The use of such codes, in which knowledge of the algorithm is all that keeps the message sate, has long been known to be poor practice.Sooner or later, someone will deduce the algorithm, and all is lost. Mono alphabetic algorithms (those using a single alphabet), like the previous code, are easily broken by using the mathematics of frequency analysis. This science relies on the fact that some letters occur more often in written language than others. If you have a large enough sample of the secret code, you can apply the knowledge of these frequencies to eventually break the code. Frequency analysis is an example of crypt analysis (the analysis of cryptographic algorithms).