I was going to say "but RC4 and AES were published well after the 6502's heyday," but NESes were completely rocking it in '87 (and I'm told 65XX cores were used as the basis for several hard drive controllers of the era.) Alas, the closest I ever came to encryption on a (less than 32-bit system) was lucifer on an IBM channel controller in the forever-ago and debugging RC5 on an 8085.
I dunno. The 6502 has been a $2 part for a long time but needs RAM and some glue logic, for a similar price you can get an AVR-8 [1] or ESP-32 [2] and get some RAM and GPIO.
[1] faster, more registers than the IBM 360, << 64k RAM
[2] much faster, 32bit, >> 64k RAM
65C02s are $8 now. That didn't stop me buying one when I was stuck at home during COVID. And a 6809 too.
But forget AVR. Yeah, for a buck or so the ATTiny85 was my go-to small MCU five years ago, and the $5 328 for bigger tasks.
But for the last three years both can be replaced by a 48 MHz 32 bit RISC-V CH32V003 for $0.10 for the 8 pin package (like ATTiny85, and also no external components needed) and $0.20 for the 20 pin package with basically the same number of GPIOs as the 328. At 2k RAM and 16K flash it's the same RAM and a little less flash than the ATMega328 -- but not as much as you'd think as RISC-V handles 16 and 32 bit values and pointers sooo much better.
And now you have the CH32V002/4/5/6 with enhanced CPU and more RAM and/or flash -- up to 8K rAM and 62K flash on the 006 -- and still for around the $0.10-$0.20 price
There are uC versions like the W65C134S: https://www.westerndesigncenter.com/wdc/w65c134s-chip.php
I'm told 65XX cores were used as the basis for several hard drive controllers of the era
Western Design Center is still (apparently) making a profit at least in part licensing 6502 core IP for embedded stuff. There's probably a 6502 buried and unrecognized in all sorts of low-cost control applications laying around you.
RC5 on an 8085
Oof. Well played.