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+<?xml version="1.0" encoding="ISO-8859-1"?>
+<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ This file is generated from xml source: DO NOT EDIT
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ -->
+<title>SSL/TLS Strong Encryption: An Introduction - Apache HTTP Server</title>
+<link href="../style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" />
+<link href="../style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" />
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+<link href="../images/favicon.ico" rel="shortcut icon" /></head>
+<body id="manual-page"><div id="page-header">
+<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p>
+<p class="apache">Apache HTTP Server Version 2.0</p>
+<img alt="" src="../images/feather.gif" /></div>
+<div class="up"><a href="./"><img title="&lt;-" alt="&lt;-" src="../images/left.gif" /></a></div>
+<div id="path">
+<a href="http://www.apache.org/">Apache</a> &gt; <a href="http://httpd.apache.org/">HTTP Server</a> &gt; <a href="http://httpd.apache.org/docs/">Documentation</a> &gt; <a href="../">Version 2.0</a> &gt; <a href="./">SSL/TLS</a></div><div id="page-content"><div id="preamble"><h1>SSL/TLS Strong Encryption: An Introduction</h1>
+<div class="toplang">
+<p><span>Available Languages: </span><a href="../en/ssl/ssl_intro.html" title="English">&nbsp;en&nbsp;</a> |
+<a href="../ja/ssl/ssl_intro.html" hreflang="ja" rel="alternate" title="Japanese">&nbsp;ja&nbsp;</a></p>
+</div>
+
+<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>
+<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
+<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>
+ <td>Name being certified</td>
+ <td>CN=Joe Average</td></tr>
+ <tr><td>Organization or Company</td>
+ <td>O</td>
+ <td>Name is associated with this<br />organization</td>
+ <td>O=Snake Oil, Ltd.</td></tr>
+ <tr><td>Organizational Unit</td>
+ <td>OU</td>
+ <td>Name is associated with this <br />organization unit, such
+ as a department</td>
+ <td>OU=Research Institute</td></tr>
+ <tr><td>City/Locality</td>
+ <td>L</td>
+ <td>Name is located in this City</td>
+ <td>L=Snake City</td></tr>
+ <tr><td>State/Province</td>
+ <td>ST</td>
+ <td>Name is located in this State/Province</td>
+ <td>ST=Desert</td></tr>
+ <tr><td>Country</td>
+ <td>C</td>
+ <td>Name is located in this Country (ISO code)</td>
+ <td>C=XZ</td></tr>
+ </table>
+
+
+ <p>A Certificate Authority may define a policy specifying which
+ distinguished field names are optional, and which are required. It
+ may also place requirements upon the field contents, as may users of
+ certificates. As an example, a Netscape browser requires that the
+ Common Name for a certificate representing a server has a name which
+ matches a wildcard pattern for the domain name of that server, such
+ as <code>*.snakeoil.com</code>.</p>
+
+ <p>The binary format of a certificate is defined using the ASN.1
+ notation [<a href="#X208">X208</a>] [<a href="#PKCS">PKCS</a>]. This
+ notation defines how to specify the contents, and encoding rules
+ define how this information is translated into binary form. The binary
+ encoding of the certificate is defined using Distinguished Encoding
+ Rules (DER), which are based on the more general Basic Encoding Rules
+ (BER). For those transmissions which cannot handle binary, the binary
+ form may be translated into an ASCII form by using Base64 encoding
+ [<a href="#MIME">MIME</a>]. This encoded version is called PEM encoded
+ (the name comes from "Privacy Enhanced Mail"), when placed between
+ begin and end delimiter lines as illustrated in the following
+ example.</p>
+
+ <div class="example"><h3>Example of a PEM-encoded certificate (snakeoil.crt)</h3><pre>-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----</pre></div>
+
+
+<h3><a name="certificateauthorities" id="certificateauthorities">Certificate Authorities</a></h3>
+
+ <p>By first verifying the information in a certificate request
+ before granting the certificate, the Certificate Authority assures
+ the identity of the private key owner of a key-pair. For instance,
+ if Alice requests a personal certificate, the Certificate Authority
+ must first make sure that Alice really is the person the certificate
+ request claims.</p>
+
+ <h4><a name="certificatechains" id="certificatechains">Certificate Chains</a></h4>
+
+ <p>A Certificate Authority may also issue a certificate for
+ another Certificate Authority. When examining a certificate,
+ Alice may need to examine the certificate of the issuer, for each
