I am studying for an exam right now. And I wanted to make sure I got this point correct.
AES is not a Feistel cipher because the operations in AES are not invertible.
Is the above statement correct? If not, why isn't it a Feistel cipher?
Well, AES is not a Feistel cipher because it's a substitution-permutation network instead. If I were taking a test that asked me why AES was not a Feistel cipher, this would be my argument: namely, that the structure of substitution-permutation networks is fundamentally different from that of Feistel networks. (Here one could elaborate on invertibility and other differences.)
That said, your statement is not correct. In a Feistel cipher, the round function is not necessarily invertible (DES's round function is not), but in AES, like any substitution-permutation network, the rounds are invertible. This is a property of the construction itself.
By definition, a Feistel network uses a series of rounds that split the input block into two sides, uses one side to permute the other side, then swaps the sides. As always, Wikipedia has a nice diagram.
AES doesn't do this. Performing a round necessarily permutes the entire state. Each round consists of the
So only the
None of that matches the "split the block into A and B and use A to permute B" style of a Feistel network.
The simple answer is "Because its an SPN cipher".
SPN operates on whole data in one round, where as Feistel divides data into N parts where N>=2 , then operate upon X parts where 0
In balanced, data is divided in Two parts i.e N = 2, and X=1 (example is camellia cipher) In Unbalanced, data is divided in more than two parts, i.e N > 2, (example is SMS4 cipher)
Now talking about invertible issue.
SPN has to be invertible, otherwise decryption will not be possible. where as in Feistel cipher, the F function can be invertible or non-invertible. And you will still be able to get encryption and decryption.
Another beauty of Feistel Cipher is that code for Encryption and Decryption is same, you only need to use the round keys in reverse order. (even if your F function is invertible, you need not to write its inversion in the code)
One more point to note is, in Feistel your F function can be a simple SPN.
From cryptography and network security principles and practices 5th edition Chapter 5 page 148
Thank you for your interest in this question.
Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).
Would you like to answer one of these unanswered questions instead?