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Substitution boxes

A subsitution table or s-box is a common component of various ciphers. It is just a table where a lookup using a few bits as an index yields some other bits as a result. For example, in an 8 by 32 s-box you go in with 8 bits and come out with 32.

DES uses 6 by 4 s-boxes. More recent ciphers such as Blowfish and CAST-128 use 8 by 32.

There has been a great deal of research into the design of good s-boxes.

Further Reading

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