Cryptographers often seek to make and distribute software that performs some action, but prevents users from carrying out that same action without having access to the software. Usually, this goal goes hand-in-hand with a desire to control the distribution and use of mass-produced commercial software -- but it sometimes pertains to other security features of the software.

In general, this goal is impossible to accomplish. If a determined attacker has access to your software, he inherently has the ability to determine what the software does. If there is a key, or an encryption algorithm, buried within the software (perhaps in obfuscated form), reverse engineering can always reveal that "secret" key/algorithm. It may well be that it is not worth an attacker's effort to find your software's secret, but cryptography never gives software the ability to perform non-replicatable magic.