Cryptography series: IDEA

Introduction

The full name of IDEA is International Data Encryption Algorithm, also called International Encryption Algorithm. Note that this is not the IDEA that we commonly use as a development tool.

Today I will give you a detailed introduction to IDEA.

Introduction to IDEA

IDEA is called International Encryption Algorithm in Chinese, and was also called Improved Proposed Encryption Standard (IPES) at first. It is a symmetric key encryption algorithm, originally proposed by James Massey and Xuejia Lai in 1991.

Its goal is to replace the DES algorithm. IDEA is a revision of the early PES. IDEA is an optional algorithm of the OpenPGP standard.

IDEA's patent has expired in 2012 and is now available for free.

IDEA principle

The length of the IDEA encryption block is 64 bits, and the length of the key is 128 bits, which is a combination of eight rounds of conversion and half-round output conversion. The process of encryption and decryption is similar. Let's look at the basic flow chart of IDEA:

In the above figure, the blue circle is the XOR exclusive OR operation, and the green box is the additive modulus 216. The red dot is the multiplication modulus 216 + 1. If the input is all 0 (0x0000), it will be converted to 216, if the input is 216, it will be converted to 0 (0x0000).

After eight rounds of operation, the following half-round output conversion is performed. The output conversion is as follows (the exchange of the two values in the middle offsets the exchange at the end of the last round, so there is no net exchange):

IDEA subkey generation

IDEA uses the Lai-Massey structure. Each round uses 6 16-bit sub-keys, and the final half-round uses 4 sub-keys, for a total of 8.5 rounds and 52 sub-keys.

The first 8 sub-keys are directly extracted from the key, because the length of the key is 128 bits, which can be divided into 8 16-bit self-keys. The K1 in the first round is the 16 bits of the key.

By moving the master key 25 bits to the left between every 8 groups, more 8 groups of subkeys are created.

This article has been included in http://www.flydean.com/idea/

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