Cryptography

Question 1

Cryptography, Cryptology, Cryptanalysis, Crypto

Answer 1

Cryptography: making secret codes
Cryptology: Making and Breaking secret codes
Cryptanalysis: Breaking secret codes
Crypto: all of the above, a black box

Question 2

Caesar Cipher

Answer 2

• extremely easy to break by shifting 3 letters left, Simple substitution
• mono alphabetic encryption
  -classical cipher
  -ve: small key space of 25 keys

Question 3

Hardening Caesar Cipher + 1-ve

Answer 3

• scramble 26 letters into other random permutations: 26!
• encrypt and decrypt with same scrambled key
• not necessarily alphabetic shift
  BUT
  English letter frequencies unbalanced, DO NOT use substitution ciphers

Question 4

cipher/cryptosystem, encryption result, decryption result, symmetric key, public key, private key

Answer 4

• cipher: encrypt plaintext
• encryption result: ciphertext
• decryption result: plaintext
• symmetric key: same key to encrypt and decrypt
• public key: to encrypt
• private key: to decrypt

Question 5

Secure cryptosystem & Broken cryptosystem

Answer 5

• Secure cryptosystem: best attack is brute force, exhaustive key search
• Broken cryptosystem: shortcut attack known without trying all keys

Question 6

Vigenere Cipher

Answer 6

• letters shifted by values defined by a key, letters that represent numbers based on position in alphabet
  eg. A:0, D: 3
• Stronger: longer keyword(less observable pattern in ciphertext, more frequency tables), shorter message(stat analysis not accurate)
• same letter to different letters
• different letters to single letter
• polyalphabetic substitution, more secure, classical cipher
• Not good today

Question 7

Vigenere Cipher decryption

Answer 7

1. Find length of key by looking at intervals between repeated text patterns. Same word encrypted with same shift values
2. Key length either whole interval and repeated text or a factor of it

Question 8

One Time Pad

Answer 8

• perfect encryption
• can only learn length of plaintext
• plaintext, random key and ciphertext all same length
• C = P XOR K
• XOR: instant encryption and decryption, XOR key twice encrypts and decrypts

Question 9

OTP encryption & decryption

Answer 9

Encryption: Add each letter by cipher until cipher ends then repeat until sequence ends
Decryption: Minus each letter by cipher until cipher ends then repeat until sequence ends
- mod26 for both. A=1, Z=26

Question 10

One Time Pad properties(3)

Answer 10

1. Random
2. Used only once
3. Known only to sender and receiver

Question 11

Problems with One Time Pad(5)

Answer 11

1. Hard to generate truly random long One Time Pad
2. Need to ensure OTP stored securely
3. Need to ensure secure encryption and decryption
4. Both parties need to sync portions of pad used
5. Need to agree on new OTP when old OTP used up or compromised

Question 12

Randomness

Answer 12

• found everywhere
  1. equally likely to get 0 or 1 bit
  2. Successive bits independent of each other
• non randomness = insecurity
• crypto randomness more strict than random used in RNG and simulations

Question 13

Errors in identifying randomness

Answer 13

1. Mistaking random for not random
2. Mistaking non random for random

Question 14

Birthday paradox

Answer 14

• only need 23 ppl to get >50% where 2 ppl same birthday
  -> 364! / 342! x 365^23 = 49.2
  -> 100 - 49.2 = 50.8%

Question 15

Crude Shannon

Answer 15

• founder of Info Theory
  1. Confusion: obscure relationship between plaintext and ciphertext
  2. Diffusion: spread plaintext statistics through ciphertext
• One Time Pad: confusion
• Double Transposition: Diffusion

Question 16

Symmetric cryptography

Answer 16

• symmetric key: same key to encrypt and decrypt
  1. Stream ciphers
  2. Block ciphers

Question 17

1. Stream cipher overview

Answer 17

• short key stretched to periodic infinite keystream
• XOR keystream with plaintext bit by bit
  eg. GRAIN: NSFR, secure non linear feedback register
• most common: Feedback Shift Registers(FSR)

Question 18

Feedback Shift Registers(FSR)

Answer 18

• LFSR based: initialise k bit seed
• Successive bits: XOR previous bits: si+1 = si-1 XOR si-2

Question 19

Stream cipher 1 +ve, 1-ve, Solution,
-ve of Solution

Answer 19

+ve: Efficient in hardware
-ve: Speed needed
Solution: fast processors today
-ve: death of linear stream ciphers due to linear lagebra

Question 20

2. Block ciphers

Answer 20

• encrypting more than one block requires padding as NOT all messages same size as block
• AES block
  –Key length: Number of rounds
  128:10
  192:12
  256:14
  1. Electronic codebook
  2. Cipher Block Chaining
  3. CTR counter mode

Question 21

ECB, Electronic codebook overview

Answer 21

• every block of plaintext encrypted independently and identically with same key k
  Encryption: C = E(k,P) = Ek(P)
  Decryption: P = D(k,C) = Dk(C)

Question 22

ECB 3 +ve, 5 -ves

Answer 22

+ves:
1. Parallel Encryption
2. Parallel Decryption
3. Random Read
-ves: Ek fixed function for fixed k so Simple Substitution
1. Fixed map for symbols
2. Patterns preserved
3. Repetition seen
4. Frequency found
5. Ciphertext leaks plaintext information
* Same issues facing deterministic cipher with fixed key

Question 23

CBC, Cipher Block Chaining + 1 -ve

Answer 23

• each block of plaintext XOR with previous block of ciphertext before encrypted using Ek
  Encryption: C0 = IV, C i = Ek(P i XOR C i-1)
  -ve: No Parallel Encryption

Question 24

CTR, Counter mode + 2 +ves

Answer 24

• use block cipher as stream cipher
• input to block cipher computes new keystream block
• Plaintext encrypted by XOR with combination of E, key and counter CTR
  Encryption: C i = P i XOR Ek(CTR i)
  Decryption: P i = C i XOR Ek(CTR i)
  +ves:
  1. Parallel Encryption
  2. Parallel Decryption

Question 25

Probability of two string of bits being the same

Answer 25

0.5^length of bit string