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An Overview of Authentication for Computer Communications


H. S. Madhusudhana

Oracle India Pvt. Ltd.


Authentication is the first and foremost security principle that involves validation of identity of an user or a machine. The successful authentication is required for authorization and secure data exchanges. A common classification of authentication system based on factors – something you know, something you own, something you did and something you are – is explained. Well-known and used password schemes employed in practice is described along with standards. A description Kerberos of authentication system based on symmetric key encryption and SSL based authentication which is based on public key encryption is given. Related Single-Sign-On technologies are explained. A brief overview of authentication for cloud computing, IoT and UIDAI is presented.

1 Introduction

It is important to know the basic principles of information security as applicable to computer science. This knowledge helps one to develop products with required security and to analyze competing security claims. The first and foremost security principle is “Authentication”, which is the process of verifying the identity of a machine or person. One has to provide valid credentials for successful authentication to get access to computer system resources. The second basic security principle is “authorization”, which controls the access to resources after successful authentication. A common tendency is to combine authentication and authorization, but it is important to understand that they are distinct. The next principles are “confidentiality” to provide data protection and “data integrity” to assure data is not modified in transit/storage. The related concept of key management is tied with these principles. The final principle is “availability” which is to make systems available despite threats and attacks.

The identity of a security principal, either you or a computer, is a declaration of who you are. This is the answer to the question “Who are you?” Some common examples of identity are user IDs, digital certificates and ATM cards. The system wants to be certain that it is indeed you and not someone else. The system will challenge the principal and expects correct responses in some way. Common examples of authenticators are passwords, private keys and PINs. Whereas identity is generally public, authentication is private: it’s a secret known only by the Principal. A typical scenario is where the client authenticates to the server to get access to service or resource. The case where both parties want to authenticate to each other is called mutual authentication.

Further, authentication is also applicable to message sent to each other. The receiver wants to be confident that the authentic message is indeed sent by the intended sender and not by someone masquerading as the sender. He also wants to be sure that the message has not been modified in transit. These authentic messages exchanged between the server and the client is the basis for arriving at the common shared key for protected data transfers.

This paper is organized as follows. A common classification of authentication into password, biometric and tokens/certificates is explained. The well-known standards for authentication is highlighted. A different way of classification based on security level is described next. This is followed by a survey of various password based authentication schemes. A description of Kerberos standards follows. The popular SSL based authentication is explained next. Related authentication technologies OpenID and OAuth are explained. A brief survey of authentication for cloud computing IoT and UIDAI is given.

The most common attacks against authentication include impersonation and message tampering. The impersonation attack means the attacker pretending to be a bonafide sender and tricking the receiver to think it has come from the sender. The second attack is to modify the sender’s message undetected by the receiver. As and when an authentication method is explained, a description of various attacks/threats handled by the scheme will be given.

2 Common Authentication Methods

A traditional way to classify is based on the following factors [2]:

Something you know – Password
Something you are – Biometric
Something you have – Access tokens, Certificates
Something you do – make a gesture, read a
text, match a CAPTCHA

The password is simple to use and remember. It has been used in military since ancient times. A person wishing to enter protected area is to supply a password and he is allowed entry only if the password is correct. Even in modern times, its use is widespread to gain entry into computers, mobile phones, ATM, etc. If the password is formed from multiple words it is called passphrase and if it is formed from numbers, it is called passcode or passkey. A good practice is to choose password which is easy to remember and type but hard to guess [2].

Biometric authentication makes use of many physiological and behavioral characteristics that are believed to be unique to individuals. The identification characteristics is something you are and uniqueness ensures difficulty of forging. A survey of physiological and behavioral characteristics useful for biometric identification are fingerprints, voice, iris, retina, hand geometry, face recognition, signature, key stroke, bio-electric signals, gait, ear shape, head resonance, odor and finger shape is given in [3]. A comparative study of these characteristics for biometric authentication system is presented.

Another type of authentication based on tokens is modeled on a physical key which is restricted to open a room, a building, etc. This type has to do with the ownership. Corporate badges, possession of king’s seal, passport and smart cards are other examples. Token verifiers may or may not have readers. In the latter case, the user has a token and can compute the reply to challenge posed by the remote host for authentication. Alternatively, a time-based token calculator, where passwords change regularly and in sync with that on the host. Yet another way is to use One-time passwords. The user has a list of passwords and uses each of them only once [2]. The certificate based authentication will be described later.

When only a single method among the preceding options is used, it is called single-factor authentication. Multi-factor authentication uses more than one of the options simultaneously during the authentication process (two-factor uses two, three-factor uses three, and so on) [2]. A familiar example of two-factor authentication is the “sign-on” process at a banking machine where the user presents a credit or debit card (“something you have”) and enters a PIN (“something you know”) to gain access to his/her bank account. Clearly, multi-factor systems are more burdensome for the user as more tasks are to be completed before the authentication process is finished. However, the security benefit is that impersonation attacks become much more difficult. Another example is online mobile banking transactions where the user is required to provide a password and One-time pseudorandom number received over the mobile (ownership factor).

A biometric system should take into account day-to-day variations in individuals bio-metric and be reliable. Though biometrics characteristics are hard to forge, it is easy to forge after a measurement is taken. An important criterion during biometric authentication is to check for liveliness of the input data. To make biometrics more effective, it is combined with a secret of some kind — a PIN, a private key on a smart card, or, yes, even a password. In other words biometric characteristic should be treated as pure identity and use other forms for authentication verification.

3 Standards for Authentication

If the security technology has not kept pace with rapid development of Information Technology (IT), IT systems, users and data, both organization and private, will be vulnerable to attacks. The attackers could be criminals or politically motivated or financially motivated. Information security standards help to prevent most of the threats, to manage security risks by making it harder for attacks to succeed and by reducing the effects of attacks.

The goal of information security standards is to improve the security of the information systems, to define functional and assurance requirements, to promote vendors to build standard-compliant product, to enable consistency among product developers and to serve as a reliable metric for purchasing products.

Request for Comments (RFC) is a collaborative publication from engineers and computer scientists in the form of a memorandum describing methods, behaviors, research, or innovations applicable to the working of the Internet and Internet-connected systems. The Internet Engineering Task Force (IETF) adopts some of the proposals published as Request for Comments (RFC) as Internet Standards. For authentication methods described here RFC’s will be mentioned where applicable.

The US National Institute of Standards and Technology (NIST) publishes white papers and other resources in its Security Management & Assurance working group. These standards are followed by mainly, US federal agencies. The recommendations are applicable to other organizations as well and referred to as NIST standards.

4 Password based Authentication Schemes

The users connect to the network via a remote connection over VPN or dial up network. In this case Password Authentication Protocol (PAP) is used [4]. After the user establishes a link, it repeatedly sends his id/password to authenticator. If he receives ACK, the connection is established. If the user receives NAK instead, the connection is terminated. The protocol is simple and the drawback is that the password is sent in the clear text.

Challenge Handshake Authentication Protocol (CHAP) is more secure than the PAP and makes use of a cryptographic hash function such as MD5 or SHA [1]. A cryptographic hash is a one-way function takes arbitrary size input and after computations it outputs 128-bit output. The security of the cryptographic hash comes from the fact that given a message “m” which hashes to digest “d”, it is computationally hard to find another message “m” that hashes to the same digest “d”.

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