+ parent Certificate Authority, until reaching one which she has
+ confidence in. She may decide to trust only certificates with a
+ limited chain of issuers, to reduce her risk of a "bad" certificate
+ in the chain.</p>
+
+
+ <h4><a name="rootlevelca" id="rootlevelca">Creating a Root-Level CA</a></h4>
+
+ <p>As noted earlier, each certificate requires an issuer to assert
+ the validity of the identity of the certificate subject, up to
+ the top-level Certificate Authority (CA). This presents a problem:
+ Since this is who vouches for the certificate of the top-level
+ authority, which has no issuer? In this unique case, the
+ certificate is "self-signed", so the issuer of the certificate is
+ the same as the subject. As a result, one must exercise extra care
+ in trusting a self-signed certificate. The wide publication of a
+ public key by the root authority reduces the risk in trusting this
+ key -- it would be obvious if someone else publicized a key
+ claiming to be the authority. Browsers are preconfigured to trust
+ well-known certificate authorities.</p>
+
+ <p>A number of companies, such as <a href="http://www.thawte.com/">Thawte</a> and <a href="http://www.verisign.com/">VeriSign</a>
+ have established themselves as Certificate Authorities. These
+ companies provide the following services:</p>
+
+ <ul>
+ <li>Verifying certificate requests</li>
+ <li>Processing certificate requests</li>
+ <li>Issuing and managing certificates</li>
+ </ul>
+
+ <p>It is also possible to create your own Certificate Authority.
+ Although risky in the Internet environment, it may be useful
+ within an Intranet where the organization can easily verify the
+ identities of individuals and servers.</p>
+
+
+ <h4><a name="certificatemanagement" id="certificatemanagement">Certificate Management</a></h4>
+
+ <p>Establishing a Certificate Authority is a responsibility which
+ requires a solid administrative, technical, and management
+ framework. Certificate Authorities not only issue certificates,
+ they also manage them -- that is, they determine how long
+ certificates are valid, they renew them, and they keep lists of
+ certificates that have already been issued but are no longer valid
+ (Certificate Revocation Lists, or CRLs). Say Alice is entitled to
+ a certificate as an employee of a company. Say too, that the
+ certificate needs to be revoked when Alice leaves the company. Since
+ certificates are objects that get passed around, it is impossible
+ to tell from the certificate alone that it has been revoked. When
+ examining certificates for validity, therefore, it is necessary to
+ contact the issuing Certificate Authority to check CRLs -- this
+ is not usually an automated part of the process.</p>
+
+ <div class="note"><h3>Note</h3>
+ <p>If you use a Certificate Authority that is not configured into
+ browsers by default, it is necessary to load the Certificate
+ Authority certificate into the browser, enabling the browser to
+ validate server certificates signed by that Certificate Authority.
+ Doing so may be dangerous, since once loaded, the browser will
+ accept all certificates signed by that Certificate Authority.</p>
+ </div>
+
+
+
+</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="ssl" id="ssl">Secure Sockets Layer (SSL)</a></h2>
+
+<p>The Secure Sockets Layer protocol is a protocol layer which may be
+placed between a reliable connection-oriented network layer protocol
+(e.g. TCP/IP) and the application protocol layer (e.g. HTTP). SSL provides
+for secure communication between client and server by allowing mutual
+authentication, the use of digital signatures for integrity, and encryption
+for privacy.</p>
+
+<p>The protocol is designed to support a range of choices for specific
+algorithms used for cryptography, digests, and signatures. This allows
+algorithm selection for specific servers to be made based on legal, export
+or other concerns, and also enables the protocol to take advantage of new
+algorithms. Choices are negotiated between client and server at the start
+of establishing a protocol session.</p>
+
+<h3><a name="table4" id="table4">Table 4: Versions of the SSL protocol</a></h3>
+
+ <table class="bordered">
+
+ <tr><th>Version</th>
+ <th>Source</th>
+ <th>Description</th>
+ <th>Browser Support</th></tr>
+ <tr><td>SSL v2.0</td>
+ <td>Vendor Standard (from Netscape Corp.) [<a href="#SSL2">SSL2</a>]</td>
+ <td>First SSL protocol for which implementations exists</td>
+ <td>- NS Navigator 1.x/2.x<br />
+ - MS IE 3.x<br />
+ - Lynx/2.8+OpenSSL</td></tr>
