Encryption

Question 1

Encryption

Answer 1

It is a process by which information is changed in a systematic way as to hide its contents from everyone but its intended recipients

Question 1

Encryption provides mechanisms for:

Answer 1

1) Confidentiality
2) Authentication
3) Integrity

Question 2

Cryptology

Answer 2

Is the branch of science concerned with the concealment of information

Question 3

Cryptography

Answer 3

The science of creating codes and ciphers

Question 4

Cryptanalysis

Answer 4

The science of breaking codes and ciphers

Question 5

What is the difference between codes and ciphers ?

Question 6

Code

Answer 6

1) A code** replaces** words, phrases or group of symbols with alternatives (code words) and its purpose is not always for secrecy and provides an alternative way of communicating information. Examples of code would be ASCII code and Morse code.
A code is a result of an encoding process and generally relies on** look-up tables (codebooks)** for the conversion processes

Question 7

Cipher

Answer 7

A cipher is the output of an encryption process that either replaces data symbols with alternative symbols, or rearranges existing data symbols and the purpose of ciphers is almost always for secrecy

Question 8

Encryption
Decryption

Answer 8

Encryption is the process of transforming data known as plain text into a cipher known as ciphertext
Decryption reverses the process by transforming ciphertext into plaintext

Question 9

There are two types of ciphers:

Answer 9

1) Substitution cipher
2) Transposition cipher

Question 10

Substitution cipher

Answer 10

The encryption process systematically manipulates a symbol (or group of symbols) in the plaintext to produce a different symbol (or group of symbols), which becomes the ciphertext.
The order of the symbols do not change.

Question 11

Transposition cipher

Answer 11

The encryption process ‘scrambles’ the symbols of the plaintext in a systematic way to produce the ciphertext.
The symbols are unchanged but their order is.

Question 12

IMPORTANT: always represent the plaintext in lower case and the ciphertext in UPPERCASE

Question 13

Explain how does the Caesar wheel work

Question 14

key

Question 15

How the Caesar cipher can be represented
as a numerical algorithm that can be
processed by a computer?

Answer 15

Caesar cipher can be represented mathematically using “Modular Arithmetic”

Question 16

Modulus

Answer 16

The number of integers in a set

Question 17

The general formula of a Caesar cipher would be expressed as:

Answer 17

p+k=c mod n

Question 18

Using the cipher wheel the decryption process simply involves:

Answer 18

• Displacing the outer wheel clockwise a number of places corresponding to the agreed key
• Translating each ciphertext letter shown on the inner wheel to its equivalent plaintext letter on the outer wheel

Question 19

Complementary pair

Answer 19

Two numbers X and Y are said to be complementary pair when X+Y =
modulus

Question 20

Mathematically, the decryption algorithm of a Caesar cipher would be expressed as:

Answer 20

p ≡ c + 𝐾 (meaning k bar)mod 26
here 𝑲 (read as K bar) is the decryption key.
And K and 𝐾 form a complementary pair (K+ 𝐾(bar)=26)

Question 21

Symmetric key system

Answer 21

only a single key is involved in the encryption and decryption processes

Question 22

A transposition cipher

Answer 22

Is an anagram of the plaintext created systematically using a method that can be shared with the recipient so that it can be decrypted

Question 23

Columnar transposition cipher

Answer 23

Encryption is performed by reordering the columns of the matrix in some systematic way and then reading of the result to produce the cipher text

Question 24

Breaking (or cracking) a cipher

Answer 24

Is to derive the plaintext out of a ciphertext without knowledge of the key and often without the knowledge of the encryption algorithm

Question 25

Strong ciphers
Weak ciphers

Answer 25

Strong ciphers take longer to break but are more difficult to use
Weak ciphers are quicker to break into but are easier to use
The choice of a cipher will be determined by the value of information it is designed to protect

Question 25

What determines the strength of a cipher?

Answer 25

The strength of a cipher is measured by how long it takes to break it

Question 26

A brute force attack

Answer 26

Trying all possible combinations in a key space

Question 27

Key space

Answer 27

The number of possible key combinations for a particular algorithm

Question 28

Brute attack notes:

Answer 28

The time taken to break a cipher is directly proportional to the key space.
It can be applied to both substitution ciphers and transposition ciphers:
1) substitution: test every key in the key space
2) transposition: test every permutation of the possible transpositions

Question 29

Weakness of a transposition cipher

Answer 29

The number of possible permutations to crack a transposition cipher depends on the content of the message.
Transposition cipher is incapable of encrypting a string of identical characters, so it’s easier to break when there are long blocks of identical characters within a string

Question 30

Maths in this section:
Caesar cipher
Calculating time that it’d take a computer to try all possible combinations