3 - Crypto Public Key Attacks

Question 1

What is the key distribution problem and its solution?

Answer 1

Need to share keys

Asymmetric cryptography is the solution

Question 2

What is Asymmetric (Public Key)?

Answer 2

Uses a different key for encryption and decryption

Depends on hard maths problems

Generate key pairs, one key is private another is public

Question 3

What are some properties of asymmetric/public keys?

Answer 3

Doesn’t matter if connection is insecure

Can prove someone encrypted something

Can combine encryption using different keys to ensure only recipient can decrypt, and prove author identity, can also prove no alterations were made

Question 4

How should keys be generated?

Answer 4

Using one way functions

Question 5

What is a one way function?

Answer 5

Function to generate keys and is computationally infeasible to get private key from public key

Question 6

What is modular arithmetic?

Answer 6

Clocks work mod 12
E.g. 14 = 2 mod 12
a ≡ b mod n if a % n = b % n
14 % 12 = 2, 2 % 12 = 2

Question 7

What is prime factorisation?

Answer 7

6 * 11 = 77

391

Question 8

What is RSA Key Generation?

Answer 8

Pick 2 large distinct random primes (p and q)

Calculate n = pq

Calculate φ(n) = (p-1)(q-1)

Pick e = number less than φ, co-prime to φ

Calculate d
- d*e mod φ(n) = 1

Public key is (e, n)
Private key is (d, n)
Computationally infeasible to compute d from e and n alone

Question 9

Factorising large numbers

Answer 9

Difficult to find the 2 numbers that make up a very large number (get better definition)

Question 10

RSA Encryption

Answer 10

Split message into blocks
e.g 070515807052 7080119270718

for each plaintext block B
bᵉ(mod n)
e.g. 1658228449402 5333403068473

Question 11

RSA Decryption

Answer 11

For each ciphertext block C

Cᵈ(mod n)

Question 12

What are important features of RSA?

Answer 12

Public key crypto

• public key is published
• private key isn’t

Relies on one way functions
- Modular arithmetic, prime factorisation

Question 13

What is Diffie-Hellman key exchange?

Answer 13

Establish a shared secret (key) between two parties

Not sharing info, but creating a key together

Question 14

What are methods for attacking a substitution cipher?

Answer 14

Known letter frequencies, common words and digraphs and trigraphs (th, er, the, ion etc)

Question 15

Does frequency analysis work on transposition ciphers?

Answer 15

No

Question 16

Classifying attacks

Answer 16

Brute force

Cipher text only

Known or chosen
- plaintext or ciphertext

Question 17

What are some applications of cryptography?

Answer 17

SSL

Encrypted email, hard disks, files, devices…

Cryptocurrencies

Question 18

What is a hash function?

Answer 18

Takes arbitrary length input and generates fixed length output

Same input always produces same hash

Question 19

Secure hash functions

Answer 19

One way
- computationally infeasible to find message from its hash

Strong collision resistance
- computationally infeasible to find two different messages that hash to same hash value

Question 20

What is MD-5?

Answer 20

128 bit hash function

Shown in 2004 to not be collision resistant

Question 21

What is SHA?

Answer 21

Another hash function

SHA-1 produces 160 bit hash code

SHA-512 produces 512 bit hash code

Question 22

Digital signatures

Answer 22

Sender combines message with their private key and sends this with signature to recipient who uses sender’s public key to decrypt message

(check this)