Proposal:
Replace all vowels with numbers, since they are redundant:
Th3s Pr4c2ss3ng pr4gr1m p2rf4rms s3mpl2 s5bst3t5t34n c3ph2r 2nc4d3ng 1nd d2c4d3ng. Th2 c3ph2r f5nct34n t1k2s 1n 3np5t str3ng 1nd c4nv2rts 21ch ch1r1ct2r f45nd 3n th2 8N str3ng (wh3ch c4nt13ns th2 1lph1b2t) t4 1 c4rr2sp4nd3ng ch1r1ct2r 3n th2 90T str3ng (wh3ch c4nt13ns sp2c31l ch1r1ct2rs 1nd n5mb2rs). Th2 d2c3ph2r f5nct34n r2v2rs2s th3s pr4c2ss.
...or literally scrap them all for zero:
Th0s Pr0c0ss0ng pr0gr0m p0rf0rms s0mpl0 s0bst0t0t00n c0ph0r 0nc0d0ng 0nd d0c0d0ng.
Th0 c0ph0r f0nct00n t0k0s 0n 0np0t str0ng 0nd c0nv0rts 00ch ch0r0ct0r f00nd 0n th0 0N str0ng (wh0ch c0nt00ns th0 0lph0b0t) t0 0 c0rr0sp0nd0ng ch0r0ct0r 0n th0 00T str0ng (wh0ch c0nt00ns sp0c00l ch0r0ct0rs 0nd n0mb0rs).
Th0 d0c0ph0r f0nct00n r0v0rs0s th0s pr0c0ss, c0nv0rt0ng 0nc0d0d ch0r0ct0rs b0ck t0 th00r 0r0g0n0l f0rm.
@ijk64 😁