S-box

In cryptography, an S-box (substitution-box) is a basic component of symmetric key algorithms which performs substitution. In block ciphers, they are typically used to obscure the relationship between the key and the ciphertext, thus ensuring Shannon's property of confusion. Mathematically, an S-box is a nonlinear[1] vectorial Boolean function.[2]

For the video game, see Sandbox (video game).

In general, an S-box takes some number of input bits, m, and transforms them into some number of output bits, n, where n is not necessarily equal to m.[3] An m×n S-box can be implemented as a lookup table with 2m words of n bits each. Fixed tables are normally used, as in the Data Encryption Standard (DES), but in some ciphers the tables are generated dynamically from the key (e.g. the Blowfish and the Twofish encryption algorithms).

Example

One good example of a fixed table is the S-box from DES (S5), mapping 6-bit input into a 4-bit output:

S5Middle 4 bits of input
0000000100100011010001010110011110001001101010111100110111101111
Outer bits 00 0010110001000001011110101011011010000101001111111101000011101001
01 1110101100101100010001111101000101010000111110100011100110000110
10 0100001000011011101011010111100011111001110001010110001100001110
11 1011100011000111000111100010110101101111000010011010010001010011

Given a 6-bit input, the 4-bit output is found by selecting the row using the outer two bits (the first and last bits), and the column using the inner four bits. For example, an input "011011" has outer bits "01" and inner bits "1101"; the corresponding output would be "1001".[4]

Analysis and properties

When DES was first published in 1977, the design criteria of its S-boxes were kept secret to avoid compromising the technique of differential cryptanalysis (which was not yet publicly known). As a result, research in what made good S-boxes was sparse at the time. Rather, the eight S-boxes of DES were the subject of intense study for many years out of a concern that a backdoor (a vulnerability known only to its designers) might have been planted in the cipher. As the S-boxes are the only nonlinear part of the cipher, compromising those would compromise the entire cipher.[5]

The S-box design criteria were eventually published (in Coppersmith 1994) after the public rediscovery of differential cryptanalysis, showing that they had been carefully tuned to increase resistance against this specific attack such that it was no better than brute force. Biham and Shamir found that even small modifications to an S-box could significantly weaken DES.[6]

Any S-box where any linear combination of output bits is produced by a bent function of the input bits is termed a perfect S-box.[7]

S-boxes can be analyzed using linear cryptanalysis and differential cryptanalysis in the form of a Linear approximation table (LAT) or Walsh transform and Difference Distribution Table (DDT) or autocorrelation table and spectrum. Its strength may be summarized by the nonlinearity (bent, almost bent) and differential uniformity (perfectly nonlinear, almost perfectly nonlinear).[8][9][10][2]

See also

References
  1. Daemen & Rijmen 2013, p. 22.
  2. Carlet, Claude (2010), Hammer, Peter L.; Crama, Yves (eds.), "Vectorial Boolean Functions for Cryptography", Boolean Models and Methods in Mathematics, Computer Science, and Engineering, Encyclopedia of Mathematics and its Applications, Cambridge: Cambridge University Press, pp. 398–470, ISBN 978-0-521-84752-0, retrieved 2021-04-30
  3. Chandrasekaran, J.; et al. (2011). "A Chaos Based Approach for Improving Non Linearity in the S-box Design of Symmetric Key Cryptosystems". In Meghanathan, N.; et al. (eds.). Advances in Networks and Communications: First International Conference on Computer Science and Information Technology, CCSIT 2011, Bangalore, India, January 2-4, 2011. Proceedings, Part 2. Springer. p. 516. ISBN 978-3-642-17877-1.
  4. Buchmann, Johannes A. (2001). "5. DES". Introduction to cryptography (Corr. 2. print. ed.). New York, NY [u.a.]: Springer. pp. 119–120. ISBN 978-0-387-95034-1.
  5. Coppersmith, D. (May 1994). "The Data Encryption Standard (DES) and its strength against attacks". IBM Journal of Research and Development. 38 (3): 243–250. doi:10.1147/rd.383.0243. ISSN 0018-8646.
  6. Gargiulo's "S-box Modifications and Their Effect in DES-like Encryption Systems" Archived 2012-05-20 at the Wayback Machine p. 9.
  7. RFC 4086. Section 5.3 "Using S-boxes for Mixing"
  8. Heys, Howard M. "A Tutorial on Linear and Differential Cryptanalysis" (PDF).
  9. "S-Boxes and Their Algebraic Representations — Sage 9.2 Reference Manual: Cryptography". doc.sagemath.org. Retrieved 2021-04-30.
  10. Saarinen, Markku-Juhani O. (2012). "Cryptographic Analysis of All 4 × 4-Bit S-Boxes". In Miri, Ali; Vaudenay, Serge (eds.). Selected Areas in Cryptography. Lecture Notes in Computer Science. Vol. 7118. Berlin, Heidelberg: Springer. pp. 118–133. doi:10.1007/978-3-642-28496-0_7. ISBN 978-3-642-28496-0.

Further reading
  • Chuck Easttom (2018). "A generalized methodology for designing non-linear elements in symmetric cryptographic primitives". 2018 IEEE 8th Annual Computing and Communication Workshop and Conference (CCWC). pp. 444–449. doi:10.1109/CCWC.2018.8301643. ISBN 978-1-5386-4649-6. S2CID 3659645.

Sources
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