Week 7: Authentication Applications Kerberos

1. Week 7: Authentication Applications Kerberos

2. Kerberos X.509

3. How To Secure Network How do you secure your network and each workstation or server with one tool?

4. How To Secure Network

5. Assume an open distributed environment in which users at workstation wish to access services on servers distributed throughout the network. Question: What kind of threats exist in this environment?

6. Assume an open distributed environment in which users at workstation wish to access services on servers distributed throughout the network. • Question: • What kind of threats exist in this environment? • Answer: • A user may gain access to a particular workstation and pretend to be another user operating from that workstation. • A user may alter the network address of a workstation so that the request sent from altered workstation appear to come from the impersonated workstation • A user may eavesdrop on exchanges and used replay attack to gain entrance to a server or to disrupt operations.

7. The Answer Is Kerberos negotiates the communication between any two points on the Internet or an intranet. It can also encrypt the communication between two points. Because of this, it provides a layer of security that is independent of either side of any firewall.

8. Problem Statement • The Internet is a vast place that connects millions of people from all corners of the globe to each other everyday. • In such a network, information can be lost, stolen, corrupted, or misused. • Another drawback of the internet is that it is difficult for individuals to confirm their identity to one another. • Confidentiality is very important for some types of information, such as information related to banking and medical. It is therefore important that a user, who wants to access this kind of information online, be able to confirm that the user is who he/she claims to be. This process is called authentication. • Kerberos plays a major role in authentication.

9. Problem Statement • Traditionally, a process was set in place called Authentication by Assertion. Authentication by assertion works as follows: • When a user runs a program that accesses a network service, the program (called the client) asserts to the service that it is running on behalf of the user. This provides a very low level of security. • Consider the example of Berkeley rlogin. If a user rlogins to an account under his own name, but on another machine, and if the user's . rhosts is set correctly, the rlogin program will assert the user's identity to the rlogin daemon on the remote machine, and the daemon does not require a password for login. This can become disastrous if an attacker is somehow able either to convince the rlogin program that he/she is the legitimate user, or to rewrite a mutant version of rlogin asserting that identity to the remote machine.

10. Problem Statement • An alternative to this situation is to require a user to enter a password each time he/she accesses a network service. This is a very time-consuming process, and it is insecure when users access services on a remote machine. When a user is logged on to a remote machine and then logs in from there to another remote machine, the password travels unencrypted through the network. • Kerberos fixes these problems because it provides single-sign-on, which lets a user log in to a system and access multiple systems or applications without the need to enter the user name and password multiple times. In addition, Kerberos is designed so that entities have to authenticate themselves by demonstrating possession of secret information. In this manner, Kerberos solves traditional problems involved with authentication.

11. Problem Statement • The problem statement discussed the problems associated with traditional authentication methods and, in particular, how passwords are vulnerable because they travel unencrypted over the network. • Password-based authentication is also inconvenient; users do not want to enter a password each time they access a network service. • Kerberos is designed to eliminate the need for users to demonstrate possession of private or secret information (the password). Instead, Kerberos disseminates this information. Kerberos Server lets entities authenticate themselves, without transmitting their passwords in clear text over the network. • Commonly used to secure particularly vulnerable network communications like ftp, telnet, and other widely used Internet protocols that normally transmit user IDs and passwords in clear text, Kerberos provides the "plumbing" for common authentication services. Its scalability means that Kerberos is ideal for large networks such as those used by governments, telecommunication networks, and major financial institutions.

12. Problem Statement • The problem statement discussed the problems associated with traditional authentication methods and, in particular, how passwords are vulnerable because they travel unencrypted over the network. • Password-based authentication is also inconvenient; users do not want to enter a password each time they access a network service. • Kerberos is designed to eliminate the need for users to demonstrate possession of private or secret information (the password). Instead, Kerberos disseminates this information. Kerberos Server lets entities authenticate themselves, without transmitting their passwords in clear text over the network. • Commonly used to secure particularly vulnerable network communications like ftp, telnet, and other widely used Internet protocols that normally transmit user IDs and passwords in clear text, Kerberos provides the "plumbing" for common authentication services. Its scalability means that Kerberos is ideal for large networks such as those used by governments, telecommunication networks, and major financial institutions.

