Ch. 3 Cryptographic Tools

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

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 pre-image resistant 5. For any given code h, it is computationally infeasible to find y does not equal 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.

List three approaches to message authentication

1. message authentication code 2. one-way hash function: public-key encryption and digital signature 3. secure hash function:

What are the two principal requirements for the secure use of symmetric encryption?

1. strong encryption algorithm. to be most secure, the opponent should be unable to decrypt the ciphertext or discover the key even if he/she is in possession of the ciphertexts together with the plaintext 2. sender and receiver must have obtained copies of the secret key in a secure fashion and must keep the key secure. if someone can discover the key and knows the algorithm, all communication using this key is readable

What are the essential ingredients of a symmetric cipher?

five ingredients: plaintext, encryption algorithm, secret key, ciphertext, decryption algorithm

How many keys are required for two people to communicate via a symmetric cipher?

one, single key

Briefly describe the three schemes illustrated in Fig 3

the message plus code are transmitted to the intended recipient. the recipient performs the same calculation on the received message, using the same secret key, to generate a new message authentication code. if only the receiver and sender know the identity of the secret key, and received code matches the calculated code, then MAC m = F(K ab, M) 3

What is a message authentication code?

the use of a secret key to generate a small block of data, message authentication code (MAC) appended to the message. two communication parties share a common secret key.