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.

Outlines

00:00

🔐 Custom Cyclical Cipher Decryption

The paragraph discusses a custom cyclical cipher and provides instructions on how to download the cipher text and encoder. It mentions that the flag format is 'pcctf{example}' and emphasizes that while flag formats are usually not secret, this cipher's encoder is. The text is a mix of uppercase and lowercase letters. The process involves looping through characters, finding their index, and using two lookup tables to compute the difference between the current and previous characters. The aim is to create a decoder script by swapping elements in the lookup tables. The paragraph also suggests that the difference between characters can be used to decrypt the text, and a Python script is proposed to achieve this. The final flag is deduced to be 'picctf{adlib}'.

Keywords

💡Cyclical Cipher

A Cyclical Cipher is a type of encryption technique where the key is based on a cycle or pattern. In the context of the video, it is used to create a custom cipher for encoding and decoding messages. The script mentions downloading a 'cipher text' and an 'encoder', which implies that the cyclical nature of the cipher is integral to the encryption process being discussed.

💡Flag

In cybersecurity, a 'flag' often refers to a specific piece of information that is the goal of a challenge, such as in a Capture The Flag (CTF) competition. The script mentions enclosing the flag for submission, indicating that the video is likely about solving a CTF challenge where the flag is the secret that needs to be discovered and submitted.

💡Encoder

An encoder is a device or program that converts data into a format that can be transmitted or stored. In the video, the encoder is mentioned in relation to the custom cyclical cipher, suggesting that it plays a role in the encryption process. The script describes the process of using the encoder to encode messages, which is a key part of the decryption puzzle.

💡Decryption

Decryption is the process of converting encrypted data back into its original form. The video's theme revolves around decryption, as the script discusses creating a decoder script to reverse the encryption process. The mention of 'decryption' is central to understanding the video's objective, which is to break the cipher and retrieve the original message.

💡Lookup

A lookup, in the context of the video, refers to a method of accessing a value in a data structure based on a key. The script describes using lookup tables to map characters for the encryption and decryption process. The concept of lookup is crucial for understanding how the cipher operates and how to reverse its effects.

💡Index

An index is a reference or pointer to a position in a data structure. In the script, characters are indexed to find their positions in the lookup tables. The concept of index is important for understanding how the characters are mapped and manipulated during the encryption and decryption processes.

💡Character

In the context of the video, a 'character' refers to the individual elements of the text that are being encrypted or decrypted. The script mentions looping through characters, which is a common step in processing text for encryption or decryption. Understanding the role of characters is essential for grasping how the cipher affects the text.

💡Python

Python is a high-level programming language that is often used for scripting and automation tasks. The video script mentions creating a Python script for decoding, which implies that Python is the tool used to implement the decryption logic. Python's role in the script highlights its utility in solving cybersecurity challenges.

💡Self Input

Self input refers to a program taking its own code as input. The script mentions a Python program that requires self input, which is an unusual and intriguing aspect of the decryption challenge. This concept is key to understanding the unique nature of the cipher and how it is intended to be solved.

💡Context

Context, in the script, refers to the surrounding text or environment that provides meaning to a term or action. The mention of context is important for understanding how the characters are interpreted within the cipher. For example, the difference in meaning between 'a' and 'A' (uppercase vs lowercase) is part of the context that the cipher and its decryption process must consider.

💡Difference

In the video, 'difference' is used to describe the mathematical operation performed on the indexed characters to achieve decryption. The script describes computing the difference from the previous letter, which is a key step in reversing the encryption. Understanding the concept of difference is crucial for grasping the decryption algorithm described.

Highlights

Introduction to the custom cyclical Cipher

Instructions to download cipher text and encoder

Details on how to enclose the flag for submission

Mention that the flag format is 'pcctf{example}'

Explanation that the flag format does not assist with decryption

Discussion on the dependency of the cipher on the encoder's secrecy

Description of the cipher text composition

Process of creating a decoder script

Attempt to swap elements in the cipher

Explanation of the lookup process in the cipher

Strategy to compute the difference from the previous letter

Observation of the pattern in the cipher text

Proposal to look up the difference and add it back

Construction of a variable to hold the character for decryption

Success in decrypting the cipher text

Requirement for a Python program to take self-input

Execution of the Python program with self-input

Revelation of the flag 'Pico CTF{adlib}'

Transcripts

C3 this is the custom cyclical Cipher uh

download the cipher text here download

the encoder here enclose the flag and

our wrapper for submission if the flag

was example you would submit pctf

example all right so um the flag

format's not going to help us with

decryption on our crypto schemes don't

depend on the encoder be secret but this

one

does right so let's grab those

all right so our safer text is a

collection of uppercase lowercase

letters let's go ahead and grab that

encoder

oops Okay so so we have a couple lookup

things we Loop through the characters we

find the

index character and lookup one we

subtract it from the

previous index that into look up

two all right

well try to create

a decoder

script Okay so

let's maybe try just swapping these

things okay

um so I'll say like

Echo ABCD pipe convert. uh

python convert.

py we'll say get rid of that new line OB

BBD Python 3 convert

to.py all right well we're gonna have to

work harder than that

so all right so we looked it

up we looked it up in the first thing

and we subtracted so we're Computing the

difference from the previous letter so

when I do a b c d

b c and d we're all one more which is

why we're getting those Capital B's in

our crypto

thing so I really do think we can just

look up the difference

here we can do but we're going to need

to

add and

then can't say previous this current

because we need it really to be

something from lookup one so we really

need like

look up one. index out BCE the whole

string though

so we'll build a

variable for this

character then we'll add it so we look

up the difference we add it back

in that's going to be the previous one

we added these things up let's try that

so uh

great so that seems to work so

now okay that gave us a Python program

asky order for to character self input

python

2

um so I guess we give this Python

program to itself because it says self

input so

cat file. py pipe and it's python 2 it

says a d l i BS so our flag is I guess

going to be Pico CF a d l i BS