13. Kerberos • Kerberos is a computer network authentication protocol which allows individuals communicating over an insecure network to prove their identity to one another in a secure manner. Kerberos prevents eavesdropping or replay attacks, and ensures the integrity of the data. Its designers aimed primarily at a client-server model, and it provides mutual authentication — both the user and the service verify each other's identity. • Kerberos builds on symmetric key cryptography and requires a trusted third party. • Kerberos is a real-world implementation of a symmetric cryptographic protocol whose propose is to provide levels of authentication and security in key exchange between users in a network. • Kerberos is a network authentication protocol which utilizes symmetric cryptography to provide authentication for client-server applications.

14. Kerberos Architecture • The Kerberos Standard Definition • Kerberos is defined in RFC 1510 - The Kerberos Network Authentication Service (V5). • The core of a Kerberos architecture is the KDC (Key Distribution Server). The KDC stores authentication information and uses it to securely authenticate users and services. • This authentication is called secure because it: • Does not occur in plaintext • Does not rely on authentication by the host operating system • Does not base trust on IP addresses • Does not require physical security of the network hosts • The KDC acts as a trusted third party in performing these authentication services. • Due to the critical function of the KDC, multiple KDC's are normally utilized. Each KDC stores a database of users, servers, and secret keys. • Kerberos clients are normal network applications which have been modified to use Kerberos for authentication. In Kerberos slang, they have been Kerberized.

15. Kerberos Protocol • Kerberos defines ten messages that make up the Kerberos protocol:

16. Kerberos Implementations • MIT Kerberos is the reference implementation. MIT Kerberos supports DEC Unix, Linux, Irix, Solaris, Windows and MacOS. • Several other commercial and non-commercial Kerberos implementations are also available. • Microsoft added a slight modified version of Kerberos v5 authentication in Windows 2000.

17. Kerberos Weaknesses • Because the KDC's store secret keys for every user and server on the network, they must be kept completely secure. If an attacker were to obtain administrative access to the KDC, he would have access to the complete resources of the Kerberos realm. • Kerberos tickets are cached on the client systems. If an attacker gains administrative access to a Kerbos client system, he can impersonate the authenticated users of that system.

18. Kerberos Encryption Protocols • Kerberos uses the DES algorithm for encryption. Kerberos also supports the CRC-32, MD4, MD5, and DES algorithms for checksums. Kerberos implementations are free to add additional algorithms for encryption and checksumming.

19. APPLICATION SERVER Authentication Process AS TGS Key Table File Secured Service Kerberos Server 1 2 4 5 6 7 Credentials Cache 3 Client Workstation

20. Authentication Process • Step 1. The user begins to use a Kerberized application by entering the user name and password. Optionally, the user can request for specific ticket flags and specify the key type to be used for constructing the secret key. The user can also accept the default, configured for the client. • The user sends the following information to the Authentication Service (AS) to obtain credentials: • Client, Server, T, N; where • Client indicates the user name, also referred to as the principal name • Server indicates the Application Server • T indicates the time stamp and • N indicates nonce AS Kerberos Server 1 Client Workstation

21. Authentication Process • Step 2. If the AS can decrypt the message successfully, it issues a temporary session key, which is encrypted with the user’s secret key (a key derived from the user password, which is stored in the KDC), and a TGT encrypted with the TGS’s secret key. The TGT contains the name of the user and a copy of the session key (a randomly generated temporary encryption key) to be used by the user and the Server for any subsequent communication. AS Kerberos Server 2 Client Workstation

22. Authentication Process • Step 3. The user decrypts the session key. The TGT and the session key are stashed in the user’s credential cache. The credentials are used to obtain tickets for each network service the principal wants to access. • This protocol exchange has two important features: • The authentication scheme does not require that the password be sent across the network, • either in encrypted form or in clear text. • The client (or any other user) cannot view or modify the contents of the TGT. Credentials Cache 3 Client Workstation

23. Authentication Process • Step 4. To obtain access to a secured network service such as rlogin, rsh, rcp, ftp, or telnet, the requesting client application uses the previously obtained TGT in a dialogue with the TGS to obtain a service ticket. The protocol is the same as used while obtaining the TGT, except that the messages contain the name of the server and a copy of the previously obtained TGT. TGS Kerberos Server 4 Client Workstation

