[Trollcat CTF] Cryptography : Write-up

Cryptography 1 - Deal Breaking [ 50 points ]

The ciphertext is made of the Caesar cipher. The Caesar online decoder allows the flag to be obtained.

https://www.dcode.fr/caesar-cipher

The decoded plain text as shown in the picture is the flag.

 

Cryptography 2 - Lost In Forest [ 50 points ]

Likewise, the ciphertext is visible. It is encrypted with base64.

https://dencode.com/

It is easy to see how it is encrypted at the site above. As with the first problem, we can authenticate the decrypted plain text with flags.

 

Cryptography 3 - Show your Dedication [ 50 points ]

 

Like the previous two problems, you can see the ciphertext. This time, the Vigenere cipher is applied.

https://www.dcode.fr/vigenere-cipher

Decode the ciphertext and you'll get the flag.

 

Cryptography 4 - Ciphers are fun [ 322 points ]

The file name is lol.png and has the following a QRcode image.

Using an online QR decoder(zxing.org/w/decode.jspx) leads to the following sites.

The password is referred to as cscodershub in a notice on the site.

However, if you enter a password, you will see a sentence saying that you solved the wrong one :(

There is a hint sentence to enter the GitHub and check it. Entering the CTF's operator GitHub, there is another related problem file. The encrypted URL is visible.

The URL with the Caesar Cipher. Let's use an online decoder to decode it.

https://www.dcode.fr/caesar-cipher

If you enter by taking a decrypted link, you can see the ciphertext as shown below.

The end "=" indicates that it is encoded in base64.

Thus, the flag can be obtained by proceeding with the decoding :)