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Exercise 2 (MAC schemes derived from block ciphers ( Let E denote the family of encryption functions for a block cipher where plaintext blocks, ciphertext blocks, and keys are each n = 80 bits in length. Let H: {0,1} {0, 1}" be a hash function. Assume that plaintext messages m all have bit-lengths that are multiples of n; we write m = (m₁, m₂,..., m.), i.e. m is composed of t blocks m, of n bits each. Consider the following MAC schemes, each using an n-bit secret key k. 1. MAC, (m) = c₁, where co= 0, and c = c₁-1 m, ⒸE(m) for 1 ≤ i ≤t. 2. MACK(m) Ek(H(m)). Are any of these MAC schemes secure? Justify your answer and clearly state any assumptions you make.

Exercise 2 (MAC schemes derived from block ciphers ( Let E denote the family of encryption functions for a block cipher where plaintext blocks, ciphertext blocks, and keys are each n = 80 bits in length. Let H: {0,1} {0, 1}" be a hash function. Assume that plaintext messages m all have bit-lengths that are multiples of n; we write m = (m₁, m₂,..., m.), i.e. m is composed of t blocks m, of n bits each. Consider the following MAC schemes, each using an n-bit secret key k. 1. MAC, (m) = c₁, where co= 0, and c = c₁-1 m, ⒸE(m) for 1 ≤ i ≤t. 2. MACK(m) Ek(H(m)). Are any of these MAC schemes secure? Justify your answer and clearly state any assumptions you make.

Computer Networking: A Top-Down Approach (7th Edition)
Computer Networking: A Top-Down Approach (7th Edition)
7th Edition
ISBN:9780133594140
Author:James Kurose, Keith Ross
Publisher:James Kurose, Keith Ross
Chapter1: Computer Networks And The Internet
Section: Chapter Questions
Problem R1RQ: What is the difference between a host and an end system? List several different types of end...
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Exercise 2 (MAC schemes derived from block ciphers ( Let E denote the family of encryption functions for a block cipher where plaintext blocks, ciphertext blocks, and keys are each n = 80 bits in length. Let H: {0,1} {0, 1}" be a hash function. Assume that plaintext messages m all have bit-lengths that are multiples of n; we write m = (m₁, m₂,...,m), i.e. m is composed of t blocks m, of n bits each. Consider the following MAC schemes, each using an n-bit secret key k. 1. MAC (m) = ct, where co= 0, and c = c-1m, E(m) for 1 ≤ i ≤t. 2. MACk (m) Ek(H(m)). = Are any of these MAC schemes secure? Justify your answer and clearly state any assumptions you make.
Transcribed Image Text:Exercise 2 (MAC schemes derived from block ciphers ( Let E denote the family of encryption functions for a block cipher where plaintext blocks, ciphertext blocks, and keys are each n = 80 bits in length. Let H: {0,1} {0, 1}" be a hash function. Assume that plaintext messages m all have bit-lengths that are multiples of n; we write m = (m₁, m₂,...,m), i.e. m is composed of t blocks m, of n bits each. Consider the following MAC schemes, each using an n-bit secret key k. 1. MAC (m) = ct, where co= 0, and c = c-1m, E(m) for 1 ≤ i ≤t. 2. MACk (m) Ek(H(m)). = Are any of these MAC schemes secure? Justify your answer and clearly state any assumptions you make.
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