Chapter 31

CryptographyAnd

Network Security

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Chapter 31: Outline

331.1 331.1 INTRODUCTIONINTRODUCTION

331.2 331.2 CONFIDENTIALITYCONFIDENTIALITY

331.3 331.3 OTHER ASPECTS OF SECURITYOTHER ASPECTS OF SECURITY

Chapter 31: Objective

The first section introduces the subject. It first describes security

goals such as confidentiality, integrity, and availability. The section shows how confidentiality is threatened by attacks such as snooping and traffic analysis. The section then shows how integrity is threatened by attacks such as modification, masquerading, replaying, and repudiation. The section mentions one attack that threatens availability, denial of service. This section ends with describing the two techniques used in security: cryptography and steganography. The chapter concentrates on the first

Chapter 31: Objective (continued)

The second section discusses confidentiality. It first describes symmetric-key ciphers and explains traditional symmetric-key ciphers such as substitution and transposition ciphers. It then moves to modern symmetric-key ciphers and explains modern block and stream ciphers. The section then shows that denial of service is an attack to availability.

The third section discusses other aspects of security: message integrity, message authentication, digital signature, entity authentication. These aspects today are part of the security system that complements confidentiality. The section also describes the topic of key management including the distribution of keys for both symmetric-key and asymmetric-key ciphers.

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31-1 INTRODUCTION31-1 INTRODUCTION

We are living in the information age. Information is an asset that has a value like any other asset. As an asset, information needs to be secured from attacks. To be secured, information needs to be hidden from unauthorized access (confidentiality), protected from unauthorized change (integrity), and available to an authorized entity when it is needed (availability).

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331.31.1 Security Goals331.31.1 Security Goals

Let us first discuss three security goals: confidentiality, integrity, and availability.

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331.31.2 Attacks331.31.2 Attacks

Our three goals of security, confidentiality, Our three goals of security, confidentiality, integrity, and availability, can be threatened by integrity, and availability, can be threatened by security attacks. Although the literature uses security attacks. Although the literature uses different approaches to categorizing the attacks, different approaches to categorizing the attacks, we divide them into three groups related to the we divide them into three groups related to the security goals. Figure 331.1 shows the taxonomy.security goals. Figure 331.1 shows the taxonomy.

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Figure 331.1: Taxonomy of attacks with relation to security goals

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331.31.3 Services and Techniques331.31.3 Services and Techniques

ITU-T defines some security services to achieve security goals and prevent attacks. Each of these services is designed to prevent one or more attacks while maintaining security goals. The actual implementation of security goals needs some techniques. Two techniques are prevalent today: one is very general (cryptography) and one is specific (steganography).

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31-2 CONFIDENTIALITY 31-2 CONFIDENTIALITY

We now look at the first goal of security, confidentiality. Confidentiality can be achieved using ciphers. Ciphers can be divided into two broad categories: symmetric-key and asymmetric-key.

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331.2.1 Symmetric-Key Ciphers331.2.1 Symmetric-Key Ciphers

A symmetric-key cipher uses the same key for both A symmetric-key cipher uses the same key for both encryption and decryption, and the key can be encryption and decryption, and the key can be used for bidirectional communication, which is used for bidirectional communication, which is why it is called symmetric. Figure 331.2 shows the why it is called symmetric. Figure 331.2 shows the general idea behind a symmetric-key cipher.general idea behind a symmetric-key cipher.

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Figure 331.2: General idea of a symmetric-key cipher

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Figure 331.3: Symmetric-key encipherment as locking and unlocking with the same key

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Figure 331.4: Representation of plaintext and ciphertext characters in modulo 26

Use the additive cipher with key = 15 to encrypt the message “hello”.

Example 331.1

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Use the additive cipher with key = 15 to decrypt the message “WTAAD”.

Example 331.2

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Figure 331.5: An example key for a monoalphabetic substitution cipher

We can use the key in Figure 10.5 to encrypt the message

Example 331.3

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Assume that Alice and Bob agreed to use an autokey cipher with initial key value k1 = 12. Now Alice wants to send Bob the message “Attack is today”. The three occurrences of “t” are encrypted differently.

Example 331.4

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Figure 331.6: Transposition cipher

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Figure 331.7: A modern block cipher

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Figure 331.8: Components of a modern block cipher

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Figure 331.9: General structure of DES

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Figure 331.10: DES function

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Figure 331.11: Key generation

We choose a random plaintext block and a random key, and determine (using a program) what the ciphertext block would be (all in hexadecimal) as shown below.

Example 331.5

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To check the effectiveness of DES when a single bit is changed in the input, we use two different plaintexts with only a single bit difference (in a program). The two ciphertexts are completely different without even changing the key. Although the two plaintext blocks differ only in the rightmost bit, the ciphertext blocks differ in 29 bits.

