Cipher's Secret Message

Introduction

Let’s read the content As you can see, we have to decrypt that to retrieve the flag

We can see the whole code here, we have to modifiy the code to get the flag

from secret import FLAG

def enc(plaintext):
    return "".join(
        chr((ord(c) - (base := ord('A') if c.isupper() else ord('a')) + i) % 26 + base) 
        if c.isalpha() else c
        for i, c in enumerate(plaintext)
    )

with open("message.txt", "w") as f:
    f.write(enc(FLAG))

1. enc() applies a Caesar cipher with incremental shift (+i)
2. . + i adds the character’s index, creating a progressive shift per character
3. chr(…) converts the resulting number back into a ciphered character.

Once we know what does the code do. we can procede to modify it As you can see I just edit the variable and the hypertext to ciphertext. but when I tried to execute it, there is an error,

But the problem here is cause, I just forgot to put the variable decrypt next to def, we can modify it just adding decrypt

message = "a_up4qr_kaiaf0_bujktaz_qm_su4ux_cpbq_ETZ_rhrudm"

def decrypt(cyphertext):
    return "".join(
        chr((ord(c) - (base := ord('A') if c.isupper() else ord('a')) + i) % 26 + base) 
        if c.isalpha() else c
        for i, c in enumerate(cyphertext)
    )

print("THM{" + decrypt(message) + "}")

I added the message variable manually with the encrypted string provided in the challenge.

I created a new function called decrypt() (instead of enc()) and changed the logic:

I replaced + i with - i to reverse the Caesar incremental shift.

This essentially undoes the encryption applied originally in enc().

Finally, I used print("THM{" + decrypt(message) + "}") to wrap the output in the expected flag format.

AND HERE IS THE FLAG

YOU HAVE TO KNOW THAT IS THERE IS A GRAMATICAL ERROR IS CAUSE IM LEARNING ENGLISH, THANK YOU