Introduction to the Problem

Question 1

cryptosystem

Answer 1

5-tuple (P,C,K,E,D)
P: possible plaintexts
C: possible cipher texts
K: possible keys
E: encryption function (unique for each key)
D: decryption function

Question 2

cipher text only attack

Answer 2

given y_1,…,y_i
deduce k, algorithm that outputs x_i+1 or x_1,…,x_i

Question 3

known plaintext attack

Answer 3

given x_1, y_1, …, x_i, y_i
deduce k or algorithm for x_i+1

Question 4

chosen plaintext attack

Answer 4

given x_1, y_1, … , x_i, y_i where attacker has chosen x
deduce k or algorithm

Question 5

chosen cipher text attack

Answer 5

given y_1, x_1, … , y_i, x_i
deduce k

Question 6

computationally secure

Answer 6

best algorithm for breaking it requires greater computational effort than the resources of the assumed attacker

Question 7

unconditionally secure

Answer 7

an attacker with infinite computational resources cannot break the system

Question 8

one-time pad

Answer 8

vernam cipher where the key stream is randomly chosen and never used again

Question 9

confidentiality

Answer 9

keeping information secret to those not authorised

Question 10

data integrity

Answer 10

ensuring information has not been altered by those not authorised

Question 11

authentication

Answer 11

confirmation of the identity

Question 12

message authentication

Answer 12

confirmation of the source of information

Question 13

signature

Answer 13

way of binding information to an entity

Question 14

certification

Answer 14

endorsement of information by trusted source

Question 15

non-repudiation

Answer 15

preventing an entity from denying previous actions

Question 16

revocation

Answer 16

retracting certification or authorisation

Question 17

substitution cipher

Answer 17

permutations of 26

Question 18

shift cipher

Answer 18

Caesar cipher

Question 19

vigenere

Answer 19

add a word

Question 20

permutation

Answer 20

permute letters