In this paper, a new secret-key block cipher called 512 bits RC6 is proposed which is an evolutionary improvement of the 128 bits RC6 designed to meet the requirements of the Advanced Encryption Standard (AES) to increase security and improve performance. The inner loop is based around the same round found in the 128 bits RC6.
The proposed algorithm includes two things the first is doubling the pervious 128 bits RC6 to 256 bits RC6 and the second adapting a Feistal network which is iterated 256 bits RC6 20 times. A desirable property of an encryption algorithm is that a small change in either the plaintext or the key should produce a significant change in the ciphertext. This is so called valanche Effect. An Avalanche Effect of 128 bit RC6 is about 43.25%. If we change the same amount of information in key for 512 bits RC6 then the Avalanche Effect is about 257.27%. The proposed algorithm is resistant to matching and a dictionary attack which increases the security of the previous 128 bits RC6 algorithm by using a block size of 512 bits instead of 128 bits.