Problem - Kevin Tafuro

obtained the public key of the server, it will need to request those credentials, generally
over an insecure channel (e.g., over the Internet). What is to stop an attacker
from replacing the server’s credentials with its own credentials?
Then, when the client tries to establish a secure connection, it could actually be talking
to an attacker, who may choose to either masquerade as the server or just sit in
the middle, communicating with the server on the client’s behalf, as shown in
Figure 7-1 (B). Such an attack is known as a man-in-the-middle attack.
A
B
Attacker
Client Server
Client
Figure 7-1. A man-in-the-middle attack
Getting a server’s key over an insecure channel is okay as long as there is some way
of determining whether the key the client gets back is actually the right one. The
most common way of establishing trust is by using a PKI, a concept we explain in
Recipe 10.1.
Another issue when it comes to public key cryptography is speed. Even the fastest
public key cryptography that’s believed to be secure is orders of magnitude slower
than traditional symmetric encryption. For example, a Pentium class machine may
encrypt data using RC4 with 128-bit keys at about 11 cycles per byte (the key size
isn’t actually a factor in RC4’s speed). The same machine can process data at only
about 2,500 cycles per byte when using an optimized version of vanilla RSA and
2,048-bit keys (the decrypt speed is the limiting factor—encryption is usually about
20 times faster). True, versions of RSA based on elliptic curves can perform better,
but they still don’t perform well for general-purpose use.
310 | Chapter 7: Public Key Cryptography
Request server credentials Request server credentials
Send attacker credentials
Actual traffic
Perceived traffic
Attacker
Actual traffic
Send server credentials
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