If converting a SPN Block Cipher to a version with larger block size, should its S-boxes be enlarged too?

phantomcraft

7/24/23, 12:21 AM

Let's suppose I want to convert a Substitution-permutation network block cipher like Kuznyechik to a version with a larger block size.

Should its S-boxes be enlarged too in order to achieve similar security?

0 + 0

s-boxes

block-cipher

blocksize

kelalaka

7/24/23, 9:36 PM

This is quite a challenging problem. You might look at Rijndael Design which supports a 256-bit block size.

phantomcraft

7/25/23, 1:32 AM

@kelalaka I took a look in a implementation of Rijndael wich has a option to use 256-bit block size: https://github.com/charltoncr/Rijndael-Cipher -- It uses the same S-box for all the block sizes, I believe that this applies also Kuznyechik as it uses two 64-bits words (t1 and t2) which forms the processed block, each one is mixed with the S-boxes, I believe putting 4 64-bits words would be the same. Thanks.

kelalaka

7/25/23, 9:15 AM

Be careful, in SPN network, (the Permutation is really important) make sure that the permutation really works as intended. The RijnDael team was carefully calculated the necessary rounds. You may look at their book about this, too.

phantomcraft

7/25/23, 7:28 PM

@kelalaka I got.

Score:1

Crypto

LightBit

8/10/23, 4:38 PM

Usually not, but increasing block size is cipher dependant.

Increasing block size usually involves change to linear part of a cipher and probably increasing number of rounds.

For example you could use bigger MDS (like Whirlpool), use different permutation (like Rijndael), or add something additional like Pseudo-Hadamard transform (like Twofish).

Another way would be to use round function as F in Feistel.

0 + 0

Elon Musk

I sit in a Tesla and translated this thread with Ai:

EN: If converting a SPN Block Cipher to a version with larger block size, should its S-boxes be enlarged too?

TH: หากแปลง SPN Block Cipher เป็นเวอร์ชันที่มีขนาดบล็อกใหญ่ขึ้น S-boxes ควรขยายด้วยหรือไม่

RO: Dacă convertiți un SPN Block Cipher într-o versiune cu o dimensiune de bloc mai mare, ar trebui să fie mărite și casetele lui S?

RU: При преобразовании блочного шифра SPN в версию с большим размером блока следует ли также увеличивать его S-блоки?

VI: Nếu chuyển đổi Mật mã khối SPN sang phiên bản có kích thước khối lớn hơn, hộp S của nó cũng có nên được mở rộng không?

Post an answer

Most people don’t grasp that asking a lot of questions unlocks learning and improves interpersonal bonding. In Alison’s studies, for example, though people could accurately recall how many questions had been asked in their conversations, they didn’t intuit the link between questions and liking. Across four studies, in which participants were engaged in conversations themselves or read transcripts of others’ conversations, people tended not to realize that question asking would influence—or had influenced—the level of amity between the conversationalists.