# File lib/aws/core/signers/version_2.rb, line 32
        def sign_request req
          req.add_param('AWSAccessKeyId', credentials.access_key_id)
          if token = credentials.session_token
            req.add_param("SecurityToken", token)
          end
          req.add_param('SignatureVersion', '2')
          req.add_param('SignatureMethod', 'HmacSHA256')
          req.add_param('Signature', signature(req))
          req.body = req.url_encoded_params
          req
        end