Kerberos

## Kerberos

Kerberos
The Kerberos protocol is An authentication system developed at the Massachusetts Institute of Technology (MIT). Kerberos is designed to enable two parties to exchange private information across an otherwise open network. It works by assigning a unique key, called a ticket, to each user that logs on to the network. The ticket is then embedded in messages to identify the sender of the message.

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.

## History and development

The Massachusetts Institute of Technology (MIT) developed Kerberos to protect network services provided by Project Athena. The protocol was named after the >Greek mythological character Kerberos (or Cerberus), known in Greek Mythology as being the 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 (obsoleted by RFC 4120 in 2005), with the intention of overcoming the limitations and security problems of version 4.

MIT makes an implementation of Kerberos freely available, under copyright permissions similar to those used for BSD.

Authorities in the United States classed Kerberos as a munition and banned its export because it used the DES encryption algorithm (with 56-bit keys). A non-US Kerberos 4 implementation, KTH-KRB developed in Sweden, made the system available outside the US before the US changed its cryptography export regulations (circa2000). The Swedish implementation was based on a 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. Australian Eric Young, the author of several cryptography libraries, put back the function calls and used his libdes encryption library. This somewhat limited Kerberos was called the eBones release. A Kerberos version 5 implementation, Heimdal, was released by basically the same group of people releasing KTH-KRB.

Windows 2000, Windows XP and Windows Server 2003 use a variant of Kerberos as their default authentication method. Some Microsoft additions to the Kerberos suite of protocols are documented in RFC 3244 "Microsoft Windows 2000 Kerberos Change Password and Set Password Protocols". Apple's Mac OS X also uses Kerberos in both its client and server versions.

As of 2005, the IETF Kerberos workgroup is updating the specifications 1. Recent updates include:

"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 GSS-API specification "The Kerberos Version 5 Generic Security Service Application Program Interface (GSS-API) Mechanism: Version 2." (RFC 4121)

## Dencription

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.

Kerberos 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 Kerberos. Knowledge of this key serves to prove an entity's identity. For communication between two entities, Kerberos generates a session key which they can use to secure their interactions.

## Uses

The following software is able to use Kerberos for authentication:

OpenSSH (with Kerberos v5 or higher
NFS (since NFSv4
PAM (with the pam_krb5 module
SOCKS (since SOCKS5
Apache (with the mod_auth_kerb module
The Dovecot IMAP4 and POP3 server.
The Kerberos software suite also comes with kerberos-enabled clients and servers for rsh, FTP, and Telnet

## The protocol

One can specify the protocol as follows in security protocol notation, where Alice (A) authenticates herself to Bob (B) using a server S:

$A \rightarrow S: A,B$

$S \rightarrow A: \{T_S, L, K_{AB}, B, \{T_S, L, K_{AB}, A\}_{K_{BS}}\}_{K_{AS}}$

$A \rightarrow B: \{T_S, L, K_{AB}, A\}_{K_{BS}}, \{A, T_A\}_{K_{AB}}$

$B \rightarrow A: \{T_A + 1\}_{K_{AB}}$

We see here that the security of the protocol relies heavily on timestamps T and lifespans L as reliable indicators of the freshness of a communication (see the BAN logic).

In relation to the following Kerberos operation, it is helpful to note that the server S here stands for both authentication service (AS), and ticket granting service (TGS). In $\{T_S, L, K_{AB}, B, \{T_S, L, K_{AB}, A\}_{K_{BS}}\}_{K_{AS}}$, KAB stands for the session key between A and B, $\{T_S, L, K_{AB}, A\}_{K_{BS}}$ is the client to server ticket, $\{A, T_A\}_{K_{AB}}$ is the authenticator, and $\{T_A + 1\}_{K_{AB}}$ confirms B's true identity and its recognition of A. This is required for mutual authentication.

## Kerberos operation

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

In one sentence: the client authenticates itself to AS, then demonstrates to the TGS that it's authorized to receive a ticket for a service (and receives it), then demonstrates to the SS that it has been approved to receive the service.

In more detail:

The client performs a one-way hash on the entered password, and this becomes the secret key of the client.
The client sends a clear-text 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.
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 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.
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 the 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.
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.
Upon receiving messages C and D, the TGS decrypts message D (Authenticator) using the client/TGS session key 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.
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 G: the client-to-server ticket, encrypted using service's secret key.
Message H: a new Authenticator, which includes the client ID, timestamp and is encrypted using client/server session key.
The server decrypts the ticket using its own secret key and sends the following message to the client to confirm its true identity and willingness to serve the client:
Message I: the timestamp found in client's recent Authenticator plus 1, encrypted using the client/server session key.
The client decrypts the confirmation using its shared key with the server 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.

Kerberos page at MIT
Kerberos explained visually
The Moron's Guide to Kerberos
The Kerberos FAQ
Heimdal Kerberos page at KTH
Shishi, a free Kerberos implementation for the GNU system
Designing an Authentication System: A Dialogue in Four Scenes. Humorous play concerning how the design of Kerberos evolved.
The Kerberos Network Authentication Service (V5). New standard.
Dencription of Kerberos 5 in the SPORE library
Kerberos Specification - RFC 1510

## References

B. Clifford Neuman and Theodore Ts'o, Kerberos: An Authentication Service for Computer Networks, IEEE Communications, 32(9) pp3338. September 1994. 2
John T. Kohl, B. Clifford Neuman, and Theodore Y. T'so, The Evolution of the Kerberos Authentication System. Distributed Open Systems, pp7894. IEEE Computer Society Press, 1994. Kerberos Documentation (Postscript format)
kerberos 2018