24. Authentication Process • Step 5. The TGS returns a new service ticket that the application client can use to authenticate the service. TGS Kerberos Server 5 Client Workstation

25. APPLICATION SERVER Authentication Process • Step 6. The application client tries to authenticate to the service on the application server using the service ticket obtained from the TGS. • The secure application validates the service ticket using the server’s service key present in the key tab file. • Using this service key, the server decrypts the authenticator and verifies the identity of the user. • It also verifies that the user’s service ticket has not expired. If the user does not have a valid service ticket, then the server will return an appropriate error code to the client. Key Table File Secured Service 6 Client Workstation

26. APPLICATION SERVER Authentication Process • Step 7. (Optional) At the client’s request, the application server can also return the time stamp the client sent encrypted in the session key. This ensures a mutual authentication between the client and the application server. Key Table File Secured Service 7 Client Workstation

27. APPLICATION SERVER Authentication Process AS TGS Key Table File Secured Service Kerberos Server 1 2 4 5 6 7 Credentials Cache 3 Client Workstation

28. Kerberos

29. Basic Kerberos System Cliff – a client Serge – a server Trent – a trusted authority (as an authentication server) Grant – a ticket-granting server

30. Kerberos Protocol Operation • Cliff to Trent • Cliff sends a message to Trent that contains his name and the name • of the ticket-granting server that he will use (in this case is Grant) • Trent to Cliff • Trent looks up Cliff’s name in his database. If he finds it, he generates • a session key KCG that will be used between Cliff and Grant. Trent also • has a secret key KC with which he can communicate with Cliff, so he • uses this to encrypt the Cliff-Grant session key: • T = e KC (KCG). • In addition, Trent creates a Ticket Granting Ticket (TGT), which will allow Cliff to authenticate himself to Grant. This ticket encrypted using Grant’s personal key KG (which Trent also has): • TGT = Grant’s name || eKC (Cliff’s name, Cliff’s Address, Timestamp1, KCG). • Here || is used to denote concatenation. The ticket that Cliff receives is • the concatenation of these two subtickets: • Ticket = T||TGT

31. Kerberos Protocol Operation • Cliff to Grant • Cliff can extract KCG using the key KC, which he shares with the Trent. • Using KCG, Cliff can now communicate securely with Grant. • Cliff now creates an authenticator, which will consist of his name, his • address, and a timestamp. He encrypts this using KCG to get: • AuthCG = eKCG (Cliff’s name, Cliff’s Address, Timestamp2). • Cliff now sends AuthCG as well as TGT to Grant so that Grant can • administer a service ticket.

32. Kerberos Protocol Operation • Grant to Cliff • Grant now has AuthCG and TGT. • (Part of TGT was encrypted using Grant’s secret key, so Grant can extract • this portion and can decrypt it. Thus he can recover • Cliff’s name, Cliff’s address, Timestamp1, as well as KCG) • Grant can now use KCG to decrypt AuthCG in order to verify the • authenticity of Cliff’s request. • That is, dKCG (AuthCG) will provide another copy of Cliff’s name, • Cliff’s address, and a different timestamp. • If two versions of Cliff’s name and address match, and if Timestamp1 • and Timestamp2 are sufficiently close to each other, then Grant will • declare Cliff valid.

33. Kerberos Protocol Operation • Grant to Cliff • Now that Cliff is approved by grant, Grant will generate a ssession key • KCS for Cliff to communicate with Serge and will also return a service • ticket. Grant has a secret key KS which he shares with Serge. The service • ticket is • ServTicket = eKS (Cliff’s name, Cliff’s Address, Timestamp3, ExpirationTime, KCS). • Here ExpirationTime is a quantity that describes the length of validity • for this service ticket. • The session key is ecrypted using a session key between Cliff and Grant: • eKCG (KCS) • Grant sends ServTicket and eKCG (KCS) to Cliff.

