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<title>SSL/TLS Strong Encryption: An Introduction - Apache HTTP Server</title>
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<a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="../">Version 2.0</a> > <a href="./">SSL/TLS</a></div><div id="page-content"><div id="preamble"><h1>SSL/TLS Strong Encryption: An Introduction</h1>
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<p><span>Available Languages: </span><a href="../en/ssl/ssl_intro.html" title="English"> en </a> |
<a href="../ja/ssl/ssl_intro.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a></p>
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<blockquote>
<p>The nice thing about standards is that there are so many to choose
from. And if you really don't like all the standards you just have to
wait another year until the one arises you are looking for.</p>
<p class="cite">-- <cite>A. Tanenbaum</cite>, "Introduction to
Computer Networks"</p>
</blockquote>
<p>As an introduction this chapter is aimed at readers who are familiar
with the Web, HTTP, and Apache, but are not security experts. It is not
intended to be a definitive guide to the SSL protocol, nor does it discuss
specific techniques for managing certificates in an organization, or the
important legal issues of patents and import and export restrictions.
Rather, it is intended to provide a common background to mod_ssl users by
pulling together various concepts, definitions, and examples as a starting
point for further exploration.</p>
<p>The presented content is mainly derived, with permission by the author,
from the article <a href="http://home.earthlink.net/~fjhirsch/Papers/wwwj/article.html">Introducing
SSL and Certificates using SSLeay</a> from <a href="http://home.earthlink.net/~fjhirsch/">Frederick J. Hirsch</a>, of The
Open Group Research Institute, which was published in <a href="http://www.ora.com/catalog/wjsum97/">Web Security: A Matter of
Trust</a>, World Wide Web Journal, Volume 2, Issue 3, Summer 1997.
Please send any positive feedback to <a href="mailto:hirsch@fjhirsch.com">Frederick Hirsch</a> (the original
article author) and all negative feedback to <a href="mailto:rse@engelschall.com">Ralf S. Engelschall</a> (the
<code class="module"><a href="../mod/mod_ssl.html">mod_ssl</a></code> author).</p>
</div>
<div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#cryptographictech">Cryptographic Techniques</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#certificates">Certificates</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#ssl">Secure Sockets Layer (SSL)</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#references">References</a></li>
</ul></div>
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<div class="section">
<h2><a name="cryptographictech" id="cryptographictech">Cryptographic Techniques</a></h2>
<p>Understanding SSL requires an understanding of cryptographic
algorithms, message digest functions (aka. one-way or hash functions), and
digital signatures. These techniques are the subject of entire books (see
for instance [<a href="#AC96">AC96</a>]) and provide the basis for privacy,
integrity, and authentication.</p>
<h3><a name="cryptographicalgo" id="cryptographicalgo">Cryptographic Algorithms</a></h3>
<p>Suppose Alice wants to send a message to her bank to transfer some
money. Alice would like the message to be private, since it will
include information such as her account number and transfer amount. One
solution is to use a cryptographic algorithm, a technique that would
transform her message into an encrypted form, unreadable except by
those it is intended for. Once in this form, the message may only be
interpreted through the use of a secret key. Without the key the
message is useless: good cryptographic algorithms make it so difficult
for intruders to decode the original text that it isn't worth their
effort.</p>
<p>There are two categories of cryptographic algorithms: conventional
and public key.</p>
<dl>
<dt>Conventional cryptography</dt>
<dd>also known as symmetric cryptography, requires the sender and
receiver to share a key: a secret piece of information that may be
used to encrypt or decrypt a message. If this key is secret, then
nobody other than the sender or receiver may read the message. If
Alice and the bank know a secret key, then they may send each other
private messages. The task of privately choosing a key before
communicating, however, can be problematic.</dd>
<dt>Public key cryptography</dt>
<dd>also known as asymmetric cryptography, solves the key exchange
problem by defining an algorithm which uses two keys, each of which
may be used to encrypt a message. If one key is used to encrypt a
message then the other must be used to decrypt it. This makes it
possible to receive secure messages by simply publishing one key
(the public key) and keeping the other secret (the private key).</dd>
</dl>
<p>Anyone may encrypt a message using the public key, but only the
owner of the private key will be able to read it. In this way, Alice
may send private messages to the owner of a key-pair (the bank), by
encrypting it using their public key. Only the bank will be able to
decrypt it.</p>
<h3><a name="messagedigests" id="messagedigests">Message Digests</a></h3>
<p>Although Alice may encrypt her message to make it private, there
is still a concern that someone might modify her original message or
substitute it with a different one, in order to transfer the money
to themselves, for instance. One way of guaranteeing the integrity
of Alice's message is to create a concise summary of her message and
send this to the bank as well. Upon receipt of the message, the bank
creates its own summary and compares it with the one Alice sent. If
they agree then the message was received intact.</p>
<p>A summary such as this is called a <dfn>message digest</dfn>, <em>one-way
function</em> or <em>hash function</em>. Message digests are used to create
short, fixed-length representations of longer, variable-length messages.
