Monday, December 26, 2016

Is double AES more secure?

Is double AES more secure? The answer is YES, but it is not much more secure as you expect.

Do you remember why to use 3DES instead of double DES? The reason is double DES can be compromised with meet-in-the-middle-attack.

Time taken for DES with a 56 bits key size is 2^56. How about double DES with different keys? The time take is not expected to 2^112. It is smaller than 2^63 by using meet-in-the middle-attack, that is developed by Merkle and Hellman.

The result can be applied to any block ciphers including AES. For example, time taken for AES with a 128 bits key size is 2^128. Time taken with double AES with different keys is much smaller than 2^256. It is,

Time = 2^128 x log (2^128) + 2^128 x log (2^128) << 2^256


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