SharifCTF-2016/srm

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srm (50 points)

Description

The file (RM.exe) is a Windows 32 bit executable:

MD5 105e5372413ad12b2fd78485d54b785b
SHA1 1dee31b54de559b5f1f948c865ad45bcd2e549fb
SHA256 78ba82e5136f3222b493b7cf2628b949a8f18d61cdcdf216f01d24ad9f9cff05
File PE32 executable (GUI) Intel 80386, for MS Windows

Analysis

Running the executable

When launched, the program shows a graphical interface with 2 fields: an email address and a serial number:

SharifCTF-2016-srm-gui.png

Attempting to put some incorrect email or serials provides us with following messages:

  • Your E-mail address is not valid
  • Registration failure

DialogFunc

Graph overview

It should be quite straightforward to identfy that the interesting code is located in the DialogFunc function at offset 0x401280. Below is the graph overview of the function:

SharifCTF-2016-srm-dialogfunc-graph-overview.png

Checks

Without detailing every line of code, the program first checks the email format. If the user input provided for the email field does not contain required characters (@, ., ...), the program jumps to the invalid email format section.

.text:00401364                 lea     eax, [ebp+my_email]
.text:0040136A                 push    offset a_       ; "."
.text:0040136F                 push    eax
.text:00401370                 call    check_email_valid
.text:00401375                 add     esp, 8
.text:00401378                 test    eax, eax
.text:0040137A                 jz      short loc_401346
.text:0040137C                 lea     eax, [ebp+my_email]
.text:00401382                 push    offset a_       ; "."
.text:00401387                 push    eax
.text:00401388                 call    check_email_valid
.text:0040138D                 add     esp, 8
.text:00401390                 cmp     byte ptr [eax+1], 0
.text:00401394                 jz      short loc_401346
.text:00401396                 lea     eax, [ebp+my_email]
.text:0040139C                 push    offset unk_410A38 ; '@'
.text:004013A1                 push    eax
.text:004013A2                 call    check_email_valid
.text:004013A7                 add     esp, 8
.text:004013AA                 cmp     byte ptr [eax+1], 2Eh
.text:004013AE                 jz      short loc_401346

Later, at offset 0x4013F7, the serial length is checked. If the serial is not 16 characters long, the program jumps to the bad boy.

.text:004013F7                 sub     ecx, edx
.text:004013F9                 cmp     ecx, 10h        ; len(my_serial) = 16
.text:004013FC                 jz      short loc_401407

Checking serial characters

Then, each of the serial characters is tested. If a test fails, the program jumps to the bad boy, which would have been brute force a good candidate. But let's perform a static analysis. The full section is commented below:

