6- crypto applications: SSL/TLS, Kerberos

Question 1

Services provided by primitives

Answer 1

Encryption - C
Hash - DI/2
MAC - DI, DOA, NR/2
Dsign - DI, DOA, NR

Question 2

Primitives that cannot be used for services

Answer 2

MAC - not C
Dsign - not C

Question 3

TLS

Answer 3

main security protocol

Question 4

Primitives of TLS

Answer 4

• PK crypto - to establish symmetric keys
• Dsign - EA through signed certificates -> EA
• MACs - DOA -> EA
• Sym Enc. -> C
• hash -> MACs, Dsigns, key derivation

Question 5

Cipher suite

Answer 5

set of primitives used for communication

Question 6

TLS parts

Answer 6

1- handshake protocol - agreement b/w entities:
- cipher suite
- establish EA
- establich keys for securing the channel
2- record protocol - implement secure channel:
- format data (blocks etc)
- compute MACs from data
- encrypt data

Question 7

TLS handshake protocol

Answer 7

1- client request
2- server response
3- pre-master secret transfer
4- client finished
5- server finished

Question 8

Client request

Answer 8

• sesion ID
• pseudorandom r
• list of ciphersuits

Question 9

Server response

Answer 9

• session ID
• pseudorandom r (server’s freshness)
• cipher-suite from client’s list
• copy of servers PK certificate (+its chain)
• Ephemeral DH for establishing keys => fresh set of params for it
• client checks validity of server’s PK certificate
• if eph DH => client verifies dsign on the DH params

Question 10

Pre-master secret transfer

Answer 10

• need to agree on secret Kp
• RSA: client pseudorandomly generates Kp, encrypts with servers PK, sends to server
• Eph. DH => client generates fresh DH key pair, sends public part to server, then both compute DH secret Kp

Question 11

Key derivation

Answer 11

1- both use key derivation function:
f (rc, Kp), f (rs, Kp) => Km
2- both use Km for MAC and encryption

Question 12

Client finished

Answer 12

1- client computes MAC (HMAC) on the has of messages sent, encrypts, sends to server

Question 13

Server finished

Answer 13

1- server checks MAC received from the client
2- server computes MAC on hash of all sent messages
3- server encrypts MAC and sends to client

client checks MAC from server

Question 14

TSL with client authentication

Answer 14

not always required

after pre-master secret transfer:
client authentication data: client sends a copy of its PK certificate to the server
- PK = verification key
- client hashes all protocol messages, signs them with client’s signature key
- server checks validity of client’s certificate
- server verifies clients Dsign at the end

Question 15

TSL record protocol

Answer 15

derivation of data:
- symmetric session keys
- symmetric MAC keys
- any required IVs

derivation: Km is input, rc and rs are inputs