34. Kerberos Protocol Operation • Cliff to Serge • Cliff is now ready to start making use of Serge’s services. • He starts by decrypting eKCG (KCS) in order to get the session key KCS • that he will use while communicating with Serge. He creates an • authenticator to use with Serge: • AuthCS = eKCS (Cliff’s name, Cliff’s Address, Timestamp4) • Cliff now sends Serge AuthCS as well as ServTicket. • Serge can decrypt ServTicket and extract from this session key KCS • that he is to use with Cliff.

35. Kerberos Protocol Operation • Cliff to Serge • Using this session key, he can decrypt AuthCS and verify that Cliff is • who says he is, and that Timestamp4 is within ExpirationTime of Stamp3. • If Timestamp4 is not within ExpirationTime of Timestamp3, then cliff’s • ticket is stale and Serge rejects his request for service. Otherwise, Cliff • and Serge may make use of KCS to perform their exchange.

36. Trent has a database of Password, information for all the clients. Trent 2. Returns a ticket, that is encrypted With clients secret password information 1. Request ticket 7. Serge provide the service 6. Cliff sends Serge the service ticket with the authentication credential to make sure it is valid. Serge 3. Cliff present ticket to grant Server 4. Grant gives a new ticket to cliff that will allow cliff to make use of serge’s service 5. Now cliff has a service ticket which he can present to serge Grant A ticket-granting server Kerberos Protocol Operation Cliff

37. Kerberos X.509

38. X.509 Authentication Service • part of CCITT X.500 directory service standards • distributed servers maintaining some info database • defines framework for authentication services • directory may store public-key certificates • with public key of user • signed by certification authority • also defines authentication protocols • uses public-key crypto & digital signatures • algorithms not standardised, but RSA recommended

39. X.509 Certificates • issued by a Certification Authority (CA), containing: • version (1, 2, or 3) • serial number (unique within CA) identifying certificate • signature algorithm identifier • issuer X.500 name (CA) • period of validity (from - to dates) • subject X.500 name (name of owner) • subject public-key info (algorithm, parameters, key) • issuer unique identifier (v2+) • subject unique identifier (v2+) • extension fields (v3) • signature (of hash of all fields in certificate) • notation CA<<A>> denotes certificate for A signed by CA

40. X.509 Certificates

41. Obtaining a Certificate • any user with access to CA can get any certificate from it • only the CA can modify a certificate • because cannot be forged, certificates can be placed in a public directory

42. CA Hierarchy • if both users share a common CA then they are assumed to know its public key • otherwise CA's must form a hierarchy • use certificates linking members of hierarchy to validate other CA's • each CA has certificates for clients (forward) and parent (backward) • each client trusts parents certificates • enable verification of any certificate from one CA by users of all other CAs in hierarchy

43. CA Hierarchy Use Y<<X>> : Cert of user X issued by CA Y

44. Certificate Revocation • certificates have a period of validity • may need to revoke before expiry, eg: • user's private key is compromised • user is no longer certified by this CA • CA's certificate is compromised • CA’s maintain list of revoked certificates • the Certificate Revocation List (CRL) • users should check certs with CA’s CRL

45. Authentication Procedures • X.509 includes three alternative authentication procedures: • One-Way Authentication • Two-Way Authentication • Three-Way Authentication • all use public-key signatures

46. One-Way Authentication • 1 message ( A->B) used to establish • the identity of A and that message is from A • message was intended for B • integrity & originality of message • message must include timestamp, nonce, B's identity and is signed by A

47. Two-Way Authentication • 2 messages (A->B, B->A) which also establishes in addition: • the identity of B and that reply is from B • that reply is intended for A • integrity & originality of reply • reply includes original nonce from A, also timestamp and nonce from B

48. Three-Way Authentication • 3 messages (A->B, B->A, A->B) which enables above authentication without synchronized clocks • has reply from A back to B containing signed copy of nonce from B • means that timestamps need not be checked or relied upon

49. X.509 Version 3 • has been recognised that additional information is needed in a certificate • email/URL, policy details, usage constraints • rather than explicitly naming new fields defined a general extension method • extensions consist of: • extension identifier • criticality indicator • extension value

50. Certificate Extensions • key and policy information • convey info about subject & issuer keys, plus indicators of certificate policy • certificate subject and issuer attributes • support alternative names, in alternative formats for certificate subject and/or issuer • certificate path constraints • allow constraints on use of certificates by other CA’s