## pcapin (forensics, 150p, 41 solves)
> We have extracted a pcap file from a network where attackers were present. We know they were using some kind of file transfer protocol on TCP port 7179. We're not sure what file or files were transferred and we need you to investigate. We do not believe any strong cryptography was employed.> > Hint: The file you are looking for is a png> pcapin_73c7fb6024b5e6eec22f5a7dcf2f5d82.pcap
### PL Version`for ENG version scroll down`
Dostajemy [plik .pcap](pcapin.pcap). Jest w nim tylko jeden interesujący stream tcp, więc wyciągamy od razu z niego dane (tylko wysyłane z serwera do klienta, chociaż wygląda na to że klient wysyła dane tym samym protokołem) do [osobnego pliku](rawdata.bin).
W tym momencie rozpoczyna się analiza protokołu. Np. na pierwszy rzut oka widać powtarzający się fragment `00440000073200010000000000` w pierwszej części, a później wariacje na temat `00D423C60732001C00010000`.
Oszczędzimy może analizy krok po kroku (bo była długa i burzliwa), ale kluczowe było zauważenie że dane dzielą się na pakiety, i pierwszy word każdego pakietu to długość tego pakietu. Wtedy możemy podzielić odpowiedź na pakiety, i widzimy dodatkowo że odpowiedź kończy sie zawsze bajtami `END`.
Z tą wiedzą dekodujemy wszystkie pakiety po kolei, używamy trochę domyślności i dochodzimy do takiej oto struktury:
    struct packet {        uint16_t length;        uint16_t hash;        uint16_t magic1;        uint16_t conn_id;        uint16_t seq_id;        uint16_t unk2;        uint8_t raw[10000];    };
Napisaliśmy mały tool do dumpowania zawartości poszczególnych pakietów z tej struktury:
    msm@andromeda /cygdrive/c/Users/msm/Code/RE/CTF/2015-09-16 csaw/forensics_200_pcapin    $ ./a.exe    PACKET 0     - size: 68 bytes     - hash:  0     - magic1: 732     - conn_id:  1     - seq_id:  0     - unk2:  0     - calculated hash: f9e9     - rawdata:        00 25 f2 a9 8d 96 8a 8c 84 9c 87 8d c7 89 8d 9f        e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9        e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9        e9 f9 e9 f9 e9 f9 60 00    PACKET 1     - size: 68 bytes     - hash:  0     - magic1: 732     - conn_id:  1     - seq_id:  0     - unk2:  0     - calculated hash: f9e9     - rawdata:        00 00 28 a9 9a 98 84 89 85 9c c7 8d 80 9f e9 f9        e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9        e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9        e9 f9 e9 f9 e9 f9 60 00    PACKET 2     - size: 68 bytes     - hash:  0     - magic1: 732     - conn_id:  1     - seq_id:  0     - unk2:  0     - calculated hash: f9e9     - rawdata:        00 00 15 c1 86 8c 9d 9f 80 95 8c d7 8d 98 9d f9        e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9        e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9        e9 f9 e9 f9 e9 f9 60 00    (...)    PACKET 9     - size: 212 bytes     - hash: 4567     - magic1: 732     - conn_id: 1c     - seq_id:  0     - unk2:  0     - calculated hash: 3f50     - rawdata:        d9 6f 1e 78 5d 35 4a 35 50 3f 50 32 19 77 14 6d        50 3f 51 78 50 3f 50 28 58 39 50 3f 50 a7 e0 b2        78 3f 50 3f 56 5d 1b 78 14 3f af 3f af 3f af 9f        ed 98 c3 3f 50 2a 26 76 14 7e 04 47 cc d2 cd 48        08 6b 87 89 90 40 63 cb 51 79 0a 2b 4b 2d 33 ef        40 ce f9 7e ff 9d 32 1e 4a 34 16 9c 96 2d 1b b3        19 77 14 90 76 5d 28 7b d9 1a 01 cb 4a 5d d9 22        73 09 7c 67 ff 5d 47 b6 75 9f 72 fe 30 28 75 1d        70 ed 6b 7c 83 a6 a7 38 63 d8 5e 05 98 3f d3 da        0d 41 8f 08 8f d8 cc 06 ab a3 e5 08 2b 92 e3 c9        0a d4 3c 7a da 97 78 3a e5 9f e4 93 dc 2a 6b 48        e2 5f b3 79 a2 35 5b 86 46 23 1c e4 e7 e1 fa f2        75 d4 d4 d4 21 4e 68 b2     (...)
Co się rzuca w oczy bardzo - powtarzający sie padding na początku (e9f9). Dalej, wiemy że dane to plik .png - pierwszy pakiet z danymi to packet 9 (domyślamy się tego, bo jest w odpowiedzi na drugi request od klienta, oraz ma troche inną strukture niż peirwsze pakiety - przypominające headery jakieś).
