Cryptography Reference
In-Depth Information
ciphers that are either published in international standards and/or adopted by
commercial applications.
3
. Find out what the difference is between
synchronous
and
self-synchronous
stream ciphers, using examples from this chapter to explain your answer.
4
. The Feistel Cipher design is very clever.
(a) Verify the claim that decryption using a Feistel Cipher is essentially the same
as encryption. One way of explaining this is by using the simple equations
that describe what happens at each round, although you can also explain
this by describing the process in words.
(b) From a practical (implementation) perspective, what are the significant
advantages of the Feistel Cipher design structure?
5
. Triple DES is a widely used block cipher whose design is motivated by practical
considerations.
(a) Explain what is meant by 'backwards compatibility'.
(b) Describe three different ways in which keys could be chosen in order to
make Triple DES backwards compatible with (single) DES.
6
. Both Triple DES and the much weaker 'Double DES' are subject to attacks
known as
meet-in-the-middle
attacks.
(a) Find out, and then explain (roughly), how this type of attack works.
(b) Explain why this type of attack is not so effective against AES.
7
. Triple DES is just one idea for creating a stronger block cipher using DES. An
alternative idea is DES-X, which is an encryption algorithm built from DES in the
following way:
• Let
K
1
and
K
2
be two 64-bit keys and let
K
be a DES key.
• Let
DES
K
(
P
) denote encryption of plaintext
P
using normal single DES with
key
K
.
• For a plaintext
P
we define the encryption of
P
using DES-X to be:
C
=
K
2
⊕
DES
K
(
P
⊕
K
1
)
.
(a) What is the effective length of a DES-X key?
(b) Explain how to decrypt a ciphertext using DES-X.
(c) Is DES-X backwards compatible with single DES?
8
. Suppose that we use AES with a 128-bit randomly generated key to encrypt
a single block of 128 bits, and use the key only once. Is this an example of a
one-time pad?
9
. AES is a very popular symmetric encryption algorithm.
(a) What applications of cryptography that you have used support the use of
AES, and which key lengths do they recommend?
