Assembly - The LODS Instruction

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In cryptography, a Caesar cipher is one of the simplest known encryption techniques. In this method, each letter in the data to be encrypted is replaced by a letter some fixed number of positions down the alphabet.

In this example, let us encrypt a data by simply replacing each alphabet in it with a shift of two alphabets, so a will be substituted by c, b with d and so on.

We use LODS to load the original string 'password' into the memory.

section .text
   global _start         ;must be declared for using gcc
_start:    ;tell linker entry point
   mov    ecx, len
   mov    esi, s1
   mov    edi, s2
loop_here:
   lodsb
   add al, 02
   stosb
   loop    loop_here          
   cld
   rep     movsb
   mov     edx,20   ;message length
   mov     ecx,s2   ;message to write
   mov     ebx,1    ;file descriptor (stdout)
   mov     eax,4    ;system call number (sys_write)
   int     0x80     ;call kernel
   mov     eax,1    ;system call number (sys_exit)
   int     0x80     ;call kernel
section .data
s1 db 'password', 0 ;source
len equ $-s1
section .bss
s2 resb 10         ;destination

When the above code is compiled and executed, it produces the following result:

rcuuyqtf


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