HackTheBox Hardware Challenge (Debug the Interface) Write-Up

File provided:

The file is in .sal extension

What’s .sal file?

files with .sal is exported by Saleae Logic 2 software-Software for displaying and analyzing signals captured by logic analyzer.

STEP 1: Determine the software

unzip the .sal file

hexdump the .bin file:

hexdump:

displays the contents of binary files in hexadecimal, decimal, octal or ASCII.

xxd digital-0.bin

—> header in the hexdump shows Saleae, the logic analyzer software. 

What is Logic analyzer software?

It is a test instrument for capturing and displaying multiple electronic signals in a digital circuit. Hardware plugged into devices and software to display the results. 

STEP 2: Determine the right analyzer tool

Analyzers:

https://www.seeedstudio.com/blog/2019/09/25/uart-vs-i2c-vs-spi-communication-protocols-and-uses/

Async Serial (also sometimes called UART?)

Any data that is transmitted one bit at a time. The serial analyzer is designed to only decode serial data that uses the standard start bit and stop bit. 

what’s the difference between asynchronous and synchronous serial communication?

synchronous—(sender and receiver are synced) graph occurs at regular intervals

asynchronous—start bit (0)—>message (i.e. 01001011)—>stop bit (1)

I2C (Inter-integrated-circut)

Not used for PC device comm, but with modules and sensors

SPI (Serial Peripheral Interface)

The choice would be async serial.

https://support.saleae.com/protocol-analyzers/analyzer-user-guides/using-async-serial#:~:text=Async%20serial%20communication%20is%20a,bit%20and%20stop%20bit%20format.

Input channel — the channel where the image is shown

Bit rate / Baud rate 

Bits per frame — number of bits per word. 

Stop bits (explained above in asynchronous serial)

Parity bit — serial transmitter includes an extra biota info but before stop bit to help receiver detect bit errors.

Significant bit — left to right / right to left

STEP 3: Determine the proper bit rate

Since it is showing frame errors, the bit rate is not right. 

In the async serial settings, the bit per frame is set to 8.

However, actually there are 31 bits per frame, (which means that bit rate is too slow.)

Bit rate = number of bits / second

Hover the mouse over the shortest interval —> 32.02 microseconds in one bit.

So, bit rate = 1/ (32.02 * 10^(-6)) = approximately 31,230

(in some areas, the shortest interval is 32.04, that number also works just fine.)

STEP 4: Get the flag

Only change the bit rate to 31,230 (do not change the bit per frame). 

Click terminal view.

Get the Flag!