1 An Introduction to Kerberos Shumon Huque ISC Networking & Telecommunications University of Pennsylvania March 19th 2003

2 What this talk is about A high-level view of how Kerberos works How Kerberos differs from some other authentication systems SSH password auth, SSH public key auth, SSL Target audience: LSPs, computing staff, others?

3 What this talk is not about Details of Penn’s Kerberos deployment plans How to get PennKeys, which Kerberos enabled applications do I need to use Writing Kerberized applications In-depth protocol details and packet formats Number Theory & Cryptography

4 What is Kerberos? Developed at M.I.T. A secret key based service for providing authentication in open networks Authentication mediated by a trusted 3rd party on the network: Key Distribution Center (KDC)

5 Kerberos: etymology The 3-headed dog that guards the entrance to Hades Originally, the 3 heads represented the 3 A’s But one A was work enough!

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7 Fluffy, the 3 headed dog, from “Harry Potter and the Sorcerers Stone”

8 Some Kerberos benefits Standards based strong authentication system Wide support in various operating systems Make strong authentication readily available for use with campus computer systems Prevents transmission of passwords over the network Provides “single-sign-on” capability Only 1 password to remember Only need to enter it once per day (typically)

9 So, what is Authentication? The act of verifying someone’s identity The process by which users prove their identity to a service Doesn’t specify what a user is allowed or not allowed to do (Authorization)

10 Password based Authentication Transmit password in clear over the network to the server Main Problem Eavesdropping/Interception

11 Cryptographic Authentication No password or secret is transferred over the network Users prove their identity to a service by performing a cryptographic operation,usually on a quantity supplied by the server Crypto operation based on user’s secret key

12 Encryption and Decryption Encryption Process of scrambling data using a cipher and a key in such a way, that it’s intelligible only to the recipient Decryption Process of unscambling encrypted data using a cipher and key (possibly the same key used to encrypt the data)

13 Symmetric Key Cryptography Aka, Secret Key cryptography The same key is used for both encryption and decryption operations (symmetry) Examples: DES, 3-DES, AES

14 Asymmetric Key Cryptography Aka Public key cryptography A pair of related keys are used: Public and Private keys Private key can’t be calculated from Public key Data encrypted with one can only be decrypted with the other Usually, a user publishes his public key widely Others use it to encrypt data intended for the user User decrypts using the private key (known only to him) Examples: RSA

15 Communicating Parties Alice and Bob Alice: initiator of the communication  Think of her as the “client” or “user” Bob: correspondent or 2nd participant  Think of him as the “server” “Alice” wants to access service “Bob” Baddies: Eve, Trudy, Mallory

16 Simple shared-secret based cryptographic authentication

17 Add mutual authentication

18 Problems with this scheme Poor scaling properties Generalizing the model for m users and n services, requires a priori distribution of m x n shared keys Possible improvement: Use trusted 3rd party, with which each user and service shares a secret key: m + n keys Also has important security advantages

19 Mediated Authentication A trusted third party mediates the authentication process Called the Key Distribution Center (KDC) Each user and service shares a secret key with the KDC KDC generates a session key, and securely distributes it to communicating parties Communicating parties prove to each other that they know the session key

20 Mediated Authentication Nomenclature: K a = Master key for “alice”, shared by alice and the KDC K ab = Session key shared by “alice” and “bob” T b = Ticket to use “bob” K{data} = “data” encrypted with key “K”

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22 Mediated Authentication

23 Mediated Authentication

24 Kerberos uses timestamps Timestamps as nonce’s are used in the mutual authentication phase of the protocol This reduces the number of total messages in the protocol But it means that Kerberos requires reasonably synchronized clocks amongst the users of the system

25 Kerberos (almost)

26 Kerberos (roughly)

27 Needham-Schroeder Protocol

28 Kerberos (detailed) Each user and service registers a secret key with the KDC Everyone trusts the KDC “Put all your eggs in one basket, and then watch that basket very carefully” - Anonymous Mark Twain The user’s key is derived from a password, by applying a hash function The service key is a large random number, and stored on the server

29 Kerberos “principal” A client of the Kerberos authentication service A user or a service Format: Examples:

30 Kerberos without TGS A simplified description of Kerberos without the concept of a TGS (Ticket Granting Service)

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34 Combining 2 previous diags …

