XDES: 以DES為基礎並加強安全性之區塊加密法

Abstract

DES和 Triple DES即將因為新標準的提出而過時，本文依照新一代加密演算法之規格提出新的加密法，我們修改DES提出新的演算法使其加密金匙和加密資料區塊長度符合現代標準，這個演算法主要是使用較多的替代盒以達成較長的加密金匙和加密資料區塊，並且將我們的演算法針對差分攻擊法和線性攻擊法做調整使其對於此兩種攻擊法有抵抗力。本文分析所提出之加密法，其安全性有一定的保證。並且我們也發現DES之原始設計對於線性攻擊法有所缺陷，我們提出之演算法則可避免此一問題。 差分攻擊法藉由特定的明文與密文組合來分析加密金匙，一般衡量一加密系統對於差分攻擊法的抵抗強度以估計在許多明文密文組合中能夠找到有用組合的機率來衡量，本文亦提出一個嘗試，使用演化式計算以更快找到所需的明文密文組合。 新一代標準加密法AES由Rijndael獲勝，本文也對於此加密法提出修改，藉由安插隨機位元進入被加密資料區塊以增強其強度。

DES and Triple DES will be out of date due to the new standards. In this thesis, a new encryption algorithm is proposed according to the new requirements of key and block sizes. Based on DES, we modify the algorithm to enhance the sizes. The modification is to use more S-boxes in a round function. We also tune our algorithm to resist differential and linear attacks. Based on our cryptanalyses, the security of the algorithm shows some promises. The weakness of DES toward linear cryptanalysis is also shown in this thesis, which is avoided in our algorithm. Differential attack makes use of specific plaintext pairs to analyze the key. The strength of an encryption system toward differential relies on a lower expected number of right pairs among huge number of plaintext pairs. In this thesis, we proposed an evolutionary computation model to try to find the right pair effectively. The new standard AES winner is Rijndael. We also modify Rijndael to improve its security by adding some random bits into the plaintext.