# How are subkeys, in Feistel block ciphers, derived for each round?

I'm bit confused about the subkeys that are used in Feistel ciphers. I know that all the subkeys ki are derived from the main key K, but how?

Say I have a simplified Feistel block cipher of i rounds, is it then correct to say that each key ki is the K/i part of the key?

So if the key is 4 bytes and there are 4 rounds, then I use the first 1 byte as subkey for the first round, then the second byte as subkey for the 2nd round and so on?

I can't seem to find this information when looking at articles/explanations of the Feistel cipher. I'm trying to solve an exercise and want to make sure that I can make the assumption that the key should be divided by the number of rounds and then use subkeys from this.

Note: It is specified in the exercise that the Feistel cipher uses 8-bit independent round keys ki. I'm not sure if this aligns with my assumption above.

• A Feistel cipher is a generic structure, many block ciphers (but not only block ciphers) are based on it and they use different round functions / key derivation mechanisms. – Aleph Jun 20 '15 at 10:44

I know that all the subkeys $k_i$ are derived from the main key $K$, but how?