# Internetwache CTF 2016## Rev50 - SPIM
### Task description:*My friend keeps telling me, that real hackers speak assembly fluently. Are you a real hacker? Decode this string: "IVyN5U3X)ZUMYCs"*
### Solution:We're given some assembly code along the task description. A quick search for "SPIM" reveals that it's a MIPS32 simulator.It doesn't take a genius to figure out this code is MIPS assembly ;).
```asmUser Text Segment [00400000]..[00440000][00400000] 8fa40000  lw $4, 0($29)            ; 183: lw $a0 0($sp) # argc [00400004] 27a50004  addiu $5, $29, 4         ; 184: addiu $a1 $sp 4 # argv [00400008] 24a60004  addiu $6, $5, 4          ; 185: addiu $a2 $a1 4 # envp [0040000c] 00041080  sll $2, $4, 2            ; 186: sll $v0 $a0 2 [00400010] 00c23021  addu $6, $6, $2          ; 187: addu $a2 $a2 $v0 [00400014] 0c100009  jal 0x00400024 [main]    ; 188: jal main [00400018] 00000000  nop                      ; 189: nop [0040001c] 3402000a  ori $2, $0, 10           ; 191: li $v0 10 [00400020] 0000000c  syscall                  ; 192: syscall # syscall 10 (exit) [00400024] 3c081001  lui $8, 4097 [flag]      ; 7: la $t0, flag [00400028] 00004821  addu $9, $0, $0          ; 8: move $t1, $0 [0040002c] 3401000f  ori $1, $0, 15           ; 11: sgt $t2, $t1, 15 [00400030] 0029502a  slt $10, $1, $9          [00400034] 34010001  ori $1, $0, 1            ; 12: beq $t2, 1, exit [00400038] 102a0007  beq $1, $10, 28 [exit-0x00400038] [0040003c] 01095020  add $10, $8, $9          ; 14: add $t2, $t0, $t1 [00400040] 81440000  lb $4, 0($10)            ; 15: lb $a0, ($t2) [00400044] 00892026  xor $4, $4, $9           ; 16: xor $a0, $a0, $t1 [00400048] a1440000  sb $4, 0($10)            ; 17: sb $a0, 0($t2) [0040004c] 21290001  addi $9, $9, 1           ; 19: add $t1, $t1, 1 [00400050] 0810000b  j 0x0040002c [for]       ; 20: j for [00400054] 00082021  addu $4, $0, $8          ; 24: move $a0, $t0 [00400058] 0c100019  jal 0x00400064 [printstring]; 25: jal printstring [0040005c] 3402000a  ori $2, $0, 10           ; 26: li $v0, 10 [00400060] 0000000c  syscall                  ; 27: syscall [00400064] 34020004  ori $2, $0, 4            ; 30: li $v0, 4 [00400068] 0000000c  syscall                  ; 31: syscall [0040006c] 03e00008  jr $31                   ; 32: jr $ra ```
So first, because I don't like looking at a page of text, let's make this prettier!I extracted the hex encoded machine code:
```8fa4000027a5000424a600040004108000c230210c100009000000003402000a0000000c3c081001000048213401000f0029502a34010001102a0007010950208144000000892026a1440000212900010810000b000820210c1000193402000a0000000c340200040000000c03e00008```and pasted it into a working mips executable. Something from uClibc did the job. After rebasing the executable to start at `0x400000` and fiddling with the comments a bit, I got this:
![Magic](PIMS.png)
Much better! :)
At `label_for` we have a loop that loops from `0` to `15` and at `0x400040` we xor the `i-th` letter with `i`.
A quick one-liner in python got us the flag:```pythonIn [4]: "".join(chr(ord("IVyN5U3X)ZUMYCs"[i]) ^ i) for i in range(15))Out[4]: 'IW{M1P5_!S_FUN}'```
### Flag:`IW{M1P5_!S_FUN}`