Symmetric encryption algorithm, inspired by one of the challenges in this (2025) year's PatriotCTF (https://competitivecyber.club/patriotctf/), and also inspired by this year's silly "6-7" viral meme.
The CTF challenge presented an encryption script, a ciphertext file created by that script, and one clue consisting of a reference to a ternary operator used in the esoteric Malbolge programming language. Competitors had to study (reverse-engineer) the script and figure out how to decrypt the file. It was my first ever successful flag earned solo in a competitive CTF, and I was so happy to finally contribute points to my teammates in one of our competitions!
Afterward, I continued to experiment with what I learned about arithmetic with ternary (base 3) numerals. After hearing friends laugh about viral growth of "six seven" slang interjections spoken in popular culture, I thought it would be fun to make a septenary (base 7) encryption algorithm as a block cipher, encrypting blocks of six septenary digits at a time. "Doot Doot 6-7!"
Right now (2025 Dec) the encryption key is hardcoded as three 7x7 matrices derived from some interesting isograms. It would be more interesting to come up with some way to apply an encryption key or password in an algorithm that deterministically scrambles the matrices' entries before they are used to encrypt and decrypt.
Regardless, this cipher is just for fun. Don't use this cipher for anything you care to keep confidential! I'm certain that it's susceptible to easy breaking through elementary cryptanalysis. Also, it uses a lot of division operations, which are expensive. Use well established and time-tested symmetric encryption algorithms that use fewer (or carefully chosen, or no) divisions. They tend to be faster, safer, and preferable.