about 9 years ago
用bochs把OS image跑起來，看到需要輸入Access Code
從MBR code可以看到它將img第二個sector載入5120 bytes到0x8000,並將前2048 bytes跟四位Access Code循環xor解碼
最後跳到0x8000執行
seg000:0000 jmp short loc_A
seg000:0000 ; ---------------------------------------------------------------------------
seg000:0002 db 42h ; B
seg000:0003 db 43h ; C
seg000:0004 db 54h ; T
seg000:0005 db 46h ; F
seg000:0006 db 4Fh ; O
seg000:0007 db 53h ; S
seg000:0008 db 4Ch ; L
seg000:0009 db 44h ; D
seg000:000A ; ---------------------------------------------------------------------------
seg000:000A
seg000:000A loc_A: ; CODE XREF: seg000:0000j
seg000:000A cli
seg000:000B mov ax, cs
seg000:000D mov ds, ax
seg000:000F mov es, ax
seg000:0011 mov ss, ax
seg000:0013 mov sp, 0FFFFh
seg000:0016 sti
seg000:0017 mov ax, 0Ch
seg000:001A push ax
seg000:001B lea ax, unk_7D34
seg000:001F push ax
seg000:0020 call sub_62
seg000:0023 mov ax, 800h
seg000:0026 mov es, ax
seg000:0028 assume es:nothing
seg000:0028 xor bx, bx
seg000:002A xor cx, cx
seg000:002C mov cl, 2
seg000:002E xor dx, dx
seg000:0030 mov dl, 80h ; 'Ç'
seg000:0032 mov ax, 20Ah
seg000:0035 int 13h ; DISK - READ SECTORS INTO MEMORY
seg000:0035 ; AL = number of sectors to read, CH = track, CL = sector
seg000:0035 ; DH = head, DL = drive, ES:BX -> buffer to fill
seg000:0035 ; Return: CF set on error, AH = status, AL = number of sectors read
seg000:0037 call sub_A0 ; 讀取Access Code，xor解碼
seg000:003A mov ax, 800h
seg000:003D push ax
seg000:003E xor ax, ax
seg000:0040
seg000:0040 loc_40: ; DATA XREF: sub_62+19r
seg000:0040 ; sub_62+29r
seg000:0040 push ax
seg000:0041 retf ; 跳轉到0x8000
seg000:0042
取出第二個sector data後，先猜測裡面有不少連續為0的區段，試著找出最常出現的重複4bytes區段。結果最常出現的區段，重複次數只有5次，用此段xor下去後還是亂碼。但是發現亂碼第一條指令向下跳轉0xA，然後這10bytes區間內有TF字樣，因此猜測這段開頭跟MBR開頭相同，有"BCTFOS"。測試後發現是正確的，Access Code剛好為"1337"
進入系統後，發現裡面實作了簡單的檔案系統，從code可以看到檔案被每26bytes切開儲存，儲存位置根據一連串的計算會亂跳。
檔案的metadata包含檔案儲存時的加密key和第一個檔案片段偏移位置，最後接上xor 0xcc後的檔名
<bochs:10> x/32bx 0xA000
[bochs]:
0x000000000000a000 <bogus+ 0>: 0x01 0x00 0x06 0x00 [0x48 0xfe] 0x01 0x0f
加密key
0x000000000000a008 <bogus+ 8>: [0x0b] 0x0f 0xad [0xad 0xad 0xad 0xad 0xad]
第一段位置 檔名"aaaaaa"
0x000000000000a010 <bogus+ 16>: 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x000000000000a018 <bogus+ 24>: 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
<bochs:11>
第一個檔案片段
<bochs:11> x/32bx 0xB160
[bochs]:
0x000000000000b160 <bogus+ 0>: 0x01 0x00 0xff 0xff 0xff 0xff [0x29 0x9e
檔案內容
0x000000000000b168 <bogus+ 8>: 0x2b 0x9c 0x2d 0x9a] 0x00 0x00 0x00 0x00
0x000000000000b170 <bogus+ 16>: 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x000000000000b178 <bogus+ 24>: 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
檔案加密方式是： 檔案byte xor 檔案offset xor (照offset奇偶輪流取key[0], key[1])
最後發現OS image中後面眾多0裡面有6個檔案片段與一個檔名為"key"的metadata，解密後得到
Dear CTFer, if you see this message, you have completely understood my OS. Congratulations!Here is what you want: BCTF{6e4636cd8bcfa93213c83f4b8314ef00}
解密code
b1 = [0x16, 0x36, 0x31, 0x23, 0x76, 0x14, 0x00, 0x13, 0x3F, 0x29, 0x74, 0x79, 0x37, 0x39, 0x7C, 0x24,
0x2D, 0x36, 0x60, 0x32, 0x23, 0x22, 0x64, 0x31, 0x22, 0x22]
b2 = [0x4E, 0x1B, 0x41, 0x56, 0x4B, 0x1F, 0x4B, 0x5C, 0x4C, 0x57, 0x1A, 0x01, 0x64, 0x64, 0x70, 0x63,
0x51, 0x1D, 0x4D, 0x1D, 0x18, 0x1C, 0x1A, 0x4E, 0xB6, 0xEB]
b3 = [0x3B, 0x69, 0x23, 0x2A, 0x3F, 0x3E, 0x13, 0x14, 0x15, 0x5D, 0x56, 0x0E, 0x1B, 0x00, 0x5A, 0x13,
0x19, 0x0F, 0x1B, 0x5F, 0x1F, 0x12, 0x0F, 0x13, 0x0C, 0x04]
b4 = [0x12, 0x02, 0x08, 0x1C, 0x4A, 0x1E, 0x06, 0x0D, 0x0B, 0x1D, 0x1F, 0x19, 0x7D, 0x7C, 0x74, 0x31,
0x7B, 0x6E, 0x34, 0x5A, 0x49, 0x35, 0x38, 0x5A, 0x71, 0x71]
b5 = [0x7B, 0x6F, 0x63, 0x77, 0x75, 0x6D, 0x67, 0x73, 0x6D, 0x6A, 0x64, 0x78, 0x29, 0x0A, 0x0D, 0x47,
0x69, 0x7F, 0x57, 0x13, 0x59, 0x42, 0x16, 0x40, 0x5C, 0x54]
bx = [0xB2, 0xB2, 0xB0, 0xB6, 0xED, 0xE6, 0xE8, 0xEA, 0xEB, 0xBA, 0xE6, 0xEC, 0xBA, 0xE9, 0xA0, 0xFB,
0xF3, 0xF0, 0xF2, 0xA2, 0xA2, 0xF5, 0xFA, 0xB6, 0x00, 0x00]
i = 0x52
j = 0x52
s = ''
l = b1 + b3 + b4 + b5 + b2 + bx
for k in range(len(l)):
if k % 2:
s += chr(l[k] ^ j ^ k)
else:
s += chr(l[k] ^ i ^ k)
print(s)
