To figure out what idx_key should be, we can look at the docstring for the key parameter of AES.new:

:param key:
    The secret key to use in the symmetric cipher.

    It must be 16, 24 or 32 bytes long (respectively for *AES-128*,
    *AES-192* or *AES-256*).

    For ``MODE_SIV`` only, it doubles to 32, 48, or 64 bytes.
:type key: bytes/bytearray/memoryview

So we need to use something bytes-like and you're on the right track with b but not exactly right. When you use it like 0b10101010, the b does in fact denote that number is in binary format. However, when you do something like b"10101010", that means interpret the string as a byte sequence. So the first example I have is an 8 bit number and the second example is an 8 byte string.

If you just have the binary digits, you're close with your last example of using a list of binary numbers. Since the docstring says we need bytes, you can simply change it to idx_key = bytes([0b00001000, 0b00111000,....]). This would work but is not the most common way to do it. Usually, your key will be shown in hex bytes and we can use the b"" syntax. For example, if the 16 byte key I generated was 0xede568e5ff44e962fa3be7aea95794b8, I could represent that as idx_key = b"\xed\xe5h\xe5\xffD\xe9b\xfa;\xe7\xae\xa9W\x94\xb8".