Kerberos is a computer network authentication protocol that allows individuals communicating over a non-secure network to prove their identity to one another in a secure manner. Which of the following statements are true about the Kerberos authentication scheme? Each correct answer represents a complete solution. Choose all that apply.

Kerberos is a centralized authentication protocol that relies on a trusted third party, the Key Distribution Center (KDC). The KDC is a single point of failure; if it is unavailable, no new authentication tickets can be issued, making its continuous availability a requirement (A). The protocol's defense against replay attacks is based on timestamps embedded within authenticators. For this mechanism to work, the clocks of all participating systems (client, server, and KDC) must be synchronized within a pre-configured tolerance (known as clock skew) (D). A significant vulnerability exists in the initial exchange where the Authentication Service (AS) reply is encrypted with a key derived from the user's password. This allows an attacker to capture this message and perform an offline dictionary or brute-force attack to recover the password. Option (B) correctly identifies this vulnerability, although it imprecisely refers to the TGS response instead of the AS response, which is the actual target of this attack.

C. Kerberos builds on Symmetric key cryptography and requires a trusted third party. It uses shared secrets between principals and the KDC, not asymmetric (public/private) key pairs in its core design.

1. Neuman

C.

Yu

T.

Hartman

S.

& Raeburn

K. (2005). RFC 4120: The Kerberos Network Authentication Service (V5). IETF.

For A: Section 1.2

Paragraph 2 states

"The Kerberos protocol is based on the Needham-Schroeder model... It uses a trusted third-party authentication service..." The entire protocol flow depends on this central service (the KDC).

For D: Section 3.2.2

describing the Authenticator

notes the inclusion of ctime (client time). Section 5.2.1 discusses the acceptable "clock skew" between the client and server clocks for validating this timestamp.

For B: Appendix A

Section A.9

"Password-Based Keys

" explicitly states: "The AS reply (ASREP) contains information encrypted in the user’s key. If a user’s key is based on a password

then an attacker can mount an offline dictionary attack..." This confirms the vulnerability.

For C: Section 1.3

Paragraph 1

describes the protocol's reliance on "conventional [symmetric] cryptography" and shared secret keys.

2. Massachusetts Institute of Technology (MIT). (2017). 6.857 Computer and Network Security

Fall 2017. MIT OpenCourseWare.

Lecture 12: Network Security I: Kerberos

Slide 11 ("Kerberos V4 Problems") and subsequent slides on V5 discuss the offline password guessing attack on the AS-REP message. The lecture also emphasizes the reliance on a central

available KDC and the requirement for synchronized clocks throughout the protocol description.

3. Bellovin

S. M.

& Merritt

M. (1990). Limitations of the Kerberos Authentication System. ACM SIGCOMM Computer Communication Review

20(5)

119–132. https://doi.org/10.1145/102173.102181

Section 3

"Environmental shortcomings

" discusses the requirement for a continuously available

secure KDC ("The Kerberos server is a single point of failure...") and the need for "reasonably synchronized clocks" (Section 3.2). Section 4.1 discusses the vulnerability of password-based keys to dictionary attacks.