Hybrid Playfair: a modified Playfair cipher combining 2D and 3D Playfair

Abstract

Existing modifications to the original (2D) Playfair cipher have made significant progress on improving its security, but little attention was given to optimize or maintain the original time and space efficiency. To close the gap, this paper proposes Hybrid Playfair, a modified Playfair cipher using a three-dimensional key and 2-character message pairs for encryption and decryption. With a schema made of five different substitution policies covering seven distinct cases, as well as an implementation guideline with sorted entry map and cross-layered key storage, Hybrid Playfair can offer not only better protection against frequency analysis attacks than 2D Playfair but also lower space and time usage than both 2D and 3D Playfair cipher. Hybrid Playfair can be used for applications on less powerful platforms with high throughput demands, such as live streaming and audio filtering devices, with its high performance-cost ratio and unimpacted security.