diff --git a/app/assets/javascripts/discourse/routes/application_route.js b/app/assets/javascripts/discourse/routes/application_route.js
index a63d9c51607..3e1cd343f3d 100644
--- a/app/assets/javascripts/discourse/routes/application_route.js
+++ b/app/assets/javascripts/discourse/routes/application_route.js
@@ -14,8 +14,12 @@ Discourse.ApplicationRoute = Em.Route.extend({
       if (Discourse.get("isReadOnly")) {
         bootbox.alert(I18n.t("read_only_mode.login_disabled"));
       } else {
-        Discourse.Route.showModal(this, 'login');
-        this.controllerFor('login').resetForm();
+        if(Discourse.SiteSettings.enable_sso) {
+          window.location = Discourse.getURL('/session/sso');
+        } else {
+          Discourse.Route.showModal(this, 'login');
+          this.controllerFor('login').resetForm();
+        }
       }
     },
 
diff --git a/app/assets/javascripts/discourse/templates/user/preferences.js.handlebars b/app/assets/javascripts/discourse/templates/user/preferences.js.handlebars
index 41f05183979..b9b2f886366 100644
--- a/app/assets/javascripts/discourse/templates/user/preferences.js.handlebars
+++ b/app/assets/javascripts/discourse/templates/user/preferences.js.handlebars
@@ -46,6 +46,7 @@
       </div>
     </div>
 
+    {{#unless Discourse.SiteSettings.enable_sso }}
     <div class="control-group">
       <label class="control-label">{{i18n user.password.title}}</label>
       <div class="controls">
@@ -59,6 +60,7 @@
         {{passwordProgress}}
       </div>
     </div>
+    {{/unless}}
 
     <div class="control-group">
       <label class="control-label">{{i18n user.avatar.title}}</label>
diff --git a/app/controllers/session_controller.rb b/app/controllers/session_controller.rb
index 371a7fd4dda..45245d520ef 100644
--- a/app/controllers/session_controller.rb
+++ b/app/controllers/session_controller.rb
@@ -1,12 +1,49 @@
 class SessionController < ApplicationController
 
   skip_before_filter :redirect_to_login_if_required
+  skip_before_filter :check_xhr, only: ['sso', 'sso_login']
 
   def csrf
     render json: {csrf: form_authenticity_token }
   end
 
+  def sso
+    if SiteSetting.enable_sso
+      redirect_to DiscourseSingleSignOn.generate_url
+    else
+      render nothing: true, status: 404
+    end
+  end
+
+  def sso_login
+    unless SiteSetting.enable_sso
+      render nothing: true, status: 404
+      return
+    end
+
+    sso = DiscourseSingleSignOn.parse(request.query_string)
+    if !sso.nonce_valid?
+      render text: "Timeout expired, please try logging in again.", status: 500
+      return
+    end
+
+    sso.expire_nonce!
+
+    if user = sso.lookup_or_create_user
+      log_on_user user
+      redirect_to sso.return_url || "/"
+    else
+      render text: "unable to log on user", status: 500
+    end
+  end
+
   def create
+
+    if SiteSetting.enable_sso
+      render nothing: true, status: 500
+      return
+    end
+
     params.require(:login)
     params.require(:password)
 
@@ -46,6 +83,11 @@ class SessionController < ApplicationController
   def forgot_password
     params.require(:login)
 
