Provided by: libsession-storage-secure-perl_0.011-1_all 
NAME
Session::Storage::Secure - Encrypted, expiring, compressed, serialized session data with integrity
VERSION
version 0.011
SYNOPSIS
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 );
DESCRIPTION
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
<http://www.cse.msu.edu/~alexliu/publications/Cookie/Cookie_COMNET.pdf> by Alex Liu and others. Liu
proposes a session cookie value as follows:
user|expiration|E(data,k)|HMAC(user|expiration|data|ssl-key,k)
where
| 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:
salt|expiration|E(data,k)|HMAC(expiration|E(data,k),k)
where
| 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.
ATTRIBUTES
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.
default_duration
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.
old_secrets
An optional array reference of strings containing old secret keys no longer used for encryption but still
supported for decrypting session data.
separator
A character used to separate fields. It defaults to "~".
sereal_encoder_options
A hash reference with constructor arguments for Sereal::Encoder. Defaults to "{ snappy => 1,
croak_on_bless => 1 }".
sereal_decoder_options
A hash reference with constructor arguments for Sereal::Decoder. Defaults to "{ refuse_objects => 1,
validate_utf8 => 1 }".
transport_encoder
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.
transport_decoder
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 MIME::Base64::decode_base64url.
METHODS
encode
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.
decode
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.
LIMITATIONS
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 <https://github.com/Sereal/Sereal/wiki/Sereal-Comparison-Graphs> 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.
SECURITY
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 rights".
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 attacks.
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") ) {
context->destroy_session;
}
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
<http://www.cl.cam.ac.uk/~sjm217/papers/protocols08cookies.pdf> is to store an iterated hash of the
hashed password in the database and use the hashed password itself within the session.
# 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 ) ) {
context->destroy_session;
}
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.
SEE ALSO
Papers on secure cookies and cookie session storage:
• Liu, Alex X., et al., A Secure Cookie Protocol
<http://www.cse.msu.edu/~alexliu/publications/Cookie/Cookie_COMNET.pdf>
• Murdoch, Stephen J., Hardened Stateless Session Cookies
<http://www.cl.cam.ac.uk/~sjm217/papers/protocols08cookies.pdf>
• Fu, Kevin, et al., Dos and Don'ts of Client Authentication on the Web
<http://pdos.csail.mit.edu/papers/webauth:sec10.pdf>
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
SUPPORT
Bugs / Feature Requests
Please report any bugs or feature requests through the issue tracker at
<https://github.com/dagolden/Session-Storage-Secure/issues>. 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.
<https://github.com/dagolden/Session-Storage-Secure>
git clone https://github.com/dagolden/Session-Storage-Secure.git
AUTHOR
David Golden <dagolden@cpan.org>
CONTRIBUTOR
Tom Hukins <tom@eborcom.com>
COPYRIGHT AND LICENSE
This software is Copyright (c) 2013 by David Golden.
This is free software, licensed under:
The Apache License, Version 2.0, January 2004
perl v5.26.1 2018-04-26 Session::Storage::Secure(3pm)