Description
Problem 1 (1 point). Given a ciphertext JSSXFEPP encrypted by Shift Cipher, compute the key of
shift cipher and the original message using brute-force attacks. In this problem, we assume the original
message “makes sense” and is human-readable. The message space includes all the lower case characters,
i.e., M = {a, b, …, z}, key space is K = {0, 1, …, 25} and ciphertext space is C = {A, B, …, Z}.
Problem 2 (1 point). Given an encryption key (i.e., a permutation) of Substitution Cipher presented
below, compute the ciphertext of a message universityofcincinnati.
a b c d e f g h i j k l m n o p q r s t u v w x y z
E X A U N D K B M V O R Q C S F H Y G W Z L J I T P
If the message space of Substitution Cipher has a number of 50 unique characters/symbols, what is the
size of the key space? In other words, how many permutations in total?
Problem 3 (1 point). Assume the key of Vigenere Cipher is cats, what is the ciphertext of a message
datasecurity encrypted by this key using Vigenere Cipher? What is the size of the key space for Vigenere
Cipher if each key is a string of 4 characters? For easy calculation, a mapping table between characters (a,
…, z) and integers (0, …, 25) is listed below.
a b c d e f g h i j k l m n o p q r s t u v w x y z
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Problem 4 (1 point). Assume we have a sequence of 100 characters, the frequency distribution of
different characters is listed below, compute the (approximated) index of coincidence (IC) of this sequence.
char a b c d e f g h i j k l m n o p q r s t u v w x y z
frequency 1 1 1 1 1 1 1 1 1 1 5 5 5 5 5 5 5 5 10 10 10 10 10 0 0 0
Problem 5 (1 point). An adversary analyzes a sequence of (ciphertext) characters, which is encrypted
by Vigenere Cipher, using Kasishi’s method. In addition, it knows the key length is at least 2. If it can find
a sub-string with a length of 4 repeated twice in the sequence, and the distance between the two repeated
sub-strings is 12, what are the possible key length of this Vigenere cipher?
2
Problem 6 (1 point). Assume an attacker knows the index of coincidence in plaintext is ICplain =
0.090. Given a long sequence of (ciphertext) characters, e.g.,
c1c2c3c4c5c6c7c8c9………
which is encrypted by Vigenere Cipher, explain how to calculate/estimate the key length by using the
index of coincidence.
