Cryptography Reference
In-Depth Information
9
. Hash functions are sometimes used as a practical means of pseudorandomly
generating numbers.
(a) Why is it reasonable to expect a hash function to generate 'random' ouput?
(b) What are the possible advantages of using a hash function as a pseudoran-
dom number generator?
10
. The SHA-3 'competition' is an AES-style process designed to standardise hash
functions.
(a) What were the SHA-3 hash function competition design criteria?
(b) Based on themost current information that you can find, write a short report
on this competition.
11
. A block cipher with an unknown key is used in ECB mode to encrypt the
plaintext:
The order is Karl, Andy, Fred and Ian. Ian and Andy have left
. All that
is known about the block cipher is that:
• it encrypts plaintext blocks consisting of two letters into ciphertext blocks
consisting of two letters;
• punctuation and spaces are ignored.
Suppose that the original ciphertext is
C
1
,
C
2
,
C
3
...,
C
23
.
(a) Find two numbers
i
and
j
such that
C
i
=
C
j
;
(b) Decrypt the ciphertext:
C
1
,
C
2
,
C
3
,
C
4
,
C
5
,
C
10
,
C
11
,
C
6
,
C
7
,
C
8
,
C
9
,
C
12
,
C
13
,
C
14
,
C
15
,
C
16
,
C
17
,
C
6
,
C
7
,
C
20
,
C
21
,
C
22
,
C
23
(c) Decrypt the ciphertext:
C
1
,
C
2
,
C
3
,
C
4
,
C
5
,
C
10
,
C
11
,
C
12
,
C
13
,
C
14
(d) Decrypt the ciphertext:
C
10
,
C
8
,
C
6
,
C
17
,
C
6
,
C
7
,
C
20
,
C
21
,
C
22
,
C
23
(e) Find another sequence of ciphertext blocks that decrypt to a plausible
plaintext.
(f) What lesson about the delivery of security services does this example
demonstrate?
12
. Explain the extent to which a MAC prevents the following attacks:
(a) unauthorised changing of part of a message;
(b) unauthorised deletion of part of a message;
(c) unauthorised sending of a false message;
(d) unauthorised replaying of a previously sent message.
13
. Alice and Charlie are flavour technicians at a food processing company. Alice
shares key
K
A
with Boss Bob and Charlie shares key
K
C
with Bob. Suppose that:
