Minus Seven

; z1_sub_const: Minus Seven. An arithmetic cipher using subtraction: 7 is
; subtracted from each input byte before the compare. The sub with immediate 7 is
; the tell. Reverse by adding 7 back to each stored byte. r0 = 1 on accept.
;
; (input[i]: 7) must equal enc[i], enc = { 0x44, 0x3e, 0x52, 0x30 }.
; enc[i] + 7 -> { 'K','E','Y','7' }. Length must be 4. Answer: KEY7

        len   r1
        cmp   r1, 4
        jnz   bad           ; length must be exactly 4

        mov   r7, 0
        ldb   r0, [r7]
        sub   r0, 7
        cmp   r0, 0x44
        jnz   bad
        inc   r7
        ldb   r0, [r7]
        sub   r0, 7
        cmp   r0, 0x3e
        jnz   bad
        inc   r7
        ldb   r0, [r7]
        sub   r0, 7
        cmp   r0, 0x52
        jnz   bad
        inc   r7
        ldb   r0, [r7]
        sub   r0, 7
        cmp   r0, 0x30
        jnz   bad

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