Więc, kierowani intuicją, xorujemy pierwsze bajty pakietu 9 z nagłówkiem .png:
    >>> ' d9 6f 1e 78 5d 35 4a 35 50 3f 50 32 19 77 14 6d'.replace(' ', '')    'd96f1e785d354a35503f50321977146d'    >>> ' d9 6f 1e 78 5d 35 4a 35 50 3f 50 32 19 77 14 6d'.replace(' ', '').decode('hex')    '\xd9o\x1ex]5J5P?P2\x19w\x14m'    >>> raw = ' d9 6f 1e 78 5d 35 4a 35 50 3f 50 32 19 77 14 6d'.replace(' ', '').decode('hex')    >>> png = '89504E470D0A1A0A0000000D49484452'.decode('hex')    >>> def xor(a, b):    ...     return ''.join(chr(ord(ac) ^ ord(bc)) for ac, bc in zip(a, b))    ...    >>> xor(raw, png)    'P?P?P?P?P?P?P?P?'    >>> xor(raw, png).encode('hex')    '503f503f503f503f503f503f503f503f'
W tym momencie możemy uścisnąć sobie dłonie - praktycznie rozwiązaliśmy zadanie. Pozostaje pytanie, skąd bierze się liczba z którą xorujemy - nie jest to stała, niestety. Ale kierowani znowu intuicją, domyślamy się że 'padding' z pierwszych pakietów to xorowane null bajty (długość się zgadza).
Ale od czego zależy ta liczba? W pakiecie mamy ciekawą daną z której jeszcze nie skorzystaliśmy - oznaczoną w strukturze jako 'hash'. Kiedy ta liczba jest równa 0, xorujemy dane z e9f9. Kiedy ta liczba jest równa 4567 xorujemy z 503f. W jaki sposób może być wyprowadzany wynikowy hash? Zgadnijmy...:
    >>> hex(0x503f + 0x4567)    '0x95a6'
Jest to proste dodawanie wartości w polu 'hash' oraz magicznej stałej. Zaiste silne szyfrowanie ;).
Pozostaje dopracować nasz parser, i mamy wynik:
    msm@andromeda /cygdrive/c/Users/msm/Code/RE/CTF/2015-09-16 csaw/forensics_200_pcapin    $ ./a.exe -tc    PACKET 0     - size: 68 bytes     - hash:  0     - magic1: 732     - conn_id:  1     - seq_id:  0     - unk2:  0     - calculated hash: f9e9     - rawdata:        e9 dc 1b 50 64 6f 63 75 6d 65 6e 74 2e 70 64 66   ...Pdocument.pdf        00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00   ................        00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00   ................        00 00 00 00 00 00 89 f9                           ........    PACKET 1     - size: 68 bytes     - hash:  0     - magic1: 732     - conn_id:  1     - seq_id:  0     - unk2:  0     - calculated hash: f9e9     - rawdata:        e9 f9 c1 50 73 61 6d 70 6c 65 2e 74 69 66 00 00   ...Psample.tif..        00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00   ................        00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00   ................        00 00 00 00 00 00 89 f9                           ........    (...)    PACKET 9     - size: 212 bytes     - hash: 4567     - magic1: 732     - conn_id: 1c     - seq_id:  0     - unk2:  0     - calculated hash: 3f50     - rawdata:        89 50 4e 47 0d 0a 1a 0a 00 00 00 0d 49 48 44 52   .PNG........IHDR        00 00 01 47 00 00 00 17 08 06 00 00 00 98 b0 8d   ...G............        28 00 00 00 06 62 4b 47 44 00 ff 00 ff 00 ff a0   (....bKGD.......        bd a7 93 00 00 15 76 49 44 41 54 78 9c ed 9d 77   ......vIDATx...w        58 54 d7 b6 c0 7f 33 f4 01 46 5a 14 1b 12 63 d0   XT....3..FZ...c.        10 f1 a9 41 af a2 62 21 1a 0b 46 a3 c6 12 4b 8c   ...A..b!..F...K.        49 48 44 af 26 62 78 44 89 25 51 f4 1a 62 89 1d   IHD.&bxD.%Q..b..        23 36 2c 58 af 62 17 89 25 a0 22 c1 60 17 25 22   #6,X.b..%.".`.%"        20 d2 3b 43 d3 99 f7 07 33 e7 0e 3a c8 00 83 e5    .;C....3..:....        5d 7e df 37 df e7 9c 39 fb 9c b5 37 7b ad b3 f6   ]~.7...9...7{...        5a eb 6c 45 8a a8 28 05 b5 a0 b4 ac 8c 15 3b 77   Z.lE..(.......;w        b2 60 e3 46 f2 0a 0b b9 16 1c 4c db b7 de aa cd   .`.F......L.....        25 eb 84 eb 71 71 38 8d                           %...qq8.
