Efficiency of public key cryptosystems as block ciphers

As block ciphers, public key cryptosystems operate on groups of plaintext characters, called blocks. Character grouping enhances efficiency and security. However, the encryption functions, which are one-way trapdoor functions defined over finite fields, restrict the sizes of the blocks. In this paper, implementations of conventional RSA and ElGamal cryptosystems together with implementations using Chebyshev polynomials are considered. In one case, in ElGamal cryptosystem using Chebyshev polynomials, we provide an implementation where, without compromising security, restriction on blocks is lifted. An empirical study compares the efficiencies of the different implementations, and, in particular, investigates the impact of dropping block size restriction on the performance of the ElGamal cryptosystems.