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Kerberos (protocol)

Kerberos is a computer network authentication protocol which allows individualscommunicating 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

Description

Uses The protocol

Kerberos operation

History and development

The Massachusetts Institute of Technology (MIT) developed Kerberos to protect networkservices provided by Project Athena. The protocol was named after the Greek

mythological characterKerberos (orCerberus), known in Greek Mythology as being themonstrous three-headed guard dog of Hades. Several versions of the protocol exist;versions 13 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 permissionssimilar 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 theUS before the US changed its cryptography export regulations (circa 2000). 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) basedon 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.

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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 ofprotocols 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 itsclient and server versions.

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

include:

"Encryption and Checksum Specifications"

"Advanced Encryption Standard (AES) Encryption for Kerberos 5"

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: Version2." (RFC 4121)

Description

Kerberos uses as its basis the Needham-Schroeder protocol. It makes use of a trusted

third party, termed aKey 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. Knowledgeof 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)

The protocol

http://www.ietf.org/rfc/rfc4120.txthttp://www.ietf.org/rfc/rfc1510.txthttp://www.ietf.org/rfc/rfc4120.txthttp://www.ietf.org/rfc/rfc1510.txt

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One can specify the protocol as follows in security protocol notation, whereAlice (A)

authenticates herself toBob (B) using a serverS:

We see here that the security of the protocol relies heavily on timestamps T and lifespansL as reliable indicators of thefreshness 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,KAB stands for the session key

between A and B, is the client to server ticket,

is the authenticator, and

confirms B's true identity and its recognition of A. This is required formutual authentication.

Kerberos operationWhat follows is a simplified description of the protocol. The following shortcuts will beused: 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:

1. A user enters a username and password on the client.2. The client performs a one-way hash on the entered password, and this becomes

the secret key of the client.

3. The client sends a clear-text message to the AS requesting services on behalf ofthe user. Sample Message: "User XYZ would like to request services". Note:

Neither the secret key nor the password is sent to the AS.

4. The AS checks to see if the client is in its database. If it is, the AS sends back thefollowing two messages to the client:

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o Message A: Client/TGS session key encrypted using the secret key of the

user.

o 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.

5. Once the client receives messages A and B, it decrypts message A to obtain theclient/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 authenticateitself to the TGS.

6. When requesting services, the client sends the following two messages to the

TGS:

o Message C: Composed of the Ticket-Granting Ticketfrom 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.

7. 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:o Message E: Client-to-server ticket(which includes the client ID, client

network address, validity period) encrypted using the service's secret key.

o Message F: Client/server session key encrypted with the client/TGS

session key.

8. 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 followingtwo messages:

o Message G: the client-to-server ticket, encrypted using service's secret

key.o Message H: a new Authenticator, which includes the client ID, timestamp

and is encrypted using client/server session key.

9. The server decrypts the ticket using its own secret key and sends the followingmessage to the client to confirm its true identity and willingness to serve the

client:

o Message I: the timestamp found in client's recent Authenticator plus 1,

encrypted using the client/server session key.

10. 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.

11. The server provides the requested services to the client