+ <tr><td>SSL v3.0</td>
+ <td>Expired Internet Draft (from Netscape Corp.) [<a href="#SSL3">SSL3</a>]</td>
+ <td>Revisions to prevent specific security attacks, add non-RSA
+ ciphers, and support for certificate chains</td>
+ <td>- NS Navigator 2.x/3.x/4.x<br />
+ - MS IE 3.x/4.x<br />
+ - Lynx/2.8+OpenSSL</td></tr>
+ <tr><td>TLS v1.0</td>
+ <td>Proposed Internet Standard (from IETF) [<a href="#TLS1">TLS1</a>]</td>
+ <td>Revision of SSL 3.0 to update the MAC layer to HMAC, add block
+ padding for block ciphers, message order standardization and more
+ alert messages.</td>
+ <td>- Lynx/2.8+OpenSSL</td></tr>
+ </table>
+
+
+<p>There are a number of versions of the SSL protocol, as shown in
+<a href="#table4">Table 4</a>. As noted there, one of the benefits in
+SSL 3.0 is that it adds support of certificate chain loading. This feature
+allows a server to pass a server certificate along with issuer certificates
+to the browser. Chain loading also permits the browser to validate the
+server certificate, even if Certificate Authority certificates are not
+installed for the intermediate issuers, since they are included in the
+certificate chain. SSL 3.0 is the basis for the Transport Layer Security
+[<a href="#TLS1">TLS</a>] protocol standard, currently in development by
+the Internet Engineering Task Force (IETF).</p>
+
+<h3><a name="session" id="session">Session Establishment</a></h3>
+
+ <p>The SSL session is established by following a handshake sequence
+ between client and server, as shown in <a href="#figure1">Figure 1</a>. This sequence may vary, depending on whether the server
+ is configured to provide a server certificate or request a client
+ certificate. Though cases exist where additional handshake steps
+ are required for management of cipher information, this article
+ summarizes one common scenario: see the SSL specification for the full
+ range of possibilities.</p>
+
+ <div class="note"><h3>Note</h3>
+ <p>Once an SSL session has been established it may be reused, thus
+ avoiding the performance penalty of repeating the many steps needed
+ to start a session. For this the server assigns each SSL session a
+ unique session identifier which is cached in the server and which the
+ client can use on forthcoming connections to reduce the handshake
+ (until the session identifer expires in the cache of the server).</p>
+ </div>
+
+ <p class="figure">
+ <img src="../images/ssl_intro_fig1.gif" alt="" width="423" height="327" /><br />
+ <a id="figure1" name="figure1"><dfn>Figure 1</dfn></a>: Simplified SSL
+ Handshake Sequence</p>
+
+ <p>The elements of the handshake sequence, as used by the client and
+ server, are listed below:</p>
+
+ <ol>
+ <li>Negotiate the Cipher Suite to be used during data transfer</li>
+ <li>Establish and share a session key between client and server</li>
+ <li>Optionally authenticate the server to the client</li>
+ <li>Optionally authenticate the client to the server</li>
+ </ol>
+
+ <p>The first step, Cipher Suite Negotiation, allows the client and
+ server to choose a Cipher Suite supportable by both of them. The SSL3.0
+ protocol specification defines 31 Cipher Suites. A Cipher Suite is
+ defined by the following components:</p>
+
+ <ul>
+ <li>Key Exchange Method</li>
+ <li>Cipher for Data Transfer</li>
+ <li>Message Digest for creating the Message Authentication Code (MAC)</li>
+ </ul>
+
+ <p>These three elements are described in the sections that follow.</p>
+
+
+<h3><a name="keyexchange" id="keyexchange">Key Exchange Method</a></h3>
+
+ <p>The key exchange method defines how the shared secret symmetric
+ cryptography key used for application data transfer will be agreed
+ upon by client and server. SSL 2.0 uses RSA key exchange only, while
+ SSL 3.0 supports a choice of key exchange algorithms including the
+ RSA key exchange when certificates are used, and Diffie-Hellman key
+ exchange for exchanging keys without certificates and without prior
+ communication between client and server.</p>
+
+ <p>One variable in the choice of key exchange methods is digital
+ signatures -- whether or not to use them, and if so, what kind of
+ signatures to use. Signing with a private key provides assurance
+ against a man-in-the-middle-attack during the information exchange
+ used in generating the shared key [<a href="#AC96">AC96</a>, p516].</p>
+
+
+<h3><a name="ciphertransfer" id="ciphertransfer">Cipher for Data Transfer</a></h3>
+
+ <p>SSL uses the conventional cryptography algorithm (symmetric
+ cryptography) described earlier for encrypting messages in a session.
