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My Reverse Engineering notes

InstructionsExplanation
dbDefine bytes (expressions)
CALLCall a function
ADDPerform an addition
SUBPerform a subtraction
CMPCompares two numeric data fields
JNZJump non zero

Reverse engineering my own programs

To start learning assembly and reverse engineering I decided to make my own simple C programs and observe the program flow through the disassemblers.

Observing basic functions

#include <stdio.h>

int variableA = 10;
int variableB = 5;
int variableC = 2;
int result;
int result_sub;

int sum()
{
    result = variableA + variableB;
    printf("The result for sum is %d \n ",result);
    return 0;
}

int sub()
{
    result_sub = variableA - variableC;
    printf("The result for sub is %d ",result_sub);
    return result;
}

int main() {

   printf("Hello, World! \n");
   sum();
   sub();
   return 0;
}

Dissemble in IDA Pro

main function:

sum function:

sub function:

Password checker

#include <stdio.h>
int main()
{
    int input;
    int password = 5678;
    printf("Please enter the password : ");
    scanf("%d",&input);
    if(input == password)
        printf("Correct password");
    else
        printf("Incorrect password");

    return 0;
}
  • 55d = hard coded variable
  • 574 = user input
  • 586 = comparison with EAX (user input) and local_c (password)
  • 589 = Jump non zero , If & else

Hello World

#include <stdio.h>

int main() {

    printf("Hello World");
    return 0;

}

Addition

#include <stdio.h>

int number1 = 5;
int number2 = 3;

int main() {
    int add = number1 + number2;
    printf("%d",add);
    return 0;

}

User Input

#include <stdio.h>

char string;

int main() {

    printf("Please enter a string : ");
    scanf("%s",&string);
    printf("Your string is %s" ,&string);
    return 0;


}