[strings + variables] Help with simple program i am writing

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thor6136

Joined: 19 Dec 2011
Posts: 2

thor6136 19 Dec 2011, 22:35

Hey guys i am writing a program where it asks you for a password and if password is right then it says accepted and if it is wrong then it says denied, so here is the code: (dont bash me i am beginner Smile

)

Code:


org 100h
        mov bx, 3                                ; used for describing how many inserted characters were equal to our password
        mov cx, 3                                ; used for describing how many loops will it take, since our password is 3 characters long it takes 3 turns
        mov ah, 09h                            
        mov dx, insert_password
        int 21h

        i=0


w_password:                                      ; this part compares if character imputed by the user is equal to character in password
        mov si, password+i                   
        mov ah, 08h
        int 21h
        mov [di], al
        mov al, [si]
        cmp al, [di]
        je t
        jne f

t:
        i=i+1                                        ; this is the part that it doesn't work, here i is not increased it always stays 0
        dec bx
        call cx_register

f:
        call cx_register                           ; since character was wrong it doesn't need to do anything

cx_register:                                        ; this part decreases cx (cx value = lenght of password) 
        dec cx
        jz compare
        jnz w_password


compare:                                           ; at the end it compares if bx is 0 (if it is all characters were equal)
        cmp bx, 0
        je true_m
        jne false_m

true_m:
        mov ah, 09h
        mov dx, true
        int 21h

        mov ah, 08h
        int 21h
        int 20h

false_m:
        mov ah, 09h
        mov dx, false
        int 21h

        mov ah, 08h
        int 21h
        int 20h


insert_password: db 'Insert password: ',24h
false: db 'false',24h
true: db 'true',24h
password: db 'asd'

So i hope you understand, my english is not really good and i am sorry. So basically i want program to check each turn if character that was inputed is the same as in password and if it is than password that was inputed by the user is accepted and if not it was denied. But i am stuck with the part where it has to check for the 2nd and 3rd character in password. It only checks the first one, if you look at the code i is not increased, it stays 0.

19 Dec 2011, 22:35

JohnFound

Joined: 16 Jun 2003
Posts: 3499
Location: Bulgaria

JohnFound 19 Dec 2011, 22:45

use "inc si" and "inc di" instead of i=i+1.
You don't need "i" here at all. There is no variables in this meaning in assembly language. "si" and "di" are your variables here.

19 Dec 2011, 22:45

DOS386

Joined: 08 Dec 2006
Posts: 1905

DOS386 13 Jan 2012, 08:45

Your "i" is a variable, but it exists at compile time only ... and has no effect ! Shocked

13 Jan 2012, 08:45

typedef

Joined: 25 Jul 2010
Posts: 2909
Location: 0x77760000

typedef 13 Jan 2012, 15:42

try this simple snippet

Code:

password db 'pass1234'
input       db 8 dup(0)

[...]    ;<-- Get the password here

mov  cx,   8  ;<---- strlen(password)
lea    esi,  [password]
lea    edi,  [input]
repe  compsb

jne    not_equal

[...]    ; valid password

13 Jan 2012, 15:42

thor6136

Joined: 19 Dec 2011
Posts: 2

thor6136 24 Feb 2012, 22:16

typedef wrote:

try this simple snippet

Code:

password db 'pass1234'
input       db 8 dup(0)

[...]    ;<-- Get the password here

mov  cx,   8  ;<---- strlen(password)
lea    esi,  [password]
lea    edi,  [input]
repe  compsb

jne    not_equal

[...]    ; valid password

Can you tell me what exactly is dup(0), and lea esi, edi

24 Feb 2012, 22:16

AsmGuru62

Joined: 28 Jan 2004
Posts: 1708
Location: Toronto, Canada

AsmGuru62 25 Feb 2012, 00:53

dup means duplicate.
Value in brackets is a value to be used to initialize memory.
The following code:

Code:

input db 8 dup(0)

is equivalent to this:

Code:

input db 0,0,0,0,0,0,0,0

LEA instruction is used to Load Executive Address into a register:

Code:

LEA ESI,[password]

In this ^^^ case ESI will have an address of a 1st byte in 'password'.

25 Feb 2012, 00:53

DOS386

Joined: 08 Dec 2006
Posts: 1905

DOS386 03 Mar 2012, 08:24

> LEA ESI,[password]
> In this ^^^ case ESI will have an address of a 1st byte in 'password'

The ZERO'th Byte Wink

LEA makes sense if your buffer is local ... so password translates into something like "EBP+8" for example ... otherwise use MOV:

Code:

MOV ESI, password ; constant
MOV ESI, EAX      ; from other register

03 Mar 2012, 08:24

shutdownall

Joined: 02 Apr 2010
Posts: 517
Location: Munich

shutdownall 03 Mar 2012, 17:54

Anyway it is not good to store a password in plain text in the program (which maybe watched with a hex editor easily). Better to just store a hash value and compare with a hash value from the input. MD5 is an often used hash mechanism.

03 Mar 2012, 17:54

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