![Chapter 2 Conventional Encryption Message Confidentiality Henric Johnson Blekinge Institute of Technology, Sweden http://www.its.bth.se/staff/hjo/ [email_address]](https://image.slidesharecdn.com/chapter-ns2-6656/85/Conventional-Encryption-NS2-1-320.jpg)
![Triple DEA Use three keys and three executions of the DES algorithm (encrypt-decrypt-encrypt) C = ciphertext P = Plaintext EK[X] = encryption of X using key K DK[Y] = decryption of Y using key K Effective key length of 168 bits C = E K3 [D K2 [E K1 [P]]]](https://image.slidesharecdn.com/chapter-ns2-6656/85/Conventional-Encryption-NS2-15-320.jpg)
Conventional Encryption NS2
- 1. Chapter 2 Conventional Encryption Message Confidentiality Henric Johnson Blekinge Institute of Technology, Sweden http://www.its.bth.se/staff/hjo/ [email_address]
- 2. Outline Conventional Encryption Principles Conventional Encryption Algorithms Cipher Block Modes of Operation Location of Encryption Devices Key Di s tribution
- 3. Conventional Encryption Principles An encryption scheme has five ingredients: Plaintext Encryption algorithm Secret Key Ciphertext Decryption algorithm Security depends on the secrecy of the key, not the secrecy of the algorithm
- 4. Conventional Encryption Principles
- 5. Cryptography Classified along three independent dimensions: The type of operations used for transforming plaintext to ciphertext The number of keys used symmetric (single key) asymmetric (two-keys, or public-key encryption) The way in which the plaintext is processed
- 6. Average time required for exhaustive key search 2.15 milliseconds 2 32 = 4.3 x 10 9 32 5.9 x 10 30 years 2 168 = 3.7 x 10 50 168 5.4 x 10 18 years 2 128 = 3.4 x 10 38 128 10 hours 2 56 = 7.2 x 10 16 56 Time required at 10 6 Decryption/ µs Number of Alternative Keys Key Size (bits)
- 7. Feistel Cipher Structure Virtually all conventional block encryption algorithms, including DES have a structure first described by Horst Feistel of IBM in 1973 The realisation of a Fesitel Network depends on the choice of the following parameters and desi gn features (see next slide) :
- 8. Feistel Cipher Structure Block size: larger block sizes mean greater security Key Size: larger key size means greater security Number of rounds: multiple rounds offer increasing security Subkey generation algorithm: greater complexity will lead to greater difficulty of cryptanalysis. Fast software encryption/decryption: the speed of execution of the algorithm becomes a concern
- 10. Conventional Encryption Algorithms Data Encryption Standard (DES) The most widely used encryption scheme The algorithm is reffered to the Data Encryption Algorithm (DEA) DES is a block cipher The plaintext is processed in 64-bit blocks The key is 56-bits in length
- 13. DES The overall processing at each iteration: L i = R i-1 R i = L i-1 F(R i-1 , K i ) Concerns about: The algorithm and the key length (56-bits)
- 14. Time to break a code (10 6 decryptions/µs)
- 15. Triple DEA Use three keys and three executions of the DES algorithm (encrypt-decrypt-encrypt) C = ciphertext P = Plaintext EK[X] = encryption of X using key K DK[Y] = decryption of Y using key K Effective key length of 168 bits C = E K3 [D K2 [E K1 [P]]]
- 16. Triple DEA
- 17. Other Symmetric Block Ciphers International Data Encryption Algorithm (IDEA) 128-bit key Used in PGP Blowfish Easy to implement High execution speed Run in less than 5K of memory
- 18. Other Symmetric Block Ciphers RC5 Suitable for hardware and software Fast, simple Adaptable to processors of different word lengths Variable number of rounds Variable-length key Low memory requirement High security Data-dependent rotations Cast-128 Key size from 40 to 128 bits The round function differs from round to round
- 19. Cipher Block Modes of Operation Cipher Block Chaining Mode (CBC) The input to the encryption algorithm is the XOR of the current plaintext block and the preceding ciphertext block. Repeating pattern of 64-bits are not exposed
- 21. Location of Encryption Device Link encryption: A lot of encryption devices High level of security Decrypt each packet at every switch End-to-end encryption The source encrypt and the receiver decrypts Payload encrypted Header in the clear High Security: Both link and end-to-end encryption are needed (see Figure 2.9)
- 23. Key Distribution A key could be selected by A and physically delivered to B. A third party could select the key and physically deliver it to A and B. If A and B have previously used a key, one party could transmit the new key to the other, encrypted using the old key. If A and B each have an encrypted connection to a third party C, C could deliver a key on the encrypted links to A and B.
- 24. Key Distribution (See Figure 2.10) Session key: Data encrypted with a one-time session key.At the conclusion of the session the key is destroyed Permanent key: Used between entities for the purpose of distributing session keys
- 26. Recommended Reading Stallings, W. Cryptography and Network Security: Principles and Practice, 2 nd edition . Prentice Hall, 1999 Scneier, B. Applied Cryptography , New York: Wiley, 1996 Mel, H.X. Baker, D. Cryptography Decrypted . Addison Wesley, 2001