Computer Security and Cryptography

9.21 THE IDEA BLOCK CIPHER

IDEA (Fig. 9.20) is a block cipher design by Xuejia Lai and James Massey. Its design was influenced by DES; it uses eight rounds to mix the key and plaintext. In each round the basic operations applied to 16-bit variables X₁, X₁, X₂, X₃ are XOR, modulo 2¹⁶ + 1 multiplication, and modulo 2¹⁶ addition. Additionally, at the end of each round there is an interchange of the processed blocks X₁ and X₂. At the end of the eighth round there is an additional combination of the key and processed plaintext.

9.21.1 The IDEA Key Schedule

The IDEA key of length 128-bits is divided into 8 blocks of 16 bits K = (K₀, K_i, …, K₇). IDEA uses six blocks of 16 bits in each of the eight rounds and four blocks for the final operation. The blocks used in each round are derived as follows:

1. K₀, K₁, …, K₅ are used in round 1; K₆, K₇ are the first two blocks in round 2.
2. The 128-bit block K is left-shifted 25 places, and the first four 16-bit blocks K₈, K₉, K₁₀, K₁₂ are used in round 2.

The process is repeated.