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assembly nasm x86-64

How to compare two strings in nasm assembler with test or cmp correctly (x64)

发布于 2020-06-28 13:45:52

I am trying to make a simple password check in nasm, but my program never jumps to correct_func, but the password is right. Am I doing something wrong?

section .data
    msg1: db "Enter your password: ", 0
    len1 equ $-msg1
    correct: db "Correct!", 0x0a
    lenc equ $-correct
    wrong: db "Wrong!", 0x0a
    lenw equ $-wrong
    passwd: db "ASD123"
    input

section .text
global _start

correct_func:
    mov rax, 0x1
    mov rdi, 1
    mov rsi, correct
    mov rdx, lenc
    syscall
    mov rax, 60
    mov rdi, 0
    syscall
    ret

_start:
    mov rax, 0x1
    mov rdi, 1
    mov rsi, msg1
    mov rdx, len1
    syscall
    mov rax, 0x0
    mov rdi, 0
    mov rsi, input
    mov rdx, 6
    syscall
    mov rax, passwd
    mov rbx, input
    cmp rax, rbx
    je correct_func
    mov rax, 0x1
    mov rdi, 1
    mov rsi, wrong
    mov rdx, lenw
    syscall
    mov rax, 60
    mov rdi, 0
    syscall
Questioner
BitFriends
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fuz 2020-04-14 01:09

To compare two strings compare each character of one string with the corresponding character of the other. If the two strings have the same length and match in all their characters, they are equal. I'll assume that you checked that the two have the same length.

You can do this with an explicit loop:

        mov ecx, 0            ; index and loop counter
.loop:  mov al, [passwd+rcx]  ; load a character from passwd
        cmp al, [input+rcx]   ; is it equal to the same character in the input?
        jne .incorrect        ; if not, the password is incorrect
        inc ecx               ; advance index
        cmp ecx, 6            ; reached the end of the string?
        jle .loop             ; loop until we do
                              ; if this line is reached, the password was correct

Alternatively, you can use the cmpsb instruction:

        mov rsi, passwd       ; load the address of passwd
        mov rdi, input        ; load the address of input
        mov ecx, 6            ; load the string length
        repe cmpsb            ; compare the strings
        je correct_func       ; jump to correct_func if they are equal
                              ; if this line is reached, the password was wrong

Refer to the instruction set reference for details.

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