In this guide, we will discuss LODS Instruction in Assembly Programming Language. 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:
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