Advanced Encryption Standard Algorithm Secret Key

Question

just a follow up question with this question..

How does the key schedule of Rijndael looks for keysizes other than 128 bit?

A 256-bit key can still encrypt a block of plaintext in 128 bit?

but in this tutorial, to encrypt 128-bit plaintext, It need 128-bit of secret key also..

can someone provide me a clear explanation about using of 192, 256 bit key to encrypt a 128-bit block of plaintext?

The only thing that I can understand is that If you will encrypt a 128-bit of plaintext, it need to use 128 bit of secret key, to encrypt a 192 bit of plaintext, it need to use 192 bit of secret key, to encrypt a 256 bit of plaintext, it need to use 256 of secret key.. that is the only thing that I know based on my research..

Answer 1

One very important thing you are missing is the idea behind AddRoundKey and the Key Schedule.

Each round, there is a new round-key. The round key is derived from the main key. So, say you have a 128-bit block (AES) and a 256-bit key. The internal state is 128-bits and the round key is 128-bits. The specification states that for this configuration, there will be 14 rounds. Since the round-key changes for each round, it is easy to see that all 256-bits of the main key will be used over the 14 rounds.

To see exactly how all 256-bits of the main key are used, you'll have to dive into the details of the key schedule.