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<li class="toctree-l1"><a class="reference internal" href="../preface.html">Preface</a></li>
<li class="toctree-l1"><a class="reference internal" href="../foundation.html">Chapter 1:  Foundation</a></li>
<li class="toctree-l1"><a class="reference internal" href="../direct.html">Chapter 2:  Direct Links</a></li>
<li class="toctree-l1"><a class="reference internal" href="../internetworking.html">Chapter 3:  Internetworking</a></li>
<li class="toctree-l1"><a class="reference internal" href="../scaling.html">Chapter 4:  Advanced Internetworking</a></li>
<li class="toctree-l1"><a class="reference internal" href="../e2e.html">Chapter 5:  End-to-End Protocols</a></li>
<li class="toctree-l1"><a class="reference internal" href="../congestion.html">Chapter 6:  Congestion Control</a></li>
<li class="toctree-l1"><a class="reference internal" href="../data.html">Chapter 7: End-to-End Data</a></li>
<li class="toctree-l1 current"><a class="reference internal" href="../security.html">Chapter 8: Network Security</a><ul class="current">
<li class="toctree-l2 current"><a class="current reference internal" href="#">Problem: Security Attacks</a></li>
<li class="toctree-l2"><a class="reference internal" href="trust.html">8.1 Trust and Threats</a></li>
<li class="toctree-l2"><a class="reference internal" href="crypto.html">8.2 Cryptographic Building Blocks</a></li>
<li class="toctree-l2"><a class="reference internal" href="key-distro.html">8.3 Key Predistribution</a></li>
<li class="toctree-l2"><a class="reference internal" href="authentication.html">8.4 Authentication Protocols</a></li>
<li class="toctree-l2"><a class="reference internal" href="systems.html">8.5 Example Systems</a></li>
<li class="toctree-l2"><a class="reference internal" href="trend.html">Perspective: Blockchain and a Decentralized Internet</a></li>
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<li class="toctree-l1"><a class="reference internal" href="../applications.html">Chapter 9: Applications</a></li>
<li class="toctree-l1"><a class="reference internal" href="../README.html">About This Book</a></li>
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  <div class="section" id="problem-security-attacks">
<h1>Problem: Security Attacks<a class="headerlink" href="#problem-security-attacks" title="Permalink to this headline">¶</a></h1>
<p>Computer networks are typically a shared resource used by many
applications representing different interests. The Internet is
particularly widely shared, being used by competing businesses, mutually
antagonistic governments, and opportunistic criminals. Unless security
measures are taken, a network conversation or a distributed application
may be compromised by an adversary.</p>
<p>Consider, for example, some threats to secure use of the web. Suppose
you are a customer using a credit card to order an item from a website.
An obvious threat is that an adversary would eavesdrop on your network
communication, reading your messages to obtain your credit card
information. How might that eavesdropping be accomplished? It is trivial
on a broadcast network such as an Ethernet or Wi-Fi, where any node can
be configured to receive all the message traffic on that network. More
elaborate approaches include wiretapping and planting spy software on
any of the chain of nodes involved. Only in the most extreme cases
(e.g.,national security) are serious measures taken to prevent such
monitoring, and the Internet is not one of those cases. It is possible
and practical, however, to encrypt messages so as to prevent an
adversary from understanding the message contents. A protocol that does
so is said to provide <em>confidentiality</em>. Taking the concept a step
farther, concealing the quantity or destination of communication is
called <em>traffic confidentiality</em>—because merely knowing how much
communication is going where can be useful to an adversary in some
situations.</p>
<p>Even with confidentiality there still remains threats for the website
customer. An adversary who can't read the contents of your encrypted
message might still be able to change a few bits in it, resulting in a
valid order for, say, a completely different item or perhaps 1000 units
of the item. There are techniques to detect, if not prevent, such
tampering. A protocol that detects such message tampering is said to
provide <em>integrity</em>.</p>
<p>Another threat to the customer is unknowingly being directed to a false
website. This can result from a Domain Name System (DNS) attack, in
which false information is entered in a DNS server or the name service
cache of the customer's computer. This leads to translating a correct
URL into an incorrect IP address—the address of a false website. A
protocol that ensures that you really are talking to whom you think
you're talking is said to provide <em>authentication</em>. Authentication
entails integrity, since it is meaningless to say that a message came
from a certain participant if it is no longer the same message.</p>
<p>The owner of the website can be attacked as well. Some websites have
been defaced; the files that make up the website content have been
remotely accessed and modified without authorization. That is an issue
of <em>access control</em>: enforcing the rules regarding who is allowed to do
what. Websites have also been subject to denial of service (DoS)
attacks, during which would-be customers are unable to access the
website because it is being overwhelmed by bogus requests. Ensuring a
degree of access is called <em>availability</em>.</p>
<p>In addition to these issues, the Internet has notably been used as a
means for deploying malicious code, generally called <em>malware</em>, that
exploits vulnerabilities in end systems. <em>Worms</em>, pieces of
self-replicating code that spread over networks, have been known for
several decades and continue to cause problems, as do their relatives,
<em>viruses</em>, which are spread by the transmission of infected files.
Infected machines can then be arranged into <em>botnets</em>, which can be used
to inflict further harm, such as launching DoS attacks.</p>
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