The first step toward securing the resources of a LAN is the ability to verify the identities of users [BNOV91]. The process of verifying a user’s identity is referred to as authentication. Authentication provides the basis for the effectiveness of other controls used on the LAN.
For example the logging mechanism provides usage information based on the userid. The access control mechanism permits access to LAN resources based on the userid. Both these controls are only effective under the assumption that the requestor of a LAN service is the valid user assigned to that specific userid.
Identification requires the user to be known by the LAN in some manner. This is usually based on an assigned userid. However the LAN cannot trust the validity that the user is in fact, who the user claims to be, without being authenticated.
The authentication is done by having the user supply something that only the user has, such as a token, something that only the user knows, such as a password, or something that makes the user unique, such as a fingerprint. The more of these that the user has to supply, the less risk in someone masquerading as the legitimate user.
A requirement specifying the need for authentication should exist in most LAN policies. The requirement may be directed implicitly in a program level policy stressing the need to effectively control access to information and LAN resources, or may be explicitly stated in a LAN specific policy that states that all users must be uniquely identified and authenticated.
On most LANs, the identification and authentication mechanism is a userid/password scheme. [BNOV91] states that "password systems can be effective if managed properly [FIPS112], but seldom are. Authentication which relies solely on passwords has often failed to provide adequate protection for systems for a number of reasons.
Users tend to create passwords that are easy to remember and hence easy to guess. On the other hand users that must use passwords generated from random characters, while difficult to guess, are also difficult to be remembered by users. This forces the user to write the password down, most likely in an area easy accessible in the work area".
Research work such as [KLEIN] detail the ease at which passwords can be guessed. Proper password selection (striking a balance between being easy-to-remember for the user but difficult-to-guess for everyone else) has always been an issue.
Password generators that produce passwords consisting of pronounceable syllables have more potential of being remembered than generators that produce purely random characters. [FIPS180] specifies an algorithm that can be used to produce random pronounceable passwords.
Password checkers are programs that enable a user to determine whether a new passwords is considered easy-to-guess, and thus unacceptable. Password-only mechanisms, especially those that transmit the password in the clear (in an unencrypted form) are susceptible to being monitored and captured.
This can become a serious problem if the LAN has any uncontrolled connections to outside networks. Agencies that are considering connecting their LANs to outside networks, particularly the Internet, should examine [BJUL93] before doing so.
If, after considering all authentication options, LAN policy determines that password-only systems are acceptable, the proper management of password creation, storage, expiration and destruction become all the more important. [FIPS 112] provides guidance on password management. [NCSC85] provides additional guidance that may be considered appropriate.
Because of the vulnerabilities that still exist with the use of password-only mechanisms, more robust mechanisms can be used. [BNOV91] discusses advances that have been made in the areas of token-based authentication and the use of biometrics.
A smartcard based or token based mechanism requires that a user be in possession of the token and additionally may require the user to know a PIN or password. These devices then perform a challenge/response authentication scheme using realtime parameters.
Using realtime parameters helps prevent an intruder from gaining unauthorized access through a login session playback. These devices may also encrypt the authentication session, preventing the compromise of the authentication information through monitoring and capturing.
Locking mechanisms for LAN devices, workstations, or PCs that require user authentication to unlock can be useful to users who must leave their work areas frequently. These locks allow users to remain logged into the LAN and leave their work areas (for an acceptable short period of time) without exposing an entry point into the LAN.
Modems that provide users with LAN access may require additional protection. An intruder that can access the modem may gain access by successfully guessing a user password. The availability of modem use to legitimate users may also become an issue if an intruder is allowed continual access to the modem.
Mechanisms that provide a user with his or her account usage information may alert the user that the account was used in an abnormal manner (e.g. multiple login failures). These mechanisms include notifications such as date, time, and location of last successful login, and number of previous login failures.
The type of security mechanisms that could be implemented to provide the identification and authentication service are listed below.
- Password based mechanism
- Smartcards/smart tokens based mechanism
- Biometrics based mechanism
- Password generator
- Password locking
- Keyboard locking
- PC or workstation locking
- Termination of connection after multiple failed logins
- User notification of ‘last successful login’ and ‘number of login failures’
- Real-time user verification mechanism
- Cryptography with unique user keys.