pico2024 C3

Summary

The video script discusses a custom cyclical cipher for encrypting and decrypting messages. It explains the process of downloading cipher text and encoder, and submitting the flag 'pcctf{example}' if the flag is 'example'. The script describes the decryption method involving character lookup and computing differences between character indices. The video concludes with a Python script that can decrypt the cipher, suggesting the flag might be 'PicoCTF{adlib}'.

Takeaways

• 🔐 The script is about a custom cyclical cipher used for encryption.
• 📄 The cipher text and encoder can be downloaded for use.
• 🏁 The flag format is 'pcctf{example}' if the flag was 'example'.
• 🔑 The encryption scheme relies on a secret encoder.
• 🔡 The cipher text consists of a mix of uppercase and lowercase letters.
• 🔄 The encoder involves a lookup process and character index manipulation.
• 🤖 A decoder script is created to reverse the encoding process.
• 🔄 The decoder computes the difference between characters and uses lookup tables.
• 🔢 The process involves subtracting and then adding back values from lookup tables.
• 💻 The final step is to run a Python program that takes itself as input to reveal the flag.
• 🏴‍☠️ The flag is revealed to be 'PicoCTF{adlib}' after running the Python program.

Q & A

What is the custom cyclical Cipher mentioned in the transcript?

The custom cyclical Cipher is a cryptographic algorithm that involves encoding text using a specific method, which in this case, involves downloading a cipher text and an encoder.

How is the flag supposed to be submitted according to the transcript?

The flag should be submitted in a specific format, such as 'pcctf{example}' if the flag was 'example'.

What is the significance of the flag format in the context of the cipher?

The flag format is important because it indicates the expected structure of the output after decryption, but the actual decryption process does not depend on the encoder being secret.

What is the nature of the cipher text mentioned in the transcript?

The cipher text consists of a collection of uppercase and lowercase letters.

What is the role of the encoder in the decryption process described in the transcript?

The encoder is used to encode the text before it is encrypted. The transcript suggests that a decoder script is needed to reverse the process.

What does the process of looking up and subtracting in the cipher involve?

The process involves finding the index of a character in a lookup table and then subtracting it from the index of the previous character to compute the difference.

Why is it necessary to swap certain elements in the decoder script according to the transcript?

Swapping elements in the decoder script is necessary to reverse the encoding process, which involves reversing the operations performed by the encoder.

What is the purpose of creating a decoder script as described in the transcript?

The purpose of creating a decoder script is to decrypt the encoded text back to its original form.

How does the difference between characters in the cipher relate to the decrypted output?

The difference between characters, when computed and added back, helps to retrieve the original character from the cipher text.

What is the significance of the 'self input' mentioned in the transcript?

The 'self input' suggests that the Python program requires its own code as input to function correctly, which is an unusual requirement and part of the challenge.

What is the final flag obtained from the Python program as per the transcript?

The final flag obtained from the Python program is 'Pico CTF{adlib}' after following the described decryption process.