+ There are nine choices, including the choice to perform no
+ encryption:</p>
+
+ <ul>
+ <li>No encryption</li>
+ <li>Stream Ciphers
+ <ul>
+ <li>RC4 with 40-bit keys</li>
+ <li>RC4 with 128-bit keys</li>
+ </ul></li>
+ <li>CBC Block Ciphers
+ <ul><li>RC2 with 40 bit key</li>
+ <li>DES with 40 bit key</li>
+ <li>DES with 56 bit key</li>
+ <li>Triple-DES with 168 bit key</li>
+ <li>Idea (128 bit key)</li>
+ <li>Fortezza (96 bit key)</li>
+ </ul></li>
+ </ul>
+
+ <p>Here "CBC" refers to Cipher Block Chaining, which means that a
+ portion of the previously encrypted cipher text is used in the
+ encryption of the current block. "DES" refers to the Data Encryption
+ Standard [<a href="#AC96">AC96</a>, ch12], which has a number of
+ variants (including DES40 and 3DES_EDE). "Idea" is one of the best
+ and cryptographically strongest available algorithms, and "RC2" is
+ a proprietary algorithm from RSA DSI [<a href="#AC96">AC96</a>,
+ ch13].</p>
+
+
+<h3><a name="digestfuntion" id="digestfuntion">Digest Function</a></h3>
+
+ <p>The choice of digest function determines how a digest is created
+ from a record unit. SSL supports the following:</p>
+
+ <ul>
+ <li>No digest (Null choice)</li>
+ <li>MD5, a 128-bit hash</li>
+ <li>Secure Hash Algorithm (SHA-1), a 160-bit hash</li>
+ </ul>
+
+ <p>The message digest is used to create a Message Authentication Code
+ (MAC) which is encrypted with the message to provide integrity and to
+ prevent against replay attacks.</p>
+
+
+<h3><a name="handshake" id="handshake">Handshake Sequence Protocol</a></h3>
+
+ <p>The handshake sequence uses three protocols:</p>
+
+ <ul>
+ <li>The <dfn>SSL Handshake Protocol</dfn>
+ for performing the client and server SSL session establishment.</li>
+ <li>The <dfn>SSL Change Cipher Spec Protocol</dfn> for actually
+ establishing agreement on the Cipher Suite for the session.</li>
+ <li>The <dfn>SSL Alert Protocol</dfn> for conveying SSL error
+ messages between client and server.</li>
+ </ul>
+
+ <p>These protocols, as well as application protocol data, are
+ encapsulated in the <dfn>SSL Record Protocol</dfn>, as shown in
+ <a href="#figure2">Figure 2</a>. An encapsulated protocol is
+ transferred as data by the lower layer protocol, which does not
+ examine the data. The encapsulated protocol has no knowledge of the
+ underlying protocol.</p>
+
+ <p class="figure">
+ <img src="../images/ssl_intro_fig2.gif" alt="" width="428" height="217" /><br />
+ <a id="figure2" name="figure2"><dfn>Figure 2</dfn></a>: SSL Protocol Stack
+ </p>
+
+ <p>The encapsulation of SSL control protocols by the record protocol
+ means that if an active session is renegotiated the control protocols
+ will be transmitted securely. If there were no session before, then
+ the Null cipher suite is used, which means there is no encryption and
+ messages have no integrity digests until the session has been
+ established.</p>
+
+
+<h3><a name="datatransfer" id="datatransfer">Data Transfer</a></h3>
+
+ <p>The SSL Record Protocol, shown in <a href="#figure3">Figure 3</a>,
+ is used to transfer application and SSL Control data between the
+ client and server, possibly fragmenting this data into smaller units,
+ or combining multiple higher level protocol data messages into single
+ units. It may compress, attach digest signatures, and encrypt these
+ units before transmitting them using the underlying reliable transport
+ protocol (Note: currently all major SSL implementations lack support
+ for compression).</p>
+
+ <p class="figure">
+ <img src="../images/ssl_intro_fig3.gif" alt="" width="423" height="323" /><br />
