Problem - Kevin Tafuro

See Also
In many circumstances, the client won’t have the server’s public key in
advance. In such a case, the server will often send a copy of its public
key (or a digital certificate containing the public key) at Step 6. In this
case, the client can’t assume that the public signing key is valid; there’s
nothing to distinguish it from an attacker’s public key! Therefore, the
key needs to be validated using a trusted third party before the client
trusts that the party on the other end is really the intended server. (We
discuss this problem in Recipes 7.1 and 10.1.)
Recipes 7.1, 8.15, 8.16, 10.1
8.19 Minimizing the Window of Vulnerability
When Authenticating Without a PKI
Problem
You have an application (typically a client) that is likely to receive from a server identifying
information such as a certificate or key that may not necessarily be able to be
automatically verified—for example, because there is no PKI.
Without a way to absolutely defend against man-in-the-middle attacks in an automated
fashion, you want to do the best that you can, either by having the user manually
do certificate validation or by limiting the window of vulnerability to the first
connection.
Solution
Either provide the user with trusted certificate information over a secure channel and
allow him to enter that information, or prompt the user the first time you see a certificate,
and remember it for subsequent connections.
These solutions push the burden of authentication off onto the user.
Discussion
It is common for small organizations to host some kind of a server that is SSLenabled
without a certificate that has been issued by a third-party CA such as Veri-
Sign. Most often, such an organization issues its own certificate using its own CA. A
prime example would be an SSL-enabled POP3 or SMTP server. Unfortunately,
when this is the case, your software needs to have some way of allowing the client to
indicate that the certificate presented by the server is acceptable.
There are two basic ways to do this:
438 | Chapter 8: Authentication and Key Exchange
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