IDEA is a symmetric block cipher that operates on 64-bit blocks with a 128-bit key. It was designed in 1991 as an improvement over DES. IDEA consists of 8 rounds plus a half round. Each round splits the block into four 16-bit quarters, applies additions and multiplications using parts of the round key, exchanges inner quarters, and XORs with the output of the previous operations. The key schedule generates subkeys by rotating the main key and combining values. IDEA is considered secure and was faster than DES, though its simple key schedule was a potential weakness.

1. IDEA Cipher Block
Presented by:
Boudour Ouissem-Eddine Amir
Bellache Nour El Islam
Responsible :
Dr. Chikouche
International Data Encryption Algorithm

2. What is IDEA ?
• IPES Improved proposed encrypting standard 1990.
• IDEA = International Data Encryption Algorithm
• Block Cipher
• Designed in 1991 by:
James Massey (cryptographer, Germany)
Xuejia Lai (cryptographer, China)
• Best Block Cipher available until
AES

3. Algorithm - Overview
• Block cipher :
- Block size : 64 bits
- Key size : 128 bits
• 8 rounds + ½ output
Transformation (half-round)
• Symmetric cipher
Cryptography
Symmetric
With Block
Ex: DES,AES,IDEA…
With flux
Asymmetric

4. Round Structure
⊕
Addition modulo 216⊞
Bitwise exclusive OR
⊙ multiplication
modulo 216+1
x1 x2 x3 x4
Encrypting

5. Round Structure
• Split into four 16 bits quarter blocks
• First Layer:
Apply two l6-bit additions and two 16-bit
multiplications to quarter blocks using appropriate
parts of the round key
Encrypting

6. Round Structure
Second Layer:
1. Calculate two intermediate
quarter blocks with 16-bit
additions and
multiplications using parts
of the round key
2. XOR intermediaite quarter
blocks with the blocks
from layar 1
3. Exchange the inner quarter
blocks
Encrypting

7. Output Transformation
• Done after the 8th round
• Exchange the inner quarter blocks
• Apply 16-bit additions and multiplications
using parts of the round key
• Recombine the quarter blocks to the final
result
Encrypting

8. Key Schedule
• 128 bit key
• 16-bit sub-keys used (52 times in total)
• Key simply divided into eight 16-bit sub-keys
Algorithm:
• Take the 8 first sub-keys
• Then rotate the key 25 bits to the left
• Then do it again

9. Decryption
• Basically same as the encryption algorithm !
• Except the sub-keys
-> Schedule for decryption key is using
combinations of multiplicative inverse, additive
inverse, and original sub-keys

10. Security
• Considered as really secure
• Best attack can break IDEA reduced To 6 rounds (full
IDEA = 8.5 rounds)
• But could be redesigned because of The very simple
key schedule
• “Weak key" problem with too many 0-bits

11. Running time measurements & analysis
• Most frequently-executed methods:
- IDEA.encrypt
- IDEA. multiplication
- IDEA.addition
• Twice the speed as DES

12. References
• Wikipedia article
https://fr.wikipedia.org/wiki/International_Data_Encryption_A
lgorithm
• Team OSIX FINAL PRESENTATION .pdf
• "Handbook of Applied Cryptography" (by Alfred J. Menezes)
http://cacr.uwaterloo.ca/hac/
• Secret-key Cryptography institute of computer technology
Vienna university of technology 2005 manfred lindner

13. Thanks!