Substitution Box

; z2_affine_sub: affine (multiply-mask) substitution cipher, per position.
; Each input byte is multiplied by 0x4D (77) and the low 8 bits are kept; the
; product is compared to a stored target at each of 7 positions. Because 77 is
; odd it has a modular inverse mod 256 (77 * 133 = 1 mod 256), so the map is a
; true reversible substitution: c = (target * 133) mod 256. Inverting the seven
; targets recovers SUBST88. r0 = 1 on accept; rule: nonzero.
;
; Maps cleanly to the 8-bit core: mul already wraps mod 256, which IS the
; "multiply and keep the low byte" the original did with mul + uxtb.
; Targets 0xF7 0x91 0xDA 0xF7 0x44 0xD8 0xD8 = (c * 0x4D) & 0xFF of "SUBST88".

        len   r2
        cmp   r2, 7
        jnz   bad

        mov   r7, 0
loop:   cmp   r7, 7
        jge   ok
        ldb   r0, [r7]
        mul   r0, 0x4D      ; multiply by 77, low byte kept (mul wraps mod 256)

        cmp   r7, 0
        jz    t0
        cmp   r7, 1
        jz    t1
        cmp   r7, 2
        jz    t2
        cmp   r7, 3
        jz    t3
        cmp   r7, 4
        jz    t4
        cmp   r7, 5
        jz    t5
        mov   r1, 0xD8      ; position 6
        jmp   chk
t0:     mov   r1, 0xF7
        jmp   chk
t1:     mov   r1, 0x91
        jmp   chk
t2:     mov   r1, 0xDA
        jmp   chk
t3:     mov   r1, 0xF7
        jmp   chk
t4:     mov   r1, 0x44
        jmp   chk
t5:     mov   r1, 0xD8
chk:    cmp   r0, r1
        jnz   bad
        inc   r7
        jmp   loop

ok:     mov   r0, 1
        ret
bad:    mov   r0, 0
        ret