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COSC362 > Lecture 17: Transport Layer Security Protocol Part 1 > Flashcards

Lecture 17: Transport Layer Security Protocol Part 1 Flashcards

1
Q

Briefly outline the history of TLS

A
  • TLS 1.0 –> 1999
  • TLS 1.1 –> 2006, fixing problems with non-random IVs and exploitation of padding error messages
  • TLS 1.2 –> 2008, allowing the use of standard authentication encryption rather than separating encryption and MAC
  • TLS 1.3 –> 2018, separating key agreement and authentication algorithms for cipher suites
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2
Q

What is TLS?

A
  • cryptographic services protocol based upon PKI and commonly used on the Internet
  • often used to allow browsers to establish secure sessions with Web servers
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3
Q

What does TLS primarily run over?

A

TCP

Variant DTLA runs over datagram protocols

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4
Q

What was TLS designed for?

A

To secure reliable end-to-end services over TCP

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5
Q

What are the three higher level TLS protocols?

A

1) TLS handshake protocol
2) TLS alert protocol
3) TLS change cipher spec protocol

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6
Q

What is the general idea of the TLS handshake protocol?

A

to set up sessions

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7
Q

What is the general idea of the TLS alert protocol?

A

to signal events, such as failures

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8
Q

What is the general idea of the TLS change cipher spec protocol?

A

to change the cryptographic algorithms

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9
Q

What does TLS record protocol provide?

A

basic services to various higher level protocols

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10
Q

Give the protocol stack of TLS

A

See slide 8 in set 17

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11
Q

What are the two TLS connection services?

A

1) message confidentiality –> ensure that the message contents cannot be read in transit
2) message integrity –> ensuring that the receiver can detect if a message is modified in transmission

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12
Q

What are the TLS connection services possibly provided by?

A

symmetric encryption alg and a MAC

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13
Q

From TLS 1.2, what are the connection services provided with?

A

authentication modes (CCM, GCM)

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14
Q

What type of keys does the TLS handshake protocol establish?

A

symmetric session keys

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15
Q

Give the format of a TLS record using in the record protocol

A

See slide 11 in set 17

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16
Q

What can the content type in the header of a TLS record i.t.o the TLS record protocol be?

A

1) change-cipher-spec
2) alert
3) handshake
4) application-data

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17
Q

What are possible protocol versions in the header of a TLS record i.t.o the TLS record protocol?

A
  • major version: 3 for TLS
  • minor version:
    • 1 for TLS 1.0
    • 2 for TLS 1.1
    • 3 for TLS 1.2
    • 4 for TLS 1.3
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18
Q

What does the length field in the header of a TLS record contain?

A

length of the data, in octets

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19
Q

What is the fragmentation operation of the TLS record protocol?

A

each application layer message is fragmented into blocks of 2^14 bytes or less

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20
Q

Comment on the compression operation of the TLS record protocol

A
  • default compression algorithm is null for TLS 1.2 (thus optionally applied)
  • removed in TLS 1.3
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21
Q

Comment on the authenticated data of the TLS record protocol

A

consisting of the (compressed) data, header and an implicit record sequence number

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22
Q

Comment on the plaintext of the TLS record protocol

A

compressed data and MAC (if present)

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23
Q

Comment on the session keys of the TLS record protocol

A

computed during handshake protocol, for either MAC and encryption algorithms, or authenticated encryption algorithm

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24
Q

Comment on the specification of the TLS record protocol

A

encryption and MAC algorithms are specified in the negotiated cipher suite

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25
Q

What type of MAC is used in all TLS versions?

A

HMAC, using a negotiated hash function

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26
Q

What versions of TLS allow SHA-2?

A

only from TLS 1.2

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27
Q

Which hash functions have been discarded from TLS 1.3?

A

MD5 and SHA-1

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28
Q

Comment on the encryption algorithm used in TLS

A

Either a negotiated block cipher in CBC mode or a stream cipher

For block ciphers, padding is applied after MAC to make a multiple of the cipher block size

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29
Q

What is the most common block cipher for TLS?

A

AES

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30
Q

Which ciphers have been discarded by TLS 1.3?

A

3DES and RC4

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31
Q

What can be used instead of encryption and MAC from TLS 1.2?

