protocols part 1 - symmetric key

Question 1

what does A -> b: ‘Im alice’ mean

Answer 1

alice is sending a message ‘im Alice’ to bob

Question 2

what type of attack can happen to a simple protocol

Answer 2

impersonation attack
E(A) -> B : ‘im Alice’

Question 3

what is an impersonation attack

Answer 3

adversary is impersonating one of the people that is is the messaging
to trick the other person

Question 4

how to get pass an impersonation attack

Answer 4

use encryption
e.g symmteric key encryption

Question 5

notation for symmetric key encyption in protocols

Answer 5

{}k_ab
k_ab -> shared symmetric key

Question 6

what attack could happen to simple symmetric key encryption protocol

Answer 6

replay attack

Question 7

what is a replay attack

Answer 7

a sends message to b
A -> B : {msg}kab
adversary hears message and then sends the same message to b
E(A) -> B : {msg}kab

Question 8

replay attack notation

Answer 8

E(A) -> B : {msg}kab

Question 9

how can we stop a replay attack

Answer 9

use a nonce

Question 10

what is a nonce

Answer 10

randomly generated number that ia only used once

Question 11

when is a nonce often used

Answer 11

for a challenge response protocol in a challenge response setting

Question 12

protocol steps for nonce symmetric key protocol

Answer 12

1. A →B : A
   a sends message/identity to b
2. B →A : {Na}Kab
   b sends encrypted nonce -> challenge response protocol
3. A →B : {Na +1}Kab , {Pay Elvis 5}Kab
   a decrypts nonce to prove it is them , modifies nonce by doing something arithmetic , sends modified nonce back encrypted and also encrypted message

Question 13

what attack is nonce protocol susceptible to

Answer 13

man in the middle attack

Question 14

describe man in the middle attack to nonce protocol

Answer 14

(also known as reflection attack?)
1. A →B : A
2. B →A : {Na1}Kab
3. A →B : {Na1 +1}Kab , {Pay Elvis
5}Kab
4. A →B : A
5. B →A : {Na2}Kab
6. E →B : {Na2 +1}Kab , {Pay Elvis 5}Kab
elvis listens , tajes new nonce, joins with previous message and sends it to bob

Question 15

how to stop man in the middle attack in nonce protocol

Answer 15

merge nonce and message together into one encryption
1. A →B : A
2. B →A : {Na}K_ab
3. A →B : {Na, Pay Elvis 5}K_ab
therefore adversary cannot edit the message

Question 16

1. A →B : A
2. B →A : {Na}K_ab
3. A →B : {Na, Pay Elvis 5}K_ab
   through this what can bob be sure of ?

Answer 16

he is talking to alice
A wants to send elvise £5
a messages are fresh