Reversing 200: be-quick-or-be-dead-1

Challenge

You find this when searching for some music, which leads you to be-quick-or-be-dead-1. Can you run it fast enough?

You can also find the executable in /problems/be-quick-or-be-dead-1_3_aeb48854203a88fb1da963f41ae06a1c.

Solution

1
2
3
4
5
6

$ ./be-quick-or-be-dead-1
Be Quick Or Be Dead 1
=====================

Calculating key...
You need a faster machine. Bye bye.

ok, sounds like we need to speed up the execution of the program. We examine it with objdump and find
this functions that wastes a lot of time:

0000000000400706 <calculate_key>:
  400706:       55                      push   %rbp
  400707:       48 89 e5                mov    %rsp,%rbp
  40070a:       c7 45 fc 3c 7e d4 6f    movl   $0x6fd47e3c,-0x4(%rbp)
  400711:       83 45 fc 01             addl   $0x1,-0x4(%rbp)
  400715:       81 7d fc 78 fc a8 df    cmpl   $0xdfa8fc78,-0x4(%rbp)
  40071c:       75 f3                   jne    400711 <calculate_key+0xb>
  40071e:       8b 45 fc                mov    -0x4(%rbp),%eax
  400721:       5d                      pop    %rbp
  400722:       c3                      retq

This function sets a value of 0x6fd47e3c, then repeatedly adds 1 to this value until
it becomes equal to 0xdfa8fc78 ..if we edit the intial value to be 0xdfa8fc77 it
will be much faster while still reaching the same final state. We can do this with
a hex editor

we save, and run the program again:

1
2
3
4
5
6
7
8

$ ./be-quick-or-be-dead-1
Be Quick Or Be Dead 1
=====================

Calculating key...
Done calculating key
Printing flag:
picoCTF{why_bother_doing_unnecessary_computation_27f28e71}

Flag