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In cryptography, a substitution box (or S-box) is a basic component of symmetric key algorithms. In block ciphers, they are typically used to obscure the relationship between the plaintext and the ciphertext — Shannon's property of confusion. In many cases, the S-boxes are carefully chosen to resist cryptanalysis.

In general, an S-box takes some number of input bits, m, and transforms them into some number of output bits, n: an m×n S-box, implemented as a lookup table. 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. Bruce Schneier describes IDEA's modular multiplication step as a key-dependent S-box.

One good example is this 6×4-bit S-box from DES (S₅):

Given a 6-bit input, the 4-bit output is found by selecting the row using the outer two 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".

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 (Don Coppersmith, 1994) after the public rediscovery of differential cryptanalysis, showing that they had been carefully tuned to increase resistance against this specific attack. Other research had already indicated that even small modifications to an S-box could significantly weaken DES.

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.

• Boolean function
• Nothing up my sleeve number
• Substitution cipher

• Kaisa Nyberg (1991). "Perfect nonlinear S-boxes" (PDF). Advances in Cryptology - EUROCRYPT '91: 378–386. Retrieved on 2007-02-20. 
• Don Coppersmith (1994). "The Data Encryption Standard (DES) and its strength against attacks" (PDF). IBM Journal of Research and Development 38 (3): 243–250. Retrieved on 2007-02-20. 
• S. Mister and C. Adams (1996). "Practical S-Box Design" (PostScript). Workshop on Selected Areas in Cryptography (SAC '96) Workshop Record: pp. 61–76. Retrieved on 2007-02-20. 
• Schneier, Bruce (1996). Applied Cryptography, Second Edition. John Wiley & Sons, 296-298, 349. ISBN 0-471-11709-9. 

• A literature survey on S-box design
• John Savard's "Questions of S-Box Design"