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 — Shannon's property of confusion.

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.[1] 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).

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

S5 Middle 4 bits of input
0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 1101 1110 1111
Outer bits 00 0010 1100 0100 0001 0111 1010 1011 0110 1000 0101 0011 1111 1101 0000 1110 1001
01 1110 1011 0010 1100 0100 0111 1101 0001 0101 0000 1111 1010 0011 1001 1000 0110
10 0100 0010 0001 1011 1010 1101 0111 1000 1111 1001 1100 0101 0110 0011 0000 1110
11 1011 1000 1100 0111 0001 1110 0010 1101 0110 1111 0000 1001 1010 0100 0101 0011

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".[2]

The 8 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. 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. Biham and Shamir found that even small modifications to an S-box could significantly weaken DES.[3]

There has been a great deal of research into the design of good S-boxes, and much more is understood about their use in block ciphers than when DES was released.

Any S-box where each output bit is produced by a bent function of the input bits, and where any linear combination of the output bits is also a bent function of the input bits, is a perfect S-box.[4]

See also

References

  1. ↑ 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. 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.
  2. ↑ Buchmann, Johannes A. (2001). "5. DES". Introduction to cryptography (Corr. 2. print. ed.). New York, NY [u.a.]: Springer. pp. 119–120. ISBN 0-387-95034-6.
  3. ↑ Gargiulo's "S-Box Modifications and Their Effect in DES-like Encryption Systems" p. 9.
  4. ↑ RFC 4086. Section 5.3 "Using S-Boxes for Mixing"

Further reading

External links