What is Kerberos?
Kerberos is a network authentication protocol which utilizes symmetric cryptography to provide authentication for client-server applications.
The Kerberos Standard Definition
Kerberos is defined in RFC 1510 - The Kerberos Network Authentication Service (V5).
The Kerberos Architecture
The core of a Kerberos architecture is the KDC (Key Distribution Server). The KDC stores authentication information and uses it to securely authenticate users and services.
This authentication is called secure because it:
- Does not occur in plaintext
- Does not rely on authentication by the host operating system
- Does not base trust on IP addresses
- Does not require physical security of the network hosts
The KDC acts as a trusted third party in performing these authentication services.
Due to the critical function of the KDC, multiple KDC's are normally utilized. Each KDC stores a database of users, servers, and secret keys.
Kerberos clients are normal network applications which have been modified to use Kerberos for authentication. In Kerberos slang, they have been Kerberized.
The Kerberos Protocol
Kerberos defines ten messages that make up the Kerberos protocol:
KRB_AS_REQ Kerberos Authentication Service Request KRBAS_REP Kerberos Authentication Service Reply KRB_AP_REQ Kerberos Application Request KRB_AP_REP Kerberos Application Reply KRB_TGS_REQ Kerberos Ticket Granting Service Request KRB_TGS_REP Kerberos Ticket Granting Service Reply KRB_SAFE Kerberos Safe (Checksummed) Application Message KRB_PRIV Kerberos Private (Encrypted) Application Message KRB_CRED Kerberos Credentiials KRB_ERROR Kerberos Error
Kerberos Implementations
MIT Kerberos is the reference implementation. MIT Kerberos supports DEC Unix, Linux, Irix, Solaris, Windows and MacOS.
Several other commercial and non-commercial Kerberos implementations are also available.
Microsoft added a slight modified version of Kerberos v5 authentication in Windows 2000.
Kerberos Weaknesses
Because the KDC's store secret keys for every user and server on the network, they must be kept completely secure. If an attacker were to obtain administrative access to the KDC, he would have access to the complete resources of the Kerberos realm.
Kerberos tickets are cached on the client systems. If an attacker gains administrative access to a Kerbos client system, he can impersonate the authenticated users of that system.
Kerberos Encryption Protocols
Kerberos uses the DES algorithm for encryption. Kerberos also supports the CRC-32, MD4, MD5, and DES algorithms for checksums. Kerberos implementations are free to add additional algorithms for encryption and checksumming.
Additional Reading on Kerberos
RFC 1510 is an excellent resource for understanding the Kerberos protocol.
The Kerberos FAQ is also very well written and answers many questions that you will have.
If you prefer a printed book, Kerberos: The Definitive Guide is available from O'Reilly.
Purchase Kerberos: The Definitive Guide at Amazon.com
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