1. Information Security and Management (Abstract) 5
Information Security and Management (Abstract) 5. Advanced Encryption Standard 6. Contemporary Symmetric Ciphers
Chih-Hung Wang
Fall 2011

2. Famous Symmetric Ciphers
AES (Advanced Encryption Standard)
Triple DES
Other symmetric block ciphers
(ex. IDEA、Blowfish、RC5、CAST-
128…etc.)

3. AES Next generation encryption standard of NIST/FIPS
It will replace the use of DES in the following 30 years
The sensitive information protected by AES cannot be revealed within 100 years
It is selected by the competition from international selection process

4. Calendar of AES

5. AES Requirements Block cipher 128-bit block 128/192/256-bit keys
It is equal to Triple DES at least on security and is more efficient
Provide descriptions and analysis
Provide three implementations in two languages (reference and optimized in C，optimized in Java)
IF selected, royalty free world wide

6. The 15 Submission for AES (1)

7. The 15 Submission for AES (2)

8. Final AES Selection Rijndael Block cipher with block size 128 bits
Accept 128-, 192-, 256-bit length keys
Easy to implement in H/W

9. The Implementation of Crypto Algorithms (W32)
Different platforms: win16, win32, linux, OS/2,…
Triple DES, Rijndael, Safer+, Blowfish, Cast-128, …
Old version
Crypto++: a C++ Class Library of Cryptographic Primitives
Version (2010/8/9)
Microsoft CryptoAPI

10. More AES Information NIST AES Homepage AES proposal
Rijndael Specification Those who are interested in the AES specification (i.e., what will be in the standard) should refer to the Draft FIPS for the AES.
Test Values
Supporting Documentation
Rijndael Developers' Contact Information
Rijndael Code: C/C++/Java/Visual Basic
FIPS 197
AES proposal
ammended.pdf

11. Practice Reading Project Programming/testing Project
Rijndael Specification : download from
Programming/testing Project
Download Rijndael source code (C/C++) from Rijndael homepage or other web sites.
Modify and refine the source code from web sites such that it can be used to encipher and decipher a message.
Write a report that includes the address of web sits, source code and testing results.

12. Triple DES
Double DES

13. Triple DES Double DES: reduction to a single key
It would be possible to find a key K3 such that
With 264 possible inputs, how may different mappings? (264)! > (101020)
It is reasonable to assume that if DES is used twice with different keys, it will produce one of the many mappings that is not defined by a single application of DES.

14. Meet-in-the-Middle Attack
Works whenever use a cipher twice [DIFF77]
Since X = EK1[P] = DK2[C]
Attack by encrypting P with all keys and store
Then decrypt C with keys and match X value
Can show takes O(256) steps

15. Triple DES Triple DES with two keys C=EK1[DK2[EK1[P]]]
P=DK1[EK2[DK1[C]]]

16. Triple DES Standardized in ANSI X9.17 & ISO 8732
No current known practical attacks
The cost of a brute-force key search on triple DES: 2112 (51033)
Has been adopted by some Internet applications, eg PGP, S/MIME

17. Known-Plaintext Attack

18. Other symmetric block ciphers
IDEA
International Data Encryption Algorithm
A symmetric block cipher developed by Xuejia Lai and James Massey of the Swiss Federal Institute of Technology.
One of a number of conventional encryption algorithms that have been proposed in recent years to replace DES.
IDEA is included in PGP.
128-bit key to encrypt data in blocks of 64 bits

19. Other symmetric block ciphers
Blowfish
A symmetric block cipher developed by Bruce Schneier.
Characteristics
Fast
Compact: can run less than 5K of memory.
Simple
Variably secure: the key length is variable and can be as long as 448 bits. This allows a tradeoff between higher speed and higher security.
64-bits block cipher

20. Other symmetric block ciphers
RC5
A symmetric encryption algorithm developed by Ron Rivest.
Adaptable to processors of different word lengths.
Variable number of rounds.
Variable-length key
Low memory requirement
Suitable for smart cards

21. Comparison