What are you using to write this application? ASP? JSP? PHP? the crypt() function (at least in PHP) accepts 2 arguments the value to be encrypted and the 'salt', both as strings. To make your encrypted number more difficult to guess you only need to choose a 'salt' value that nobody knows, like this (price=100): $encryptedString = crypt("100", "this will never be guessed"); this way, to guess the number you must know what the 'salt' is, making almost impossible to break it with brute force. Do a 'man crypt', this unix command should accept a 'salt' too... ----- Original Message ----- From: "Eduardo Dominguez" <lalo_dominguez at yahoo.com> > Even using unix crypt() function solves nothing since a simple > for() loop will get the price in less than a minute. :( > The problem is that prices are numbers, so they are easily > manipulated. How can I mix letters/numbers so that even they > guy that coded the function that checked the price cant know > it ?