  In this paper, a word based chaotic image encryption scheme for gray images
is proposed, that can be used in both synchronous and self-synchronous modes.
The encryption scheme operates in a finite field where we have also analyzed
its performance according to numerical precision used in implementation. We
show that the scheme not only passes a variety of security tests, but also it
is verified that the proposed scheme operates faster than other existing
schemes of the same type even when using lightweight short key sizes.
