Assembly Programming Assembly LODS Instruction - Assembly Programming

What is Assembly LODS Instruction?

In cryptography, a Caesar cipher is one of the only known encryption techniques. on this method, each letter in the facts to be encrypted is changed 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 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