In this chapter we consider Dual RSA, a variant of RSA, that is designed to reduce the space requirements for storing the public and private keys when two instances of RSA are required. Dual RSA, introduced by Sun et al. , is essentially two instances of RSA that share the same public and private exponents but have diﬀerent moduli. Since the public and private exponents are the same, only one copy of each is needed to be stored. For certain parameter choices, the total space requirement for Dual RSA is less than that for other alternatives, such as using two instances of RSA with compressed moduli or using Twin RSA , another variant of RSA. A comparison of memory requirements will be made at the end of the chapter.