Randomized testing aka fuzz testing is the process of generating random inputs and feeding them to a program or a function to see whether the program behaves correctly. The immediate issue is how to determine what the correct output is for a given input. If a reference implementation is available—that is, an implementation that is believed to be correct but in some other way does not suffice (e.g., its performance is too slow, or it is in a different language)—then the outputs of the two implementations can be compared. Otherwise, perhaps some property of the output could be checked. For example,
"not crashing" is a property of interest in user interfaces;
adding elements to a data collection then removing those elements, and ending up with an empty collection, is a property of interest in data structures; and
encrypting a string under a key then decrypting it under that key and getting back the original string is a property of interest in an encryption scheme like Enigma.
Randomized testing is an incredibly powerful technique. It is often
used in testing programs for security vulnerabilities. The
qcheck package for OCaml supports randomized testing.
We'll look at it, next, after we discuss random number generation.