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An Overview of Authentication for Computer Communications

DIAMETER uses TCP or SCTP unlike RADIUS which uses UDP, therefore delegating detection and handling of communication problems to those protocols. DIAMETER does not include encryption, but can be protected by transport level security IPSEC or TLS. Diameter has enhanced features to support many different interfaces defined by 3rd Generation Partnership Project (3GPP) IP Multimedia Subsystem (IMS).

Both RADIUS and Diameter support authentication CHAP and EAP (Extensible Authentication Protocol), and PAP (Password Authentication Protocol). However, RADIUS has some limitations: Its CHAP authentication is subject to dictionary attacks, and it protects clear-text passwords (PAP) only on a hop-by-hop basis.

5 Kerberos Authentication Protocol

Kerberos is an authentication server developed as a part of Project Athena, MIT. According to Greek mythology, Kerberos is a ferocious 3-headed dog guarding the Gates to the Underworld. Since Kerberos authentication requires 3 entities to authenticate and has an excellent track record of making computing safer, the naming is appropriate.

Kerberos model is based on Needham-Schroeder trusted third party protocol [1]. It uses symmetric key cryptography and requires trusted third-party authorization to verify user identities. It provides centralized private-key third-party authentication in a distributed network. The latest Kerberos Version 5 is specified in RFC 4120 [12].

It is highly reliable and employs a distributed server architecture. As it is scalable, the system should support large number of clients and servers. This technology is used by Microsoft Windows, Apple OS, FreeBSD, UNIX and Linux. Kerberos protocol messages are protected against eavesdropping and replay attacks. The strong cryptography and third-party ticket authorization make it much more difficult for attackers to infiltrate the network.

Kerberos system has two main parts Authentication Server (AS) and Ticket Granting Server (TGS) as shown in Figure 3. Users interact with AS to identify self and negotiate a ticket granting ticket (TGT) which is a non-corruptible authentication credential. Users can subsequently request access to other services from TGS based on the TGT.

Figure 3: Kerberos Authentication Mechanism
Figure 3: Kerberos Authentication Mechanism

Kerberos V5 Messages:

  1. 1. The client sends a clear text message consisting of the user ID and the TGS server name to the AS.

  2. 2. The AS checks to see if the client is in its database. If it is, the AS generates and sends back the following two messages to the client:

    Message A: Client/TGS Session Key encrypted using the secret key of the client

    Message B: Ticket-Granting-Ticket (TGT), which includes the client ID, network address, the server name, a time-stamp and the client/TGS session key encrypted using the secret key of the TGS.

    The client decrypts the first message and retrieves the session key. Only the legitimate client with the correct knowledge of the password is able to decrypt the message.

    The client saves the session key and TGT for the future use. It erases the password and its one way hash to reduce the chance of compromise.

  3. 3. A client sends a request to the TGS to obtain separate tickets for each of the services she wants to use from TGT. The request consists of an authenticator encrypted with the shared session key between the client and the server and TGT. The authenticator consists of the client name, time stamp and optional key. The TGS, upon receiving the request, decrypts the TGT and retrieves the shared session key. Then the shared key is used to decrypt the authenticator. After due validations of the client information from the ticket and the authenticator, the request is allowed to proceed.

    Checking timestamps assumes all machines in Kerberos authentication network have synchronized clocks, at least to within several minutes. If the timestamp in the request is too far from the current time, TGS treats the request as an attempt to replay.

  4. 4. TGS responds to the valid request by returning a valid ticket for the client to present to the app server along with the new session key for the client and the app server.

  5. 5. The client similar to step 3 creates an authenticator for the app server and sends authenticator encrypted with the shared key and the app server ticket.

  6. 6. The server decrypts the ticket to retrieve the shared session key and then uses it to decrypt the authenticator. It compares client credentials and timestamp in the ticket as well as authenticator. If everything checks out, the app server grants service access to the client.

Kerberos may be susceptible to replay attacks from old and cached authenticators. Although, the timestamps are used to prevent this, replays are possible during ticket’s lifetime. The servers are supposed to store all live tickets to stop this but this is not always practicable. Another requirement is that all the clocks in the network are time synchronized. If a host is fooled about the correct time, old authenticator replay is possible. Yet another vulnerability is password cracking attacks. If the intruder collects enough tickets his chances of success are good.

6 Public Key Cryptography based Authentication

Though the use of shared key or symmetric key encryptions is widespread because of its moderate computations, the key distribution and management becomes more unwieldy and complex. If “n” users want to securely communicate with each other, this would require “$(n^2-n)/2$” secret keys. It is difficult to arrange in advance secure physical means of sharing secret keys for large “n”. Public key cryptography (PKC) is invented as a solution by Whitefield Diffie and Martin Hellman and independently by Merkle [1], [13]. In a PKC system, each user will have only one private key, which should be kept secret and the related public key which can be shared with others.

The generation of private and public keys depends on cryptographic algorithms based on mathematical problems to produce one-way functions. A typical invertible one-way function “$f$” be a function defined over integers modulo a large number N (which can be a prime or product of two large primes) such that computing $f(x)=y$, given ‘$x$’ is easy; however, given $y=f(x)$, computing ‘$x$’ from ‘$y$’ is difficult or hard. If “N” is a single prime, the computation of $x$ given $y$ is termed the discrete logarithm problem [1]. If “N” is the product of 2 prime numbers, the computation of x given y is equivalent to finding the factors of N, termed the factorization problem [1]. The well-known Rivest-Shamir-Adelman (RSA) PKC system, named after its inventors, is based on a number which is product of two large primes [14]. Typical operations of encryption and decryption are shown in Figure 4.

Two of the best-known uses of public key cryptography are to ensure confidentiality and signature. In the first case, if any one wants to send a message to user A, the message has to be encrypted with the user-A’s public key. This encrypted message cannot be decrypted by anyone other than user-A, who does not possess the matching private key. Only the user-A can decrypt the message who is the owner of that key and the person associated with the public key. This is used in an attempt to ensure confidentiality.

Figure 4: Typical RSA Encryption and Decryption
Figure 4: Typical RSA Encryption and Decryption

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