SECURE COMMUNICATIONS (SEC150900-2010FA) Jamie L. Lawson

Chapter 3 Review Questions

3.1 List three approaches to message authentication.

1. Message authentication code (MAC)2. One-way hash function3. HMAC

3.2 What is a message authentication code?

An authentication technique that involves the use of a secret key to generate a small block of data that is appended to the message.

3.3 Briefly describe the three schemes illustrated in Figure 3.2.

The message digest is encrypted using conventional encryption, public-key encryption and a hash function to be authenticated.

3.4 What properties must a hash function have to be useful for message authentication?

To be useful for message authentication, a hash function H must have the following properties:

1. H can be applied to a block of data of any size.2. H produces a fixed-length output.3. H(x) is relatively easy to compute for any given x, making both

hardware and software implementations practical.4. For any given code h, it is computationally infeasible to find x such

that H(x) = h. A hash function with this property is referred to as one-way or preimage resistant

5. For any given block x, it is computationally infeasible to find y != x with H(y) = H(x). A hash function with this property is referred to as second preimage resistant. This is sometimes referred to as weak collision resistant.

6. It is computationally infeasible to find any pair (x, y) such that H(x) = H(y).A hash function with this property is referred to as collision resistant. This is sometimes referred to as strong collision resistant.

3.5 In the context of a hash function, what is a compression function?

A compression function takes a fixed-length input and returns a shorter, fixed-length output.

3.6 What are the principal ingredients of a public-key cryptosystem?

SECURE COMMUNICATIONS (SEC150900-2010FA) Jamie L. Lawson

Plaintext: This is the readable message or data that is fed into the algorithm input.

Encryption algorithm: The encryption algorithm performs various transformationson the plaintext.

Public and private key : This is a pair of keys that have been selected so that ifone is used for encryption, the other is used for decryption. The exact transformations performed by the encryption algorithm depend on the public or private key that is provided as input

Ciphertext: This is the scrambled message produced as output. It depends onthe plaintext and the key. For a given message, two different keys will producetwo different ciphertexts.

Decryption algorithm: This algorithm accepts the ciphertext and the matchingkey and produces the original plaintext.

3.7 List and briefly define three uses of a public-key cryptosystem.

1. Encryption/decryption2. Digital signature3. Key exchange

3.8 What is the difference between a private key and a secret key?

The difference is the private key remains private to be used by one person to encrypt and decrypt and the public key is for anyone to use to that has it.

3.9 What is digital signature?

A digital signature is an electronic signature that can be used to authenticate the identity of the sender of a message or the signer of a document, and possibly to ensure that the original content of the message or document that has been sent is unchanged.