Concepts

Question 1

Security

Answer 1

Possibility of protecting information, stored or being transmitted on a network.
Parties must be sure a set of requirements was met.

Question 2

Objectives to met to ensure Security

Answer 2

ACIAN
- Authenticity (Entity Authentication/Identification)
- Confidentiality (Privacy)
- Integrity
- Availability
- Non repudiation

Question 3

Authenticity

Answer 3

An entity should be correctly identified

Question 4

Confidentiality/Secrecy

Answer 4

• Information should only be accessed by authorized entities
• (Privacy) Individuals control what information can be collected and stored and to whom it may be disclosed.

Question 5

Availability

Answer 5

Information should be available/usable by authorized users.

Question 6

Non-repudiation

Answer 6

An entity should not be able to deny an event.

Question 7

Accountability

Answer 7

Tracing back an action to an entity

Question 8

Typical attacks

Answer 8

Passive:
- Eavesdropping (Interception) -> Confidentiality

Active:
- Interruption -> Availability + System integrity
- Modification -> System integrity + Confidentiality
- Forging -> System integrity + Authenticity, Accountability

Question 9

Classical Cryptography Ciphers/Cryptosystem

Answer 9

Monoalphabetic:
- Caesar Cipher
- Shift Cipher
- Substitution Cipher
- Affine Cipher

Polyalphabetic:
- Vigenère
- Hill Cipher
- Permutation/Transposition Cipher

Vulnerable to:
- Cryptanalysis
- Brute-force attack

Question 10

Cipher or Cryptosystem

Answer 10

5-tuple consisting of (P,C,K,E,D)
- P: Set of plaintexts
- C: Set of ciphertexts
- K: Set of keys
- E: k x P -> C, encryption function
- D: k x C -> P, decryption function

Question 11

A cryptosystem is of practical use if…

Answer 11

• Each encryption function and each decryption function should be efficiently computable
• An opponent, upon seeing a ciphertext string y, should be unable to determine the key K that was used, or the plaintext string x.

Question 12

Kerchkoff’s principle is…

Answer 12

• A cipher should remain secure even if the algorithm becomes public
• The system should be, in practice, unbreakable

Question 13

Attacks to Shift Ciphers

Answer 13

Brute force on the key space (26 keys)

Question 14

Relation between classical ciphers

Answer 14

• Caesar cipher is a subcase of Shift cipher with k = 3
• Shift cipher is a subcase of Substitution cipher where only 1 permutation is used the alphabetically ordered one
• Shift cipher is a subcase of Affine cipher where a equals 1
• Permutation/Transposition Cipher is a subcase of Hill Cipher where the K matrix is a modified identity matrix which does the transposition of places

Question 15

Attacks to Substitution Cipher

Answer 15

As it is a monoalphabetic cipher, statistical reconstruction of keys can be used to decrypt messages.
Making replacements and guesses on single characters, digrams and trigraphs

Question 16

Attacks to Vigenère

Answer 16

Friedman method. Statistical measures to recover the length m of the key.
Index of coincidence = sum_i=1^26( f_i * f_i -1) / n * n-1

If IoC ~ 0.038 -> polyalphabetic cipher
else if IoC ~ 0.065 -> monoalphabetic cipher
-> calculate that a given m length IoC evaluation matches the plaintext language

Using the Friedman Test

Question 17

Types of cyptanalysis attacks

Answer 17

• Ciphertext Only / Known ciphertext (weakes type of attack)
  
  • attacker has a string of ciphertext, y
• Known plaintext
  
  • attacker has plaintext, x and corresponding ciphertext, y
• Chosen plaintext (CPA)
  
  • attacker has temp access to the encryption machinery. He can choose plaintext , x and construct ciphertext y
• Chosen ciphertext (CCA)
  
  • attacker has temp access to decryption machinery. He can choose ciphertext y and construct plaintext x
• Chosen text
  
  • attacker has temp access to decryption+encryption algorithm.

Question 18

Attack to affine cipher

Answer 18

Letter frequency can be used in addition with hypothesis on assignment of most common letters. Until a equation system is constructed such as:
- C_1 * a + b = P_1
- C_2 * a + b = P_2
being C chosen ciphertext chars and P plaintexts chars

The idea would be to get the a and b used in the encryption and decryption function.

Question 19

Inverse of matrix

Answer 19

2X2
det(K) = 1/ (ad-bc)
det-1(K) mod 26 = find multiplicative inverse mod 26 such that det(K) mod 26 =1
= det^-1(K) * [d -b,
-c a]

3X3
- find det(k) = for i,j in first row block column j and row i and compute det of remaining matrix
+ det matrix [1,2] - det matrix [0,2] + det matrix[0,1]
- find multiplicative inverse of det(k)
k^-1 mod 26 = 1
- find adj(k)
duplicate two columns + duplicate two rows
block row i and column j
cross multiply each 2x2 mini matrix

Question 20

Diff between Block ciphers and Stream ciphers

Answer 20

• Block ciphers reuse the same key to encrypt letters or blocks

Block cipher can be a stream cipher with constant keystream z_i = K for all i
* Operates block by block (f.e. 64bits)
* Uses confusion + diffusion principle
* Slower
* Encrypt: Electronic Code Book (ECB) and Cipher Block Chaining (CBC)
* Decrypt: Reverse of encryption
* DES, AES

• Stream ciphers use a stream of keys z_1 up to z_n instead of just one
• Operates bit by bit or byte by byte
• Uses confusion principle
• Faster
• Encrypt: CFB (Cipher Feedback) and OFB (Output Feedback)
• Decrypt: XOR
• Vernam Cipher

Question 21

Synchronous stream cipher

Answer 21

5-tuple (P, C, K, L, E, D)
- P possible plaintexts
- C possible ciphertexts
- K keyspace, possible keys
- L keystream alphabet
- g keystream generator. g takes K as input and generates z_i where i e L
- for each z_i there is a Encryption rule and a Decryption rule

Question 22

Asynchronous stream cipher

Answer 22

In contrast to sync, the keystream generator function is defined in terms on the previous plaintext or ciphertext at each step i

Question 23

Autokey cipher

Answer 23

Async stream cipher.
Uses the plaintext to construct the keystream

Question 24

Stream cipher

Answer 24

Using a keystream z = z_1z_2… used to encrypt a plaintext string as
y = y_1y_2 … = ez_1(x_1)ez_2(x_2)….

Question 25

Periodic Async stream cipher

Answer 25

Has a period d if z_i+d = z_i for all int i >= 1
Like vigenère with keyword length m.
Vigenère can be a periodic sync stream cipher with period d meaning
the keystream generator function will return the sequence k_1,…,k_m and will continue again with k_1,…,k_m in case the plaintext sequence continues.

Question 26

Attacks on Hill Cipher

Answer 26

Easy with known plaintext attacks using the multiple encoded messages it can be obtained that:

X^-1*Y = K

if X is not invertible, it will be needed to try other pairs of plaintext-ciphertext

Question 27

Perfect cipher / Perfect secrecy

Answer 27

• a cipher that can never be broken, even after unlimited time
• such that on seeing the ciphertext the interceptor gets no extra information about the plaintext in comparison on what he had before it was observed

if Pr[x|y] = Pr[x] for all x in P, y in C.