+    if SiteSetting.enable_sso
+      render nothing: true, status: 500
+      return
+    end
+
     user = User.find_by_username_or_email(params[:login])
     if user.present?
       email_token = user.email_tokens.create(email: user.email)
diff --git a/app/models/discourse_single_sign_on.rb b/app/models/discourse_single_sign_on.rb
new file mode 100644
index 00000000000..b3644efcc5f
--- /dev/null
+++ b/app/models/discourse_single_sign_on.rb
@@ -0,0 +1,69 @@
+require_dependency 'single_sign_on'
+class DiscourseSingleSignOn < SingleSignOn
+  def self.sso_url
+    SiteSetting.sso_url
+  end
+
+  def self.sso_secret
+    SiteSetting.sso_secret
+  end
+
+  def self.generate_url(return_url="/")
+    sso = new
+    sso.return_url = return_url
+    sso.nonce = SecureRandom.hex
+    sso.register_nonce
+    sso.to_url
+  end
+
+  def register_nonce
+    if nonce
+      $redis.setex(nonce_key, NONCE_EXPIRY_TIME, payload)
+    end
+  end
+
+  def nonce_valid?
+    nonce && $redis.get(nonce_key).present?
+  end
+
+  def expire_nonce!
+    if nonce
+      $redis.del nonce_key
+    end
+  end
+
+  def nonce_key
+    "SSO_NONCE_#{nonce}"
+  end
+
+
+  def lookup_or_create_user
+    sso_record = SingleSignOnRecord.where(external_id: external_id).first
+    if sso_record && sso_record.user
+      sso_record.last_payload = unsigned_payload
+      sso_record.save
+    else
+      user = User.where(email: Email.downcase(email)).first
+
+      user_params = {
+          email: email,
+          name:  User.suggest_name(name || username || email),
+          username: UserNameSuggester.suggest(username || name || email),
+      }
+
+      if user || user = User.create(user_params)
+        if sso_record = user.single_sign_on_record
+          sso_record.last_payload = unsigned_payload
+          sso_record.external_id = external_id
+          sso_record.save!
+        else
+          sso_record = user.create_single_sign_on_record(last_payload: unsigned_payload,
+                                            external_id: external_id)
+        end
+      end
+    end
+
+    sso_record && sso_record.user
+  end
+end
+
diff --git a/app/models/single_sign_on_record.rb b/app/models/single_sign_on_record.rb
new file mode 100644
index 00000000000..7b9e7ecffd2
--- /dev/null
+++ b/app/models/single_sign_on_record.rb
@@ -0,0 +1,3 @@
+class SingleSignOnRecord < ActiveRecord::Base
+  belongs_to :user
+end
diff --git a/app/models/user.rb b/app/models/user.rb
index 6fe3d9a35ac..d82be171581 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -37,6 +37,7 @@ class User < ActiveRecord::Base
   has_one :github_user_info, dependent: :destroy
   has_one :oauth2_user_info, dependent: :destroy
   has_one :user_stat, dependent: :destroy
+  has_one :single_sign_on_record, dependent: :destroy
   belongs_to :approved_by, class_name: 'User'
 
   has_many :group_users, dependent: :destroy
diff --git a/config/locales/server.en.yml b/config/locales/server.en.yml
index 31be4d01fe1..0a602c86269 100644
--- a/config/locales/server.en.yml
+++ b/config/locales/server.en.yml
@@ -664,6 +664,11 @@ en:
 
     min_password_length: "Minimum password length."
     block_common_passwords: "Don't allow passwords that are in the 5000 most common passwords."
+
+    enable_sso: "Enable single sign on via an external site"
+    sso_url: "URL of single sign on endpoint"
+    sso_secret: "Secret string used to encrypt/decrypt SSO information, be sure it is 10 chars or longer"
+
     enable_local_logins: "Enable traditional, local username and password authentication"
     enable_local_account_create: "Enable creating new local accounts"
     enable_google_logins: "Enable Google authentication"
diff --git a/config/routes.rb b/config/routes.rb
index 2576531637a..d42072a6e37 100644
--- a/config/routes.rb
+++ b/config/routes.rb
@@ -140,6 +140,8 @@ Discourse::Application.routes.draw do
     end
   end
 
+  get "session/sso" => "session#sso"
+  get "session/sso_login" => "session#sso_login"
   get "session/current" => "session#current"
   get "session/csrf" => "session#csrf"
   get "composer-messages" => "composer_messages#index"
diff --git a/config/site_settings.yml b/config/site_settings.yml
index 50f89ea7fd9..24101afccb2 100644
--- a/config/site_settings.yml
+++ b/config/site_settings.yml
@@ -71,6 +71,13 @@ basic:
     default: 50
 