.text:00401407                 mov     ecx, dword ptr [ebp+my_serial_00] ;
.text:00401407                                         ; ecx = my_serial
.text:0040140D                 cmp     cl, 43h         ; my_serial[0] = 'C'
.text:00401410                 jnz     short invalid_serial
;-----------------------------------------------------------------------------
.text:00401412                 movsx   eax, [ebp+my_serial_15]
.text:00401419                 add     eax, 43h
.text:0040141C                 cmp     eax, 9Bh        ; my_serial[15] = 0x9B - 0x43 = 'X'
.text:00401421                 jnz     short invalid_serial
;-----------------------------------------------------------------------------
.text:00401423                 movsx   ecx, ch         ; ch = my_serial[1]
.text:00401426                 lea     eax, [ecx-3]
.text:00401429                 cmp     eax, 57h        ; my_serial[1] = 0x57 + 3 = 'Z'
.text:0040142C                 jnz     short invalid_serial
;-----------------------------------------------------------------------------
.text:0040142E                 movsx   eax, [ebp+my_serial_14] ; eax = my_serial[14]
.text:00401435                 add     eax, ecx        ; ecx = my_serial[1] = 'Z' (0x5A)
.text:00401437                 cmp     eax, 9Bh        ; my_serial[14] = 0x9B - 0x5a = 'A'
.text:0040143C                 jnz     short invalid_serial
;-----------------------------------------------------------------------------
.text:0040143E                 movsx   ecx, [ebp+my_serial_02] ; ecx = my_serial[2]
.text:00401445                 lea     eax, [ecx+1]
.text:00401448                 cmp     eax, 3Ah        ; my_serial[2] = 0x3A - 0x1 = '9'
.text:0040144B                 jnz     short invalid_serial
;-----------------------------------------------------------------------------
.text:0040144D                 movsx   eax, [ebp+my_serial_13] ; eax = my_serial[13]
.text:00401454                 add     eax, ecx        ; ecx = my_serial[2] = '9' (0x39)
.text:00401456                 cmp     eax, 9Bh        ; my_serial[13] = 0x9B - 0x39 = 'b'
.text:0040145B                 jnz     short invalid_serial
;-----------------------------------------------------------------------------
.text:0040145D                 cmp     [ebp+my_serial_03], 64h ; my_serial[3] = 'd'
.text:00401464                 jnz     short invalid_serial
;-----------------------------------------------------------------------------
.text:00401466                 movsx   eax, [ebp+my_serial_12]
.text:0040146D                 add     eax, 64h
.text:00401470                 cmp     eax, 9Bh        ; my_serial[12] = 0x9B - 0x64 = '7'
.text:00401475                 jnz     short invalid_serial
;-----------------------------------------------------------------------------
.text:00401477                 cmp     [ebp+my_serial_04], 6Dh ; my_serial[4] = 'm'
.text:0040147E                 jnz     invalid_serial
;-----------------------------------------------------------------------------
.text:00401484                 movsx   eax, [ebp+my_serial_11]
.text:0040148B                 add     eax, 81h
.text:00401490                 cmp     eax, 0C8h       ; my_serial[11] = 0xC8 - 0x81 = 'G'
.text:00401495                 jnz     invalid_serial
;-----------------------------------------------------------------------------
.text:0040149B                 movsx   ecx, [ebp+my_serial_05]
.text:004014A2                 lea     eax, [ecx-2Dh]
.text:004014A5                 cmp     eax, 44h        ; my_serial[5] = 0x44 + 0x2D = 'q'
.text:004014A8                 jnz     invalid_serial
;-----------------------------------------------------------------------------
.text:004014AE                 movsx   eax, [ebp+my_serial_10]
.text:004014B5                 add     eax, ecx        ; ecx = my_serial[5] = 'q' (0x71)
.text:004014B7                 cmp     eax, 0AAh       ; my_serial[10] = 0xAA - 0x71 = '9'
.text:004014BC                 jnz     invalid_serial
;-----------------------------------------------------------------------------
.text:004014C2                 cmp     [ebp+my_serial_06], 34h ; my_serial[6] = '4'
.text:004014C9                 jnz     invalid_serial
;-----------------------------------------------------------------------------
.text:004014CF                 movsx   eax, [ebp+my_serial_09]
.text:004014D6                 add     eax, 34h
.text:004014D9                 cmp     eax, 9Bh        ; my_serial[9] = 0x9B - 0x34 = 'g'
.text:004014DE                 jnz     invalid_serial
;-----------------------------------------------------------------------------
.text:004014E4                 cmp     [ebp+my_serial_07], 63h ; my_serial[7] = 'c'
.text:004014EB                 jnz     invalid_serial
;-----------------------------------------------------------------------------
.text:004014F1                 movsx   eax, [ebp+my_serial_08]
.text:004014F8                 add     eax, 63h
.text:004014FB                 cmp     eax, 9Bh        ; my_serial[8] = 0x9B - 0x63 = '8'
.text:00401500                 jnz     invalid_serial

Solution

The above code eventually leads to the following serial:

CZ9dmq4c8g9G7bAX

It is our flag!

SharifCTF-2016-srm-solution.png

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Keywords: sharif 2016 challenge reversing