Jak widać wszystkie dane z png zostały pięknie przeczytane. Pozostaje zapisać pakiety od 9 do końca w pliku i odczytać znajdujacy się tam [obrazek png](pcapin.png).
Zadanie rozwiązane.
Źródła całego dekodera (nie wiem po co napisanego, skoro prawdopodobnie żaden program na świecie nie używa takiego formatu do komunikacji, ale lubimy pisać parsery) znajdują się w pliku [parser.c](parser.c)
### ENG Version
We get [.pcap file](pcapin.pcap). There is only a single interesting tcp stream inside so we extract data from it (only those sent from server to the client, however it seems that the client was sending data using the same protocol) to [separate file](rawdata.bin).
Now the protocol analysis starts. For example at the first glance we can notice a repeating data pattern `00440000073200010000000000` in the first part, and then some variations of `00D423C60732001C00010000`.
We will spare the reader the step-by-step analysis (since it was long and turbulent), but the key point was to notice that the data are paritioned into packets and first word of each one of them is the packet length. This was we can split the server response into packets and we can also see that a response ends with `END`.
With this knowledge we decode all packets in the sequence and using some guessing we end up with a structure as follows:
    struct packet {        uint16_t length;        uint16_t hash;        uint16_t magic1;        uint16_t conn_id;        uint16_t seq_id;        uint16_t unk2;        uint8_t raw[10000];    };
We wrote a small tool for dumping the contents of packets with given structure:	
    msm@andromeda /cygdrive/c/Users/msm/Code/RE/CTF/2015-09-16 csaw/forensics_200_pcapin    $ ./a.exe    PACKET 0     - size: 68 bytes     - hash:  0     - magic1: 732     - conn_id:  1     - seq_id:  0     - unk2:  0     - calculated hash: f9e9     - rawdata:        00 25 f2 a9 8d 96 8a 8c 84 9c 87 8d c7 89 8d 9f        e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9        e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9        e9 f9 e9 f9 e9 f9 60 00    PACKET 1     - size: 68 bytes     - hash:  0     - magic1: 732     - conn_id:  1     - seq_id:  0     - unk2:  0     - calculated hash: f9e9     - rawdata:        00 00 28 a9 9a 98 84 89 85 9c c7 8d 80 9f e9 f9        e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9        e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9        e9 f9 e9 f9 e9 f9 60 00    PACKET 2     - size: 68 bytes     - hash:  0     - magic1: 732     - conn_id:  1     - seq_id:  0     - unk2:  0     - calculated hash: f9e9     - rawdata:        00 00 15 c1 86 8c 9d 9f 80 95 8c d7 8d 98 9d f9        e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9        e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9 e9 f9        e9 f9 e9 f9 e9 f9 60 00    (...)    PACKET 9     - size: 212 bytes     - hash: 4567     - magic1: 732     - conn_id: 1c     - seq_id:  0     - unk2:  0     - calculated hash: 3f50     - rawdata:        d9 6f 1e 78 5d 35 4a 35 50 3f 50 32 19 77 14 6d        50 3f 51 78 50 3f 50 28 58 39 50 3f 50 a7 e0 b2        78 3f 50 3f 56 5d 1b 78 14 3f af 3f af 3f af 9f        ed 98 c3 3f 50 2a 26 76 14 7e 04 47 cc d2 cd 48        08 6b 87 89 90 40 63 cb 51 79 0a 2b 4b 2d 33 ef        40 ce f9 7e ff 9d 32 1e 4a 34 16 9c 96 2d 1b b3        19 77 14 90 76 5d 28 7b d9 1a 01 cb 4a 5d d9 22        73 09 7c 67 ff 5d 47 b6 75 9f 72 fe 30 28 75 1d        70 ed 6b 7c 83 a6 a7 38 63 d8 5e 05 98 3f d3 da        0d 41 8f 08 8f d8 cc 06 ab a3 e5 08 2b 92 e3 c9        0a d4 3c 7a da 97 78 3a e5 9f e4 93 dc 2a 6b 48        e2 5f b3 79 a2 35 5b 86 46 23 1c e4 e7 e1 fa f2        75 d4 d4 d4 21 4e 68 b2     (...)