 users:
+  enable_sso:
+    client: true
+    default: false
+  sso_url:
+    default: ''
+  sso_secret:
+    defalt: ''
   enable_local_logins:
     client: true
     default: true
diff --git a/db/migrate/20140224232913_add_single_sign_on_records.rb b/db/migrate/20140224232913_add_single_sign_on_records.rb
new file mode 100644
index 00000000000..8daa750a7f0
--- /dev/null
+++ b/db/migrate/20140224232913_add_single_sign_on_records.rb
@@ -0,0 +1,12 @@
+class AddSingleSignOnRecords < ActiveRecord::Migration
+  def change
+    create_table :single_sign_on_records do |t|
+      t.integer :user_id, null: false
+      t.string :external_id, null: false, length: 255
+      t.text :last_payload, null: false
+      t.timestamps
+    end
+
+    add_index :single_sign_on_records, :external_id, unique: true
+  end
+end
diff --git a/lib/single_sign_on.rb b/lib/single_sign_on.rb
new file mode 100644
index 00000000000..4c7c3dd1553
--- /dev/null
+++ b/lib/single_sign_on.rb
@@ -0,0 +1,70 @@
+class SingleSignOn
+  ACCESSORS = [:nonce, :return_url, :name, :username, :email, :about_me, :external_id]
+  FIXNUMS = []
+  NONCE_EXPIRY_TIME = 10.minutes
+
+  attr_accessor(*ACCESSORS)
+  attr_accessor :sso_secret, :sso_url
+
+  def self.sso_secret
+    raise RuntimeError, "sso_secret not implemented on class, be sure to set it on instance"
+  end
+
+  def self.sso_url
+    raise RuntimeError, "sso_url not implemented on class, be sure to set it on instance"
+  end
+
+  def sso_secret
+    @sso_secret || self.class.sso_secret
+  end
+
+  def sso_url
+    @sso_url || self.class.sso_url
+  end
+
+  def self.parse(payload, sso_secret = nil)
+    sso = new
+    sso.sso_secret = sso_secret if sso_secret
+
+    parsed = Rack::Utils.parse_query(payload)
+    if sso.sign(parsed["sso"]) != parsed["sig"]
+      raise RuntimeError, "Bad signature for payload"
+    end
+
+    decoded = Base64.decode64(parsed["sso"])
+    decoded_hash = Rack::Utils.parse_query(decoded)
+
+    ACCESSORS.each do |k|
+      val = decoded_hash[k.to_s]
+      val = val.to_i if FIXNUMS.include? k
+      sso.send("#{k}=", val)
+    end
+    sso
+  end
+
+  def sign(payload)
+    Digest::SHA2.hexdigest(payload + sso_secret)
+  end
+
+
+  def to_url(base_url=nil)
+    "#{base_url || sso_url}?#{payload}"
+  end
+
+  def payload
+    payload = Base64.encode64(unsigned_payload)
+    "sso=#{CGI::escape(payload)}&sig=#{sign(payload)}"
+  end
+
+  def unsigned_payload
+    payload = {}
+    ACCESSORS.each do |k|
+     next unless (val = send k)
+
+     payload[k] = val
+    end
+
+    Rack::Utils.build_query(payload)
+  end
+
+end
diff --git a/spec/controllers/session_controller_spec.rb b/spec/controllers/session_controller_spec.rb
index 4959351ce65..94d798d7586 100644
--- a/spec/controllers/session_controller_spec.rb
+++ b/spec/controllers/session_controller_spec.rb
@@ -2,6 +2,89 @@ require 'spec_helper'
 
