interencdec

• Description: Can you get the real meaning from this file.
• Difficulty: Easy

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🔎 Solution

Upon inspecting the contents of the file, we quickly recognize that it is a Base64-encoded string.

┌──(kali㉿kali)-[~/Desktop]
└─$ cat enc_flag                        
YidkM0JxZGtwQlRYdHFhR3g2YUhsZmF6TnFlVGwzWVROclgyZzBOMm8yYXpZNWZRPT0nCg==

Decoding it once reveals yet another Base64-encoded string.

┌──(kali㉿kali)-[~/Desktop]
└─$ cat enc_flag | base64 -d            
b'd3BqdkpBTXtqaGx6aHlfazNqeTl3YTNrX2g0N2o2azY5fQ=='

After a second round of decoding, we finally obtain another string. However, the output doesn't immediately resemble the expected flag format.

┌──(kali㉿kali)-[~/Desktop]
└─$ echo "d3BqdkpBTXtqaGx6aHlfazNqeTl3YTNrX2g0N2o2azY5fQ==" | base64 -d
wpjvJAM{jhlzhy_k3jy9wa3k_h47j6k69}

Looking closely, we notice that the structure and length of the string suggest it might be the flag — just obfuscated. For example, the standard flag format picoCTF{...} appears as something like wpjvJAM{...}. The number of characters remains unchanged, but the letters have been shifted.

This behavior points to the use of a Caesar Cipher, a classical encryption technique. In a Caesar Cipher, each letter in the plaintext is shifted a fixed number of positions down (or up) the alphabet. For example, shifting the alphabet by 3 turns A into D, B into E, and so on. It's a simple yet historically significant method of encryption.

Using this tool, we test various shift values. Eventually, with a shift of 7 characters, the scrambled text decodes properly and reveals the original flag.

🚩Flag

picoCTF{caesar_d3cr9pt3d_a47c6d69}