Note: I did not solve this challenge during the CTF, but my teammate Volker did.

For this challenge, we were given a zip containing an encoded syslog (sysloc.enc), and an auth.json file:

    "manufact": [
        "Apple Inc.",
        "Linux 5.10.0-kali8-amd64 xhci-hcd",
    "pid": [
    "prod": [
        "USB2.0 HD UVC WebCam",
        "USB2.0 Hub",
        "xHCI Host Controller"
    "serial": [
    "vid": [

The goal of the challenge is to find the serial number of the rubber ducky which was connected to the machine for which we have the syslog.

$ file syslog.enc
syslog.enc: data

The file doesn’t look like any well known format, so we can assume (quite safely) that the encryption is a simple XOR. We’ll use the known plaintext XOR tool from Didier Stevens.

A good candidate for the plaintext is Linux 5.10.0-kali8-amd64 xhci-hcd, which will surely appear.

$ python2 -d <(echo 'Linux 5.10.0-kali8-amd64 xhci-hcd') syslog.enc > syslog
$ head -n 5 syslog
Jun  7 22:53:25 kali usbmuxd[117991]: [22:53:25.287][3] Connecting to new device on location 0x10004 as ID 1
Jun  7 22:53:25 kali usbmuxd[117991]: [22:53:25.287][3] Connected to v2.0 device 1 on location 0x10004 with serial number 4b8d611f609abfc5471ae40d08d0c8b785eab9d4
Jun  7 22:53:27 kali ModemManager[830]: <info>  [base-manager] couldn't check support for device '/sys/devices/pci0000:00/0000:00:08.1/0000:03:00.3/usb1/1-2': not supported by any plugin
Jun  8 00:16:16 kali lynis[144261]: - usbmuxd.service: [ UNSAFE ]
Jun  8 00:16:22 kali lynis[118776]: - Checking usb-storage driver (modprobe config) [ NON DESACTIVÉ ]

That seems like a valid syslog! After looking for a short while, we find:

Apr 12 20:21:25 kali kernel: [    1.830063] usb 1-2: Product: Fake_KeyBoard
Apr 12 20:21:25 kali kernel: [    1.830064] usb 1-2: Manufacturer: APT43.
Apr 12 20:21:25 kali kernel: [    1.830065] usb 1-2: SerialNumber: 854be9ee57ef47ce74e73904998d61c8846e9239

The flag is CYBERTF{854be9ee57ef47ce74e73904998d61c8846e9239}.

You can view the sources on github or read other writeups.