picoCTF new_caesar writeup

We are given a ciphertext and a python program for encryption.
There were two functions in the program: 1) b16_encode(plain) which takes the plaintext are an argument and, 2) shift(c,k) used for shifting the letters.
After reading the code, I understood that the encoding function converts each character in plaintext into 8-bit binary form then split into two 4-bit chunks. Each 4-bit chunk (i.e., 0–15) is mapped to a letter in
ALPHABET = string.ascii_lowercase\[:16\] (i.e., 'a' to 'p').

In the shift function:
Letters in the range 'a' to 'p' are shifted using modular addition with the key (1-letter key also from 'a' to 'p').
So the final output is a modular shift of the base16-encoded string.

I knew that I had to first unshift the ciphertext and then decode the values.
This was the script I wrote:

import string

LOWERCASE_OFFSET = ord("a")
ALPHABET = string.ascii_lowercase[:16]  # 'a' to 'p'

def unshift(c, k):
    t1 = ord(c) - LOWERCASE_OFFSET
    t2 = ord(k) - LOWERCASE_OFFSET
    return ALPHABET[(t1 - t2) % len(ALPHABET)]

def b16_decode(encoded):
    binary = ""
    for c in encoded:
        val = ALPHABET.index(c)
        binary += "{0:04b}".format(val)

    plain = ""
    for i in range(0, len(binary), 8):
        byte = binary[i:i+8]
        plain += chr(int(byte, 2))
    return plain

encrypted = "ihjghbjgjhfbhbfcfjflfjiifdfgffihfeigidfligigffihfjfhfhfhigfjfffjfeihihfdieieih"  # Replace this
key = "c"  

assert all(c in ALPHABET for c in encrypted)
assert key in ALPHABET
assert len(key) == 1

unshifted = ""
for i, c in enumerate(encrypted):
    unshifted += unshift(c, key[i % len(key)])

decoded = b16_decode(unshifted)
print("Decoded flag:", decoded)

I started with key="a" and when I found "et_tu" in the decoded plaintext at key="c", that was a humorous moment for me.
(For those who don't know, Julius Caesar utters these words "Et tu Brute, then fall Caesar" to Marcus Brutus, his close friend and a conspirator, at the moment of his assassination, expressing his shock and despair at being betrayed by someone he trusted.)