#crypto joke

This is a favorite of mine. Nearly every programmer with any interest in crypto gets this idea and it’s usually some variant of the following. Below is a complete C program with a custom PRNG (pseudo-random number generator). It is more specifically a Linear Congruential Generator. The main purpose of the custom generator is it shows where those “random” numbers are coming from. It turns out they are not so random. You could be using a built in PRNG function such as BASIC’s RND() or C library rand() and while the specific generator used may not be the same, it can also be broken just as easily.

I use this type of PRNG because it’s easy to code and it’s pretty straight forward math with no major tricks. Some simple multiplication and addition with a special type of division. The most unfamiliar math used is a Modulus. In C it is the “%” symbol. The easiest way to define a modulus is the remainder of integer division. So if you have 7 mod(4) you have 1 remainder 3 and we throw out the 1 while keeping the 3. It makes circular alphabets or tables easy. Just the thing to make a virtual code wheel or slide. When used with an alphabet, the modulus will normally be 26.

About the program:

It was written using the Dev-C++ IDE and compiled using mingw which is a Windows port of GCC. If you use a different compiler, you may get various warnings and errors. The variables for the key and the PRNG may need to be changed to long int types for old (ancient?) 16 bit compilers. Most 32 and 64 bit compilers should be fine. This should compile and run under Linux as well. These are all simple command line or console programs using basic C library functions so that they should work for nearly anyone with a C/C++ compiler. The above system that I use is free as is GCC and Linux.

Most of the programs I post here will be some variant of the code below. To make a Caesar Cipher, you would change the key section and file read/write some then omit the entire PRNG function. A multi-purpose polyalphabetic program would be a further modification of the Caesar Cipher. Forgive the way I code some things. If I write something out, it’s still legal C and it is easier to understand. I also tend to use a compact style that can be harder to read but fits better in a post.