I'm starting to tackle the challenges of PicoCTF, a website/platform to test/challenge myself on Capture the Flag puzzles, learning the basics of cybersecurity and maybe make it useful for game development, web development and life in general.

So the first ones are very basic, the objective is to learn along, and for that, I'll be starting with the Playlists.

As it says in the webpage : "Playlists are collections of challenges, sometimes with readings or games, that are curated to help students learn a particular topic."

And I'll start with Low Level Binary Intro, that can be helpful with my current job (Game Development).

The first one is actually a game and a sanity check called "Obedient Cat"

Sanity Check is a simple test to check if everything is happening as expected on the basic level. In this case, I just need to download the flag in a file and read the content inside. There's a flag for this one.

After the sanity check, we reach the main challenge of this post, called "Warmed Up".

This challenge is really easy, just using the DuckDuckGo web search as "0x3D in decimal" should give us the answer. But I know that I'll need to use python in this playlist. So, I'll go further and create a Hexadecimal to Decimal converter in python.

I currently have Python 3.10.8 version. So let's begin.

The idea is to make a Hex string and each char value multiply by 16.

For example: 0xA9D = A*16*16 + 9*16 + D
That's the same as = 10*(16^2) + 9*(16^1) + 13*(16^0) = 2717

So first, I'll make a method to convert the single Hex char to a decimal number:

#List/Dictionary of numbers above 9
#To be called from a loop
associationList = [
            ['A', 10],
            ['B', 11],
            ['C', 12],
            ['D', 13],
            ['E', 14],
            ['F', 15],
        ]

def ToDecimal(hexChar):

    #First try the string/char range of numbers
    #If there's a match of the key, return value
    for item in associationList:
        if item[0] == hexChar.upper():
            return item[1]

    #Try to return a valid number, if not, return -1
    try:
        return int(hexChar)
    except:
        return 0

The Method ToDecimal(char) will make my value 'F' to be converted to 15, but also make my value '6' to be converted to 6 (as an integer)

Now I need to create a method that get the Hex value inputted from the user and convert it to decimal values and sum them.

import sys

def HexToDec():

    #Total Sum after calculations
    total = 0

    #Try to parse the user input, if cannot, show error
    try:
        #Go through all input chars backwards
        for x,char in enumerate(reversed(sys.argv[1])):

            #Multiply the respective position Hexadecimal
            #To the decimal value
            multiplier = ToDecimal(char) * pow(16,x)

            #Sum/Add to the final result
            total += multiplier

        print(total)
    except:
        print("Not valid arguments or missing argument")

HexToDec()

With this code, I can pass the Hexadecimal value through the parameters calling something like : python HexToDec.py 2e87 and it will return me 11911.

But after making many things that does not solve the original problem, let's go back and solve it:

I need to know the value of 0x3D in Decimal, and putting it in the python code it returns me...

> 61

So I just need to append the 61 to the format of CTF Flag picoCTF{61} and:

And that's it, a very super complicated solution to a simple problem.

Here's the final Python File