ROT13 is a C program which reads a file and makes a copy in which all characters have been "rotated" by 13 positions, and all digits have been "rotated" by 5 positions.
Applying ROT13 to the transformed file returns the original. Thus, ROT13 can be used as a very simply method of encoding and decoding text files. Only an idiot would be fooled by this encoding, but nonetheless the encoding serves a number of useful purposes. And often, it's only idiots you have to fool, after all.
The computer code and data files described and made available on this web page are distributed under the GNU LGPL license.
ROT13 is available in a C version and a C++ version and a FORTRAN90 version and a MATLAB version and a Python version.
CAESAR, a C library which can apply a Caesar Shift Cipher to a string of text.
CHRPAK, a C library which works with characters and strings.
FILUM, a C library which performs various operations on files.
PIG_LATIN, a C program which makes a copy of a file, translating the text into Pig Latin.
UUDECODE, a C program which reads a UU encoded text file and translates it back to its original form.
UUENCODE, a C program which translates a file (possibly in binary format) to the UU encoding.
You can go up one level to the C source codes.