Crypto Protocols

Question 1

Reflection Attack

Answer 1

• message is encrypted
• attacker can send the original message back to the sender
• if m = “pay peter 200 €”; peter would get 400

Question 2

Defence Reflection Attack

Answer 2

• put the original sender in the encryption

- attacker can no longer reflect to original person

Question 3

Replay Attack

Answer 3

• attacker copies message and send several time to a person

- if m = hans, “pay peter 200 €”; peter would get from receiver 400

Question 4

Defence Replay Attack

Answer 4

1. with timestamps
   - needs global clock
2. Nonces
   - > b sends a nonces; a puts nonces in message
   - A nonce (“Number used ONCE”)

Question 5

PC (plain-cipher)
CP (cipher-plain)
CC (cipher-cipher)

Answer 5

PC:
b to a: nonce
a to b: {A, m, nonces}_k_AB
CP:
b to a: {A, m, nonces}_k_AB
a to b: nonces -> as acknowledgment
CC:
b to a: {B, m1, nonces}_k_AB
a to b: {A, m2, nonces}_k_AB

all encrypted m can just be decrypted by a and b

Question 6

TLS: Transport Layer Security - Record Protocol

Answer 6

• en-/decrypts and validates data

Question 7

TLS Handshake Protocol

Answer 7

C -> S: highest TLS + nonces + session ID
S -> C: chooses TLS + nonces + copied session ID from C
S -> C: server certificate + DH parameters + ask C for certificate
C -> S: certificate + DH parameter + {pre-master secret}_enc pub key of S + CV -> signs handshake with certificate
C -> S: sends changed_cipher_specs
S -> C: finished

Question 8

Why ID’s in TSL?

Answer 8

• multiple TLS connection share a session

- resume a session or add one to it; very easy

Question 9

Why DH in TLS and not RSA?

Answer 9

• if RSA key leaked whole conversation can be leaked

- with DH -> perfect forward security

Question 10

SSL 2.0 weaknesses

Answer 10

• no support for non-RSA alg. and certificates
• attacker could delete padding of MAC
• DOWNGRADE ATTACK (give server options with venerability in it)

Question 11

TLS traffic analysis

Answer 11

• if content on server is know and m has certain size we can assume that this was dowloaded

Question 12

Common Name (CN)

Answer 12

determines domain validity
e.g. for www.google.com
or *.brainscape.com
-> wildcard, also a.b.brainscape.com

Question 13

PKI / Certificate Hierarchy

Answer 13

Public Key Infrastructure (PKI)

1. Root CA: trusted by everyone
2. Intermidiary CA: root CA trust tehem to issu certificates
3. End entities: Users, system, devices

-> chain of trust