title: Why do we only take the 1st and last bits together for SBox substitution?
body: <p>Today, in class we were discussing about how s-box is used in feistal ciphers. Our sir went on to say that for s-box substitution, we take the 1st and last bits as one part and the remaining bits as another, following which we find the matching 4 bits.</p>
  <p><a href="https://i.stack.imgur.com/Ta2yx.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Ta2yx.png" alt="enter image description here" /></a></p>
  <p>So if we were to have the 6bits as: &quot;100100&quot;, it would map to 1110 (left column: 10 , right column: 0010)</p>
  <p>My doubt now is if it possible for us to take different positions of bits together, like the 1st and 3rd bit? If not, is there a reason why we can only take the 1st and last bits?</p>