Example 331.6

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Figure 331.12: One-time pad

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331.2.2 Asymmetric-Key Ciphers331.2.2 Asymmetric-Key Ciphers

In previous sections we discussed symmetric-key In previous sections we discussed symmetric-key ciphers. In this section, we start the discussion of ciphers. In this section, we start the discussion of asymmetric-key ciphers. Symmetric- and asymmetric-key ciphers. Symmetric- and asymmetric-key ciphers will exist in parallel and asymmetric-key ciphers will exist in parallel and continue to serve the community. We actually continue to serve the community. We actually believe that they are complements of each other; believe that they are complements of each other; the advantages of one can compensate for the the advantages of one can compensate for the disadvantages of the other.disadvantages of the other.

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Figure 331.13: Locking and unlocking in asymmetric-key cryptosystem

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Figure 331.14: General idea of asymmetric-key cryptosystem

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Figure 331.15: Encryption, decryption, and key generation in RSA

For the sake of demonstration, let Bob choose 7 and 11 as p and q and calculate n = 7 × 11 = 77, φ(n) = (7 − 1)(11 − 1), or 60. If he chooses e to be 13, then d is 37. Note that e × d mod 60 = 31. Now imagine that Alice wants to send the plaintext 5 to Bob. She uses the public exponent 13 to encrypt 5. This system is not safe because p and q are small.

Example 331.7

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Here is a more realistic example calculated using a computer program in Java. We choose a 512-bit p and q, calculate n and φ(n). We then choose e and calculate d. Finally, we show the results of encryption and decryption. The integer p is a 159-digit number.

Example 331.8

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Example 331.8 (continued)

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Example 331.8 (continued)

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Example 331.8 (continued)

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31-3 OTHER ASPECTS OF SECURITY31-3 OTHER ASPECTS OF SECURITY

The cryptography systems that we have studied so far provide confidentiality. However, in modern communication, we need to take care of other aspects of security, such as integrity, message and entity authentication, nonrepudiation, and key management. We briefly discuss these issues in this section.

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331.3.1 Message Integrity331.3.1 Message Integrity

There are occasions where we may not even need secrecy but instead must have integrity: the message should remain unchanged. For example, Alice may write a will to distribute her estate upon her death. The will does not need to be encrypted. After her death, anyone can examine the will. The integrity of the will, however, needs to be preserved. Alice does not want the contents of the will to be changed.

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Figure 331.16: Message and digest

Insecure channel

Channel immune to change

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331.3.2 Message Authentication331.3.2 Message Authentication

A digest can be used to check the integrity of a message—that the message has not been changed. To ensure the integrity of the message and the data origin authentication—that Alice, not somebody else, is the originator of the message—we need to include a secret shared by Alice and Bob (that Eve does not possess) in the process; we need to create a message authentication code (MAC). Figure 331.17 shows the idea.

Figure 10.17: Message authentication code

M + MAC

Insecure channel

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331.3.3 Digital Signature331.3.3 Digital Signature

Another way to provide message integrity and message authentication (and some more security services, as we will see shortly) is a digital signature. A MAC uses a secret key to protect the digest; a digital signature uses a pair of private-public keys.

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Figure 331.18: Digital signature process

(M, S)

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Figure 331.19: Signing the digest

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Figure 331.20: Using a trusted center for non-repudiation

(M, SA) (M, ST)

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Figure 331.21: The RSA signature on the message digest

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331.3.4 Entity Authentication331.3.4 Entity Authentication

Entity authentication is a technique designed to let one party verify the identity of another party. An entity can be a person, a process, a client, or a server. The entity whose identity needs to be proven is called the claimant; the party that tries to verify the identity of the claimant is called the verifier.

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Figure 331.22: Unidirectional, symmetric-key authentication

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Figure 331.23: Unidirectional, asymmetric-key authentication

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Figure 331.24: Digital signature, unidirectional authentication

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331.3.5 Key Management331.3.5 Key Management

We discussed symmetric-key and asymmetric-key cryptography in the previous sections. However, we have not yet discussed how secret keys in symmetric-key cryptography, and public keys in asymmetric-key cryptography, are distributed and maintained. This section touches on these two issues.

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Figure 331.25: Multiple KDCs

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Figure 331.26: Creating a session key using KDC

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Figure 331.27: Diffie-Hellman method

Example 331.9

Let us give a trivial example to make the procedure clear. Our example uses small numbers, but note that in a real situation, the numbers are very large. Assume that g = 7 and p = 23. The steps are as follows:

31. Alice chooses x = 3 and calculates R1 = 73 mod 23 = 231.

Bob chooses y = 6 and calculates R2 = 76 mod 23 = 4.

2. Alice sends the number 21 to Bob.

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Example 331.9 (continued)

3. Bob sends the number 4 to Alice.

4. Alice calculates the symmetric key K = 43 mod 23 = 18. Bob calculates the symmetric key K = 216 mod 23 = 18.

Conclusion:The value of K is the same for both Alice and Bob; gxy mod p = 718 mod 23 = 18.

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Figure 331.28: Certification authority