1 files changed, 92 insertions, 0 deletions
diff --git a/app/lib/sso/from_discourse.rb b/app/lib/sso/from_discourse.rb
new file mode 100644
index 0000000..66742e2
--- /dev/null
+++ b/app/lib/sso/from_discourse.rb
@@ -0,0 +1,92 @@
+# frozen_string_literal: true
+
+module SSO
+  class FromDiscourse
+    attr_accessor :nonce, :token, :user_info, :status
+
+    class << self
+      # See config/initializers/sso.rb
+      # This is a hash:
+      # SSO::FromDiscourse.config = {
+      #   sso_url: 'https://talk.incommon.cc/session/sso_provider',
+      #   return_url: "#{API_ROOT_URL}/my/account",
+      #   sso_secret: Rails.application.credentials.sso_secret,
+      # }
+      # In config/routes.rb:
+      # ...
+      # get 'my/account/:token' => 'authentications#sso_login'
+      attr_accessor :config
+    end
+
+    def initialize(options = {})
+      @nonce     = options[:nonce] || SecureRandom.hex
+      @token     = options[:token] || SecureRandom.hex
+      @user_info = nil
+      @status    = :unauthorized
+    end
+
+    def parse(params)
+      params[:sig] =~ %r{\A[a-f0-9]{64}\z}      || raise(ArgumentError, 'HMAC invalid')
+      hmac_matches?(params[:sso], params[:sig]) || raise(ArgumentError, 'HMAC mismatch')
+      base64?(params[:sso])                     || raise(ArgumentError, 'Encoding invalid')
+
+      response = Rack::Utils.parse_query(Base64.decode64(params[:sso]))
+
+      # Of course, the nonces must match!
+      ActiveSupport::SecurityUtils
+        .fixed_length_secure_compare(response['nonce'], @nonce) ||
+        raise(ArgumentError, 'Nonce mismatch')
+
+      response.delete('nonce')
+
+      @user_info = response.symbolize_keys
+      @status    = :ok
+
+      self
+    end
+
+    def request_uri
+      format('%<url>s?sso=%<payload>s&sig=%<hmac>s',
+             url: config[:sso_url].to_s,
+             payload: Rack::Utils.escape(b64_payload),
+             hmac: mac_signature)
+    end
+
+    def success?
+      @status == :ok
+    end
+
+    private
+
+    def config
+      self.class.config
+    end
+
+    # Estimate whether given data is encoded in Base64.
+    # This is not fool-proof but good enough for our purpose.
+    def base64?(data)
+      data.is_a?(String) &&
+        (data.length % 4).zero? &&
+        data.match?(%r{\A[a-zA-Z\d+/]+={,2}\z})
+    end
+
+    def hmac_matches?(payload, signature)
+      hmac = mac_signature(payload)
+      ActiveSupport::SecurityUtils.fixed_length_secure_compare(hmac, signature)
+    end
+
+    def payload
+      format('nonce=%<n>s&return_sso_url=%<u>s',
+             n: @nonce,
+             u: "#{config[:return_url]}/#{token}")
+    end
+
+    def b64_payload
+      Base64.encode64(payload)
+    end
+
+    def mac_signature(payload = b64_payload)
+      OpenSSL::HMAC.hexdigest('SHA256', self.class.config[:sso_secret], payload)
+    end
+  end
+end