Welcome to my series of writeups for n00bzunit3d 2024 capture-the-flag competition. In this post, we look at the Rev/brain challenge. The challenge gives us a text document called bf.txt and hints that this may be a “language”

bf.txt

>+++++++++++[<++++++++++>-]<[-]>++++++++[<++++++>-]<[-]>++++++++[<++++++>-]<[-]>++++++++++++++[<+++++++>-]<[-]>+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++[<++>-]<[-]>+++++++++++++++++++++++++++++++++++++++++[<+++>-]<[-]>+++++++[<+++++++>-]<[-]>+++++++++++++++++++[<+++++>-]<[-]>+++++++++++[<+++++++++>-]<[-]>+++++++++++++[<++++>-]<[-]>+++++++++++[<++++++++++>-]<[-]>+++++++++++++++++++[<+++++>-]<[-]>+++++++++++[<+++++++++>-]<[-]>++++++++[<++++++>-]<[-]>++++++++++[<++++++++++>-]<[-]>+++++++++++++++++[<+++>-]<[-]>+++++++++++++++++++[<+++++>-]<[-]>+++++++[<+++++++>-]<[-]>+++++++++++[<++++++++++>-]<[-]>+++++++++++++++++++[<+++++>-]<[-]>++++++++++++++[<+++++++>-]<[-]>+++++++++++++++++++[<++++++>-]<[-]>+++++++++++++[<++++>-]<[-]>+++++++[<+++++++>-]<[-]>+++++++++++[<++++++++++>-]<[-]>+++++++++++++++++[<++++++>-]<[-]>+++++++[<++++++>-]<[-]>+++++++++++[<+++++++++>-]<[-]>+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++[<+>-]<[-]>+++++++++++[<+++>-]<[-]>+++++++++++++++++++++++++[<+++++>-]<[-]

For the uninitiated, this is brainfuck, an esoteric programming language that serves no real purpose. The language uses a tape and pointer much like a Turing Machine to make computations. Pasting the string into an online brainfuck decoder yields no results as the above code prints one character at a time and deletes it after printing it. The solution requires us to split the string into subparts and get the output of each part individually. The [-] in the string are the pieces of code to delete the character after printing. Splitting the string at those points and then decoding gives us the real flag.