Cryptography Reference
In-Depth Information
11
. The Vigenère Cipher is of historical importance.
(a) Encrypt MY HOME IS NEW ZEALAND using a Vigenère Cipher with
keyword KEA.
(b) The ciphertext JNHYSMCDJOPwas obtained by encrypting an English word
using a Vigenère Cipher:
i. The first and ninth letters of the plaintext are identical. What does this tell you about
the key?
ii. Given that the keyword length is less than 7, the third and fourth letters of the plaintext
are F and O, respectively, and that A is a letter in the keyword, find the keyword and
the plaintext.
(c) If the keyword of a Vigenère Cipher has repeated letters, does this make it
any easier to break?
12
.
In this chapter we suggested a technique for cryptanalysing the Vigenère
Cipher. Discuss at least two different situations where it will be hard to break
the Vigenère Cipher using the technique that we described.
13
.
In this exercise we consider the Simple Substitution Cipher as a 5-bit block
cipher. The letters A
=
1, B
=
2, C
=
3,
...
,Z
=
26 are first represented as
binary strings (5 bits per letter) as follows: A
=
00001, B
=
00010, C
=
00011,
...
11010. Let the key of a Simple SubstitutionCipher be given by (plaintext
in top row above is replaced by bold ciphertext in bottom row):
,Z
=
ABCDEFGHI J KLM
GRKZLABXTNMEQ
NOPQRSTUVWXYZ
WFVYUSPJHODI C
(a) Write the plaintext TODAY as a binary string.
(b) Encrypt TODAY using the above cryptosystem and write the ciphertext as
a binary string.
(c) Change the second bit of the above ciphertext and then decrypt this altered
ciphertext.
(d) How many bits of the resulting plaintext are incorrect?
14
. What are the implications of a known-plaintext attack on:
(a) The Caesar Cipher?
(b) The Simple Substitution Cipher?
(c) The Vigenère Cipher?
15
. Although the Simple Substitution Cipher and the Playfair Cipher are simple
enough to conduct encryption and decryption by 'hand', their keys are not
straightforward to memorise.
