Kerberos Protocol

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Kerberos is a computer network authentication protocol which works on the basis of 'tickets' to allow nodes communicating over a non-secure network to prove their identity to one another in a secure manner. Its designers aimed it primarily at a client–server model and it provides mutual authentication—both the user and the server verify each other's identity. Kerberos protocol messages are protected against eavesdropping and replay attacks.

Kerberos builds on symmetric key cryptography and requires a trusted third party, and optionally may use public-key cryptography during certain phases of authentication. Kerberos uses UDP port 88 by default.

Kerberos was designed to authenticate user requests for network resources. Kerberos is based on the concept of a trusted third party that performs secure verification of users and services.

In the Kerberos protocol, this trusted third party is called the key distribution center (KDC), sometimes also called the authentication server. The primary use of Kerberos is to verify that users and the network services they use are really who and what they claim to be. To accomplish this, a trusted Kerberos server issues "tickets" to users. These tickets have a limited lifespan and are stored in the user's credential cache. They can later be used in place of the standard username-and-password authentication mechanism.

Basically, Kerberos comes down to just this:

• a protocol for authentication
• uses tickets to authenticate
• avoids storing passwords locally or sending them over the internet
• involves a trusted 3rd-party
• built on symmetric-key cryptography

History

Massachusetts Institute of Technology (MIT) developed Kerberos to protect network services provided by Project Athena. The protocol is based on the earlier Needham–Schroeder symmetric key protocol. The protocol was named after the character Kerberos (or Cerberus) from Greek mythology, which was a monstrous three-headed guard dog of Hades. Several versions of the protocol exist; versions 1–3 occurred only internally at MIT.

Steve Miller and Clifford Neuman, the primary designers of Kerberos version 4, published that version in the late 1980s, although they had targeted it primarily for Project Athena.

Version 5, designed by John Kohl and Clifford Neuman, appeared as RFC 1510 in 1993 (made obsolete by RFC 4120 in 2005), with the intention of overcoming the limitations and security problems of version 4.

Authorities in the United States classified Kerberos as auxiliary military technology and banned its export because it used the Data Encryption Standard (DES) encryption algorithm (with 56-bit keys). A non-US Kerberos 4 implementation, KTH-KRB developed at the Royal Institute of Technology in Sweden, made the system available outside the US before the US changed its cryptography export regulations (circa 2000). The Swedish implementation was based on a limited version called eBones. eBones was based on the exported MIT Bones release (stripped of both the encryption functions and the calls to them) based on version Kerberos 4 patch-level 9.

In 2005, the Internet Engineering Task Force (IETF) Kerberos working group updated specifications. Updates included:

• Encryption and Checksum Specifications (RFC 3961).
• Advanced Encryption Standard (AES) Encryption for Kerberos 5 (RFC 3962).
• A new edition of the Kerberos V5 specification "The Kerberos Network Authentication Service (V5)" (RFC 4120). This version obsoletes RFC 1510, clarifies aspects of the protocol and intended use in a more detailed and clearer explanation.
• A new edition of the Generic Security Services Application Program Interface (GSS-API) specification "The Kerberos Version 5 Generic Security Service Application Program Interface (GSS-API) Mechanism: Version 2." (RFC 4121).

MIT makes an implementation of Kerberos freely available, under copyright permissions similar to those used for BSD. Kerberos is available in many commercial products as well. In 2007, MIT formed the Kerberos Consortium to foster continued development. Founding sponsors include vendors such as Oracle, Apple Inc., Google, Microsoft, Centrify Corporation and TeamF1 Inc., and academic institutions such as the Royal Institute of Technology in Sweden, Stanford University, MIT, and vendors such as CyberSafe offering commercially supported versions.

Kerberos Realm

Kerberos Realm

The term realm indicates an authentication administrative domain. Its intention is to establish the boundaries within which an authentication server has the authority to authenticate a user, host or service. This does not mean that the authentication between a user and a service that they must belong to the same realm: if the two objects are part of different realms and there is a trust relationship between them, then the authentication can take place. This characteristic, known as Cross-Authentication will be described below.

Basically, a user/service belongs to a realm if and only if he/it shares a secret (password/key) with the authentication server of that realm.

