Mumbai University > EXTC > Sem 7 > Data Compression and Encryption

Marks: 5 M

Year: Dec 2013

$$\text{Figure 4.9 S Box Design}$$

1. The substitution function of a set of 8 S- boxes, each of which accepts 6 bits as input and produces 4 bits as output.
2. The main characteristics of S box is its size.
3. The n × m S box has n inputs and m outputs bits. e.g DES has 6 × 4 S boxes.
4. Larger the S box more is it resistant to the crypto analysis. However if n is large, the look up table is large, the look up table is large. Hence a limit of n equals to about 8 to 10 is usually imposed.
5. Also if S box is large, it is more difficult to design it properly.

The S- Box Design Criteria

1. No output bits of any S box should be close to a linear function of input bits.
2. Each row of a S box should include all possible bit combinations.
3. If 2 inputs to an S box differs in exactly, the output must in exactly in at least 2 bits.
4. If 2 inputs to a S box differ in two middle bits, the output must differ in at least 2 bits.
5. If 2 inputs to a S box differ in 2 first bits are identical in their last 2 bits, the 2 outputs must not be same.
6. The above criteria make the algorithm robust and can’t be easily broken. If it also gives good confusion properties.

The S-Box Design Approaches

1. Random: It was some Pseudo random number generation or some table of random digits to generate the entries in ‘S’ box.
2. Random with Testing: Choose S box entries randomly then let the results against various criteria and throw away those that do not pass.
3. Human Made: This is a manual approach with only simple maths to support it ( Not Suitable for large S box)
4. Math Made: Generate S box according to mathematical principles.