Kerberos (iletişim kuralı)

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Kerberos, bir bilgisayar ağı kimlik doğrulama protokolünün adıdır. Güvenli olmayan bir ağ içinde kullanıcıların kimliklerini güvenli şekilde ispat etmelerini sağlayan, MIT (Massachusetts Institute of Technology - Massachusetts Teknoloji Enstitüsü) tarafından geliştirilmiş yazılımdır. Kerberos mesajın gizlice dinlenmesini ve bazı saldırıları engelleyerek veri bütünlüğü sağlar. Tasarımcıları öncelikle hem kullanıcı hem de sunucunun birbirinin kimliğini kontrol ettiği istemci-sunucu modelini amaçladılar. Kerberos simetrik anahtarla şifreleme yöntemini kullanır.

[değiştir] Tarihçe ve Gelişim

MIT, Kerberos'u Project Athena isimli projenin sunduğu ağ servislerini korumak için geliştirdi ve mitolojideki Kerberos'tan esinlenerek protokole bu isim verildi. Protokolün birkaç versiyonu bulunmaktadır, 1-3 arası versiyonları sadece MIT'de kullanıldı.

Kerberos versiyon 4'ün başlıca tasarımcıları olan Steve Miller ve Clifford Neuman, sözkonusu versiyonu "Project Athena" projesine hedeflemelerine rağmen 1980'lerin sonlarında çıkardılar.

John Kohl ve Clifford Neuman tarafından tasarlanan Versiyon 5, Versiyon 4'ün bazı kısıtlamalarını ve güvenlik problemlerini aşarak 1993'te RFC 1510 olarak ortaya çıkarıldı.

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

Authorities in the United States classified 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 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 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. 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 ve Windows Server 2003 kimlik doğrulama metodu olarak Kerberos'un bir çeşidini kullanırlar. 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 Mac OS X de Kerberos'u hem istemci hem sunucu tarafta kullanır.

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).

[değiştir] Tanım

Kerberos, Needham-Schroeder protokolünü baz alır. 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.

[değiştir] Kullanımları

Aşağıdaki yazılımlar kimlik doğrulama işlemi için Kerberos kullanırlar:

AFS
Apache (with the mod_auth_kerb module)
Apache 2 (using libapache-mod-auth-kerb)
Cisco routers and switches running IOS
Coda File System
Eudora
Mac OS X
Microsoft Windows (2000 and later) uses as default authentication protocol
Mulberry, an e-mail client developed by Cyrusoft, Inc.
NFS (since NFSv3)
OpenSSH (with Kerberos v5 or higher)
PAM (with the pam_krb5 module)
Samba since v3.x
SOCKS (since SOCKS5)
Netatalk
The X Window System implementations
Indirectly, any software that allows the use of SASL for authentication, such as OpenLDAP, Dovecot IMAP4 and POP3 server, Postfix mail server
The Kerberos software suite also comes with kerberos-enabled clients and servers for rsh, FTP, and Telnet

[değiştir] Protokol

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

K_AS is a pre-established secret key known only to A and S
Likewise, K_BS is known only to B and S
K_AB is a session key between A and B, freshly generated for each run of the protocol
T_S and T_A are timestamps generated by S and A, respectively
L is a 'lifespan' value defining the validity of a timestamp

$A \rightarrow S: A,B$

A asks S to initiate communication with B

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

S generates a fresh K_AB, and sends it to A together with a timestamp and the same data encrypted for B.

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

A passes on the message to B, obtains a new T_A and passes it under the new session key

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

B confirms receipt of the session key by returning a modified version of the timestamp to A

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 $\{T_S, L, K_{AB}, B, \{T_S, L, K_{AB}, A\}_{K_{BS}}\}_{K_{AS}}$ , $\{T_S, L, K_{AB}, A\}_{K_{BS}}$ 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.

[değiştir] Kerberos işlemleri

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.

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.

Daha detaylı olarak:

Kullanıcı istemcide bir kullanıcı adı ve şifre girer.
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 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 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 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.

[değiştir] Kerberos dezavantajları

Single point of failure: It requires continuous availability of a central server. When Kerberos server is down, no one can log in. This can be mitigated by using multiple Kerberos servers.
Kerberos requires the clocks of the involved hosts to be synchronized. The tickets have time availability period and, if the host clock is not synchronized with the clock of Kerberos server, the authentication will fail. The default configuration requires that clock times are no more than 10 minutes apart. In practice, NTP daemons are usually employed to keep the host clocks synchronized.
Password changing is not standardized, and differs between server implementations.