Secure Programming Cookbook for C and C++

8.15. Performing Password-Based Authentication and Key Exchange

Problem

You want to establish a secure channel without using public key cryptography at all. You want to avoid tunneling a traditional authentication protocol over a protocol like SSL, instead preferring to build your own secure channel with a good protocol.

Solution

SAX (Symmetric Authenticated eXchange) is a protocol for creating a secure channel that does not use public key cryptography.

PAX (Public key Authenticated eXchange) is similar to SAX, but it uses public key cryptography to prevent against client spoofing if the attacker manages to get the server-side authentication database. The public key cryptography also makes PAX a bit slower.

Discussion

The SAX and PAX protocols both perform authentication and key exchange. The protocols are generic, so they work in any environment. However, in this recipe we’ll show you how to use SAX and PAX in the context of the Authenticated eXchange (AX) library, available from http://www.zork.org/ax/. This library implements SAX and PAX over TCP/IP using a single API.

Let’s take a look at how these protocols are supposed to work from the user’s point of view. The server needs to have authentication information associated with the user. The account setup must be done over a preexisting secure channel. Perhaps the user sits down at a console, or the system administrator might do the setup on behalf of the user while they are talking over the phone.

Account setup requires the user’s ...