This is pretty much a Cryptopals challenge. Solution for those who understand Vietnamese is available at this link. Understanding how to solve this challenge in the Cryptopals intended way requires a deep understanding into how group theory works. A cheating way to solve this is to search the name of the challenge on Github, and there should be Python/Sage solutions. Leaving this challenge to the future me who “hopefully” understand in the future....
The description is mentioning something about Infinite Garble Extension, which is used in Telegram (searching the name of the Python class, aesige), hence the name “Paper Plane” - Telegram’s logo. The only observation needed to solve this challenge is that, decryption on a single block can be done without including the entire ciphertext. We only need the first block of ciphertext, and the m0 and c0 corresponding to that block. The very first block will have m0 and c0 provided, and consequent blocks will use the previous plaintext block decrypted for m0, and its corresponding ciphertext block for c0....