Google CTF 2016 - For2
We are given a PCAPNG file with a bunch of USB packets. By scanning through the PCAP, I noticed that there appears to be a large amount of
URB_INTERRUPT packets after some initial configuration and setup. On a hunch, I immediately suspected either a USB keyboard or mouse due to the amount of data and the fact that all incoming packets were created from interrupts (i.e some kinda of slow I/O device driven by a human).
While looking at the packets, I initially assumed this was a keyboard as it would be straight forward to hide a flag in keyboard data. I tried the obvious choice and googled for some Python that already solved this problem and I came across this keyboard PCAP parser. I noticed that the offsets used in the file were not close to the size of the packets I was seeing, so I did some more research on USB HID devices and I came across this great page on USB mice packets.
The packet structure described here matched what I was seeing in the PCAP data. I confirmed this by looked for a USB descriptor packet from the device. I found the right packet at number 84 (below).
No. | Time | Source | Dest | Protocol | Length | Info ----+----------+--------+------+----------+--------+------------------------------ 84 | 6.505211 | 1.3.0 | host | USB | 46 | GET DESCRIPTOR Response DEVICE
The device descriptor decoding showed that this was definitely a mouse.
DEVICE DESCRIPTOR bLength: 18 bDescriptorType: 0x01 (DEVICE) bcdUSB: 0x0200 bDeviceClass: Device (0x00) bDeviceSubClass: 0 bDeviceProtocol: 0 (Use class code info from Interface Descriptors) bMaxPacketSize0: 8 idVendor: Logitech, Inc. (0x046d) idProduct: M90/M100 Optical Mouse (0xc05a) bcdDevice: 0x6300 iManufacturer: 1 iProduct: 2 iSerialNumber: 0 bNumConfigurations: 1
At this point, I modified the keyboard Python to extract out the mouse data which is in the binary format of
BYTE[Key State] BYTE[Signed X Offset] BYTE[Signed Y Offset]. With all of the mouse movements extracted, I needed a way to visualize this data. My first thought was to draw the mouse movements on a canvas with a pen (or turtle). I have experience with Python’s Image Library (PIL) so I immediately reached for that.
Using the X, Y, and key state, I came up with the code below to draw and display an image:
Here is what it produced: