Objective

This program resides in /home/flag14/flag14. It encrypts input and writes it to standard output. An encrypted token file is also in that home directory, decrypt it :)

To do this level, log in as the level14 account with the password level14. Files for this level can be found in /home/flag14.

Getting the flag

We don't have any source code this time.

Let's navigate to the folder and see what's inside:

If we open a flag14 binary, it will notify as the -e flag is required

The binary is taking input, making some changes, and returning the result:

Let's check how it's reacting on some other characters, like numbers

At this point, we can assume that the binary is taking symbols as input, and doing something like +i, where i would be a position starting from the beginning of the input (aaa turns into abc, etc.).

We also have a token in this directory:

So, the token is 857:g67?5ABBo:BtDA?tIvLDKL{MQPSRQWW. and we need to decrypt it.

It sounds like quite basic cryptography that can be solved without any technical skills, but I decided to write a script. I would intentionally not share the source code of it, as it's the main takeaway of this level - sometimes you need to write simple stuff to test your theories, and you'd need some proficiency in the scripting languages (python, bash, etc.)

Let's run the script and test the result:

Woohoo, just five levels to go!