The Numbers PicoCTF Walkthrough – Decoding Numeric Patterns

 

The Numbers

CTF's Author: Danny

Description

The numbers... what do they mean?

 

Hint is-The flag is in the format PICOCTF{}

 

So in this challenge we have been a an image which has numbers in it and we need to decode this.

We can observe that between the numbers there are two {}.So its very clear that we will have to check the alphabets corresponding to the numbers.

But wait, should we do it manually? Of course not, we are Hackers. So we will use a python script for decoding this.

Python Script is-

def numbers_to_text(numbers):

    return ''.join(chr(int(num) + 64) for num in numbers)

 

# Numbers in the flag part

number_string = "20 8 5 14 21 13 2 5 18 19 13 1 19 15 14"

numbers = number_string.split()

 

decoded = numbers_to_text(numbers)

 

flag = f"PICOCTF{{{decoded.lower()}}}"  # .lower() if flags are usually lowercase

print("Decoded Flag:", flag)

 

Just run this script in any Python compiler and you will get the flag.

Flag-PICOCTF{thenumbersmason}

You can find this CTF on picoCTF official website-https://picoctf.org/

Hey everyone! Back with another CTF writeup, this time tackling the "InterEncDec" challenge. The description was pretty straightforward: "Can you get the real meaning from this file" with a hint that said "Engaging in various decoding processes is of utmost importance."

That hint about "various decoding processes" was key - it told me right away this wasn't going to be a simple one-step decode.

Initial Analysis

Downloaded the file and found enc_flag containing:

YidkM0JxZGtwQlRYdHFhR3g2YUhsZmF6TnFlVGwzWVROclgyeG9OakJzTURCcGZRPT0nCg==

The string had all the hallmarks of Base64 encoding - alphanumeric characters with some equals signs for padding at the end.

Step-by-Step Solution

Step 1: Base64 Decoding

First layer was definitely Base64. Running it through a decoder:

echo "YidkM0JxZGtwQlRYdHFhR3g2YUhsZmF6TnFlVGwzWVROclgyeG9OakJzTURCcGZRPT0nCg==" | base64 -d

Got back:

b'd3BqdkpBTXtqaGx6aHlfazNqeTl3YTNrX2xoNjBsMDBpfQ=='

Interesting! This looked like a Python bytes string with those b'...' markers around it. The content inside still looked like Base64 though.

Step 2: Clean Up and Decode Again

Stripped off the b' from the beginning and the ' from the end, leaving:

d3BqdkpBTXtqaGx6aHlfazNqeTl3YTNrX2xoNjBsMDBpfQ==

Running this through Base64 again:

echo "d3BqdkpBTXtqaGx6aHlfazNqeTl3YTNrX2xoNjBsMDBpfQ==" | base64 -d

Result:

wpjvJAM{jhlzhy_k3jy9wa3k_lh60l00i}

Now this was looking more like a flag format! I could see the curly braces that are typical in CTF flags, but the content inside was still scrambled.

Step 3: Caesar Cipher Recognition

The scrambled text had a pattern that screamed Caesar cipher to me. The structure looked right for "picoCTF" at the beginning if I shifted the letters around.

Let me work through the Caesar shift:

• w shifted becomes p
• p shifted becomes i
• j shifted becomes c
• v shifted becomes o

This was looking like a Caesar cipher with a specific shift value. After trying different shift values, I found that shifting by 13 (ROT13) gave me readable text.

Applying ROT13 to wpjvJAM{jhlzhy_k3jy9wa3k_lh60l00i}:

• wpjvJAM becomes picoCTF
• jhlzhy_k3jy9wa3k_lh60l00i becomes caesar_d3cr9pt3d_ea60e00b

Final Flag

After applying both decoding steps:

picoCTF{caesar_d3cr9pt3d_ea60e00b}

Key Lessons

This challenge was a great reminder that:

1. Always look for clues in the decoded output - those b'...' markers were telling me the format
2. Don't ignore classic cipher patterns - the scrambled but structured text was a dead giveaway for Caesar cipher
3. The challenge name "InterEncDec" hinted at multiple encoding/decoding steps
4. Sometimes you need to clean up formatting characters between decoding steps

Tools Used

• Base64 decoder (command line or online)
• Caesar cipher decoder/ROT13 tool

This was a solid challenge that combined modern encoding (Base64) with classic cryptography (Caesar cipher). The hint about "various decoding processes" was spot-on - we really did need two completely different approaches to crack it!

Final Flag: picoCTF{caesar_d3cr9pt3d_ea60e00b}