1. Simplified IDEA
Cryptography and
Network Security

2. Components of IDEA Key generation Three operations
Multiple rounds and an output transformation

3. IDEA Simplified IDEA 128-bit key 64-bit plaintext 8 rounds
8*6 + 4 = bit subkeys
64-bit plaintext
8 rounds
Operations: modulo 216 and 216+1
16-bit key
2*6 + 4 = 16 2-bit subkeys
8-bit plaintext
2 rounds
Operations: modulo 22 and 22+1

4. Key Generation 16-bit key: Z[1..16]
Z1 = Z[1,2] , Z2 = Z[3,4], …, Z8 = Z[15,16]
Z9 = Z[4,5], Z10 = Z[5,6], …, Z13 = Z[12,13], Z14 = Z[14,15] Z15 = Z[16,1], Z16= Z[2,3]
(rotate 3 bits to the left before extracting second set of 8 subkeys)

5. Two-bit Operands
Plaintext divided into 4 2-bit pieces and subjected to the two round and final output transformation
2-bit subkeys used as follows:
Z1..Z6 for first round
Z7..Z12 for second round
Z13..Z16 for final transformation

6. Operations
XOR
Addition modulo 4
Multiplication modulo 5

7. Encryption Decryption Subkeys Subkeys
Z1, Z2, Z3, Z4, Z5, Z6
Z7, Z8, Z9, Z10, Z11, Z12
Z13, Z14, Z15, Z16
Z13-1, -Z14, -Z15 Z16-1, Z11, Z12
Z7-1, -Z9, -Z8, Z10-1, Z5, Z6
Z1-1, -Z2, -Z3, Z4-1