|
In cryptography, Madryga is a block cipher created in 1984 by W. E. Madryga. It was designed to be easy and efficient for implementation in software. Serious weaknesses have since been found in the algorithm, but it was one of the first encryption algorithms to make use of data-dependent rotations, later used in other ciphers, such as RC5 and RC6. In his proposal, Madryga set forth twelve design objectives that are generally considered to be good goals in the design of a block cipher. DES had already fulfilled nine of them. The three that DES did not fulfill were: # Any possible key should produce a strong cipher. (Meaning no weak keys, which DES has.) # The length of the key and the text should be adjustable to meet varying security requirements. # The algorithm should be efficiently implementable in software on large mainframes, minicomputers, and microcomputers, and in discrete logic. (DES has a large amount of bitwise permutations, which are very inefficient in software implementations.) ==The algorithm== Madryga met the objective of being efficient in software: the only operations it uses are XOR and rotations, both operating only on whole bytes. Madryga has a variable-length key, with no upper limit on its length. Madryga is specified with eight rounds, but this can be increased to provide more security if need be. In each round, the algorithm passes over the entire plaintext ''n'' times, where ''n'' is the length of the plaintext in bytes. The algorithm looks at three bytes at a time, so Madryga is a 24-bit block cipher. It XORs a key byte with the rightmost byte, and rotates the other two as one block. The rotation varies with the output of the XOR. Then, the algorithm moves to the right by one byte. So if it were working on bytes 2, 3 and 4, after it finished rotating and XORing them, it would repeat the process on bytes 3, 4 and 5. The key schedule is very simple. To start with, the entire key is XORed with a random constant of the same length as the key, then rotated to the left by 3 bits. It is rotated again after each iteration of rotation and XOR. The rightmost byte of it is used in each iteration to XOR with the rightmost byte of the data block. The decryption algorithm is simply the reverse of the encryption algorithm. Due to the nature of the XOR operation, it is reversible. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Madryga」の詳細全文を読む スポンサード リンク
|