforced expiration timestamp (optional)

       ·   Integrity protected with a message authentication code (MAC)

       The storage protocol used in this module is based heavily on A Secure Cookie Protocol
       <> by Alex Liu and
       others.  Liu proposes a session cookie value as follows:



           | denotes concatenation with a separator character
           E(p,q) is a symmetric encryption of p with key q
           HMAC(p,q) is a keyed message hash of p with key q
           k is HMAC(user|expiration, sk)
           sk is a secret key shared by all servers
           ssl-key is an SSL session key

       Because SSL session keys are not readily available (and SSL termination may happen prior
       to the application server), we omit "ssl-key".  This weakens protection against replay
       attacks if an attacker can break the SSL session key and intercept messages.

       Using "user" and "expiration" to generate the encryption and MAC keys was a method
       proposed to ensure unique keys to defeat volume attacks against the secret key.  Rather
       than rely on those for uniqueness (with the unfortunate side effect of revealing user
       names and prohibiting anonymous sessions), we replace "user" with a cryptographically-
       strong random salt value.

       The original proposal also calculates a MAC based on unencrypted data.  We instead
       calculate the MAC based on the encrypted data.  This avoids an extra step decrypting
       invalid messages.  Because the salt is already encoded into the key, we omit it from the
       MAC input.

       Therefore, the session storage protocol used by this module is as follows:



           | denotes concatenation with a separator character
           E(p,q) is a symmetric encryption of p with key q
           HMAC(p,q) is a keyed message hash of p with key q
           k is HMAC(salt, sk)
           sk is a secret key shared by all servers

       The salt value is generated using Math::Random::ISAAC::XS, seeded from Crypt::URandom.

       The HMAC algorithm is "hmac_sha256" from Digest::SHA.  Encryption is done by Crypt::CBC
       using Crypt::Rijndael (AES).  The ciphertext and MAC's in the cookie are Base64 encoded by
       MIME::Base64 by default.

       During session retrieval, if the MAC does not authenticate or if the expiration is set and
       in the past, the session will be discarded.


   secret_key (required)
       This is used to secure the session data.  The encryption and message authentication key is
       derived from this using a one-way function.  Changing it will invalidate all sessions.

       Number of seconds for which the session may be considered valid.  If an expiration is not
       provided to "encode", this is used instead to expire the session after a period of time.
       It is unset by default, meaning that session expiration is not capped.

       An optional array reference of strings containing old secret keys no longer used for
       encryption but still supported for decrypting session data.

       A character used to separate fields.  It defaults to "~".

       A hash reference with constructor arguments for Sereal::Encoder. Defaults to "{ snappy =>
       1, croak_on_bless => 1 }".

       A hash reference with constructor arguments for Sereal::Decoder. Defaults to "{
       refuse_objects => 1, validate_utf8  => 1 }".

       A code reference to convert binary data elements (the encrypted data and the MAC) into a
       transport-safe form.  Defaults to MIME::Base64::encode_base64url.  The output must not
       include the "separator" attribute used to delimit fields.

       A code reference to extract binary data (the encrypted data and the MAC) from a transport-
       safe form.  It must be the complement to "encode".  Defaults to


         my $string = $store->encode( $data, $expires );

       The $data argument should be a reference to a data structure.  By default, it must not
       contain objects.  (See "Objects not stored by default" for rationale and alternatives.) If
       it is undefined, an empty hash reference will be encoded instead.

       The optional $expires argument should be the session expiration time expressed as epoch
       seconds.  If the $expires time is in the past, the $data argument is cleared and an empty
       hash reference is encoded and returned.  If no $expires is given, then if the
       "default_duration" attribute is set, it will be used to calculate an expiration time.

       The method returns a string that securely encodes the session data.  All binary components
       are protected via the "transport_encoder" attribute.

       An exception is thrown on any errors.

         my $data = $store->decode( $string );

       The $string argument must be the output of "encode".

       If the message integrity check fails or if expiration exists and is in the past, the
       method returns undef or an empty list (depending on context).

       An exception is thrown on any errors.


   Secret key
       You must protect the secret key, of course.  Rekeying periodically would improve security.
       Rekeying also invalidates all existing sessions unless the "old_secrets" attribute
       contains old encryption keys still used for decryption.  In a multi-node application, all
       nodes must share the same secret key.

   Session size
       If storing the encoded session in a cookie, keep in mind that cookies must fit within 4k,
       so don't store too much data.  This module uses Sereal for serialization and enables the
       "snappy" compression option.  Sereal plus Snappy appears to be one of the fastest and most
       compact serialization options for Perl, according to the Sereal benchmarks
       <> page.

