CAESAR is a FORTRAN90 library which can apply a Caesar shift cipher to a string of text.
The computer code and data files described and made available on this web page are distributed under the GNU LGPL license.
CAESAR is available in a C version and a C++ version and a FORTRAN90 version and a MATLAB version and a Python version.
CHRPAK, a FORTRAN90 library which works with characters and strings.
FILUM, a FORTRAN90 library which can work with information in text files.
ROT13, a FORTRAN90 library which can encipher a string using the ROT13 cipher for letters, and the ROT5 cipher for digits.
You can go up one level to the FORTRAN90 source codes.