What is very evident, there is a repeating padding in the beginning (e9f9). Next, we know that contents are a .png file - first data packet is the one with number 9 (we figure this out based on the fact that it comes after second request from user and has a slightly different structure than the rest of the packets - it looks as it contains some header)
Guided by intuition we xor first bytes from packet 9 with .png file header:
    >>> ' d9 6f 1e 78 5d 35 4a 35 50 3f 50 32 19 77 14 6d'.replace(' ', '')    'd96f1e785d354a35503f50321977146d'    >>> ' d9 6f 1e 78 5d 35 4a 35 50 3f 50 32 19 77 14 6d'.replace(' ', '').decode('hex')    '\xd9o\x1ex]5J5P?P2\x19w\x14m'    >>> raw = ' d9 6f 1e 78 5d 35 4a 35 50 3f 50 32 19 77 14 6d'.replace(' ', '').decode('hex')    >>> png = '89504E470D0A1A0A0000000D49484452'.decode('hex')    >>> def xor(a, b):    ...     return ''.join(chr(ord(ac) ^ ord(bc)) for ac, bc in zip(a, b))    ...    >>> xor(raw, png)    'P?P?P?P?P?P?P?P?'    >>> xor(raw, png).encode('hex')    '503f503f503f503f503f503f503f503f'
At this point we can all shake hands - we basically solved the task. The only question is where does the number we use for XOR comes from - it's not a constant unfortunately. But still guided by intuition we guess that the 'padding' from the first packets are xored null bytes (length is matching).
But what does this number depends on? In the packet there is still data we haven't used - we marked it as 'hash' in the structure. When this number is equal to 0 we xor data with e9f9. When it's equal to 4567 we xor with 503f. How can we devise the hash value? Let's guess...:
    >>> hex(0x503f + 0x4567)    '0x95a6'
It's a simple addition of the 'hash' field value and a magic constant. A strong cipher indeed ;).
The only thing left is to work a little bit on the parser and we have the result:
    msm@andromeda /cygdrive/c/Users/msm/Code/RE/CTF/2015-09-16 csaw/forensics_200_pcapin    $ ./a.exe -tc    PACKET 0     - size: 68 bytes     - hash:  0     - magic1: 732     - conn_id:  1     - seq_id:  0     - unk2:  0     - calculated hash: f9e9     - rawdata:        e9 dc 1b 50 64 6f 63 75 6d 65 6e 74 2e 70 64 66   ...Pdocument.pdf        00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00   ................        00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00   ................        00 00 00 00 00 00 89 f9                           ........    PACKET 1     - size: 68 bytes     - hash:  0     - magic1: 732     - conn_id:  1     - seq_id:  0     - unk2:  0     - calculated hash: f9e9     - rawdata:        e9 f9 c1 50 73 61 6d 70 6c 65 2e 74 69 66 00 00   ...Psample.tif..        00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00   ................        00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00   ................        00 00 00 00 00 00 89 f9                           ........    (...)    PACKET 9     - size: 212 bytes     - hash: 4567     - magic1: 732     - conn_id: 1c     - seq_id:  0     - unk2:  0     - calculated hash: 3f50     - rawdata:        89 50 4e 47 0d 0a 1a 0a 00 00 00 0d 49 48 44 52   .PNG........IHDR        00 00 01 47 00 00 00 17 08 06 00 00 00 98 b0 8d   ...G............        28 00 00 00 06 62 4b 47 44 00 ff 00 ff 00 ff a0   (....bKGD.......        bd a7 93 00 00 15 76 49 44 41 54 78 9c ed 9d 77   ......vIDATx...w        58 54 d7 b6 c0 7f 33 f4 01 46 5a 14 1b 12 63 d0   XT....3..FZ...c.        10 f1 a9 41 af a2 62 21 1a 0b 46 a3 c6 12 4b 8c   ...A..b!..F...K.        49 48 44 af 26 62 78 44 89 25 51 f4 1a 62 89 1d   IHD.&bxD.%Q..b..        23 36 2c 58 af 62 17 89 25 a0 22 c1 60 17 25 22   #6,X.b..%.".`.%"        20 d2 3b 43 d3 99 f7 07 33 e7 0e 3a c8 00 83 e5    .;C....3..:....        5d 7e df 37 df e7 9c 39 fb 9c b5 37 7b ad b3 f6   ]~.7...9...7{...        5a eb 6c 45 8a a8 28 05 b5 a0 b4 ac 8c 15 3b 77   Z.lE..(.......;w        b2 60 e3 46 f2 0a 0b b9 16 1c 4c db b7 de aa cd   .`.F......L.....        25 eb 84 eb 71 71 38 8d                           %...qq8.
As we can see all the data from png files were read correctly. Now we only need to save the packets from 9 until end of fie and read the result [png picture](pcapin.png).
Task solved.
Source codes of the decoder (we're not really sure why we actually spent time on this, since most likely there is no software that uses this format, but some of us like to write parsers) are in the [parser.c](parser.c)