Which one of the following is not a possible key length for the Advance Encryption Standard Rijndael cipher?56128192256

To determine which one of the given key lengths is not possible for the Advanced Encryption Standard (AES) Rijndael cipher, we need to compare the options with the valid key lengths for AES.

The AES Rijndael cipher supports three key lengths: 128 bits, 192 bits, and 256 bits.

Let's analyze each option:

1. 56 bits: This is not a valid key length for AES. AES only supports key lengths of 128, 192, and 256 bits.

2. 128 bits: This is a valid key length for AES.

3. 192 bits: This is also a valid key length for AES.

4. 256 bits: This is the maximum key length supported by AES and is also valid.

Therefore, the key length that is not possible for the AES Rijndael cipher is 56 bits.