CS260 Study Guide

5a. Explain authentication, confidentiality, and non-repudiation

• What is authentication?
• What is confidentiality?
• What is non-repudiation?

The goal of confidentiality is to ensure the contents of a message cannot be determined by unintended receivers. This course emphasizes encryption techniques to ensure confidentiality. Authentication means that a receiver can verify the source of a message. Here, the terminology has the potential to be confusing. Integrity refers to when a message can be verified that it has not been modified. To be sure, sender authentication attempts to verify the source of a message, and message integrity attempts to authenticate message content. Message authentication codes (MAC) are codes derived from a message that can be used to achieve integrity and authentication. Finally, non-repudiation means that a sender cannot deny sending a message (it can also mean a receiver cannot deny receiving a message). Digital signatures can be used to achieve non-repudiation. Make sure to understand the importance of these definitions in network security applications. Additionally, make sure you understand how hash functions, symmetric cryptography, and public key cryptography in various configurations can be used to achieve authentication, confidentiality, and non-repudiation.

To review, see:

• Introduction to IT Security

5b. Apply architectures for message integrity

• What is a message authentication code?
• How are MACs used in practice?
• How are MACs implemented?

With an understanding of how to implement hash functions, symmetric cryptography, and asymmetric cryptography, it is now sensible to apply them in a larger context. Given the definitions of confidentiality, integrity, and authentication, it is important to understand how to devise configurations (or "architectures") that can achieve these characteristics in network security applications. One major step to going beyond encryption for confidentiality is to understand how to implement a system to achieve message integrity and sender authentication. This is because messages need not always be confidential, but you would still like to be able to verify the message contents have not been altered. For example, you could send a message M along with an encrypted version E(K,M) using a symmetric system. Assuming the receiver has the key K, the decrypted contents D(K,M) can be compared with the received message. Thus, E(K,M) can be used as a message authentication code (MAC) where E(K,M) is the authenticator. This scheme authenticates the message and authenticates the sender (assuming only the sender and receiver have the key, K). You should understand how different architectures can be used to construct MACs.

To review, see:

• Introduction to MAC Architectures and Configuration

5c. Explain keyed hash functions

• What is a keyed hash function?
• Why are keyed hash functions useful?
• How are keyed hash functions implemented?

When designing MACs, the prevailing school of thought is that hash functions run faster than typical encryption methods for use as the authenticator. Furthermore, as you have already seen, libraries for implementing hash functions are readily available. Keyed hash functions are simply hash functions that require extra secret knowledge (such as a key) to encode the message hash. Such an approach can be as simple as encrypting the hash of a message. The HMAC is a standard devised by NIST for designing keyed hash functions. A popular example of a standard keyed hash function is HMAC RFC 2104. You should understand how keyed hash function implementations can be used to achieve authentication.

To review, see:

• Hash Functions in Authentications

5d. Explain digital signatures

• What is a digital signature?
• How do digital signatures help to achieve non-repudiation?
• How are digital signatures implemented?

Even if the sender of a message can be authenticated, the sender can still deny having sent the message. In other words, the message's source can be known, but the sender's actions must still be verified. Digital signatures are a way of achieving non-repudiation. A digital signature is a way of signing a message so that the sender cannot deny sending the message. One common way to achieve this is to use a public key cryptosystem. For example, a sender encrypting with their private key so that the public key can be used for decryption will not achieve confidentiality, but the private key can act as a digital signature because only the sender has the private key. You should be familiar with architectures that achieve non-repudiation.

To review, see:

• Overview
• More on Digital Signatures

Unit 5 Vocabulary

This vocabulary list includes terms you will need to know to successfully complete the final exam.

• digital signature
• HMAC
• keyed hash function
• message authentication code (MAC)
• non-repudiation