Lecture Notes

Question 1

What is a cipher?

Answer 1

A pair of functions (Encryption & Decryption) that satisfy the consistency equation.

Question 2

What is the consistency equation?

Answer 2

It is where you pass the plaintext into a function, you have some form of key that you plan
to use for the encryption, and out the other side should come the ciphertext. Then the converse
has to work as well, whereby you pass the ciphertext into a function, then key is used to perform
the decryption and out comes the plaintext.

P = plaintext
K = key
C = ciphertext
E = encryption

Algorithm = C = E(k,p)

Question 3

What is a substitution cipher?

Answer 3

It is a method of encyption. Units of plaintext are replaced by ciphertext in a defined manner with the help
of a key. The letters are changed, but the position does not change.

Question 4

Name three types of substitution ciphers

Answer 4

1. Monoalphabetic
2. Polyalphabetic
3. Polygraphic

Question 5

What is a mono-alphabetic substitution cipher?

Answer 5

It uses a simple 1 to 1 mapping. It uses a fixed key which consists of 26 letters of a shuffled alphabet.

Question 6

What is a Polyalphabetic cipher?

Answer 6

A polyalphabetic cipher substitution, using multiple substitution alphabets. The Vigenère cipher is probably the best-known example of a polyalphabetic cipher, though it is a simplified special case.

Question 7

What is a transpositional cipher?

Answer 7

No substitution takes place. Ciphertext is an anagram of the original message.

Question 8

Name two types of transpositional cipher

Answer 8

1. Rail Fence
2. Columnar Transposition Cipher

Question 9

List five types of substitution ciphers

Answer 9

1. Rotational
2. Symbol substitution
3. Homophonic
4. Polyalphabetic
5. Polygraphic

Question 10

Rot13 is an example of what type of substitution cipher

Answer 10

Rotational / shift cipher

Question 11

Pigpen is an example of what type of substitution cipher

Answer 11

Symbol substitution

Question 12

Zodiac killer is an example of what type of substitution cipher

Answer 12

Homophonic

Question 13

Vigenere is an example of what type of substitution cipher

Answer 13

Polyalphabetic

Question 14

Playfair is an example of what type of substitution cipher

Answer 14

Polygraphic

Question 15

What is frequency analysis in cryptography

Answer 15

It is the study of the frequency of letters or groups of letters in a ciphertext

Question 16

Name three fundamentals elements that must be present for a one-time pad

Answer 16

1. The key must be truly random
2. The key must never be reused
3. The key must be the same size as the message being sent.

Question 17

Is a Pseudo Random Number Generator (PRNG) suitable for one-time pads?

Answer 17

No - as the number will repeat after a given period.

Question 18

True random number generators are used for what purposes?

Answer 18

Cryptography, lotteries, and games

Question 19

Name the two types of encryption

Answer 19

Secret-key encryption
Public-key encryption

Question 20

Describe the attributes of secret-key encryption

Answer 20

Uses the same key for encryption and decryption, otherwise known as symmetric encryption

Question 21

Describe the attributes of public key encryption

Answer 21

Uses different keys for encryption and decryption, otherwise known as asymmetric encryption