Problem - Kevin Tafuro

Use across applications
For some people, it is important to manage authentication centrally across a
series of applications. In such a situation, authentication should involve a separate
server that manages credentials. Kerberos is the popular technology for
meeting this requirement, but a privately run public key infrastructure can be
used to do the same thing.
Patents
Many people also want to avoid any algorithms that are likely to be covered by
patent.
Efficiency
Other people may be concerned about efficiency, particularly on a server that
might need to process many connections in a short period of time. In that situation,
it could be important to avoid public key cryptography altogether, or to
find some other way to minimize the impact on the server, to prevent against
denial of service.
Common mechanism
It may also be a requirement to have authentication and key exchange be done
by the same mechanism. This can improve ease of development if you pick the
right solution.
Economy of expression
An authentication protocol should use a minimal number of messages to do
work. Generally, three messages are considered the target to hit, even when
authentication and key exchange are combined. This is usually not such a big
deal, however. A few extra messages generally will not noticeably impact performance.
Protocol designers like to strive to minimize the number of messages,
because it makes their work more elegant and less ad hoc. Of course, simplicity
should be a considered requirement, but then again, we have seen simple fivemessage
protocols, and ridiculously complex three-message protocols!
Security
Security is an obvious requirement at the highest level, but there are many different
security properties you might care about, as we’ll describe in the rest of this
section.
In terms of the security of your mechanism, you might require a mechanism that
effectively provides its own secure channel, resisting sniffing attacks, man-in-themiddle
attacks, and so on that might lead to password compromise, or even just the
attacker’s somehow masquerading as either the client or server without compromising
the password. (This could happen, for example, if the attacker manages to get the
server password database.)
On the other hand, you might want to require something that does not build its own
secure channel. For example, if you are writing something that will be used only on
the console, you will already be assuming a trusted path from the user to your code,
Choosing an Authentication Method | 365
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