# File lib/active_merchant/billing/integrations/world_pay/helper.rb, line 62
          def encrypt(secret, fields = [:amount, :currency, :account, :order])
            signature_fields = fields.collect{ |field| mappings[field] }
            add_field('signatureFields', signature_fields.join(':'))

            field_values = fields.collect{ |field| form_fields[mappings[field]] }
            signature    = "#{secret}:#{field_values.join(':')}"            
            add_field('signature', Digest::MD5.hexdigest(signature))
          end