Kerberos Protocol

Kerberos Protocol
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.

Kerberos Protocol
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 Protocol
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
Figure 1 Kerberos Realm

Kerberos Protocol
belongs to the DNS domain example.com, it is appropriate that the related Kerberos realm is
EXAMPLE.COM.
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)
Figure 2 Kerberos Operation

Kerberos Protocol
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.
Figure 3 Kerberos Negotiations

Kerberos Protocol
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:
o Message A: Client/TGS Session Key encrypted using the secret key of the client/user.
o 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:
o Message C: Composed of the TGT from message B and the ID of the requested service.
o 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.

Kerberos Protocol
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:
o Message E from the previous step (the client-to-server ticket, encrypted using service's
secret key).
o 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:
o 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 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 Protocol
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.
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.
Figure 4 Kerberos Keys
Figure 5 Kerberos Authentication Request and Reply

Kerberos Protocol
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
Fig Kerberos Application Request and Reply

Kerberos Protocol
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
Figure 6 Kerberos Application Request and Reply

Kerberos Protocol
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

Kerberos Protocol
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.

Kerberos Protocol

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