  Differential Power Analysis (DPA) presents a major challenge to
mathematically-secure cryptographic protocols. Attackers can break the
encryption by measuring the energy consumed in the working digital circuit. To
prevent this type of attack, this paper proposes the use of reversible logic
for designing the ALU of a cryptosystem. Ideally, reversible circuits dissipate
zero energy. Thus, it would be of great significance to apply reversible logic
to designing secure cryptosystems. As far as is known, this is the first
attempt to apply reversible logic to developing secure cryptosystems. In a
prototype of a reversible ALU for a crypto-processor, reversible designs of
adders and Montgomery multipliers are presented. The reversible designs of a
carry propagate adder, four-to-two and five-to-two carry save adders are
presented using a reversible TSG gate. One of the important properties of the
TSG gate is that it can work singly as a reversible full adder. In order to
design the reversible Montgomery multiplier, novel reversible sequential
circuits are also proposed which are integrated with the proposed adders to
design a reversible modulo multiplier. It is intended that this paper will
provide a starting point for developing cryptosystems secure against DPA
attacks.
