Keystream

; z3_keystream: "Keystream". Each byte is XOR'd with a different key, but the
; keys aren't stored: a tiny LCG generates them. k0 = 0x6B, and after each byte
; k = (k*5 + 0x1F) & 0xFF. The masked byte is compared to a stored target;
; recover the keystream and peel it off. Technique: LCG keystream XOR. This fits
; the 8-bit core exactly (the LCG already runs mod 256). r0 = 1 on accept.
; Accept rule: nonzero.
;
; keystream = 6B 5A AD 22 71 8A 39 ; targets = 27 70 18 52 54 FF 98 ; answer LF5RKEY.
;
; The seven targets are inline immediates (no key table to read); the keystream
; is regenerated in r4 by the LCG and XOR'd into each byte before the compare.

        len   r2
        cmp   r2, 7
        jnz   bad           ; exactly 7 bytes

        mov   r4, 0x6B      ; r4 = current key (LCG state, starts at the seed)

        ; position 0
        mov   r7, 0
        ldb   r0, [r7]
        xor   r0, r4
        cmp   r0, 0x27
        jnz   bad
        mov   r5, r4
        shl   r5, 2
        add   r5, r4
        add   r5, 0x1F
        mov   r4, r5        ; k = k*5 + 0x1F  -> 0x5A

        ; position 1
        inc   r7
        ldb   r0, [r7]
        xor   r0, r4
        cmp   r0, 0x70
        jnz   bad
        mov   r5, r4
        shl   r5, 2
        add   r5, r4
        add   r5, 0x1F
        mov   r4, r5        ; -> 0xAD

        ; position 2
        inc   r7
        ldb   r0, [r7]
        xor   r0, r4
        cmp   r0, 0x18
        jnz   bad
        mov   r5, r4
        shl   r5, 2
        add   r5, r4
        add   r5, 0x1F
        mov   r4, r5        ; -> 0x22

        ; position 3
        inc   r7
        ldb   r0, [r7]
        xor   r0, r4
        cmp   r0, 0x52
        jnz   bad
        mov   r5, r4
        shl   r5, 2
        add   r5, r4
        add   r5, 0x1F
        mov   r4, r5        ; -> 0x71

        ; position 4
        inc   r7
        ldb   r0, [r7]
        xor   r0, r4
        cmp   r0, 0x54
        jnz   bad
        mov   r5, r4
        shl   r5, 2
        add   r5, r4
        add   r5, 0x1F
        mov   r4, r5        ; -> 0x8A

        ; position 5
        inc   r7
        ldb   r0, [r7]
        xor   r0, r4
        cmp   r0, 0xFF
        jnz   bad
        mov   r5, r4
        shl   r5, 2
        add   r5, r4
        add   r5, 0x1F
        mov   r4, r5        ; -> 0x39

        ; position 6 (last; no further LCG step needed)
        inc   r7
        ldb   r0, [r7]
        xor   r0, r4
        cmp   r0, 0x98
        jnz   bad

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