python - シーザー暗号の復号化を試みていますが、取得し続けます: ord() は文字が必要ですが、長さ 10 の文字列が見つかりました

Question

shift = 3

encrypted_text = open("book.txt", "r", encoding="utf-8")

plain_text = ""

for c in encrypted_text:
    
    if c.isupper():
        c_unicode = ord(c)

        c_index = ord(c) - ord("A")

        new_index = (c_index - shift) % 26

        new_unicode = new_index + ord("A")

        new_character = chr(new_unicode)

        plain_text = plain_text + new_character

    else:

        plain_text += c

encrypted_text.close()

print("Decrypted text:", plain_text)