Digest algorithms are designed to produce unique digests for different
messages. Message digests are designed to make it too difficult to determine
the message from the digest, and also impossible to find two different
messages which create the same digest -- thus eliminating the possibility of
substituting one message for another while maintaining the same digest.</p>
<p>Another challenge that Alice faces is finding a way to send the digest to the
bank securely; when this is achieved, the integrity of the associated message
is assured. One way to do this is to include the digest in a digital
signature.</p>
<h3><a name="digitalsignatures" id="digitalsignatures">Digital Signatures</a></h3>
<p>When Alice sends a message to the bank, the bank needs to ensure that the
message is really from her, so an intruder does not request a transaction
involving her account. A <em>digital signature</em>, created by Alice and
included with the message, serves this purpose.</p>
<p>Digital signatures are created by encrypting a digest of the message,
and other information (such as a sequence number) with the sender's
private key. Though anyone may <em>decrypt</em> the signature using the public
key, only the signer knows the private key. This means that only they may
have signed it. Including the digest in the signature means the signature is
only good for that message; it also ensures the integrity of the message since
no one can change the digest and still sign it.</p>
<p>To guard against interception and reuse of the signature by an intruder at a
later date, the signature contains a unique sequence number. This protects
the bank from a fraudulent claim from Alice that she did not send the message
-- only she could have signed it (non-repudiation).</p>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="certificates" id="certificates">Certificates</a></h2>
<p>Although Alice could have sent a private message to the bank, signed
it, and ensured the integrity of the message, she still needs to be sure
that she is really communicating with the bank. This means that she needs
to be sure that the public key she is using corresponds to the bank's
private key. Similarly, the bank also needs to verify that the message
signature really corresponds to Alice's signature.</p>
<p>If each party has a certificate which validates the other's identity,
confirms the public key, and is signed by a trusted agency, then they both
will be assured that they are communicating with whom they think they are.
Such a trusted agency is called a <em>Certificate Authority</em>, and
certificates are used for authentication.</p>
<h3><a name="certificatecontents" id="certificatecontents">Certificate Contents</a></h3>
<p>A certificate associates a public key with the real identity of
an individual, server, or other entity, known as the subject. As
shown in <a href="#table1">Table 1</a>, information about the subject
includes identifying information (the distinguished name), and the
public key. It also includes the identification and signature of the
Certificate Authority that issued the certificate, and the period of
time during which the certificate is valid. It may have additional
information (or extensions) as well as administrative information
for the Certificate Authority's use, such as a serial number.</p>
<h4><a name="table1" id="table1">Table 1: Certificate Information</a></h4>
<table>
<tr><th>Subject</th>
<td>Distinguished Name, Public Key</td></tr>
<tr><th>Issuer</th>
<td>Distinguished Name, Signature</td></tr>
<tr><th>Period of Validity</th>
<td>Not Before Date, Not After Date</td></tr>
<tr><th>Administrative Information</th>
<td>Version, Serial Number</td></tr>
<tr><th>Extended Information</th>
<td>Basic Constraints, Netscape Flags, etc.</td></tr>
</table>
<p>A distinguished name is used to provide an identity in a specific
context -- for instance, an individual might have a personal
certificate as well as one for their identity as an employee.
Distinguished names are defined by the X.509 standard [<a href="#X509">X509</a>], which defines the fields, field names, and
abbreviations used to refer to the fields (see <a href="#table2">Table
2</a>).</p>
<h4><a name="table2" id="table2">Table 2: Distinguished Name Information</a></h4>
<table class="bordered">
<tr><th>DN Field</th>
<th>Abbrev.</th>
<th>Description</th>
<th>Example</th></tr>
<tr><td>Common Name</td>
<td>CN</td>
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