writeup | redecorate.lol

the problem

in this file:

main.go

package main

import (
    *snip*
)

var CharSet = map[rune]string{
    *snip*
}

func catify(input string, keys []int) string {
	var keyedText string
	var result string
	for i, char := range input {
		keyedText += string(rune(int(char) + keys[i]))
	}
	fmt.Printf("I2Keyed: %s\n", keyedText)
	hexEncoded := strings.ToUpper(hex.EncodeToString([]byte(keyedText)))
	fmt.Printf("K2Hex: %s\n", hexEncoded)

	for _, rune := range hexEncoded {
		result += CharSet[rune]
	}
	return result
}

func savePair(name, input, output string) {
	inputFile, err := os.OpenFile(name+"_input.txt", os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0644)
	if err != nil {
		fmt.Println(err)
		return
	}
	defer inputFile.Close()
	outputFile, err := os.OpenFile(name+"_output.txt", os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0644)
	if err != nil {
		fmt.Println(err)
		return
	}
	defer outputFile.Close()
	if _, err := inputFile.Write([]byte(input)); err != nil {
		fmt.Println(err)
		return
	}
	if _, err := outputFile.Write([]byte(output)); err != nil {
		fmt.Println(err)
		return
	}
}

func getKeys(length int) []int {
	var keys = []int{}
	keyFileName := fmt.Sprintf("keys_%d.json", length)
	file, err := os.Open(keyFileName)
	if err != nil {

		for i := 0; i < length; i++ {
			num, _ := rand.Int(rand.Reader, big.NewInt(60000))
			keys = append(keys, int(num.Int64()))
		}
		keyFile, err := os.OpenFile(keyFileName, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0644)
		if err != nil {
			fmt.Println(err)
			return []int{}
		}
		defer keyFile.Close()
		encoded, _ := json.Marshal(keys)
		keyFile.Write(encoded)
		return keys
	}
	json.NewDecoder(file).Decode(&keys)
	return keys
}

func main() {
	input := "You fools! You will never get my catnip!!!!!!!"
	keys := getKeys(len(input))
	encoded := catify(input, keys)
	savePair("example", input, encoded)
}

a message is obfuscated through a series of character value shifts. each character is shifted using a set of random key values, then hex-encoded, and finally mapped to emojis. while it hides the original text, it lacks actual cryptographic security.

solving the obfuscation

to reverse the encoding, we need to undo each transformation step.

solve.py

*snip*

def emojis_to_hex(ciphertext_emoji: str) -> str:
    return pattern.sub(lambda m: emoji_to_hex_map[m.group()], ciphertext_emoji)


def hex_to_keyed_text(hex_string: str) -> str:
    return binascii.unhexlify(hex_string).decode('utf-8')


# use known plaintext before the obsfucation 
example_hex = emojis_to_hex(example_cipher_emoji)
example_keyed_text = hex_to_keyed_text(example_hex)
keys = [ord(k) - ord(p) for p, k in zip(plaintext, example_keyed_text)]

# find the flag
flag_hex = emojis_to_hex(flag_cipher_emoji)
flag_keyed_text = hex_to_keyed_text(flag_hex)
flag_plaintext = ''.join([chr(ord(k) - key) for k, key in zip(flag_keyed_text, keys)])

print("recovered keys:", keys)
print("flag:", flag_plaintext)

firstly, we map the emojis back to their corresponding hex values. then, we convert the hex back into an intermediate "keyed" text. since we have that known plaintext example, we can determine the character shift values used in the encoding. with the keys, we subtract the shifts from the encoded flag text to determine the original message. the lack of cryptographic security makes this decryption easy, once the pattern is identified, reversing it is just a matter of applying character operations.

afterword

this write-up was done in part for IRISCTF 2025. It was a fun challenge that required a mix of cryptography, reverse engineering, and programming skills.

the decoding process demonstrated here emphasizes the limitations of security through obscurity, and that simpler, isn't always better.

i hope this write-up brought you insight into the "complexity" of string obfuscation algorithms and how easily some of them can be defeated.