AngstromCTF_2018: Rev2

Category: Reverse Points: 80 Description:

It's time for Rev2! This one is pretty similar to the first: once you get the inputs right to the program, you will get the flag. You don't need the shell server for this one, but the binary can be found at /problems/rev2/ if you would like to run it there.

Write-up

This challenge involves reading some assembly,

│           0x0804853b      e890feffff     call sym.imp.__isoc99_scanf
│           0x08048540      83c410         add esp, 0x10
│           0x08048543      8b45e4         mov eax, dword [local_1ch]
│           0x08048546      3dd7110000     cmp eax, 0x11d7

This part above looks for a number equals to 0x11d7 or 4567.

│      │    0x0804858b      e840feffff     call sym.imp.__isoc99_scanf
│      │    0x08048590      83c410         add esp, 0x10
│      │    0x08048593      8b45e8         mov eax, dword [local_18h]
│      │    0x08048596      83f863         cmp eax, 0x63               ; 'c' ; 99
│      │┌─< 0x08048599      7f22           jg 0x80485bd
│      ││   0x0804859b      8b45e8         mov eax, dword [local_18h]
│      ││   0x0804859e      83f809         cmp eax, 9                  ; 9
│     ┌───< 0x080485a1      7e1a           jle 0x80485bd
│     │││   0x080485a3      8b45ec         mov eax, dword [local_14h]
│     │││   0x080485a6      83f863         cmp eax, 0x63               ; 'c' ; 99
│    ┌────< 0x080485a9      7f12           jg 0x80485bd
│    ││││   0x080485ab      8b45ec         mov eax, dword [local_14h]
│    ││││   0x080485ae      83f809         cmp eax, 9                  ; 9
│   ┌─────< 0x080485b1      7e0a           jle 0x80485bd
│   │││││   0x080485b3      8b55e8         mov edx, dword [local_18h]
│   │││││   0x080485b6      8b45ec         mov eax, dword [local_14h]
│   │││││   0x080485b9      39c2           cmp edx, eax

The next part aboves checks that both numbers on part 2, is smaller than or equals to 99 but bigger than 9.

│      ││   0x080485da      0fafc2         imul eax, edx
│      ││   0x080485dd      8945f0         mov dword [local_10h], eax
│      ││   0x080485e0      817df0670d00.  cmp dword [local_10h], 0xd67 ; [0xd67:4]=-1 ; 3431

This part checks that our lovely second input has to multiple together to form 0xd67 or 3431. This factors out nicely to 47 and 73. Therefore, by now running it with all of our gathered inputs, we get

$ ./rev2_32 
Welcome to Rev2! You'll probably want to use a dissassembler or gdb.
Level 1: What number am I thinking of: 4567
Level 2: Which two two-digit numbers will solve this level. Enter the two numbers separated by a single space (num1 should be the lesser of the two): 47 73
Congrats, you passed Rev2! The flag is: actf{4567_47_73}

Therefore, the flag is actf{4567_47_73}.

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