Network Security (NetSec)
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1 Chair of Network Architectures and Services Department of Informatics Technical University of Munich Network Security (NetSec) IN2101 WS 17/18 Prof. Dr.-Ing. Georg Carle Dr. Heiko Niedermayer Cornelius Diekmann Chair of Network Architectures and Services Department of Informatics Technical University of Munich
2 Chapter 15: Kerberos and other Frameworks for Client Authentication Introduction Learning Goals Frameworks for Client Authentication Kerberos Conclusions What have we learned Chapter 15: Kerberos and other Frameworks for Client Authentication 15-1
3 Chapter 15: Kerberos and other Frameworks for Client Authentication This course is based to some extend on slides provided by Günter Schäfer, author of the book "Netzsicherheit - Algorithmische Grundlagen und Protokolle", available in German from dpunkt Verlag. The English version of the book is entitled Security in Fixed and Wireless Networks: An Introduction to Securing Data Communications and is published by Wiley is also available. We gratefully acknowledge his support. The slide set has been reworked by Heiko Niedermayer, Ali Fessi, Ralph Holz, Cornelius Diekmann, and Georg Carle. Chapter 15: Kerberos and other Frameworks for Client Authentication 15-2
4 Chapter 15: Kerberos and other Frameworks for Client Authentication Slides called "- Explanation" and usually marked with are not for the lecture, but they contain further explanations for your learning at home. Parts called "Exercise" are voluntary exercises for discussion in lecture as well as for your reworking of the slides and learning at home. Chapter 15: Kerberos and other Frameworks for Client Authentication 15-3
5 Chapter 15: Kerberos and other Frameworks for Client Authentication Agenda Introduction Learning Goals Frameworks for Client Authentication Kerberos Conclusions What have we learned Chapter 15: Kerberos and other Frameworks for Client Authentication 15-4
6 Chapter 15: Kerberos and other Frameworks for Client Authentication Introduction Learning Goals Frameworks for Client Authentication Kerberos Conclusions What have we learned Chapter 15: Kerberos and other Frameworks for Client Authentication 15-5
7 Introduction User Authentication Chapter 15: Kerberos and other Frameworks for Client Authentication Introduction 15-6
8 Introduction User Authentication Username Credential Credential usually: Password / Shared Key Signature (less frequent) Multiple Factors Know password Possess card... Benefit of multiple factors is to have multiple lines of defense. Makes attacks more complicated, not impossible. Chapter 15: Kerberos and other Frameworks for Client Authentication Introduction 15-7
9 Login to Services on the Network The service needs to know user and a credential. In this chapter, we usually assume the use of symmetric cryptography. Keys derived from password or other factor. Chapter 15: Kerberos and other Frameworks for Client Authentication Introduction 15-8
10 Chapter 15: Kerberos and other Frameworks for Client Authentication Introduction Learning Goals Frameworks for Client Authentication Kerberos Conclusions What have we learned Chapter 15: Kerberos and other Frameworks for Client Authentication 15-9
11 Learning Goals Basic understanding of the Kerberos protocol Remember and explain Apply what you learned in crypto prots on it and modified protocols Ticket concept Remember and explain Apply in modified setting Inclusion of Authentication Backends Know of concept, know names of some backends Where does user authentication come from? Chapter 15: Kerberos and other Frameworks for Client Authentication Learning Goals 15-10
12 Chapter 15: Kerberos and other Frameworks for Client Authentication Introduction Learning Goals Frameworks for Client Authentication Kerberos Conclusions What have we learned Chapter 15: Kerberos and other Frameworks for Client Authentication 15-11
13 Authenticating to a Server Client (A) Server (S1) User name and Password protected in some form. A client has accounts at each of the servers it uses. Each server has a user database and operates as authentication server. Disadvantages: hard to manage, number of passwords,... Chapter 15: Kerberos and other Frameworks for Client Authentication Frameworks for Client Authentication 15-12
14 Authenticating to a Server with Authentication Server Client (A) Server (S1) User name and Password protected in some form. User name and Password protected in some form. Authentication Server (AS) An Authentication Server (AS) manages the accounts. Authentication has to happen via the AS. Chapter 15: Kerberos and other Frameworks for Client Authentication Frameworks for Client Authentication 15-13
15 Authentication with external AS Option: S1 uses the credentials provided by A to run a cryptographic protocol with AS. Common practice for local authentication within one infrastructure (Kerberos might be used). Alternative (better): End-to-end Authentication between A and AS. S1 relays messages and AS informs S1 about outcome (ACCEPT / DENY). Found in more public infrastructures. Link Layer Access: Extensible Authentication Protocol (EAP),... Alternative (better): A runs protocol with AS, interaction results in information A and S1 can use to mutually authenticate. Can happen before A and S1 interact or in-between. OpenID, OAuth,... Kerberos (original idea) Chapter 15: Kerberos and other Frameworks for Client Authentication Frameworks for Client Authentication 15-14
