Authentication

Security Handshake Pitfalls 2

1 IT352 | Network Security

|Najwa AlGhamdi

Mediated Authentication ( With KDC)

• KDC has a database consists of all users’ keys.

• Simple Protocol

Problems • Potential delayed key delivery to

Bob. – Alice could send message to Bob as

soon as she got KAB from KDC , while Bob didn’t receive the key yet.

IT352 | Network Security |Najwa AlGhamdi

2

Alic

e

Bo

b KDC

Alice wants Bob

KA{Bob, KAB}

KB{Alice, KAB}

Mediated Authentication ( With KDC)

• Since Alice is going to communicate with Bob any , KDC gives Alice the information KDC would pass it to Bob using Ticket.

.

• This protocol should be followed by mutual authentication.

IT352 | Network Security |Najwa AlGhamdi

3

Alic

e

Bo

b KDC

Alice wants Bob

KA{Bob, KAB}, ticketB

where ticketB= KB{Alice, KAB}

Alice, ticketB

Mediated Authentication ( With KDC)

Needham-Schroeder

• A classic protocol for authentication using KDC.

• Nonce is used.

• Nonce : a number that is used only once. It could be

1. a sequence number.

2. Large random number

3. Timestamp.

IT352 | Network Security |Najwa AlGhamdi

4

Mediated Authentication ( With KDC)

Needham-Schroeder

IT352 | Network Security |Najwa AlGhamdi

5

Alic

e

Bo

b

KDC

N1, Alice, Bob

KA{N1, Bob, KAB, ticketB} where ticketB= KB{KAB, Alice}

ticketB, KAB{N2}

KAB{N2-1, N3}

KAB{N3-1}

Mediated Authentication ( With KDC)

Needham-Schroeder

• The purpose of Nonce N1 is to assure Alice that she is really talking to KDC

IT352 | Network Security |Najwa AlGhamdi

6

Alic

e

Bo

b

KDC

N1, Alice, Bob

KA{N1, Bob, KAB, ticketB} where ticketB= KB{KAB, Alice}

ticketB, KAB{N2}

KAB{N2-1, N3}

KAB{N3-1}

Mediated Authentication ( With KDC)

Needham-Schroeder

• KDC will give back generated to Alice and Bob.

• “Bob” is added to insure that this ticket is issued

for Bob.

IT352 | Network Security |Najwa AlGhamdi

7

Alic

e

Bo

b

KDC

N1, Alice, Bob

KA{N1, Bob, KAB, ticketB} where ticketB= KB{KAB, Alice}

ticketB, KAB{N2}

KAB{N2-1, N3}

KAB{N3-1}

Mediated Authentication ( With KDC)

Needham-Schroeder

• Alice Send an encrypted challenge N2 with

ticket to Bob.

IT352 | Network Security |Najwa AlGhamdi

8

Alic

e

Bo

b

KDC

N1, Alice, Bob

KA{N1, Bob, KAB, ticketB} where ticketB= KB{KAB, Alice}

ticketB, KAB{N2}

KAB{N2-1, N3}

KAB{N3-1}

Mediated Authentication ( With KDC)

Needham-Schroeder

• Bob proves he know KAB because he use it to extract N2.

IT352 | Network Security |Najwa AlGhamdi

9

Alic

e

Bo

b

KDC

N1, Alice, Bob

KA{N1, Bob, KAB, ticketB} where ticketB= KB{KAB, Alice}

ticketB, KAB{N2}

KAB{N2-1, N3}

KAB{N3-1}

Mediated Authentication ( With KDC)

Needham-Schroeder

• Alice proves he know KAB because he use it to extract N3.

IT352 | Network Security |Najwa AlGhamdi

10

Alic

e

Bo

b

KDC

N1, Alice, Bob

KA{N1, Bob, KAB, ticketB} where ticketB= KB{KAB, Alice}

ticketB, KAB{N2}

KAB{N2-1, N3}

KAB{N3-1}

Mediated Authentication ( With KDC)

Security Vulnerability in Needham-Schroeder

• Happens when Trudy manages to capture a previous key used by Alice. Then Trudy will simply discover the shared key.

Expanded Needham-Schroeder

• Two Additional messages will be added

1. Alice will request a nonce from Bob .

2. Alice will pass this nonce to KDC , then KDC will package this nonce in the ticket to Bob.

Mediated Authentication ( With KDC)

Expanded Needham-Schroeder

• Two Additional messages will be added

1. Alice will request a nonce from Bob .

2. Alice will pass this nonce to KDC , then KDC will package this nonce in the ticket to Bob.

Alic

e

Bo

b KDC

N1, Alice, Bob, KB{NB}

KA{N1, Bob, KAB, ticketB} where ticketB= KB{KAB, Alice, NB}

ticketB, KAB{N2}

KAB{N2-1, N3}

KAB{N3-1}

hello

KB{NB}

Mediated Authentication ( With KDC)

Otway-Rees

• Alice will use two nonces .

Alic

e

Bo

b KDC

NC, KA{NA, KAB}, KB{NB, KAB}

KA{NA, NC, “Alice”, “Bob”} KB{NB, NC, “Alice”, “Bob”}

NC, “Alice”, “Bob”, KA{NA, NC, “Alice”, “Bob”}

KA{NA, KAB}

KAB{anything recognizable}