Kerberos as an Authentication Service

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If you’re planning on taking the Security+ exam, you should have a good understanding of Kerberos as one of the authentication services. For example, can you answer this question?

Q. Which of the following is a feature of Kerberos?

A. It does not require time synchronization

B. It provides centralized authentication for remote access servers

C. It uses tickets

D. It uses SAML for SSO

More, do you know why the correct answer is correct and the incorrect answers are incorrect? The answer and explanation is available at the end of this post.

How Kerberos Works

Kerberos is a network authentication mechanism used within Windows Active Directory domains and some Unix environments known as realms. It was originally developed at MIT (the Massachusetts Institute of Technology) for Unix systems and later released as a request for comments (RFC). Kerberos provides mutual authentication that can help prevent man-in-the-middle attacks and uses tickets to help prevent replay attacks.

Kerberos includes several requirements for it to work properly. They are:

  • A method of issuing tickets used for authentication. The Key Distribution Center (KDC) uses a complex process of issuing ticket-granting tickets (TGTs) and other tickets. The KDC (or TGT server) packages user credentials within a ticket. Tickets provide authentication for users when they access resources such as files on a file server. These tickets are sometimes referred to as tokens, but they are logical tokens, not a key-fob type of token.
  • Time synchronization. Kerberos version 5 requires all systems to be synchronized and within five minutes of each other. The clock that provides the time synchronization is used to timestamp tickets, ensuring they expire correctly. This helps prevent replay attacks. In a replay attack, a third party attempts to impersonate a client after intercepting data captured in a session. However, if an attacker intercepts a ticket, the timestamp limits the amount of time an attacker can use the ticket.
  • A database of subjects or users. In a Microsoft environment, this is Active Directory, but it could be any database of users.

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When a user logs on with Kerberos, the KDC issues the user a ticket-granting ticket, which typically has a lifetime of 10 hours to be useful for a single workday. When the user tries to access a resource, the ticket-granting ticket is presented as authentication, and the user is issued a ticket for the resource. However, the ticket expires if users stay logged on for an extended period, such as longer than 10 hours. This prevents them from accessing network resources. In this case, users may be prompted to provide a password to renew the ticket-granting ticket, or they may need to log off and back on to generate a new ticket-granting ticket.

Kerberos

Additionally, Kerberos uses symmetric-key cryptography to prevent unauthorized disclosure and to ensure confidentiality. In short, symmetric-key cryptography uses a single key for both encryption and decryption of the same data.

In contrast, asymmetric encryption uses two keys: one key to encrypt and one key to decrypt. Asymmetric encryption requires a PKI to issue certificates. The two keys used in a PKI are a public key and a private key created as matched pairs. Information encrypted with the public key can only be decrypted with the matching private key. Similarly, information encrypted with the private key can only be decrypted with the matching public key.

As a memory trick, you might want to remember this: Symmetric encryption uses one key. Asymmetric adds a syllable (“a”), and it also adds a key, using two keys.

Remember this

Kerberos is a network authentication protocol within a Microsoft Windows Active Directory domain or a Unix realm. It uses a database of objects such as Active Directory and a KDC (or TGT server) to issue timestamped tickets that expire after a certain time period.

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Q. Which of the following is a feature of Kerberos?

A. It does not require time synchronization

B. It provides centralized authentication for remote access servers

C. It uses tickets

D. It uses SAML for SSO

Answer is C. Kerberos uses a ticket-granting ticket server to create tickets for users. These tickets include user credentials for authentication.

Kerberos does require time synchronization. Kerberos version 5 requires all systems to be within five minutes of each other.

Remote Authentication Dial-In User Service (RADIUS) and Diameter provide authentication authorization, accounting for remote access servers.

Security Assertion Markup Language (SAML) is an Extensible Markup Language (XML)–based data format used for single sign-on on web browsers. While Kerberos provides SSO, it does not use SAML.

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See Chapter 1 of the CompTIA Security+: Get Certified Get Ahead: SY0-401 Study Guide for more information on authentication services.

 

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