picoCTF2018

This challenge is give we a file like this:

This is a file ELF 32-bit LSB executable, ok..try run it.

The program is request a parameter . This parameter must lenght is 16 byte . So we need input 16 character for this file. if this input correct then program will show flag. Open file with IDA pro to see souce code. Found to main function

Summary program active : Firstly -> program will check parameter . if parameter < 1 program will show “Please Provide a VALID 16 byte Product Key.” else it will call check_valid_key(). This is check_valid_key() code :

The main purpose of check_valid_key() is check characters in the range “0”->”9” and “A” -> “Z”. Next validate_key() function:

The main purpose of this function is check sum of (key[i] * index+i) (i in range 0->14) then take compare sum % 0x24 with key[15]. if equal will show flag. Ok.. I writed a python script to show Product Key

Result: