The practice of traitor tracing is most often implemented with computer software, and evolved from the previous method of activation codes. In this model, each box of software ships with a unique activation number on a sticker or label that can only be read after the package is opened, separate from the CD-ROM or a DVD-ROM. This number is an encoded serial number, expanded to a usually large number or string of letters, digits, and hyphens. When the software is being installed, or the first time it is run, the user is prompted to type in the license code. This code is then decoded back to its base serial number. This process reduces the number in complexity, and the additional information removed by this process is used to verify the authenticity of the serial number. If the user mistypes a single character in what is sometimes a very long code, the software will refuse to install and require the number to be retyped until it is correct.