1 - Classical Symmetric Algorithms

Question 1

Caesar/Shift cipher

Answer 1

key K 1 to 25

Replace each plaintext character with letter K positions further

Inverse = backwards

Question 2

Components of a cryptosystem

Answer 2

Set of possible plaintexts
… ciphertexts
… keys (keyspace)
the encryption function E(K,m) (K key is parameter)
the decryption function D(K,c)

Question 3

The encryption (E) and decryption function (D) much be such that..

w/r to a fixed K (key)

Answer 3

• For fixed K, different messages are encrypted to different cipher texts. injective/one to one
• For fixed K, E/D are inverses

Question 4

Exhaustive Keysearch Attack

Answer 4

Try decrypting c using all possible keys in turn, until result makes sense.

Combatted by large keyspace

Question 5

Key is string of n bits. How many keys?

Answer 5

2 to the power of n

Question 6

How many keys in a 128bit key

Answer 6

2 to the power of 128

Question 7

Absolute minimum number of bits

Answer 7

80bits. Most common 128

Question 8

Substitution Cipher

Answer 8

Plaintext: alphabet
ciphertext: 26 characters
Key: A substitution table of two rows (top - OG, bottom - cipher)
Encryption/Decryption: Substitute the letters

Question 9

How can context be used to crack a cipher?

Answer 9

The environment or known patterns of the plaintext can be used.

Question 10

How many possible keys in a substitution cipher

Answer 10

26!

26 factorial = 403291461126605635584000000

26 possibilities on first char, 25 on second, 24 on third

Question 11

How can letter frequencies be used to crack a cipher?

Answer 11

Some letters are more frequent in languages and so, patterns can be recognised using this fact.

Question 12

Vigenere cipher

Answer 12

Key: sequence of n numbers from 0 to 26.

Shifts applied corresponding to each number in the key

Question 13

Vigenere Cipher: Number of keys

Answer 13

26 to the power of n

Question 14

Permutation cipher: Concept

Answer 14

anagrams

Split message into fixed length blocks and apply same permutation in each block

Question 15

Permutation Example Key

Answer 15

1 2 3 4 5
3 1 5 2 4

Plaintext: nobod y here
ciphertext: oondb hryee

Inverse:
1 2 3 4 5
2 4 1 5 3

Question 16

Permutation Number of keys

Answer 16

n!

n factorial (reasonably large)

Question 17

Does the hacker know the algorithm?

Answer 17

Assume yes

Question 18

Ciphertext only attack

Answer 18

Eve only has ciphertexts. Obtan plain or key

Question 19

Known plaintext attack

Answer 19

Eve has plaintext(s) and corresponding ciphertext(s)

Question 20

Chosen plaintext attack

Answer 20

Eve obtained temporary access to encryption and encrypted a custom plaintext

Question 21

Chosen ciphertext attack

Answer 21

Eve has temporary access to decryption and decrypts a chosen ciphertext