Problem - Kevin Tafuro

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 password for that server. The password is used to
compute some secret information stored on the server; then the actual password is
thrown away.
At account creation time, the server picks a salt value that is used to thwart a number
of attacks. The server can choose to do one of two things with this salt:
• Tell it to the user, and have the user type it in the first time she logs in from any
new machine (the machine can then cache the salt value for subsequent connections).
This solution prevents attackers from learning anything significant by
guessing a password, because the attacker has to guess the salt as well. The salt
effectively becomes part of the password.
422 | Chapter 8: Authentication and Key Exchange
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