This document introduces cryptography and discusses symmetric-key and asymmetric-key cryptography. Symmetric-key cryptography uses the same key for encryption and decryption, while asymmetric-key cryptography uses two keys: a private key and a public key. Traditional ciphers like the shift cipher and transposition cipher are described. Modern symmetric ciphers include the XOR cipher, rotation cipher, and DES. Asymmetric ciphers like RSA are also introduced, which use a public key and private key based on large prime numbers. AES is presented as the replacement for the DES standard.

1. 30.1
Chapter 30
Cryptography
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. 30.2
30-1 INTRODUCTION
Let us introduce the issues involved in cryptography.
First, we need to define some terms; then we give some
taxonomies.
Definitions
Two Categories
Topics discussed in this section:

3. 30.3
Figure 30.1 Cryptography components

4. 30.4
Figure 30.2 Categories of cryptography

5. 30.5
Figure 30.3 Symmetric-key cryptography

6. 30.6
In symmetric-key cryptography, the
same key is used by the sender
(for encryption)
and the receiver (for decryption).
The key is shared.
Note

7. 30.7
Figure 30.4 Asymmetric-key cryptography

8. 30.8
Figure 30.6 Comparison between two categories of cryptography

9. 30.9
30-2 SYMMETRIC-KEY CRYPTOGRAPHY
Symmetric-key cryptography started thousands of years
ago when people needed to exchange secrets (for
example, in a war). We still mainly use symmetric-key
cryptography in our network security.
Traditional Ciphers
Simple Modern Ciphers
Modern Round Ciphers
Mode of Operation
Topics discussed in this section:

10. 30.10
Figure 30.7 Traditional ciphers

11. 30.11
A substitution cipher replaces one
symbol with another.
Monoalphabetic replaces the same symbol
with the same another symbol.
Polyalphabetic replaces the same symbol
with different symbols at each occurrence.
Note

12. 30.12
The shift cipher is sometimes referred to
as the Caesar cipher. (monoalphabetic)
Note

13. 30.13
Use the shift cipher with key = 15 to encrypt the message
“HELLO.”
Solution
We encrypt one character at a time. Each character is
shifted 15 characters “down”. Letter H is encrypted to W.
Letter E is encrypted to T. The first L is encrypted to A.
The second L is also encrypted to A. And O is encrypted to
D. The cipher text is WTAAD.
Example 30.3

14. 30.14
Use the shift cipher with key = 15 to decrypt the message
“WTAAD.”
Solution
We decrypt one character at a time. Each character is
shifted 15 characters “up”. Letter W is decrypted to H.
Letter T is decrypted to E. The first A is decrypted to L.
The second A is decrypted to L. And, finally, D is
decrypted to O. The plaintext is HELLO.
Example 30.4

15. 30.15
A transposition cipher reorders
(permutes) symbols in a block of
symbols (shuffle poker cards)
Note

16. 30.16
Figure 30.8 Transposition cipher

17. 30.17
Encrypt the message “HELLO MY DEAR,” using the key
shown in Figure 30.8.
Solution
We first remove the spaces in the message. We then divide
the text into blocks of four characters. We add a bogus
character Z at the end of the third block. The result is
HELL OMYD EARZ. We create a three-block ciphertext
ELHLMDOYAZER.
Example 30.5

18. 30.18
Using Example 30.5, decrypt the message
“ELHLMDOYAZER”.
Solution
The result is HELL OMYD EARZ. After removing the
bogus character and combining the characters, we get the
original message “HELLO MY DEAR.”
Example 30.6

19. 30.19
Modern Cryptography

20. 30.20
Figure 30.9 XOR cipher

21. 30.21
Figure 30.10 Rotation cipher

22. 30.22
Figure 30.11 S-box (substitution box)

23. 30.23
Figure 30.12 P-boxes (permutation box): straight, expansion, and compression

24. 30.24
Figure 30.13 DES (Data Encryption Standard)

25. 30.25
Figure 30.14 One round in DES ciphers

26. 30.26
Figure 30.16 Triple DES (to resolve the short key issue for DES)

27. 30.27
Table 30.1 AES (advanced encryption standard) configuration
AES is the replacement of DES
AES has three different configurations
with respect to the number of rounds
and key size.
Note

28. 30.28
30-3 ASYMMETRIC-KEY CRYPTOGRAPHY
An asymmetric-key (or public-key) cipher uses two
keys: one private and one public. We discuss two
algorithms: RSA and Diffie-Hellman.
RSA
Diffie-Hellman
Topics discussed in this section:

29. 30.29
Figure 30.24 RSA

30. RSA: Choosing keys
1. Choose two large prime numbers p, q.
(e.g., 1024 bits each)
2. Compute n = pq, F = (p-1)(q-1)
3. Choose e (with e<n) that has no common factors
with F. (e, F are “relatively prime”).
4. Choose d such that ed-1 is exactly divisible by F.
(in other words: ed mod F = 1 ).
5. Public key is (n,e). Private key is (n,d).
KB
+
KB
-

31. RSA: Encryption, decryption
0. Given (n,e) and (n,d) as computed above
1. To encrypt bit pattern, m, compute
c = m mod n
e
(i.e., remainder when m is divided by n)
e
2. To decrypt received bit pattern, c, compute
m = c mod n
d
(i.e., remainder when c is divided by n)
d
m = (m mod n)
e mod n
d
Magic
happens! c

32. RSA example:
Bob chooses p=5, q=7. Then n=35, F =24.
e=5 (so e, F relatively prime).
d=29 (so ed-1 exactly divisible by F).
letter m m
e c = m mod n
e
l 12 1524832 17
c m = c mod n
d
17 481968572106750915091411825223071697
12
cd letter
l
encrypt:
decrypt:
Computational very extensive

33. 30.33
In RSA, e and n are announced to the
public; d and F are kept secret.
Public cryptography is very
computational expensive.
Note