In Rivest's description of RC5 [17], a round consists of two equations, and in
each equation, either A or B is modied while the other remains unchanged.
We will refer to each equation as a half-round. So one half-round of RC5 is
similar to a full round in DES [16]. For ease of discussions, we adopt the
common notation for Feistel ciphers2 and rewrite RC5 as follows