> In my 6502 hacking days, the presence of an exclusive OR was a sure-fire indicator you’d either found the encryption part of the code, or some kind of sprite routine.

Correct. Most ciphers of that era should be Feistel cipher in the likes of DES/3DES, or even RC4 uses XOR too. Later AES/Rijndael, CRC and ECC (Elliptic Curve Cryptography) also make heavy use of XOR but in finite field terms which is based on modular arithmetic over GF(2), that effectively reduces to XOR (while in theory should be mod 2).