Kerberos

Kerberos is a computer network authentication protocol, which allows individuals communicating over a non-secure network to prove their identity to one another in a secure manner. It is also a suite of free software published by Massachusetts Institute of Technology (MIT) that implements this protocol. Its designers aimed 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. Extensions to Kerberos can provide for the use of public-key cryptography during certain phases of authentication.


Description

Kerberos uses as its basis the Needham-Schroeder protocol. It makes use of a trusted third party, termed a key distribution center (KDC), which consists of two logically separate parts: an Authentication Server (AS) and a Ticket Granting Server (TGS). Kerberos works on the basis of "tickets" which serve to prove the identity of users.

The KDC maintains a database of secret keys; each entity on the network — whether a client or a server — shares a secret key known only to itself and to the KDC. Knowledge of this key serves to prove an entity's identity. For communication between two entities, the KDC generates a session key which they can use to secure their interactions.


Protocol

The security of the protocol relies heavily on participants maintaining loosely synchronized time and on short-lived assertions of authenticity called Kerberos tickets.

What follows is a simplified description of the protocol. The following abbreviations will be used:

• AS = Authentication Server
• TGS = Ticket Granting Server
• SS = Service Server
• TGT = Ticket Granting Ticket
  

Briefly, the client authenticates to AS using a long-term shared secret and receives a ticket from the AS. Later the client can use this ticket to get additional tickets for SS without resorting to using the shared secret. These tickets can be used to prove authentication to SS.

In more detail:

User Client-based Logon Steps:

1. A user enters a username and password on the client machine.
2. The client performs a one-way function (Hash mostly) on the entered password, and this becomes the secret key of the client/user.
   

Client Authentication Steps:

1. The client sends a cleartext message to the AS requesting services on behalf of the user. Sample message: "User XYZ would like to request services". Note: Neither the secret key nor the password is sent to the AS.
2. 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:
• Message A: Client/TGS Session Key encrypted using the secret key of the client/user.
• Message B: Ticket-Granting Ticket (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.
3. Once the client receives messages A and B, it decrypts message A to obtain the Client/TGS Session Key. This session key is used for further communications with 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 Steps:

1. When requesting services, the client sends the following two messages to the TGS:
• Message C: Composed of the Ticket-Granting Ticket from message B and the ID of the requested service.
• Message D: Authenticator (which is composed of the client ID and the timestamp), encrypted using the Client/TGS Session Key.
2. 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 Steps:

1. 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:
• Message E from the previous step (the client-to-server ticket, encrypted using service's secret key).
• Message G: a new Authenticator, which includes the client ID, timestamp and is encrypted using client/server session key.
2. 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:
• Message H: the timestamp found in client's Authenticator plus 1, encrypted using the Client/Server Session Key.
3. 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.
4. The server provides the requested services to the client.
   

Drawbacks

• Single point of failure: It requires continuous availability of a central server. When the Kerberos server is down, no one can log in. This can be mitigated by using multiple Kerberos servers and fallback authentication mechanisms.
• Kerberos requires the clocks of the involved hosts to be synchronized. 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 requires that clock times are no more than 10 minutes apart. In practice Network Time Protocol daemons are usually used to keep the host clocks synchronized.
• The administration protocol is not standardized and differs between server implementations. Password changes are described in RFC 3244.
• Since the secret keys for all users are stored on the central server, a compromise of that server will compromise all users' secret keys.
• A compromised client will compromise the user's password