10/11 - Transport Layer Security

Question 1

SSL

Answer 1

Secure Sockets Layer

Question 2

TLS

Answer 2

Transport Layer Security

Question 3

WHich of SSL and TLS is recommended?

Answer 3

TLS

SSL is deprecated.

Question 4

Services provided by TLS

Answer 4

• Confidentiality (symmetric crypto; keys negotiated with public-key)
• Server authentication (certificate, signature)
• Client authentication (optional)
• Data integrity (MAC included)
• Replay prevention

Question 5

TLS is where in the protocol stack?

Answer 5

Between the application and transport layers

Question 6

TLS sub-protocols

Answer 6

• Handshake
• Change Cipher
• Alert
• Record (below the other three in layers)

Question 7

TLS Handshake Protocol

Answer 7

At the beginning:
- client/server agree on which crypto algo to use
- establish secret keys
- authenticate server
- authenticate client (optional)

Question 8

TLS Handshake clientHello

Answer 8

• Protocol version
• Random number (nonce): to prevent replay
• Cipher suite, algos supported in order of pref:
• • key exchange
• • for authenticated symmetric encryption
• • for signatures
• key (DH key shares or labels of pre-shared keys)

Question 9

TLS Handshake ServerHello

Answer 9

• highest protocol version supported by both
• random number (diff from ClientHello)
• Ciphersuite: algos chosen by server
• Key (One DH key share w/ same params or pre-shared)

Question 10

TLS Handshake Auth phase

Answer 10

• Server sends X.509 certificate(s) of server. Client checks.
• Server sends signature on all data so far
• finished: MAC on all handshake data so far

Question 11

Example of ephemeral DH in TLS Handshake

Answer 11

ClientHello
- send id from list of acceptable primes/primitive elements from spec
- chooses a private exponent a (each session) then sends g^a mod p to the server

ServerHello
- Server chooses a private exponent b and sends g^b mod p to client

Question 12

TLS Handshake secret keys required

Answer 12

• Client and server keys for MAC
• client write key, client write IV
• server write key, server write IV
  ( two symmetric pairs)

Question 13

HKDF

Answer 13

Hash based Key Derivation Function

Input:
- Shared secret (from DH exch or PSK pre-shared key)
- Data from client hello and server hello
- pre-define strings specific to each key or IV
- For IV for app data packets, the sequence number of the packet

Apply the hash function several times

Question 14

Replay Prevention (TLS)

Answer 14

Server creates new keys on new connection so replays wont work.

Question 15

TLS Record Protocol provides

Answer 15

• confidentiality (symmetric encryption)
• message integrity by using auth encryption

Question 16

TLS 1.3 vs 1.2

Answer 16

• Weaker ciphers no longer allowed
• Forward secrecy compulsory (DH required)
• Handshake reorganised for speed
• record protocol now authenticated encryption; compression not allowed