1. Lecture 2.2: Private Key Cryptography II
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Lecture 2.2: Private Key Cryptography II
CS 436/636/736
Spring 2013
Nitesh Saxena

2. Today’s fun/informative bit – The Smudge Attack
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Today’s fun/informative bit – The Smudge Attack
See:
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Lecture Private Key Cryptography II

3. Course Administration
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Course Administration
Everything seems to be under control 
Any questions?
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Lecture Private Key Cryptography II

4. Outline of today’s lecture
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Outline of today’s lecture
Block Ciphers
Data Encryption Standard (DES)
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Lecture Private Key Cryptography II

5. Block Ciphers and Stream Ciphers
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Block Ciphers and Stream Ciphers
Block ciphers partition plaintext into blocks and encrypt each block independently (with the same key) to produce ciphertext blocks.
A stream cipher generates a keystream and encrypts by combining the keystream with the plaintext, usually with the bitwise XOR operation.
We will focus mostly on Block Ciphers
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Lecture Private Key Cryptography II

6. DES – Data Encryption Standard
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DES – Data Encryption Standard
Encrypts by series of substitution and transpositions.
Based on Feistel Structure
Worldwide standard for more than 20 years.
Designed by IBM (Lucifer) with later help from NSA.
No longer considered secure for highly sensitive applications.
Replacement standard AES (advanced encryption standard) recently completed.
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Lecture Private Key Cryptography II

7. DES – Overview (Block Operation)
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DES – Overview (Block Operation)
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Lecture Private Key Cryptography II

8. 9/19/2018
DES – Each Round
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9. Lecture 2.2 - Private Key Cryptography II
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DES – Function F
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Lecture Private Key Cryptography II

10. Lecture 2.2 - Private Key Cryptography II
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DES – Key Schedule (KS)
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Lecture Private Key Cryptography II

11. Operation Tables of DES: Key Schedule, PC-1, PC-2
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Operation Tables of DES: Key Schedule, PC-1, PC-2
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Lecture Private Key Cryptography II

12. Operation Tables (IP, IP-1, E and P)
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Operation Tables (IP, IP-1, E and P)
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Lecture Private Key Cryptography II

13. S-boxes: S1 (as an example)
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S-boxes: S1 (as an example)
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111 00 01 10 11
Is the table entry from
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Lecture Private Key Cryptography II

14. Lecture 2.2 - Private Key Cryptography II
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DES Decryption
Same as the encryption algorithm with the “reversed” key schedule – NEXT!
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Lecture Private Key Cryptography II

15. Initial permutation (IP)
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Plain text
Initial permutation (IP)
Round-1 (key K1)
Rounds 2-15
Round-16 (key K16)
swap
IP inverse
Cipher text
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16. Lecture 2.2 - Private Key Cryptography II
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encrypt
IP inverse
Cipher text IP
Round-1 (K16)
decrypt =
Since
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Lecture Private Key Cryptography II

17. Lecture 2.2 - Private Key Cryptography II
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DES Example
We choose a random plaintext block and a random key, and determine what the ciphertext block would be (all in hexadecimal):
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Lecture Private Key Cryptography II

18. Example (contd) -- encryption
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Example (contd) -- encryption
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Lecture Private Key Cryptography II

19. Example (contd) -- decryption
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Example (contd) -- decryption
Let us see how Bob, at the destination, can decipher the ciphertext received from Alice using the same key. Table 6.16 shows some interesting points.
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Lecture Private Key Cryptography II

20. DES Security: Avalanche Effect
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DES Security: Avalanche Effect
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Lecture Private Key Cryptography II

21. Lecture 2.2 - Private Key Cryptography II
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Further Reading
Chapter 7.4 of HAC
Chapter 3 of Stallings
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Lecture Private Key Cryptography II