A subsitution table
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.
- S. Mister and C. Adams, "Practical S-Box Design," Workshop on Selected Areas in Cryptography (SAC '96) Workshop Record, Queens University, 1996, pp. 61--76