16 Authentication with external AS Local infrastructures may want to use different authentication services Make authentication services accessible via a generic API, e.g. PAM, GSSAPI,... Many authentication services themselves operate primarily as generic transport protocols for cryptographic protocol messages as well as authentication and authorization results (e.g. EAP, Radius,...). Chapter 15: Kerberos and other Frameworks for Client Authentication Frameworks for Client Authentication 15-15
17 Chapter 15: Kerberos and other Frameworks for Client Authentication Introduction Learning Goals Frameworks for Client Authentication Kerberos Conclusions What have we learned Chapter 15: Kerberos and other Frameworks for Client Authentication 15-16
18 Kerberos Kerberos is an authentication and access control service for work-station clusters that was designed at the MIT during the late 1980s Design goals: Security: eavesdroppers or active attackers should not be able to obtain the necessary information to impersonate a user when accessing a service Reliability: as every use of a service requires prior authentication, Kerberos should be highly reliable and available Transparency: the authentication process should be transparent to the user beyond the requirement to enter a password Scalability: the system should be able to support a large number of clients and servers Chapter 15: Kerberos and other Frameworks for Client Authentication Kerberos 15-17
19 Kerberos Concept Key Distribution Center (KDC) Client (A) Authentication Server (AS) 1. I am Alice, give me a ticket. 2. Key and ticket. Provides Authentication, only Alice can use ticket 3. Here is my ticket, give me a ticket for S1. Ticket Granting Server (TGS) 4. Key and ticket for S1. Provides Authorization, Alice allowed to use S1? 5. Dear S1, here is my ticket and authentication. 6. S1 authenticates. Server (S1) Chapter 15: Kerberos and other Frameworks for Client Authentication Kerberos 15-18
20 Kerberos Concept Kerberos Realm of A, AS, TGS, and S1 Key Distribution Center (KDC) Client (A) Authentication Server (AS) 1. I am Alice, give me a ticket. 2. Key and ticket. 3. Here is my ticket, give me a ticket for S1. Ticket Granting Server (TGS) 4. Key and ticket for S1. 5. Dear S1, here is my ticket and authentication. 6. S1 authenticates. Server (S1) Chapter 15: Kerberos and other Frameworks for Client Authentication Kerberos 15-18
21 Kerberos Needham-Schroeder Conceptually, Kerberos is based on the Needham-Schroeder Symmetric Key Protocol uses timestamps instead of nonces (random numbers) Key Distribution Center Ticket Realm Password provides authentication and authorization generates and provides the keys for the next steps like in Needham-Schroeder binds key and identity of client binds ticket to IP address of client Kerberos operates in organizational realms Operation is limited to realm Multi-realm possible if realms cooperate Shared key with AS derived from user password, traditionally k A,AS = md5(password A ) Chapter 15: Kerberos and other Frameworks for Client Authentication Kerberos 15-19
22 Kerberos Concept Client (A) 0. know each other, have longterm shared key k AS,A = h(password A ) Authentication Server (AS) Ticket Granting Server (TGS) 0. know each other, have shared key k TGS,S1 Server (S1) Chapter 15: Kerberos and other Frameworks for Client Authentication Kerberos 15-20
23 Kerberos - Protocol Client (A) Authentication Server (AS) 1. A, t A, TGS, RequestedTicketLifetime TGS 2. {K A,TGS, TGS, t AS, LifetimeTicket TGS, Ticket TGS } KA,AS with Ticket TGS = {K A,TGS, A, Addr A, TGS, t AS, LifetimeTicket TGS } KAS,TGS Ticket Granting Server (TGS) 3. S1, Ticket TGS, Authenticator A,TGS with Authenticator A,TGS = {A, Addr A, t1 A } KA,TGS 4. {K A,S1, S1, t TGS, Ticket S1 } KA,TGS with Ticket S1 = {K A,S1, A, Addr A, S1, t TGS, LifetimeTicket S1 } KTGS,S1 5. Ticket S1, Authenticator A,S1 Server (S1) with Authenticator A,S1 = {A, Addr A, t2 A } KA,S1 6. {t2 A + 1} KA,S1 Chapter 15: Kerberos and other Frameworks for Client Authentication Kerberos 15-21
24 Kerberos Protocol 1. A AS : A, t A, TGS, RequestedTicketLifetime TGS The first message does not use cryptography. Fields are the user name, timestamp, a valid Ticket Granting Server, and the requested lifetime to the tickets. 2. AS A : {K A,TGS, TGS, t AS, LifetimeTicket TGS, Ticket TGS } KA,AS with Ticket TGS = {K A,TGS, A, Addr A, TGS, t A This message is protected with the shared key of A and AS ("password of A"), which is part of the user database of the AS. The first part of the message is information Alice needs to use the ticket, e.g. the new shared key K A,TGS with the TGS. The second part of the message is the ticket, which Alice cannot decrypt or modify. In general, the ticket concepts needs to give the ticket holder enough information to be able to use the ticket. The ticket is protected from the ticket holder. The ticket itself contains similar information, yet for the server that verifies the ticket. Chapter 15: Kerberos and other Frameworks for Client Authentication Kerberos 15-22
