I wrote this simple substitution-permutation network (SPN) as part of my Junior Independent Study project on block ciphers. Most modern block ciphers, notably Advanced Encryption Standard (AES), are designed as a substitution-permutation network. This implementation serves to illustrate the components of an SPN without confusing beginners in cryptography and theoretical mathematics. The debug version (named SPN-1-0-debug.cpp) of the code prints out intermediate values during the encryption process, and is used in the test for string inputs. The other version (named SPN-1-0.cpp) does not print out intermediate values and is used in the test for image inputs. The code in both versions are almost identical except for a few modifications in memory allocations due to larger storage required to encrypt images.

Instructions: If you want to run the test on images, you need to install OpenCV at http://opencv.org per the instructions there. Otherwise, comment out the testSPN_image() and all the code to load OpenCV libraries and namespace in SPN-1-0-test.cpp. In the terminal, go to the folder containing all the source code:

• To compile the source code, type: $ ./build.sh
• To run the binary file, type: $ ./SPN

Follow the documentation and example usage in SPN-1-0-test.cpp on how to use the SPN class.

Future Improvements:

• More secure key schedule, S-boxes, and permutation tables using Galois Fields theory.
• More efficient implementation of calculations.