4- Hash, MAC, Dsign, freshness, passwords

Question 1

Data integrity problems

Answer 1

1- accidental errors
-> parity checks, CRC
2- active attack:
prevent attacker from creating fake integrity digest
-> include DOA

Question 2

Hash function

Answer 2

accepts variable size message and creates fixed-size message

Question 3

Hash function uses (5)

Answer 3

1- one-way function
2- component of crypto primitive
3- bind documents
4- source of pseudorandomness
5- bitcoin

Question 4

Hash functions requirements

Answer 4

1- preimage resistance
h(x)=z - it’s impossible to find input x
2- 2nd preimage resistance
impossible to find h(x1)=z so that h(x1)=h(x)=z
3- collision resistance
impossible to find x1 such that h(x1)=h(x2)

Question 5

Applications of hash resistances

Answer 5

1- preimage:
pw check and storage
2- 2nd preimage:
software downloading
3- collision:
contractual commitment

Question 6

Hash security

Answer 6

birthday paradox 2^(L/2)

Question 7

MAC - message authentication codes

Answer 7

cryptographic checksum sent along with a message to provide assurance of DOA

Question 8

MAC process

Answer 8

MAC(m) = f (k, m)
k(ab)

Question 9

MAC security (2)

Answer 9

assures:
- data was not altered
- is from the alleged sender

Question 10

MAC algortihms

Answer 10

1) CBC-MAC (use last block of CBC), no IV
2) HMAC - hash based MAC
hash+key

Question 11

HMAC process

Answer 11

0- have K1 and K2
1- h (K2 | m)
2- h (K1 | h (K2 | m))

Question 12

HMAC security

Answer 12

• key security
• h-function security
• length of MAC output

Question 13

MAC + confidentiality

Answer 13

1- MAC -> encrypt
2- encrypt -> MAC

Question 14

Digital signature services

Answer 14

• DOA
• NR

Question 15

Dsign computed with…

Answer 15

data and signer’s parameter

Question 16

Dsign requirements

Answer 16

• easy to sign
• easy to verify
• hard to forge

Question 17

RSA-based Dsigns process

Answer 17

1- h (doc)
2- apply key and sign. algo to the h
3- send doc and sign
4- receiver:
- computes h (doc)
- runs sign through verification algo with a verification key
5- compares results of both (same+>ok)

Question 18

Reasons for hashing before signing

Answer 18

• efficiency
• prevention of modification attacks
• prevention of existential forgeries

Question 19

RSA Dsign process with formulas

Answer 19

1- h=h(P)
2- s=h^d mod n
3- A sends (P,s) to B
4- B computes h(P)
- B computes s^e mod n
- B compares h(P) to s^e mod n

Question 20

Additional steps after Dsign

Answer 20

• A encrypts data P and signarture s with B’s public key
• A adds A identity to encryption
• A adds B identity to signature
  // => avoid impersonator’s attacks

Question 21

Entity Authentication

Answer 21

assurance that entity is involved and currently active in session

Question 22

EA dangers and mechanisms

Answer 22

• danger: impersonator’s attack
• mech: freshness

Question 23

Freshness mechanisms

Answer 23

• clock-based (synchronisation)
• sequence numbers (storage)
• nonces (rng)

Question 24

Pw problems

Answer 24

• length
• complexity
• repeatability
• vulnerabillity

Question 25

Dynamic password schemes

Answer 25

combine pw and freshness

Question 26

Dynamic password schemes mechanisms

Answer 26

1- synchronisation
2- challenge
3- counter