CRACK Solution

XOR Door

CRACK XOR cipher

In-game screenshot of XOR Door
In-game view
FamilyCRACK Graph0.080 DifficultyTutorial Ring01 IDt03_xor_intro

Prerequisites

DELAY LINEPATCH BAY

Unlocks

SHORT PULSESTALE SUM

Accepted input exit
TechniqueXOR cipher Rulenonzero Sampleexit

Walkthrough

Recover the accepted input below, then submit it to the CRACK verifier.

Verifier Listing 
; t03_xor_intro: XOR door. Each input byte is XOR'd with the single key 0x20
; (the ASCII case bit) and compared to a stored uppercase byte. XOR is its own
; inverse: XOR each stored byte by 0x20 to recover the answer. Technique: single
; key XOR. r0 = 1 on accept. Accept rule: nonzero.
;
; stored (uppercase) = 0x45 0x58 0x49 0x54 -> each ^0x20 -> "exit".

        len   r1
        cmp   r1, 4
        jnz   bad           ; exactly 4 bytes

        mov   r7, 0
        ldb   r0, [r7]
        xor   r0, 0x20
        cmp   r0, 0x45      ; 'E'
        jnz   bad
        inc   r7
        ldb   r0, [r7]
        xor   r0, 0x20
        cmp   r0, 0x58      ; 'X'
        jnz   bad
        inc   r7
        ldb   r0, [r7]
        xor   r0, 0x20
        cmp   r0, 0x49      ; 'I'
        jnz   bad
        inc   r7
        ldb   r0, [r7]
        xor   r0, 0x20
        cmp   r0, 0x54      ; 'T'
        jnz   bad

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