Copyright (c) 2009-2015 RightScale, Inc. <support@rightscale.com>; see LICENSE for more details.

= Introduction

GlobalSession enables multiple heterogeneous Web applications to share
session state in a cryptographically secure way, facilitating single sign-on
and enabling easier development of distributed applications that make use of
architectural strategies such as sharding or separation of concerns.

In other words: it glues your Web apps together by letting them share session state.
This is done by putting the session itself into a cookie and adding some crypto to
protect against tampering.

Maintained by
 - [RightScale Engineering](https://github.com/rightscale)

Merge to master whitelist
 - @tony-spataro-rs

== What Is It Not?

This gem does not provide a complete solution for identity management. In
particular, it does not provide any of the following:

* <b>federation</b> -- aka cross-domain single sign-on -- use SAML for that.

* <b>authentication</b> -- the application must authenticate the user.

* <b>authorization</b> -- the application is responsible for using the contents
  of the global session to make authorization decisions.

* <b>secrecy</b> -- global session attributes can be signed but never encrypted.
  Protect against third-party snooping using SSL. Group secrecy is expensive;
  if you don't want your users to see their session state, put it in a database,
  or in an encrypted local session cookie.

* <b>replication</b> -- the session authorities must have some way to
  share information about the database of users in order to authenticate
  them and place identifying information into the global session.

* <b>single sign-out</b> -- the authorities must have some way to broadcast a
  notification when sessions are invalidated; they can override the default
  Directory implementation to do realtime revocation checking.

= Examples

== Integration with Rails

1) Create a basic config file and edit it to suit your needs:
    $ script/generate global_session_config mycoolapp.com

2) Create an authentication authority:
    $ script/generate global_session_authority mycoolapp

3) Declare that some or all of your controllers will use the global session:
    class ApplicationController < ActionController::Base
      has_global_session
    end

4) Make use of the global session hash in your controllers:
    global_session['user'] = @user.id
    ...
    @current_user = User.find(global_session['user'])

== Integration with Other Ruby Web Frameworks

Install the GlobalSession middleware into your Rack stack; pass a config and a directory
object to its initializer. For instance, in config.ru:

    configuration = GlobalSession::Configuration.new('path/to/config.yml', RACK_ENV)
    directory = GlobalSession::Directory.new(configuration)
    use ::GlobalSession::Rack::Middleware, configuration, directory

= Global Session Contents

Global session state is stored as a cookie in the user's browser and/or sent
with every request as an HTTP Authorization header. If your app uses the
Authorization header, then it's responsible for communicating new or changed
header values to clients out-of-band (i.e. as part of an OAuth refresh-token
operation). If your app uses the cookie, GlobalSession will take care of
updating the cookie whenever session values change.

Data-wise, the session is a JSON dictionary containing the following stuff:
* session metadata (UUID, created at, expires at, signing authority)
* signed session attributes (e.g. the authenticated user ID)
* insecure session attributes (e.g. the last-visited URL)
* a cryptographic signature of the metadata and signed attributes

The global session is unserialized and its signature is verified whenever
Rack receives a request. The cookie's value is updated whenever attributes
change. As an optimization, the signature is only recomputed when the metadata
or signed attributes have changed; insecure attributes can change "for free."

Because the security properties of attributes can vary, GlobalSession
requires all _possible_ attributes to be declared up-front in the config
file. The 'attributes' section of the config file defines the _schema_
for the global session: which attributes can be used, which can be trusted
to make authorization decisions (because they are signed), and which are
insecure and act only as "hints" about the session.

Since the session is serialized as JSON, only a limited range of object
types can be stored in it: nil, strings, numbers, lists, hashes, booleans
and other Ruby primitives.

= Detailed Information

== Global Session Domain

We refer to collection of _all_ Web application instances capable of using the
global session as the "domain." The global session domain may consist of any
number of distinct nodes, possibly hidden behind load balancers or proxies.
The nodes within the domain may all be running the same Rails application,
or they may be running different codebases that represent different parts of
a distributed application. (They may also be using app frameworks other than
Rails.)