 describe SessionController do
 
+  describe '.sso_login' do
+
+    before do
+      @sso_url = "http://somesite.com/discourse_sso"
+      @sso_secret = "shjkfdhsfkjh"
+
+      SiteSetting.stubs("enable_sso").returns(true)
+      SiteSetting.stubs("sso_url").returns(@sso_url)
+      SiteSetting.stubs("sso_secret").returns(@sso_secret)
+    end
+
+    def get_sso
+      nonce = SecureRandom.hex
+      dso = DiscourseSingleSignOn.new
+      dso.nonce = nonce
+      dso.register_nonce
+
+      sso = SingleSignOn.new
+      sso.nonce = nonce
+      sso.sso_secret = @sso_secret
+      sso
+    end
+
+    it 'can take over an account' do
+      sso = get_sso
+      user = Fabricate(:user)
+      sso.email = user.email
+      sso.external_id = "abc"
+
+      get :sso_login, Rack::Utils.parse_query(sso.payload)
+
+      response.should redirect_to('/')
+      logged_on_user = Discourse.current_user_provider.new(request.env).current_user
+      logged_on_user.email.should == user.email
+
+      logged_on_user.single_sign_on_record.external_id.should == "abc"
+    end
+
+    it 'allows you to create an account' do
+      sso = get_sso
+      sso.external_id = '666' # the number of the beast
+      sso.email = 'bob@bob.com'
+      sso.name = 'Sam Saffron'
+      sso.username = 'sam'
+
+      get :sso_login, Rack::Utils.parse_query(sso.payload)
+      response.should redirect_to('/')
+
+      logged_on_user = Discourse.current_user_provider.new(request.env).current_user
+
+      logged_on_user.email.should == 'bob@bob.com'
+      logged_on_user.name.should == 'Sam Saffron'
+      logged_on_user.username.should == 'sam'
+
+      logged_on_user.single_sign_on_record.external_id.should == "666"
+    end
+
+    it 'allows login to existing account with valid nonce' do
+
+      sso = get_sso
+      sso.external_id = '997'
+      sso.return_url = '/hello/world'
+
+      user = Fabricate(:user)
+      user.create_single_sign_on_record(external_id: '997', last_payload: '')
+
+      get :sso_login, Rack::Utils.parse_query(sso.payload)
+
+      user.single_sign_on_record.reload
+      user.single_sign_on_record.last_payload.should == sso.unsigned_payload
+
+      response.should redirect_to('/hello/world')
+      logged_on_user = Discourse.current_user_provider.new(request.env).current_user
+
+      user.id.should == logged_on_user.id
+
+      # nonce is bad now
+      get :sso_login, Rack::Utils.parse_query(sso.payload)
+      response.code.should == '500'
+
+    end
+  end
+
   describe '.create' do
 
     let(:user) { Fabricate(:user) }
diff --git a/spec/models/discourse_single_sign_on_spec.rb b/spec/models/discourse_single_sign_on_spec.rb
new file mode 100644
index 00000000000..41c844c898e
--- /dev/null
+++ b/spec/models/discourse_single_sign_on_spec.rb
@@ -0,0 +1,55 @@
+require "spec_helper"
+
+describe DiscourseSingleSignOn do
+  before do
+    @sso_url = "http://somesite.com/discourse_sso"
+    @sso_secret = "shjkfdhsfkjh"
+
+    SiteSetting.stubs("enable_sso").returns(true)
+    SiteSetting.stubs("sso_url").returns(@sso_url)
+    SiteSetting.stubs("sso_secret").returns(@sso_secret)
+  end
+
+  it "can fill in data on way back" do
+    sso = SingleSignOn.new
+    sso.sso_url = "http://meta.discorse.org/topics/111"
+    sso.sso_secret = "supersecret"
+    sso.nonce = "testing"
+    sso.email = "some@email.com"
+    sso.username = "sam"
+    sso.name = "sam saffron"
+    sso.external_id = "100"
+
+    url, payload = sso.to_url.split("?")
+    url.should == sso.sso_url
+    parsed = SingleSignOn.parse(payload, "supersecret")
+
+    parsed.nonce.should == sso.nonce
+    parsed.email.should == sso.email
+    parsed.username.should == sso.username
+    parsed.name.should == sso.name
+    parsed.external_id.should == sso.external_id
+
+  end
+
+  it "validates nonce" do
+    _ , payload = DiscourseSingleSignOn.generate_url.split("?")
+
+    sso = DiscourseSingleSignOn.parse(payload)
+    sso.nonce_valid?.should == true
+
+    sso.expire_nonce!
+
+    sso.nonce_valid?.should == false
+
+  end
+
+  it "generates a correct sso url" do
+
+    url, payload = DiscourseSingleSignOn.generate_url.split("?")
+    url.should == @sso_url
+
+    sso = DiscourseSingleSignOn.parse(payload)
+    sso.nonce.should_not be_nil
+  end
+end