Simple VPN authentication choices
Tom Lancaster
When configuring VPNs, you are often presented with a list of simple and standard choices for authentication in addition to what are commonly called "strong authentication" choices such as smart cards and digital certificates. Although they're not as secure, these simple choices are much easier to deploy and usually included for free in almost any modern operating system. The choices typically include:
PAP - Password Authentication Protocol
CHAP - Challenge Handshake Authentication Protocol
MS-CHAP - MicroSoft's CHAP
MS-CHAPv2 - MicroSoft's CHAP version 2
and more recently, EAP, which is the Extensible Authentication Protocol.
The purpose of each of these protocols is to verify the identity of a client when it connects to a network and, occasionally, to allow the client to verify the identity of the network, although this option is frequently skipped.
Briefly, the major differences between PAP and the others is that PAP sends its password in clear text across the line. Generally, this shouldn't even be considered as an appropriate method of authentication for any environment.
CHAP is a pretty big step forward. It uses the message digest (MD5) hashing algorithm so that the only thing sent across the line is a hashed value, and not the actual password. CHAP is also unidirectional, so a server can challenge a client for a password without the client
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challenging the server.
MS-CHAP, in an uncharacteristic move for Microsoft, is actually more secure than the standard CHAP, while it also adds a few features for Windows users that support domain login and others.
And MS-CHAP version 2 goes even further, by increasing the initial size of the encryption keys. Version 2 also requires a challenge in both directions, which is very important these days, given the alarming frequency at which servers are hacked and hijacked.
When choosing an authentication method, the best decision is to choose the most secure algorithm that all your devices support and that you can afford, in terms of dollars and effort to deploy and maintain.
Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.
This was first published in March 2002