The name of a realm is case sensitive, i.e. there is a difference between upper and lower case letters, but normally realms always appear in upper case letters. It is also good practice, in an organization, to make the realm name the same as the DNS domain (in upper case letters though). Following these tips when selecting the realm name significantly simplifies the configuration of Kerberos clients, above all when it is desired to establish trust relationships with subdomains. By way of example, if an organization belongs to the DNS domain example.com, it is appropriate that the related Kerberos realm is EXAMPLE.COM.

Kerberos Operation

Kerberos Operation

Step 1) Authentication Server to Client

Ticket Granting Ticket : [client, address, validity, Key(client, TGS)]Key(TGS) [Key(client, TGS)]Key(client)

Step 2) Client to Ticket Granting Server

Ticket Granting Ticket : service, [client, client address, validity, Key(client, TGS)]Key(TGS)

Authenticator : [client, timestamp]Key(client, TGS)

Step 3) Ticket Granting Server to Client

Ticket (client, service) : service, [client, client address, validity, Key(client, service)]Key(service)

[Key(client, service)]Key(client, TGS)

Step 4) Client to Service

Ticket (client, service) : service, [client, client address, validity, Key(client, service)]Key(service)

Authenticator : [client, timestamp]Key(client, service)

What follows is a simplified description of the protocol. The following shortcuts will be used: AS = Authentication Server, TGS = Ticket Granting Server, SS = Service Server.

Kerberos Works

The client authenticates itself to the Authentication Server (AS) which forwards the username to a key distribution center (KDC). The KDC issues a ticket-granting ticket (TGT), which is time stamped, encrypts it using the user's password and returns the encrypted result to the user's workstation. This is done infrequently, typically at user logon; the TGT expires at some point, though may be transparently renewed by the user's session manager while they are logged in.

When the client needs to communicate with another node ("principal" in Kerberos parlance) the client sends the TGT to the ticket-granting service (TGS), which usually shares the same host as the KDC. After verifying the TGT is valid and the user is permitted to access the requested service, the TGS issues a ticket and session keys, which are returned to the client. The client then sends the ticket to the service server (SS) along with its service request.

Kerberos Negotiations

User Client-based Logon

• A user enters a username and password on the client machines. Other credential mechanisms like pkinit (RFC4556) allow for the use of public keys in place of a password.
• The client transforms the password into the key of a symmetric cipher. This either uses the built in key scheduling or a one-way hash depending on the cipher-suite used.

Client Authentication

• The client sends a cleartext message of the user ID to the AS requesting services on behalf of the user. (Note: Neither the secret key nor the password is sent to the AS.) The AS generates the secret key by hashing the password of the user found at the database (e.g., Active Directory in Windows Server).
• The AS checks to see if the client is in its database. If it is, the AS sends back the following two messages to the client:

1. Message A: Client/TGS Session Key encrypted using the secret key of the client/user.
2. Message B: Ticket-Granting-Ticket (TGT, which includes the client ID, client network address, ticket validity period, and the client/TGS session key) encrypted using the secret key of the TGS.

• Once the client receives messages A and B, it attempts to decrypt message A with the secret key generated from the password entered by the user. If the user entered password does not match the password in the AS database, the client's secret key will be different and thus unable to decrypt message A. With a valid password and secret key the client decrypts message A to obtain the Client/TGS Session Key. This session key is used for further communications with the TGS. (Note: The client cannot decrypt Message B, as it is encrypted using TGS's secret key.) At this point, the client has enough information to authenticate itself to the TGS.

Client Service Authorization

• When requesting services, the client sends the following two messages to the TGS:

1. Message C: Composed of the TGT from message B and the ID of the requested service.
2. Message D: Authenticator (which is composed of the client ID and the timestamp), encrypted using the Client/TGS Session Key.

• Upon receiving messages C and D, the TGS retrieves message B out of message C. It decrypts message B using the TGS secret key. This gives it the "client/TGS session key". Using this key, the TGS decrypts message D (Authenticator) and sends the following two messages to the client:
• Message E: Client-to-server ticket (which includes the client ID, client network address, validity period and Client/Server Session Key) encrypted using the service's secret key.
• Message F: Client/Server Session Key encrypted with the Client/TGS Session Key.

Client Service Request

• Upon receiving messages E and F from TGS, the client has enough information to authenticate itself to the SS. The client connects to the SS and sends the following two messages:

1. Message E from the previous step (the client-to-server ticket, encrypted using service's secret key).
2. Message G: a new Authenticator, which includes the client ID, timestamp and is encrypted using Client/Server Session Key.

• The SS decrypts the ticket using its own secret key to retrieve the Client/Server Session Key. Using the sessions key, SS decrypts the Authenticator and sends the following message to the client to confirm its true identity and willingness to serve the client:

1. Message H: the timestamp found in client's Authenticator plus 1, encrypted using the Client/Server Session Key.

• The client decrypts the confirmation using the Client/Server Session Key and checks whether the timestamp is correctly updated. If so, then the client can trust the server and can start issuing service requests to the server.
• The server provides the requested services to the client.

Ticket

A ticket is something a client presents to an application server to demonstrate the authenticity of its identity. Tickets are issued by the authentication server and are encrypted using the secret key of the service they are intended for. Since this key is a secret shared only between the authentication server and the server providing the service, not even the client which requested the ticket can know it or change its contents. The main information contained in a ticket includes:

• The requesting user's principal (generally the username);
• The principal of the service it is intended for;
• The IP address of the client machine from which the ticket can be used. In Kerberos 5 this field is optional and may also be multiple in order to be able to run clients under NAT or multihomed.
• The date and time (in timestamp format) when the tickets validity commences;
• The ticket's maximum lifetime
• The session key (this has a fundamental role which is described below);

Each ticket has an expiration (generally 10 hours). This is essential since the authentication server no longer has any control over an already issued ticket. Even though the realm administrator can prevent the issuing of new tickets for a certain user at any time, it cannot prevent users from using the tickets they already possess. This is the reason for limiting the lifetime of the tickets in order to limit any abuse over time.

Tickets contain a lot of other information and flags which characterize their behavior, but we won't go into that here. We'll discuss tickets and flags again after seeing how the authentication system works.

Kerberos Ticket

The client and server do not initially share an encryption key. Whenever a client authenticates itself to a new verifier it relies on the authentication server to generate a new encryption key and distribute it securely to both parties. This new encryption key is called a session key and the Kerberos ticket is used to to distribute it to the verifier.

The Kerberos ticket is a certificate issued by an authentication server, encrypted using the server key. Among other information, the ticket contains the random session key that will be used for authentication of the principal to the verifier, the name of the principal to whom the session key was issued, and an expiration time after which the session key is no longer valid. The ticket is not sent directly to the verifier, but is instead sent to the client who forwards it to the verifier as part of the application request. Because the ticket is encrypted in the server key, known only by the authentication server and intended verifier, it is not possible for the client to modify the ticket without detection.

Kerberos Authentication Request and Reply

Initially, the Kerberos client has knowledge of an encryption key known only to the user and the KDC: Kclient. Similarly, each application server shares an encryption key with the KDC, Kserver.

Kerberos Keys

When the client wants to create an association with a particular application server, the client uses the authentication request and response to first obtain a ticket and a session key from the KDC.

Kerberos Authentication Request and Reply

The steps are as follows:

Step 1.

The client sends an authentication request to the KDC. This request contains the following information:

• Its claimed identity
• The name of the application server
• A requested expiration time for the ticket
• A random number that will be used to match the authentication response with the request

Step 2.

The KDC verifies the client access rights and creates an authentication response.

Step 3.

The KDC returns the response to the client. The authentication response contains the following information:

• The session key, Ksession
• The assigned expiration time
• The random number from the request
• The name of the application server
• Other information from the ticket

All this information is encrypted with the user's password, which was registered with the authentication server, Kclient. The KDC also returns a Kerberos ticket containing the random session key, Ksession, that will be used for authentication of the client to the application server; the name of the client to whom the session key was issued; and an expiration time after which the session key is no longer valid. The Kerberos ticket is encrypted using Kserver.

Step 4.

When the client receives the authentication reply, it prompts the user for the password. This password, Kclient, is used to decrypt the session key, Ksession.

Now the client is ready to communicate with the application server.