A

authenticated encryption algorithm

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32
Q

What are the allowed authentication methods in TLS 1.3?

A

Only AES with either CCM or GCM modes in TLS 1.3

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33
Q

Where else is authenticated additional data i.t.o TLS record protocol?

A

header and implicit record sequence number

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34
Q

What are the four purposes of the TLS handshake protocol?

A
  • negotiating the TLS version and cryptographic algorithms to be used
  • establishing a shared session key for use in the record protocol
  • authenticating the server, and optionally authenticating the client
  • completing the session establishment
35
Q

What variations is TLS handshake used with?

A

1) RSA
2) Diffie-Hellman
3) Pre-shared keys
4) Mutual authentication
5) server-only (unilateral) authentication

36
Q

What is the general idea of phase 1 of the TLS handshake protocol?

A

initiating the logical connection and establishing its security capabilities

37
Q

What is the general idea of phase 2 and 3 of the TLS handshake protocol?

A

performing key exchange

–> messages and their contents depend on the handshake variant negotiated in phase 1

38
Q

What is the general idea of phase 4 of the TLS handshake protocol?

A

completing the setting up of a secure connection

39
Q

What do cipher suites specify i.t.o the TLS handshake protocol?

A

1) public key algorithm used for key establishment

2) symmetric algorithms used for providing authentication encryption and key computation

40
Q

How many standardised cipher suites are there i.t.o the TLS handshake protocol?

A

over 300
BUT
many are weak and many have been discarded in TLS 1.3

41
Q

What is the big change in TLS 1.3 i.t.o cipher suites?

A

All supported cipher suites must be Authenticated Encryption with Associated Data (AEAD)

42
Q

Explain the cipher suite example on slide 21 in set 17

A

See slide 21 in set 17

43
Q

What are the possible handshake algorithms i.t.o the TLS handshake protocol?

A

DHE-DSS
DHE-RSA
ECDHE-RSA
ECDHE-ECDSA

44
Q

Describe the algorithm DHE-DSS and which TLS version it can be used in i.t.o the TLS handshake protocol

A

DHE with Digital Signature Standard

TLS 1.2

45
Q

Describe the algorithm DHE-RSA and which TLS version it can be used in i.t.o the TLS handshake protocol

A

Ephemeral Diffie-Hellman with RSA signatures

1.2 and 1.3

46
Q

Describe the algorithm ECDHE-RSA and which TLS version it can be used in i.t.o the TLS handshake protocol

A

Elliptic curve DHE with RSA signatures

1.2 and 1.3

47
Q

Describe the algorithm ECDHE-ECDSA and which TLS version it can be used in i.t.o the TLS handshake protocol

A

Elliptic curve DHE with elliptic curve Digital Signature Algorithm

1.2 and 1.3

48
Q

What are the possible record algorithms i.t.o the TLS record protocol?

A

AES-CBC-SHA256
AES-GCM
CHACHA20-POLY1305

49
Q

Describe the algorithm AES-CBC-SHA256 and which TLS version it can be used in i.t.o the TLS record protocol

A

AES in CBC mode with HMAC from SHA256

1.2

50
Q

Describe the algorithm AES-GCM and which TLS version it can be used in i.t.o the TLS record protocol

A

AES with GCM mode

1.2 and 1.3

51
Q

Describe the algorithm CHACHA20-POLY1305 and which TLS version it can be used in i.t.o the TLS record protocol

A

ChaCha stream cipher with Poly1305 MAC

1.2 and 1.3

52
Q

Explain phase 1 of the TLS handshake protocol and give the diagram

A

client and server negotiate version, cipher suite and compression, and exchange nonces

See slide 24 in set 17 for diagram

53
Q

Explain phase 2 of the TLS handshake protocol and give the diagram

A

server sends certificate and key exchange message (if needed)

See slide 24 in set 17 for diagram

54
Q

Explain phase 3 of the TLS handshake protocol and give the diagram

A

client sends certificate and key exchange message

See slide 25 in set 17 for diagram

55
Q

Explain phase 4 of the TLS handshake protocol and give the diagram

A

client and server start secure communications. Finished messages include a check value (pseudorandom function) of all the previous messages

See slide 25 in set 17 for diagram

56
Q

What are the TLS handshake protocol messages?

A

1) client hello
2) server hello
3) server key exchange
4) client key exchange
5) change cipher suite

57
Q

Outline the client hello message in the TLS handshake protocol

A
  • Stating the highest TLS version available
  • Advertising cipher suites available to the client
  • Sending the client’s nonce N_C
58
Q

Outline the server hello message in the TLS handshake protocol

A
  • Returning the selected version and cipher suite

* Sending the server’s nonce N_S

59
Q

Outline the server key exchange message in the TLS handshake protocol

A

server’s inputs to key exchange

60
Q

Outline the client key exchange message in the TLS handshake protocol

A

client’s inputs to key exchange

61
Q

Outline the change cipher suite message in the TLS handshake protocol

A

switching to newly negotiated cipher suite for record layer

62
Q

Outline the server key exchange for the ephemeral Diffie-Hellman handshake variant (TLS handshake protocol)

A

inputs are the Diffie-Hellman generator and group parameters, along with the server’s ephemeral Diffie-Hellman value, all signed by the server

63
Q

Outline the client key exchange for the ephemeral Diffie-Hellman handshake variant (TLS handshake protocol)

A

inputs are client’s ephemeral Diffie-Hellman value

–> optionally signed by the client if the client’s certificate is used

64
Q

In terms of the ephemeral Diffie-Hellman TLS handshake protocol variant, what is the pre-master secrete pms?

A

the shared Diffie-Hellman secrete (from key agreement)

65
Q

In terms of the RSA handshake variant of the TLS handshake protocol, comment on the server key exchange

A

not required

66
Q

In terms of the RSA handshake variant of the TLS handshake protocol, explain the client key exchange

A

key transport of pre-master secret pms:

  • client randomly selects the pre-master secret pms
  • client encrypts pms with the server’s public key and sends the ciphertext to the server
  • server decrypts using its secret key to recover pms
67
Q

How is the master secret ms defined i.t.o session key generation for the TLS handshake protocol?

A

See slide 29 in set 17

68
Q

How is the key material generated i.t.o session key generation for the TLS handshake protocol?

A

See slide 29 in set 17

69
Q

Explain the session key generation process for the TLS handshake protocol

A

See slide 29 in set 17

70
Q

What can the key material include i.t.o session key generation and the TLS handshake protocol?

A

Depending on the agreed cipher suite:
• encryption key
• MAC key
• IV

71
Q

Comment on the pseudorandom function used in the TLS handshake protocol

A

PRF build from HMAC with a specified hash function

  • -> TLS 1.0 and 1.1: based on a combo of MD5 and SHA-1
  • -> TLS 1.2: based on SHA-2
72
Q

Explain the pseudorandom function example in TLS 1.2 i.t.o the handshake function on slide 30 in set 17

A

See slide 30 in set 17

73
Q

What are the other 2 handshake variants for the TLS handshake protocol?

A

1) Diffie-Hellman

2) Anonymous Diffie-Hellman

74
Q

Outline the Diffie-Hellman variant of the TLS handshake protocol

A

client and server used static/fixed Diffie-Hellman with certified keys
–> when the client does not have a certification (usual on the Internet), she uses an ephemeral Diffie-Hellman key

75
Q

Outline the anonymous Diffie-Hellman variant of the TLS handshake protocol

A

the ephemeral Diffie-Hellman keys are not signed at all

–> it only protects against passive eavesdropping

76
Q

Outline the alert protocol of TLS

A

Handling connection by sending an alert message of various degrees of severity

77
Q

What are types of alerts sent in the alert protocol of TLS?

A

1) Warning alerts
2) close_notify alerts
3) Fatal alerts

78
Q

What is the consequence of improperly handling alert messages i.t.o TLS’s alert protocol?

A

truncation attacks

79
Q

Comment on the Diffie-Hellman key exchange achieving forward secrecy i.t.o TLS’s handshake protocol

A
  • Exchange is authenticated using signatures from the long-term keys
  • Diffie-Hellman-based cipher suites provide forward secrecy
80
Q

Does RSA-based handshakes offer forward secrecy?

A

no

81
Q

Does Diffie-Hellman key exchange handshakes offer forward secrecy?

A

yes

82
Q

Does TLS 1.3 allow static RSA?

A

no

83
Q

What does TLS assume?

A

reliable message delivery, provided by TCP.

COSC362 (21 decks)

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