# NcN CTF 2k13: Canada (Base - 1200 pts)


# gunzip howtobasic.gz
# file howtobasic
howtobasic: ELF 32-bit LSB executable, Intel 80386, version 1 (GNU/Linux), statically linked, for GNU/Linux 2.6.26, BuildID[sha1]=0x1a8f284f3d67ad66c7510bc5353556e8a01db18b, stripped
# chmod +x howtobasic
# gdb --quiet ./howtobasic
(gdb) set disassembly-flavor intel
(gdb) info file
        Entry point: 0x80481c0
(gdb) run
Starting program: /root/ctf/howtobasic
Facebook CTF
Enter flag: ^C
Program received signal SIGINT, Interrupt.
0xf7ffd430 in __kernel_vsyscall ()
(gdb) finish
1234567890
0x080577a2 in ?? ()
(gdb) finish
0x0806d9a6 in ?? ()
(gdb) finish
0x08049b26 in ?? ()
(gdb) finish
0x0804932d in ?? ()
(gdb) finish
0x08049435 in ?? ()
(gdb) finish
0x0804906b in ?? ()
(gdb) finish
0x080483b6 in ?? ()
(gdb) finish
Sorry, that is not correct.
(gdb) b *0x080483b6
(gdb) run
Starting program: /root/ctf/howtobasic
Facebook CTF
Enter flag: 1234567890

(gdb) x/2i 0x080483b6
=> 0x80483b6:   test   eax,eax
   0x80483b8:   jne    0x80483f5
(gdb) x/s $eax
0x80d5298:      "1234567890\n"
(gdb) b *0x80483f5
(gdb) continue
(gdb) x/3i 0x080483f5
=> 0x80483f5:   push   eax
   0x80483f6:   xor    eax,eax
   0x80483f8:   je     0x80483fd
(gdb) b *0x80483fd
(gdb) continue
(gdb) x/2i 0x080483fd
=> 0x80483fd:   pop    eax
   0x80483fe:   jmp    0x8048486
(gdb) b *0x8048486
(gdb) continue
(gdb) x/4i 0x08048486
=> 0x8048486:   mov    eax,DWORD PTR [esp+0x14]
   0x804848a:   sub    eax,0x2
   0x804848d:   cmp    eax,DWORD PTR [esp+0x1c]
   0x8048491:   ja     0x8048403
(gdb) x/xw $esp+0x14
0xffffd604:     0x00000042
(gdb) x/xw $esp+0x1c
0xffffd60c:     0x00000000
(gdb) b *0x8048403
(gdb) continue
(gdb) x/20i 0x08048403
=> 0x8048403:   mov    eax,DWORD PTR [esp+0x1c] // eax = 0x0
   0x8048407:   and    eax,0x7   // eax = 0x0 
   0x804840a:   movzx  eax,BYTE PTR [eax+0x80d108c] // eax = 0x4d [0x80d108c] = "MOVEFAST"
   0x8048411:   not    eax    // eax = 0xffffffb2
   0x8048413:   mov    BYTE PTR [esp+0x1b],al  // [esp+0x1b] = 0x000000b2
   0x8048417:   mov    eax,DWORD PTR [esp+0x1c] // eax = 0x0 
   0x804841b:   mov    edx,DWORD PTR [esp+0x10] // edx = 0x080d5298 [0x080d5298] = "1234567890\n"
   0x804841f:   add    eax,edx   // eax = 0x080d5298
   0x8048421:   movzx  eax,BYTE PTR [eax]  // eax = 0x31
   0x8048424:   not    eax    // eax = 0xffffffce
   0x8048426:   mov    BYTE PTR [esp+0x1a],al  // [esp+0x1a] = 0x0000b2ce
   0x804842a:   mov    edx,DWORD PTR ds:0x80d1088 // edx = 0x80b21e8 [0x80b21e8] = "{\177gtsyjg,xorut21.zb \"t65t~0'\"#5d,-g|t$f1(.dq\177te2.}o##%`lyy`$ x2f"
   0x8048430:   mov    eax,DWORD PTR [esp+0x1c] // eax = 0x0
   0x8048434:   add    eax,edx   // eax = 0x80b21e8
   0x8048436:   movzx  edx,BYTE PTR [eax]  // edx = 0x7b
   0x8048439:   movzx  eax,BYTE PTR [esp+0x1a] // eax = 0xce
   0x804843e:   movzx  ecx,BYTE PTR [esp+0x1b] // ecx = 0xb2
   0x8048443:   xor    eax,ecx   // eax = 0x7c
   0x8048445:   cmp    dl,al   // dl != al
   0x8048447:   je     0x8048481
(gdb) x/5i 0x8048481
   0x8048481:   add    DWORD PTR [esp+0x1c],0x1
   0x8048486:   mov    eax,DWORD PTR [esp+0x14]
   0x804848a:   sub    eax,0x2
   0x804848d:   cmp    eax,DWORD PTR [esp+0x1c]
   0x8048491:   ja     0x8048403
(gdb) quit
# cat canada.py
#!/usr/bin/python

constraint = "{\177gtsyjg,xorut21.zb \"t65t~0'\"#5d,-g|t$f1(.dq\177te2.}o##%`lyy`$ x2f"
xorkey = "MOVEFAST"
key = ""

for i in range(len(constraint)):
        c = ord(constraint[i])
        x = ord(xorkey[i%8])
        #~k ^ ~x = c
        k = ~(c^~x)
        key += hex(k)[2:].decode("hex")
print key
# ./canada.py
60115893a79735aec54ed5ea91fbdbf0ab192e5eea24956fc29fed38466af9a2
# ./howtobasic
Facebook CTF
Enter flag: 60115893a79735aec54ed5ea91fbdbf0ab192e5eea24956fc29fed38466af9a2
Winner! Post your flag.

1 comment:

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