       However, nothing prevents the encoded output from exceeding 4k.  Applications must check
       for this condition and handle it appropriately with an error or by splitting the value
       across multiple cookies.

   Objects not stored by default
       The default Sereal options do not allow storing objects because object deserialization can
       have undesirable side effects, including potentially fatal errors if a class is not
       available at deserialization time or if internal class structures changed from when the
       session data was serialized to when it was deserialized.  Applications should take steps
       to deflate/inflate objects before storing them in session data.

       Alternatively, applications can change "sereal_encoder_options" and
       "sereal_decoder_options" to allow object serialization or other object transformations and
       accept the risks of doing so.


       Storing encrypted session data within a browser cookie avoids latency and overhead of
       backend session storage, but has several additional security considerations.

   Transport security
       If using cookies to store session data, an attacker could intercept cookies and replay
       them to impersonate a valid user regardless of encryption.  SSL encryption of the
       transport channel is strongly recommended.

   Cookie replay
       Because all session state is maintained in the session cookie, an attacker or malicious
       user could replay an old cookie to return to a previous state.  Cookie-based sessions
       should not be used for recording incremental steps in a transaction or to record "negative

       Because cookie expiration happens on the client-side, an attacker or malicious user could
       replay a cookie after its scheduled expiration date.  It is strongly recommended to set
       "cookie_duration" or "default_duration" to limit the window of opportunity for such replay

   Session authentication
       A compromised secret key could be used to construct valid messages appearing to be from
       any user.  Applications should take extra steps in their use of session data to ensure
       that sessions are authenticated to the user.

       One simple approach could be to store a hash of the user's hashed password in the session
       on login and to verify it on each request.

         # on login
         my $hashed_pw = bcrypt( $password, $salt );
         if ( $hashed_pw eq $hashed_pw_from_db ) {
           session user => $user;
           session auth => bcrypt( $hashed_pw, $salt ) );

         # on each request
         if ( bcrypt( $hashed_pw_from_db, $salt ) ne session("auth") ) {

       The downside of this is that if there is a read-only attack against the database (SQL
       injection or leaked backup dump) and the secret key is compromised, then an attacker can
       forge a cookie to impersonate any user.

       A more secure approach suggested by Stephen Murdoch in Hardened Stateless Session Cookies
       <> is to store an iterated
       hash of the hashed password in the database and use the hashed password itself within the

         # on login
         my $hashed_pw = bcrypt( $password, $salt );
         if ( bcrypt( $hashed_pw, $salt ) eq $double_hashed_pw_from_db ) {
           session user => $user;
           session auth => $hashed_pw;

         # on each request
         if ( $double_hashed_pw_from_db ne bcrypt( session("auth"), $salt ) ) {

       This latter approach means that even a compromise of the secret key and the database
       contents can't be used to impersonate a user because doing so would requiring reversing a
       one-way hash to determine the correct authenticator to put into the forged cookie.

       Both methods require an additional database read per request. This diminishes some of the
       scalability benefits of storing session data in a cookie, but the read could be cached and
       there is still no database write needed to store session data.


       Papers on secure cookies and cookie session storage:

       ·   Liu, Alex X., et al., A Secure Cookie Protocol

       ·   Murdoch, Stephen J., Hardened Stateless Session Cookies

       ·   Fu, Kevin, et al., Dos and Don'ts of Client Authentication on the Web

       CPAN modules implementing cookie session storage:

       ·   Catalyst::Plugin::CookiedSession -- encryption only

       ·   Dancer::Session::Cookie -- Dancer 1, encryption only

       ·   Dancer::SessionFactory::Cookie -- Dancer 2, forthcoming, based on this module

       ·   HTTP::CryptoCookie -- encryption only

       ·   Mojolicious::Sessions -- MAC only

       ·   Plack::Middleware::Session::Cookie -- MAC only

       ·   Plack::Middleware::Session::SerializedCookie -- really just a framework and you
           provide the guts with callbacks

       Related CPAN modules that offer frameworks for serializing and encrypting data, but
       without features relevant for sessions like expiration and unique keying.

       ·   Crypt::Util

       ·   Data::Serializer


   Bugs / Feature Requests
       Please report any bugs or feature requests through the issue tracker at
       <>.  You will be notified
       automatically of any progress on your issue.

   Source Code
       This is open source software.  The code repository is available for public review and
       contribution under the terms of the license.


         git clone


       David Golden <>


       Tom Hukins <>


       This software is Copyright (c) 2013 by David Golden.

       This is free software, licensed under:

         The Apache License, Version 2.0, January 2004