Lecture 14: 30th October 2019

Question 1

What is key exchange?

Answer 1

Methods by which cryptographic keys are exchanged between two parties, allowing use of a cryptographic algorithm.

Question 2

What is PKI?

Answer 2

PKI = Public Key Infrastructure = A set of roles, policies, hardware, software, and procedures needed to create, manage, distribute, use, store, and revoke digital certificates and manage public-key encryption to aid trusted communication by verifying the authenticity, integrity, and configuration of messages.

Question 3

How does key exchange work?

Answer 3

Alice asks Cathy to talk to Bob and uses a nonce. Cathy replies with a packet for Bob introducing Alice to Bob. Cathy then passes messages with Alice contacting Bob and Bob replying with a nonce.

Question 4

What is Needham-Schroeder?

Answer 4

A protocol that uses PKI to exchange cryptographic keys on an insecure network.

Question 5

How does Needham-Schroeder key exchange work?

Answer 5

A wants to talk to B and C is a TTP (server).

1. A ->C: A, B, NA; NA is a nonce, a number used once, e.g. clock time.
2. C->A: {NA, B, KAB, {KAB, A}KBC }KAC; KAB = key for A to use with B, KAC = key for A to use with C, A = identity of A, B = identity of B
3. A->B: {KAB, A}KBC: KBC is key for B to use with C, given by C
4. B->A: {NB}KAB: challenge response - use a ley and expect a nonce back to verify you can exchange data
5. A->B: {NB-1}KAB: respond to the above challenge; note -1 is just a known operation in the protocol. After B verifies the response, secure comms and trust has been established between A and B via the TTP C.

Question 6

What is a network session?

Answer 6

A temporary series of interactions and information exchanges between two or more communicating devices, or between a computer and user, during a single connection.

Question 7

What is a nonce?

Answer 7

“number only once”: random number used to identify a communications exchange and establish secure communications; ~ a session key

Question 8

How and why are nonces used in Needham-Schroeder key exchange?

Answer 8

They are given by the opening host because they mitigate replay attacks in which people pretend to be a TTP.

Question 9

What are the limitations of Needham-Schroeder key exchange?

Answer 9

Alice may stockpile keys for communication with various people. Cathy has no way to revoke those keys. If KAC is compromised, the attacker can obtain more keys or authority. Cathy must keep records of all keys issued.

Malicious attackers can be in the middle of the opening host and TTP, and then pose as the other host trying to be contacted. They return the response from the TTP as if the opening host was trying to contact them before or instead of the one to contact the other host they actually want to.

Question 10

How can you authenticate users and control access in distributed systems?

Answer 10

With a user identity, user network address, or an access operation

Question 11

What is a LSA?

Answer 11

LSA = Local Security Authority = A Microsoft Windows protected subsystem that is part of the Windows Client Authentication Architecture which authenticates users and creates logon session to the local computer.

Question 12

Why are LSAs used over password prompts in modern systems?

Answer 12

They are more secure.

Question 13

Why do modern protocols use asymetric encryption over Needham-Schroeder?

Answer 13

They are more secure. There are many issues: aliveness, freshness, replay, message integrity, hijacking (MITM), header and body encryption, cipher suites, strength, old standards.

Question 14

What is Kerberos?

Answer 14

The distributed Access Control system which was the default option in Windows 2000.

Question 15

What are the types of TTPs in Kerberos?

Answer 15

Kerberos has 2 types of TTP: authentication servers to which users log on and ticket granting servers that issues time-limited tickets for access to resources

Question 16

What are ticket granting servers?

Answer 16

Specialised computers that issue time-restricted tokens granting users access to resources.

Question 17

What are some general issues with asymetric key cryptography?

Answer 17

There are many issues: aliveness, freshness, replay, message integrity, hijacking (MITM), header and body encryption, cipher suites, strength, standards.

Question 18

What are authentication servers?

Answer 18

A computer running applications which verify the identities of users before they are allowed to access resources in a session.

Question 19

How does ticket granting work in Kerberos?

Answer 19

The protocol for ticket granting is a variant of Needham-Schroeder:

Alice logs onto server Sam using a password and requests resource B:

1. A ->S: A, B

The client software in her PC fetches a ticket encrypted under her password and which contains the key KAS. For access to resource B, the ticket KAB with timestamp TS and lifetime L is given.

2. S->A: {TS, L, KAB, B, {TS, L, KAB, A}KBS }KAS

Alice gets a copy of the ticket to read encrypted under KAS. She verifies the ticket by sending the timestamp TA to B which confirms its liveness by sending back the timestamp incremented by 1.

3. A->B: {TS, L, KAB , A}KBS , {A, TA}KAB
4. B->A: {TA + 1}KAB

Question 20

Why were nonces replaced with timestamps in Needham-Schroeder?

Answer 20

The random nonce of the Needham-Shroeder are replaced by timestamps to ensure freshness and aliveness.

Question 21

What are some problems with timestamps and time synchronisation that may explain why they weren’t included in Needham-Schroeder?

Answer 21

time zones giving different timestamps, timeouts, clock mismatches, and clocks changing every time

Question 22

What is DSSA?

Answer 22

DSSA = Distributed System Security Architecture = a computer security architecture that provides a suite of functions including login, authentication, and access control in a distributed system. The architecture covers user and system authentication, mandatory and discretionary security, secure initialization and loading, and delegation in a general-purpose computing environment of heterogeneous systems where there are no central authorities, no global trust, and no central controls.

Question 23

What is DSSA/SPX?

Answer 23

DSSA/SPX is the authentication protocol of DSSA.

Question 24

What is DASS?

Answer 24

DASS = Distributed Authentication Security Service = the name DSSA/SPX was adopted under.

Question 25

What is GSS-API?

Answer 25

An application programming interface for programs to access security services. For a distributed system the service layer is the most appropriate location for security enforcement.

Question 26

What is CORBA?

Answer 26

CORBA = Common Object Request Broker Architecture = a standard defined by the Object Management Group designed to facilitate the communication of heterogeneous systems.

Question 27

What is ORB?

Answer 27

ORB = Object Request Broker = a middleware which allows program calls to be made from one computer to another via a computer network, providing location transparency through remote procedure calls. It handles interactions between users and objects, and objects themselves. The ORB library contains object services such as naming, concurrency, querying, trading etc.

Question 28

What is RADIUS protocol?

Answer 28

A networking protocol that provides centralized Authentication, Authorization, and Accounting (AAA or Triple A) management for users who connect and use a network service. It carries information related to authentication, authorization, and configuration between a Network Access Server that desires to authenticate its links and a shared Authentication Server.

Question 29

What are some issues with complex exchange mechanisms?

Answer 29

issues with key handling; freshness; CA vulnerabilities; etc.

Question 30

What is a CDC?

Answer 30

CDC = Certificate Distribution Centre = The CDC is a certificate granting server within DSSA/SPX. Certificates are tickets signed by CA which contains the public key of the party being certified. Since the CDC is merely distributing previously signed certificates, it is not necessary for it to be trusted.

Question 31

What is a TTP?

Answer 31

A trusted third party (TTP) is an entity which facilitates interactions between two parties who both trust the third party. It establishes trust between them and this allows them to begin secure communications.