+ <a id="figure3" name="figure3"><dfn>Figure 3</dfn></a>: SSL Record Protocol
+ </p>
+
+
+<h3><a name="securehttp" id="securehttp">Securing HTTP Communication</a></h3>
+
+ <p>One common use of SSL is to secure Web HTTP communication between
+ a browser and a webserver. This case does not preclude the use of
+ non-secured HTTP. The secure version is mainly plain HTTP over SSL
+ (named HTTPS), but with one major difference: it uses the URL scheme
+ <code>https</code> rather than <code>http</code> and a different
+ server port (by default 443). This mainly is what <code class="module"><a href="../mod/mod_ssl.html">mod_ssl</a></code> provides to you for the Apache webserver...</p>
+
+</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="references" id="references">References</a></h2>
+
+<dl>
+<dt><a id="AC96" name="AC96">[AC96]</a></dt>
+<dd>Bruce Schneier, <q>Applied Cryptography</q>, 2nd Edition, Wiley,
+1996. See <a href="http://www.counterpane.com/">http://www.counterpane.com/</a> for various other materials by Bruce
+Schneier.</dd>
+
+<dt><a id="X208" name="X208">[X208]</a></dt>
+<dd>ITU-T Recommendation X.208, <q>Specification of Abstract Syntax Notation
+One (ASN.1)</q>, 1988. See for instance <a href="http://www.itu.int/rec/recommendation.asp?type=items&amp;lang=e&amp;parent=T-REC-X.208-198811-I">http://www.itu.int/rec/recommendation.asp?type=items&amp;lang=e&amp;parent=T-REC-X.208-198811-I</a>.
+</dd>
+
+<dt><a id="X509" name="X509">[X509]</a></dt>
+<dd>ITU-T Recommendation X.509, <q>The Directory - Authentication
+Framework</q>. See for instance <a href="http://www.itu.int/rec/recommendation.asp?type=folders&amp;lang=e&amp;parent=T-REC-X.509">http://www.itu.int/rec/recommendation.asp?type=folders&amp;lang=e&amp;parent=T-REC-X.509</a>.
+</dd>
+
+<dt><a id="PKCS" name="PKCS">[PKCS]</a></dt>
+<dd><q>Public Key Cryptography Standards (PKCS)</q>,
+RSA Laboratories Technical Notes, See <a href="http://www.rsasecurity.com/rsalabs/pkcs/">http://www.rsasecurity.com/rsalabs/pkcs/</a>.</dd>
+
+<dt><a id="MIME" name="MIME">[MIME]</a></dt>
+<dd>N. Freed, N. Borenstein, <q>Multipurpose Internet Mail Extensions
+(MIME) Part One: Format of Internet Message Bodies</q>, RFC2045.
+See for instance <a href="http://ietf.org/rfc/rfc2045.txt">http://ietf.org/rfc/rfc2045.txt</a>.</dd>
+
+<dt><a id="SSL2" name="SSL2">[SSL2]</a></dt>
+<dd>Kipp E.B. Hickman, <q>The SSL Protocol</q>, 1995. See <a href="http://www.netscape.com/eng/security/SSL_2.html">http://www.netscape.com/eng/security/SSL_2.html</a>.</dd>
+
+<dt><a id="SSL3" name="SSL3">[SSL3]</a></dt>
+<dd>Alan O. Freier, Philip Karlton, Paul C. Kocher, <q>The SSL Protocol
+Version 3.0</q>, 1996. See <a href="http://www.netscape.com/eng/ssl3/draft302.txt">http://www.netscape.com/eng/ssl3/draft302.txt</a>.</dd>
+
+<dt><a id="TLS1" name="TLS1">[TLS1]</a></dt>
+<dd>Tim Dierks, Christopher Allen, <q>The TLS Protocol Version 1.0</q>,
+1999. See <a href="http://ietf.org/rfc/rfc2246.txt">http://ietf.org/rfc/rfc2246.txt</a>.</dd>
+</dl>
+</div></div>
+<div class="bottomlang">
+<p><span>Available Languages: </span><a href="../en/ssl/ssl_intro.html" title="English">&nbsp;en&nbsp;</a> |
+<a href="../ja/ssl/ssl_intro.html" hreflang="ja" rel="alternate" title="Japanese">&nbsp;ja&nbsp;</a></p>
+</div><div id="footer">
+<p class="apache">Copyright 2009 The Apache Software Foundation.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p>
+<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p></div>
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