DeconstruCT.F 2023 - Very Basic

Description

Category: Cryptography

Sometimes, we need to oscillate To and fro? things become tough when interwoven together.

Key.txt

Hint

Interwoven cipher? Omg I knew it.

Resolution

Note: I got very lucky to solve this challenge. Even if there was some hints, it was too guessy in my opinion especially for an easy challenge.

1. Vigenere Cipher

We have an base32 encoded ciphertext in the Key.txt:

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

The hint from the description was interwoven but I had no clue about its meaning.

With the real hint (Interwoven cipher? Omg I knew it.), I searched on Google Interwoven cipher and I found out that it was about Vigenere cipher.

Ok, now we know that we need to decrypt Vigenere cipher but we are still missing the key.

Since the file name was Key.txt, I decided to use KEY as the key and it was indeed the right key (I had no way to verify that this key was the right one until the end).

I used dcode to decipher:

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

2. Base32 and Base64

After completing the challenge, I finally understand the meaning of we need to oscillate To and fro.

When we decode from base32, we can get either a base64 text or a base32 text (if there are lowercase letters then it’s base64).

So we decode with base64 and base32 with CyberChef until we find the flag:

We get the flag: DSC{V17_P0L1CY}.

Note

The exact order was:

Base32 -> Base32 -> Base64 -> Base32 -> Base64 -> Base32 -> Base64 -> Base32 -> Base32 -> Base64.

So if you used the wrong key during the previous step you will have different order and you need to decode from scratch.

I was very lucky to use the right key from the start.