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36 Review: Kerberos Credentials Ticket Allows user to use a service (actually authenticate to it) Used to securely pass the identity of the user to which the ticket is issued between the KDC and the application server K b {“alice”, K ab, lifetime} Authenticator Proves that the user presenting the ticket is the user to which the ticket was issued Proof that user knows the session key Prevents ticket theft from being useful Prevents replay attacks (timestamp encrypted with the session key): K ab {timestamp}, in combination with a replay cache on the server

37 Ticket Granting Service (TGS) Motivation

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40 Kerberos with TGS Ticket Granting Service (TGS): A Kerberos authenticated service, that allows user to obtain tickets for other services Co-located at the KDC Ticket Granting Ticket (TGT): Ticket used to access the TGS and obtain service tickets Limited-lifetime session key: TGS sessionkey Shared by user and the TGS TGT and TGS session-key cached on Alice’s workstation

41 TGS Benefits Single Sign-on (SSO) capability Limits exposure of user’s password Alice’s workstation can forget the password immediately after using it in the early stages of the protocol Less data encrypted with the user’s secret key travels over the network, limiting attacker’s access to data that could be used in an offline dictionary attack

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46 Levels of Session Protection Initial Authentication only Safe messages: Authentication of every message  Keyed hashing with session key Private messages: + Encryption of every message  With session key, or mutually negotiated subsession keys Note: Application can choose other methods

47 Pre-authentication Kerberos 5 added pre-authentication Client is required to prove it’s identity to the Kerberos AS in the first step By supplying an encrypted timestamp (encrypted with users secret key) This prevents an active attacker being able to easily obtain data from the KDC encrypted with any user’s key  Then able to mount an offline dictionary attack

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49 Kerberos & Two-factor auth In addition to a secret password, user is required to present a physical item: A small electronic device: h/w authentication token Generates non-reusable numeric responses Called 2-factor authentication, because it requires 2 things: Something the user knows (password) Something the user has (hardware token)

50 Cross Realm Authentication

51 Hierarchy/Chain of Realms

52 Kerberos and PubKey Crypto Proposed enhancements Public key crypto for Initial Authentication  “PKINIT” Public key crypto for Cross-realm Authentication  “PKCROSS”

53 Kerberos: summary Authentication method: User’s enter password on local machine only Authenticated via central KDC once per day No passwords travel over the network Single Sign-on (via TGS): KDC gives you a special “ticket”, the TGT, usually good for rest of the day TGT can be used to get other service tickets allowing user to access them (when presented along with authenticators)

54 Advantages of Kerberos (1) Passwords aren’t exposed to eavesdropping Password is only typed to the local workstation It never travels over the network It is never transmitted to a remote server Password guessing more difficult Single Sign-on More convenient: only one password, entered once Users may be less likely to store passwords Stolen tickets hard to reuse Need authenticator as well, which can’t be reused Much easier to effectively secure a small set of limited access machines (the KDC’s)

55 Advantages of Kerberos (2) Easier to recover from host compromises Centralized user account administration

56 Kerberos caveats Kerberos server can impersonate anyone KDC is a single point of failure Can have replicated KDC’s KDC could be a performance bottleneck Everyone needs to communicate with it frequently Not a practical concern these days Having multiple KDC’s alleviates the problem If local workstation is compromised, user’s password could be stolen by a trojan horse Only use a desktop machine or laptop that you trust Use hardware token pre-authentication

57 Kerberos caveats (2) Kerberos vulnerable to password guessing attacks Choose good passwords! Use hardware pre-authentication  Hardware tokens, Smart cards etc

58 References Kerberos: An Authentication Service for Open Network Systems Steiner, Neuman, Schiller, 1988, Winter USENIX Kerberos: An Authentication Service for Computer Networks Neuman and Ts’o, IEEE Communications, Sep 1994 A Moron’s guide to Kerberos - Brian Tung Designing an Authentication System: A Dialogue in Four Scenes Bill Bryant,

59 References (cont) RFC 1510: The Kerberos Network Authentication Service (v5) Kohl and Neuman, September 1993 draft-ietf-krb-wg-kerberos-clarifications-03.txt IETF Kerberos Working Group: rfc1510 revision Using Encryption for Authentication in Large Networks of Computers Roger Needham, Michael D. Schroeder CACM, Volume 21, December 1978, pp

60 Questions or comments? Shumon Huque