История и разработване
MIT Разработен в МИТ,Kerberos е създаден да защитава мрежовите ресурси предоставени от
Project Athena. Протоколът и кръстен на името на митичното създание
Kerberos (or
Cerberus) гръцката митология което е било триглаво чудовище пазещо вратите към хадес
.
Steve Miller и
Clifford Neuman, са основните дизайнери на версия 4 на Kerberos,публикувана в края на 80-те
.
Версия 5, е създадена от John Kohl и Clifford Neuman, през 1993.
Властите в САЩ класифицират Kerberos като технология с възможна военна употреба и ограничават експорта й поради използването на DES алгоритъмът за криптиране( 56-битов ключ).
Windows 2000 и лседващите версии използват Kerberos като метод за удостоверяване по подразбиране.
Теория
Kerberos използва като своя основа
symmetric Needham-Schroeder protocol. It makes use of a
trusted third party, termed a
key distribution center (KDC), който се състои от две теоритично независими роли: Удостоверителн Сървър Authentication Server (AS) и Билето предоставящ сървър -Ticket Granting Server (TGS).
The KDC подържа база данни от тайни ключове; всеки елемент от мрежата, без значение клиент или сървър, споделя таен ключ известен само на елемента и на KDC. Притежаването на този ключ служи за доказване на идентичността на елемнта. For communication purposes the KDC generates a
session key which communicating parties use to encrypt their transmissions.
Сигурността на протокола се основава на краткосрочни удостоверители на автентичност, наречени
Kerberos билети.
Описание
The client authenticates itself to the 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. If
successful, this gives the user desktop access.
When the client needs to communicate with another node ("principal"
in Kerberos parlance) it sends the TGT to the Ticket Granting Service
(TGS), which shares the same host as the TGT. 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 and keys to the service (SS).
Here is another description.
The client authenticates to the AS
once using a long-term
shared secret
(e.g. a password) and receives a Ticket Granting Ticket (TGT) from the
AS. Later, when the client wants to contact some SS, it can (re)use this
ticket to get additional tickets from TGS, for SS, without resorting to
using the shared secret. The latter tickets can be used to prove
authentication to the SS.
The phases are detailed below.
User Client-based Logon
- A user enters a username and password on the client machine.
- The client performs a one-way function (hash usually) on the entered password, and this becomes the secret key of the client/user.
Клиентско удостоверяване
- 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:
- 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.
- 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:
- Message C: Composed of the TGT 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 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:
- 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.
- 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.
- 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.
Недостатъци и ограничения
- Единична точка та провал: изисква непрекъснат достъп до централен сървър. Когато централният сървър Kerberos неработи, никой неможе да използва системата. Този проблем може да бъде решен, чрез използването на множество сървъри Kerberos и ризирвиране на механизмите за удостоверяване.
- протоколът Kerberos има строги изсквания за време, което значи, че часовниците на участващите хостове трябва да са синхронизирани. Билетите са активни през определен период от време и ако часовниците не са синхронизирани с часовника на сървъра Kerberos, удостоверяването ще се провали.Стандартната конфигурация изисква показанията на часовниците да се отклоняват с не повече от 5 минути.
- Since all authentication is controlled by a centralized KDC, compromise of this authentication infrastructure will allow an attacker to impersonate any user.