The only constraint imposed by GlobalSession is that all nodes within the
domain must have end-user-facing URLs within the same second-level DNS domain.
This is due to limitations imposed by the HTTP cookie mechanism: for privacy
reasons, cookies will only be sent to nodes within the same domain as the
node that first created them.

For example, in my GlobalSession configuration file I might specify that my
cookie's domain is "example.com". My app nodes at app1.example.com and
app2.example.com would be part of the global session domain, but my business
partner's application at app3.partner.com could not participate.

If your app uses an Authorization header instead of cookies, the domain-name
constraint does not apply!

== Authorities and Relying Parties

A node that can create or update the global session is said to be an "authority"
(because it's trusted by other parties to make assertions about global session
state). An application that can read the global session is said to be a "relying
party." In practice, every application is a relying party, but not all of them
need to be authorities.

There is an RSA key pair associated with each authority. The authority's
public key is distribued to all relying parties, but the private key must
remain a secret to that authority (which may consist of many individual
nodes).

This system allows for significant flexibility when configuring a distributed
app's global session. There must be at least one authority, but for many apps
one authority (plus an arbitrary number of relying parties, which do not need
a key pair) will be sufficient.

In general, two systems should be part of the same authority if there is no
trust boundary between them -- that is to say, trust between the two systems
is unlimited in both directions.

Here are some reasons you might consider dividing your systems into different
authorities:
* beta/staging system vs. production system
* system hosted by a third party vs. system hosted internally
* e-commerce app vs. storefront app vs. admin app

== The Keystore

The Directory is a Ruby object that provides lookups of public and private
keys. Given an authority name (as found in a session cookie), the Directory
can find the corresponding RSA public key.

If the local system is an authority itself, #private_key_name will
return the authority name and #private_key will return an RSA private key
suitable for signing session attributes.

The Keystore implementation included with GlobalSession uses the filesystem
as the backing store for its key pairs. Its #initialize method accepts a
filesystem path that will be searched for files containing PEM-encoded public
and private keys (the same format used by OpenSSH). This simple Directory
implementation relies on the following conventions:
* Public keys have a *.pub extension.
* Private keys have a *.key extension.
* If a node is an authority, then one (and *only* one) *.key file should exist.
* The local node's authority name is inferred from the name of the private key
  file.

When used with a Rails app, GlobalSession expects to find its keystore in
config/authorities. You can use the global_session generator to create new key
pairs. Remember never to check a *.key file into a public repository!! (*.pub
files can be checked into source control and distributed freely.)

If you wish all of the systems to stop trusting an authority, simply delete
its public key from config/authorities and re-deploy your app.

The keystore needs to be told where to find its keys. This is accomplished by
setting some configuration attributes like so:

    keystore:
      public:
        - config/authorities
        - config/more_authorities
      private: config/authorities/my_private.key

The filename of keys is relevant; after stripping the *.pub or *.key extension,
the remainder of the file's basename is taken to be the authority name. For
instance, "production.pub" is the public key of the authority named "production"
and "development.key" is the private key of the authority named "development."

== The Directory

The Directory is a Ruby object that performs session management operations,
including:
* Checking whether sessions have become invalid (e.g. after sign-out)
* Creating new sessions
* Unserializing existing sessions
* Renewing sessions if they will expire
* Updating session signatures

In GlobalSession v3, Directory also presents methods for key management, but
these are all delegated to the Keystore class. In v4, the concerns of Directory
and Keystore will be fully separated.

=== Implementing Your Own Directory Provider

To replace or enhance the built-in Directory, simply create a new class that
extends Directory and put the class somewhere in your app (the lib directory
is a good choice). In the GlobalSession configuration file, specify the
class name of the directory under the 'common' section, like so:

    common:
      directory:
        class: MyCoolDirectory