25 Kerberos Protocol 3. A TGS : S1, Ticket TGS, Authenticator A,TGS with Authenticator A,TGS = {A, Addr A, t1 A } KA,TGS With the Authenticator Alice shows to the TGS that she is the legitimate ticket holder. She uses the relevant key K A,TGS, which is part of the ticket. She has the right IP address Addr A. Authenticator and Ticket are fresh due to fresh enough timestamps. 4. TGS A : {K A,S1, S1, t TGS, Ticket S1} KA,TGS with Ticket S1 = {K A,S1, A, Addr A, S1, t TGS, LifetimeTicket S1} KT Similar to A S1 : Ticket S1, Authenticator A,S1 with Authenticator A,S1 = {A, Addr A, t2 A } KA,S1 Similar to S1 A : {t2 A + 1} KA,S1 S1 uses the relevant shared key and answers with Alice s timestamp as nonce. Alice knows she uses the right server. Chapter 15: Kerberos and other Frameworks for Client Authentication Kerberos 15-23
26 Kerberos Multi-Realm Realm 1 Client (A) AS TGS AS TGS have longterm shared key k TGS1,TGS2 0. know each other, Server S2 Realm 2 Chapter 15: Kerberos and other Frameworks for Client Authentication Kerberos 15-24
27 Kerberos Multi-Realm Realm 1 Client (A)... AS 4. Key and Ticket for TGS2 TGS 5. Here is my ticket, give me ticket for S2 6. Ticket for S2 7. Here is my ticket and authentication 8. S2 authenticates AS have longterm shared key k TGS1,TGS2 0. know each other, TGS Server S2 Realm 2 Chapter 15: Kerberos and other Frameworks for Client Authentication Kerberos 15-24
28 Kerberos Pre-Authentication Preliminaries Weakness Password Authentication Remember message 1: A, t A, TGS, RequestedTicketLifetime TGS Message 1 is not protected. An attacker can request a ticket for someone else. The AS will send message 2 to the attacker. Remember Message 2: {K A,TGS, TGS, t AS, LifetimeTicket TGS, Ticket TGS } KA,AS with K A,AS = h(password A ) Now the attacker has ciphertext encrypted with a low-entropy key derived from the password. Attack the key with suitable attack (e.g. dictionary attack) Chapter 15: Kerberos and other Frameworks for Client Authentication Kerberos 15-25
29 Kerberos Pre-Authentication Kerberos Pre-Authentication Pre-Authentication is a concept developed for Kerberos 1 to extend the protocol messages (optional). Protocol principles prove their identity before their message is further processed. To avoid the attack above, PA-ENC-TIMESTAMP was proposed in Kerberos 5. Pre-Authentication as generic concept supports all kinds of authentication concepts. PA-ENC-TIMESTAMP Add {t A } KA,AS as pre-authentication in message 1. AS will only reply if a current timestamp protected with Alice s key was sent. Thus, ciphertext using key K A,AS will not be sent to the attacker. 2 1 IETF RFC Our Dolev-Yao attacker will see Alice s communication and see such ciphertext nonetheless. Chapter 15: Kerberos and other Frameworks for Client Authentication Kerberos 15-26
30 Kerberos Design Goals? Remember the design goals: Security: eavesdroppers or active attackers should not be able to obtain the necessary information to impersonate a user when accessing a service Modern versions of Kerberos use state-of-the-art cryptography, (optional) pre-authentication helps with password issues Reliability: as every use of a service requires prior authentication, Kerberos should be highly reliable and available The design allows redundant servers and tickets can be reused within their lifetime. Transparency: the authentication process should be transparent to the user beyond the requirement to enter a password Kerberos is a single-sign-on solution. Applications can use tickets within their lifetime. General APIs like PAM help with Kerberos integration in applications. Scalability: the system should be able to support a large number of clients and servers The design allows redundant servers. Chapter 15: Kerberos and other Frameworks for Client Authentication Kerberos 15-27
31 Kerberos Reality Check In most environments, Kerberos is used in the backend between Server and Key Distribution Center, but not on client side. Pluggable Authentication Modules (PAM) in Linux demonstrate the situation: The services of the system use PAM to do their authentication. PAM provides an API for these services with. Kerberos is one PAM authentication service module, but there are others like LDAP, RADIUS,... FTP Server Web Server SSH Server IMAP Server PAM Configuration PAM API Kerberos LDAP RADIUS Chapter 15: Kerberos and other Frameworks for Client Authentication Kerberos 15-28
32 Chapter 15: Kerberos and other Frameworks for Client Authentication Introduction Learning Goals Frameworks for Client Authentication Kerberos Conclusions What have we learned Chapter 15: Kerberos and other Frameworks for Client Authentication 15-29
33 Conclusions What have we learned Authentication Backends Kerberos User Authentication APIs, Credentials, Transport of Credentials, Protocols Example for password authentication Ticket Concept Today s use usually differs from original idea. Chapter 15: Kerberos and other Frameworks for Client Authentication Conclusions What have we learned 15-30
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