Authentication Protocols - Computer and Network Security - Lecture Slides, Slides of Computer Science

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Part 3  Protocols 1

Authentication Protocols

Part 3  Protocols 2

Protocol

 Human protocols  the rules followed in

human interactions

o Example: Asking a question in class

 Networking protocols  rules followed in

networked communication systems

o Examples: HTTP, FTP, etc.

 Security protocol  the (communication)

rules followed in a security application

o Examples: SSL, IPSec, Kerberos, etc.

Part 3  Protocols 4

Ideal Security Protocol

 Satisfies security requirements

o Requirements must be precise

 Efficient

o Minimize computational requirement  in

particular, costly public key operations

o Minimize delays/bandwidth

 Not fragile

o Must work when attacker tries to break it

o Works even if environment changes

 Easy to use and implement, flexible, etc.

 Very difficult to satisfy all of these!

Part 3  Protocols 5

Simple Security Protocols

Part 3  Protocols 7

ATM Machine Protocol

1. Insert ATM card

2. Enter PIN

3. Correct PIN?

Yes? Conduct your transaction(s)

No? Machine eats card

Part 3  Protocols 8

Identify Friend or Foe (IFF)

Namibia

Angola

1. N

SAAF^ 2.^ E(N,K)

Impala

Russian

MIG

Part 3  Protocols 10

Authentication Protocols

Part 3  Protocols 11

Authentication

 Alice must prove her identity to Bob

o Alice and Bob can be humans or computers

 May also require Bob to prove he’s Bob

(mutual authentication)

 May also need to establish a session key

 May have other requirements, such as

o Use only public keys

o Use only symmetric keys

o Use only a hash function

o Anonymity, plausible deniability, etc., etc.

Part 3  Protocols 13

Simple Authentication

Alice Bob

“I’m Alice”

Prove it

My password is “frank”

 Simple and may be OK for standalone system

 But insecure for networked system

o Subject to a replay attack (next 2 slides)

o Bob must know Alice’s password

Part 3  Protocols 14

Authentication Attack

Alice Bob

“I’m Alice”

Prove it

My password is “frank”

Trudy

Part 3  Protocols 16

Simple Authentication

Alice Bob

I’m Alice, My password is “frank”

 More efficient…

 But same problem as previous version

Part 3  Protocols 17

Better Authentication

Alice Bob

“I’m Alice”

Prove it

h(Alice’s password)

 Better since it hides Alice’s password

o From both Bob and attackers

 But still subject to replay

Part 3  Protocols 19

Challenge-Response

Bob

“I’m Alice”

Nonce

h(Alice’s password, Nonce)

 Nonce is the challenge

 The hash is the response

 Nonce prevents replay, insures freshness

 Password is something Alice knows

 Note that Bob must know Alice’s password

Alice

Part 3  Protocols 20

Challenge-Response

Bob

“I’m Alice”

Nonce

Something that could only be

Alice from Alice (and Bob can verify)

 What can we use to achieve this?

 Hashed pwd works, crypto might be better