Problem - Kevin Tafuro

Chapter 7 CHAPTER 7
Public Key Cryptography
Many of the recipes in this chapter are too low-level for general-purpose
use. We recommend that you first try to find what you need in
Chapter 9 before resorting to building solutions yourself. If you do use
this chapter, please be careful, read all of our warnings, and do consider
the higher-level constructs we suggest.
Public key cryptography offers a number of important advantages over traditional, or
symmetric, cryptography:
Key agreement
Traditional cryptography is done with a single shared key. There are obvious
limitations to that kind of cryptography, though. The biggest one is the key
agreement problem: how do two parties that wish to communicate do so
securely? One option is to use a more secure out-of-band medium for transport,
such as telephone or postal mail. Such a solution is rarely practical, however,
considering that we might want to do business securely with an online merchant
we’ve never previously encountered. Public key cryptography can help solve the
key agreement problem, although doing so is not as easy as one might hope. We
touch upon this issue throughout this chapter and expand upon it in Chapter 8.
Digital signatures
Another useful service that public key cryptography can provide is digital signatures,
which allow for message integrity checks without a shared secret. In a
symmetric environment with message authentication codes (MACs) for message
authentication, a user can determine that someone with the MAC key sent a particular
message, but it isn’t possible to provide third parties any assurance as to
who signed a message (this ability is called non-repudiation). That is, if Alice and
Bob exchange messages using a MAC, and somehow Charlie has been given a
copy of the message and the MAC key, Charlie will be able to determine only
that someone who had the MAC key at some point before him generated the
message. Using only symmetric cryptography, he cannot distinguish between
messages created by Alice and messages created by Bob in a secure manner.
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