ROLLING KEY

; t04_rolling: rolling additive cipher, checked in a LOOP. Each input byte has
; had its own position added to a constant base: input[i] = (BASE + i) & 0xFF.
; The check subtracts the position back off and confirms every byte decodes to
; the same BASE (0x41 = 'A'). Technique: position-dependent (rolling) key + a
; counted loop over the input. r0 = 1 on accept. Accept rule: nonzero.
;
; sample "ABCDEF": 0x41 0x42 ... 0x46, each (byte: index) == 0x41.

        len   r2            ; r2 = length
        cmp   r2, 6
        jnz   bad           ; fixed-length puzzle: exactly 6 bytes

        mov   r7, 0         ; r7 = index / input offset
loop:   cmp   r7, r2
        jge   ok            ; reached the end with no mismatch -> accept
        ldb   r0, [r7]      ; r0 = input[r7]
        sub   r0, r7        ; subtract the position (the rolling key)
        cmp   r0, 0x41      ; must decode to BASE
        jnz   bad
        inc   r7
        jmp   loop

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