CRACK Solution

SUM CHECK

CRACK byte-sum checksum

In-game screenshot of SUM CHECK
In-game view
FamilyCRACK Graph0.259 DifficultyEasy Ring02 IDg12_easy5

Prerequisites

Silent Entry

Unlocks

RECTIFIERCROSSBAR

Accepted input DUMP
Techniquebyte-sum checksum Rulenonzero SampleDUMP

Walkthrough

Four bytes. The check just adds them all up and wants the low byte of the total to be 0x36. Any four chars that sum right will pass; the clean read is a real word.

Byte-sum checksum: the check compares the sum of all bytes (mod 256) to a target, so pick bytes whose total lands on it.

Hints

  • HINT 1: Four bytes. There is no per-byte transform, only one final compare against 0x36.
  • HINT 2: The loop accumulates a running sum. You need the low byte of the total to equal 0x36 (decimal 54).
  • HINT 3: D U M P sums to 310 and 310 & 0xFF is 0x36.

Reject Samples

  • DUM
  • DUMPS
  • AAAA
Verifier Listing 
; g12_easy5: byte-sum checksum. All four input bytes are added together and the
; running total (mod 256) is matched against a stored target. Technique: additive
; checksum (sum of bytes). Many inputs share a sum, so this is not an exact-match
; cipher. r0 = 1 on accept. Accept rule: nonzero. Four bytes long.
;
; target = 0x36 (310 & 0xFF).  A clean sample is "DUMP":
;   'D'(68)+'U'(85)+'M'(77)+'P'(80) = 310; 310 & 0xFF = 0x36.

        len   r2
        cmp   r2, 4
        jnz   bad           ; exactly 4 bytes

        mov   r3, 0         ; r3 = running sum
        mov   r7, 0         ; r7 = index
loop:   cmp   r7, r2
        jge   tally         ; summed all four -> compare
        ldb   r0, [r7]
        add   r3, r0        ; sum += input[r7]  (mod 256)
        inc   r7
        jmp   loop

tally:  mov   r0, r3
        cmp   r0, 0x36      ; 310 & 0xFF
        jnz   bad

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