Image Encryption Algorithm Based on Chaotic Mapping

Abstract

Imej digunakan dalam pelbagai bidang seperti perubatan, ketenteraan, sains, kejuruteraan, kesenian, hiburan, pengiklanan, pendidikan dan latihan. Dengan bertambahnya penggunaan teknik digital bagi penghantaran dan penyimpanan imej, isu asas untuk melindungi kerahsiaan, keutuhan dan kesahihan imej perlu dititikberatkan. Kertas–kerja ini membincangkan algoritma alternatif penyulitan kekunci simetri, iaitu Secure Image Encryption (SIP), bagi melindungi keselamatan imej, Algoritma ini direka bentuk berdasarkan teknik chaos. SIP mengolah piksel, bukannya bit sebagaimana yang dilakukan oleh algoritma penyulitan yang popular seperti Triple–DES dan IDEA. Pada umumnya, SIP terdiri daripada tiga komponen: (1) fungsi transformasi mendatar–menegak (HVT); (2) fungsi anjakan (S), dan (3) fungsi skala kelabu (GS). Fungsi HVT adalah berdasarkan pemetaan chaos yang digunakan dalam algoritma pemetaan Baker. Fungsi GS pula melanjutkan algoritma ini ke tiga–dimensi dengan dimensi ketiga merujuk kepada aras skala kelabu piksel. Algoritma ini menyokong dua mod operasi, iaitu Electronic Code Book (ECB) dan Cipher Block Chaining (CBC). Analisis yang dilakukan terhadap SIP menunjukkan aras keselamatan masih memuaskan walaupun kekunci yang digunakan terdiri daripada kekunci lemah jika digunakan dalam algoritma pemetaan Baker. Kata kunci: Kerahsiaan, chaos, kriptografi, penyulitan imej Images are routinely used in diverse areas such as medical, military, science, engineering, art, entertainment, advertising, education as well as training. With the increasing use of digital techniques for transmitting and storing images, the fundamental issue of protecting the confidentiality, integrity as well as the authenticity of images has become a major concern. This paper discusses an alternative symmetric–key encryption algorithm for securing images, namely Secure Image Encryption (SIP) that is based on chaos. Unlike other popular encryption algorithms such as Triple–DES and IDEA, SIP manipulates pixels rather than bits. Generally, SIP comprises of three main components: (1) horizontal–vertical transformation function (HVT); (2) shift function (S), and (3) gray scale function (GS). HVT function is based on a two–dimensional chaotic map that utilized Baker´s map algorithm. GS function extends the algorithm to three–dimension, whereby, the third dimension refers to the level of the gray–scale of a pixel. The algorithm supports two modes of operation namely Electronic Code Book (ECB) and Cipher Block Chaining (CBC). From the analysis done, SIP manage to encrypt images of various sizes even with the usage of weak keys that exist in Baker´s map encryption algorithm. Key words: Confidentiality, chaos, cryptography, image encryption