CIS 3362 Quiz #7 – Message Authentication and Hash Functions
Date: 11/18/2009
Name: ________________________________
Directions: Show your work and clearly mark your final answer.
1) (5 pts) Give an example of sequencing modification that distorts the original intention of a set
of messages. In particular, first list the original set of messages sent between A and B, then
describe the sequence modification of those messages and how it distorts the original intent.
2) (2 pts) Circle the correct answer for each part.
(a) MACs use secret keys.
TRUE
FALSE
(b) Hash functions use secret keys.
TRUE
FALSE
3) (12 pts) Each of the following shows a message sent from A to B, using the textbook’s
notation. Determine whether each of these messages provides confidentiality, authentication and
a signature. Circle each choice that applies.
(a) E(K, M)
Confidentiality
Authentication
Signature
(b) E(PUb, M)
Confidentiality
Authentication
Signature
(c) E(PRa, M)
Confidentiality
Authentication
Signature
(d) E(PUb, E(PRa, M))
Confidentiality
Authentication
Signature
4) (6 pts) Give two reasons why a MAC might be preferred for authentication over regular
symmetric encryption.
5) (8 pts) Consider a situation where the key used for a MAC has 128 bits and the output of the
MAC function is 32 bits. How many matching plaintext-MAC pairs would be necessary (on
average), for someone to use brute force to determine the key used? How much time and how
much memory (approximately) would such an attack entail? (Leave your answers to the last
question in powers of 2, in seconds, and assume each key check takes 1 second.)
6) (6 pts) Describe the difference between weak and strong collision resistance for hash
functions.
7) (8 pts) A hash function, h, produces a 10 bit output. Given that we compute the seven values
h(x1), h(x2), h(x3), h(x4), h(x5), h(x6), h(x7), and each of the seven values x1, x2, x3, x4, x5, x6, x7
are distinct, what is the probability that the at least two of the seven output values (h(xi), 1 ≤ i ≤
7) are exactly the same, assuming that the function is an ideal hash function?
8) (3 pts) On what day of the week does, Black Friday, the busiest shopping day of the year,
occur?