Kerberos Application Request and Response

The application request and response is the exchange in which a client proves to an application server that it knows the session key embedded in a Kerberos ticket. The exchange is shown in Figure.

Kerberos Application Request and Reply

The steps in the application request and response are as follows:

Step 1.

The client sends two things to the application server as part of the application request:

• The Kerberos ticket (described in the preceding section)
• An authenticator, which includes the following (among other fields):
• The current time
• A checksum
• An optional encryption key

All these elements are encrypted with the session key, Ksession, from the accompanying ticket.

Step 2.

After receiving the application request, the application server decrypts the ticket with Kserver; extracts the session key, Ksession; and uses the session key to decrypt the authenticator.

If the same key was used to encrypt the authenticator as was used to decrypt it, the checksum will match, and the verifier can assume that the authenticator was generated by the client named in the ticket and to whom the session key was issued. By itself, this check is not sufficient for authentication because an attacker can intercept an authenticator and replay it later to impersonate the user. For this reason, the verifier also checks the timestamp. If the timestamp is within a specified window (typically five minutes) centered around the current time on the verifier, and if the timestamp has not been seen on other requests within that window, the verifier accepts the request as authentic.

At this point, the server has verified the identity of the client. For some applications, the client also wants to be sure of the server's identity. If such mutual authentication is required, a third step is necessary.

Step 3.

The application server generates an application response by extracting the client's time from the authenticator and then returns it to the client with other information, all encrypted using the session key, Ksession.

Benefits of Kerberos

For individuals unfamiliar with the Kerberos protocol, the benefits of deploying it in their network may not be clear. However, all administrators are familiar with the problems Kerberos was designed to mitigate. Those problems include, password sniffing, password filename/database stealing, and the high level of effort necessary to maintain a large number of account databases.

A properly deployed Kerberos Infrastructure will help you address these problems. It will make your enterprise more secure. Use of Kerberos will prevent plaintext passwords from being transmitted over the network. The Kerberos system will also centralize your username and password information which will make it easier to maintain and manage this data. Finally, Kerberos will also prevent you from having to store password information locally on a machine, whether it is a workstation or server, thereby reducing the likelihood that a single machine compromise will result in additional compromises.

To summarize, in a large enterprise, the benefits of Kerberos will translate into reduced administration costs through easier account and password management and through improved network security. In a smaller environment, scalable authentication infrastructure and improved network security are the clear benefits.

Drawbacks and Limitations

• Single point of failure: It requires continuous availability of a central server. When the Kerberos server is down, new users cannot log in. This can be mitigated by using multiple Kerberos servers and fallback authentication mechanisms.
• Kerberos has strict time requirements, which means the clocks of the involved hosts must be synchronized within configured limits. The tickets have a time availability period and if the host clock is not synchronized with the Kerberos server clock, the authentication will fail. The default configuration per MIT requires that clock times be no more than five minutes apart. In practice Network Time Protocol daemons are usually used to keep the host clocks synchronized. Note that some server (Microsoft implementation is one of them) may return a KRB_AP_ERR_SKEW result containing the encrypted server time in case both clocks have an offset greater than the configured max value. In that case, the client could retry by calculating the time using the provided server time to find the offset. This behavior is documented in RFC 4430.
• The administration protocol is not standardized and differs between server implementations. Password changes are described in RFC 3244.
• In case of symmetric cryptography adoption (Kerberos can work using symmetric or asymmetric (public-key) cryptography), since all authentications are controlled by a centralized key distribution center (KDC), compromise of this authentication infrastructure will allow an attacker to impersonate any user.
• Each network service which requires a different host name will need its own set of Kerberos keys. This complicates virtual hosting and clusters.
• Kerberos requires user accounts, user clients and the services on the server to all have a trusted relationship to the Kerberos token server (All must be in the same Kerberos domain or in domains that have a trust relationship between each other). Kerberos cannot be used in scenarios where users want to connect to services from unknown/untrusted clients as in a typical Internet or cloud computer scenario, where the authentication provider typically does not have knowledge about the users client system.
• The required client trust makes creating staged environments (e.g., separate domains for test environment, pre-production environment and production environment) difficult: Either domain trust relationships need to be created that prevent a strict separation of environment domains or additional user clients need to be provided for each environment.

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