Design and implementation of Threefish cipher algorithm in PNG file

Abstract

This paper is presenting design and implementation of Threefish block cipher on grayscale images. Despite the fact that Threefish block cipher is one of the best secure algorithms, most studies concerning Threefish have focused on hardware implementation and have not commonly been applied on image encryption due to huge amount of data. The main contribution here was to reduce the time and the amount of data to be encrypted while maintaining encryption performance. This objective was achieved by encrypting just the most significant bits of image pixels. A 256-bit plain text blocks of the Threefish was constructed from 2n most significant bits of the pixels, where 0<n<3. Furthermore, Threefish block cipher was applied when n=3 to analyze the impact of uninvolving some bits in encryption process on the encryption performance. The results indicated that the encryption achieved good encryption quality when n=1, but it might cause some loss in decryption. In contrast, the encryption achieved high encryption quality when n=2, almost as good as the encryption of the whole pixel bits. Furthermore, the encryption time and the amount of data to be encrypted were decreased 50% as n decreased by 1. It was concluded that encrypting half of the pixel bits reduces both time and data, as well as significantly preserves the encryption quality. Finally, although the proposed method passed the statistical analysis, further work is needed to find a method resistant to the differential analysis.