Problem

My friend keeps telling me, that real hackers speak assembly fluently. Are you a real hacker? Decode this string: “IVyN5U3X)ZUMYCs”

Attachment: https://ctf.internetwache.org/files/rev50.zip

Solved by 235 teams

Solution

We unpack the file and get some MIPS assembly:

  1. User Text Segment [00400000]..[00440000]
  2. [00400000] 8fa40000  lw $4, 0($29)            ; 183: lw $a0 0($sp) # argc 
  3. [00400004] 27a50004  addiu $5, $29, 4         ; 184: addiu $a1 $sp 4 # argv 
  4. [00400008] 24a60004  addiu $6, $5, 4          ; 185: addiu $a2 $a1 4 # envp 
  5. [0040000c] 00041080  sll $2, $4, 2            ; 186: sll $v0 $a0 2 
  6. [00400010] 00c23021  addu $6, $6, $2          ; 187: addu $a2 $a2 $v0 
  7. [00400014] 0c100009  jal 0x00400024 [main]    ; 188: jal main 
  8. [00400018] 00000000  nop                      ; 189: nop 
  9. [0040001c] 3402000a  ori $2, $0, 10           ; 191: li $v0 10 
  10. [00400020] 0000000c  syscall                  ; 192: syscall # syscall 10 (exit) 
  11. [00400024] 3c081001  lui $8, 4097 [flag]      ; 7: la $t0, flag 
  12. [00400028] 00004821  addu $9, $0, $0          ; 8: move $t1, $0 
  13. [0040002c] 3401000f  ori $1, $0, 15           ; 11: sgt $t2, $t1, 15 
  14. [00400030] 0029502a  slt $10, $1, $9          
  15. [00400034] 34010001  ori $1, $0, 1            ; 12: beq $t2, 1, exit 
  16. [00400038] 102a0007  beq $1, $10, 28 [exit-0x00400038] 
  17. [0040003c] 01095020  add $10, $8, $9          ; 14: add $t2, $t0, $t1 
  18. [00400040] 81440000  lb $4, 0($10)            ; 15: lb $a0, ($t2) 
  19. [00400044] 00892026  xor $4, $4, $9           ; 16: xor $a0, $a0, $t1 
  20. [00400048] a1440000  sb $4, 0($10)            ; 17: sb $a0, 0($t2) 
  21. [0040004c] 21290001  addi $9, $9, 1           ; 19: add $t1, $t1, 1 
  22. [00400050] 0810000b  j 0x0040002c [for]       ; 20: j for 
  23. [00400054] 00082021  addu $4, $0, $8          ; 24: move $a0, $t0 
  24. [00400058] 0c100019  jal 0x00400064 [printstring]; 25: jal printstring 
  25. [0040005c] 3402000a  ori $2, $0, 10           ; 26: li $v0, 10 
  26. [00400060] 0000000c  syscall                  ; 27: syscall 
  27. [00400064] 34020004  ori $2, $0, 4            ; 30: li $v0, 4 
  28. [00400068] 0000000c  syscall                  ; 31: syscall 
  29. [0040006c] 03e00008  jr $31                   ; 32: jr $ra

I never did any MIPS work before, but armed with MIPS Instruction Reference, I started working on it.

It didn’t take long to get a hang of what was going on, the program does a very simple xor, it just xors each character in string with its position (0, 1, 2 …). So we reverse that in python:

  1. # Short, ain't it?
  2. s = 'IVyN5U3X)ZUMYCs'
  3. print ''.join([chr(i ^ ord(s[i])) for i in range(0, len(s))])

Flag: IW{M1P5_!S_FUN}

Sidenote: The whole MIPS assembly-reading could have been avoided, if after noticing the xor in the assembly, we assumed that the result will start with IW{ (as it did) and then we ran IVy xor IW{, we would have gotten 012 and we could figure out the algorithm. But reading assembly is rewarding in itself, so I’m not complaining :)