1

Authentication Applications

Digital SignaturesSecurity ConcernsX.509 Authentication ServiceKerberos

Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

2

Security Concerns

• key concerns are confidentiality and timeliness

• to provide confidentiality must encrypt identification and session key info

• which requires the use of previously shared private or public keys

• need to prove ownership of public keys• need timeliness to prevent replay

attacks • provided by using sequence numbers or

timestamps or challenge/response Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

Digital signatures

• A digital signature is an encryption of a document with the creator’s private key

• It is attached to a document that validates the creator of the document

• Any one can validate it by decrypting the signature with the claimed creator’s public key

3Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

Some Misconceptions

• Some misconceptions, corrected– Public key cryptography is NOT more

secure than symmetric key– Public key cryptography does NOT

Makes symmetric key obsolete– Central agent is needed for both

public key and conventional cryptography

4Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

Alternatively…. Digital signatures on

hashes• A more efficient way for a digital signature is by creating

an authenticator of the document first (a hash)

• Then sign the hash (i.e. encrypt the hash using private key)

• If M is the message (or document) and Alice wants Bob to be certain that M is from her.

• M is not confidential

– H = Hash(M) – Authenticator = EKRA

• i.e. encrypting H with Alice’s private key5Based on slides by Dr. Lawrie Brown of the Australian Defence Force

Academy, University College, UNSW

Digital Signatures: The basic idea

? private key

public key

public key

Alice Bob

6Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

Key management

• Distribution of public keys– Well, what’s the issue? – Can’t we just trust Mallory if she

claims a key as her public key?

? private key

public key

public key

Alice Bob

Mallory

7Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

Public keys to exchange secret keys

• Using public-keys to exchange secret keys– why exchange secret keys?– aren’t public keys sufficient?

8Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

Authenticity of public keys

?

Problem: How does Alice know that the public key she received is really Bob’s public key?

private key

AliceBob

public key

Bob’s key

9Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

Public-key certificates

• Anyone can forge public-keys

• Therefore, use public-key certificates

• A public-key certificate is a public-key that was signed by a trusted third party (called a certificate authority or CA)

• See figure on next slide

10Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

Key Management Public-Key Certificate Use

11Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

12

X.509 Authentication Service

• Distributed set of servers that maintains a database about users.

• Each certificate contains the public key of a user and is signed with the private key of a CA.

• Is used in S/MIME, IP Security, SSL/TLS and SET.

• RSA is recommended to use.

Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

13

X.509 Formats

Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

14

Typical Digital Signature Approach

Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

15

Obtaining a User’s Certificate

• Characteristics of certificates generated by CA:– Any user with access to the public key

of the CA can recover the user public key that was certified.

– No party other than the CA can modify the certificate without this being detected.

Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

16

X.509 CA Hierarchy

Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

17

Revocation of Certificates

• Reasons for revocation:– The users secret key is assumed to be

compromised.– The user is no longer certified by this

CA.– The CA’s certificate is assumed to be

compromised.

Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

18

Authentication Procedures

Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

19

KERBEROS

• Users wish to access services on servers.

• Three threats exist:– User pretend to be another user.– User alter the network address of a

workstation.– User eavesdrop on exchanges and

use a replay attack.

Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

20

KERBEROS

• Provides a centralized authentication server to authenticate users to servers and servers to users.

• Relies on conventional encryption, making no use of public-key encryption

• Two versions: version 4 and 5• Version 4 makes use of DES

Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

21

Kerberos Version 4

• Terms:– C = Client– AS = authentication server– V = server– IDc = identifier of user on C– IDv = identifier of V– Pc = password of user on C– ADc = network address of C– Kv = secret encryption key shared by AS an

V– TS = timestamp– || = concatenation

Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

22

A Simple Authentication

Dialogue(1)C AS: IDc || Pc || IDv

(2) AS C: Ticket

(3) C V: IDc || Ticket

Ticket = EKv[IDc || ADc || IDv]

Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

23

Version 4 Authentication

Dialogue• Problems:

– Lifetime associated with the ticket-granting ticket

– If too short repeatedly asked for password– If too long greater opportunity to replay

• The threat is that an opponent will steal the ticket and use it before it expires

Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

24

Version 4 Authentication Dialogue

Authentication Service Exhange: To obtain Ticket-Granting Ticket

(1) C AS: IDc || IDtgs ||TS1

(2) AS C: EKc [Kc,tgs|| IDtgs || TS2 || Lifetime2 || Tickettgs]

Ticket-Granting Service Echange: To obtain Service-Granting Ticket

(3) C TGS: IDv ||Tickettgs ||Authenticatorc

(4) TGS C: EKc [Kc,¨v|| IDv || TS4 || Ticketv]

Client/Server Authentication Exhange: To Obtain Service

(5) C V: Ticketv || Authenticatorc

(6) V C: EKc,v[TS5 +1]

Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

25

Overview of Kerberos

Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

26

Request for Service in Another Realm

Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

27

Difference Between Version 4 and 5

• Encryption system dependence (V.4 DES)

• Internet protocol dependence• Message byte ordering• Ticket lifetime• Authentication forwarding• Interrealm authentication

Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

28

Kerberos - in practice • Currently have two Kerberos versions:

• 4 : restricted to a single realm • 5 : allows inter-realm authentication, in beta test • Kerberos v5 is an Internet standard • specified in RFC1510, and used by many utilities

• To use Kerberos: • need to have a KDC on your network • need to have Kerberised applications running on all

participating systems • major problem - US export restrictions though

these have been relaxed in recent years

Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW