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+<title>SSL/TLS Strong Encryption: An Introduction - Apache HTTP Server Version 2.4</title>
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+<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="http://wiki.apache.org/httpd/FAQ">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p>
+<p class="apache">Apache HTTP Server Version 2.4</p>
+<img alt="" src="../images/feather.png" /></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 2.4</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="../fr/ssl/ssl_intro.html" hreflang="fr" rel="alternate" title="Français">&nbsp;fr&nbsp;</a> |
+<a href="../ja/ssl/ssl_intro.html" hreflang="ja" rel="alternate" title="Japanese">&nbsp;ja&nbsp;</a></p>
+</div>
+
+
+<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 <code class="module"><a href="../mod/mod_ssl.html">mod_ssl</a></code> users by pulling together various concepts, definitions,
+and examples as a starting point for further exploration.</p>
+</div>
+<div id="quickview"><a href="https://www.apache.org/foundation/contributing.html" class="badge"><img src="https://www.apache.org/images/SupportApache-small.png" alt="Support Apache!" /></a><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><h3>See also</h3><ul class="seealso"><li><a href="#comments_section">Comments</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 until it is
+ decrypted. Once in this form, the message can only be
+ decrypted by using 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. As long as this key is kept
+ secret, nobody other than the sender or recipient can read the message.
+ If Alice and the bank know a secret key, then they can send each other
+ private messages. The task of sharing a key between sender and recipient
+ before communicating, while also keeping it secret from others, 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 can 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
+ can send private messages to the owner of a key-pair (the bank), by
+ encrypting them using their public key. Only the bank will be able to
+ decrypt them.</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 for her 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 the summaries are the same then the message has been 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
+ a short, fixed-length representation of a longer, variable-length message.
+ Digest algorithms are designed to produce a unique digest for each
+ message. Message digests are designed to make it impractically difficult
+ to determine the message from the digest and (in theory) 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; if the digest is not sent securely, its integrity may
+ be compromised and with it the possibility for the bank to determine the
+ integrity of the original message. Only if the digest is sent securely can
+ the integrity of the associated message be determined.</p>
+
+ <p>One way to send the digest securely is to include it 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 cannot 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 can <em>decrypt</em> the signature using the public key, only the
+sender knows the private key. This means that only the sender can have signed
+the message. 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 is part of the bank's key-pair,
+and not an intruder's. Similarly, the bank needs to verify that the message
+signature really was signed by the private key that belongs to Alice.</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 both
+can 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. For example, a Netscape browser requires that the
+ Common Name for a certificate representing a server 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="#ASN1">ASN1</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>]. When placed between begin and end delimiter
+ lines (as below), this encoded version is called a PEM ("Privacy Enhanced
+ Mail") encoded certificate.</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 verifying the information in a certificate request
+ before granting the certificate, the Certificate Authority assures
+ itself of 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 she is.</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:
+ who can vouch 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. Browsers are preconfigured to trust well-known
+ certificate authorities, but it is important to 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.</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 for how long
+ certificates remain valid, they renew them and keep lists of
+ certificates that were issued in the past but are no longer valid
+ (Certificate Revocation Lists, or CRLs).</p>
+
+ <p>For example, if Alice is entitled to a certificate as an
+ employee of a company but has now left
+ that company, her certificate may need to be revoked.
+ Because certificates are only issued after the subject's identity has
+ been verified and can then be passed around to all those with whom
+ the subject may communicate, it is impossible to tell from the
+ certificate alone that it has been revoked.
+ Therefore when examining certificates for validity
+ it is necessary to contact the issuing Certificate Authority to
+ check CRLs -- this is usually not an automated part of the process.</p>
+
+ <div class="note"><h3>Note</h3>
+ <p>If you use a Certificate Authority that browsers are not configured
+ to trust 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 when
+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>
+ </tr>
+ <tr><td>SSL v2.0</td>
+ <td>Vendor Standard (from Netscape Corp.)</td>
+ <td>First SSL protocol for which implementations exist</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>
+ </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>
+ </tr>
+ <tr><td>TLS v1.1</td>
+ <td>Proposed Internet Standard (from IETF) [<a href="#TLS11">TLS11</a>]</td>
+ <td>Update of TLS 1.0 to add protection against Cipher block chaining
+ (CBC) attacks.</td>
+ </tr>
+ <tr><td>TLS v1.2</td>
+ <td>Proposed Internet Standard (from IETF) [<a href="#TLS12">TLS12</a>]</td>
+ <td>Update of TLS 1.1 deprecating MD5 as hash, and adding incompatibility
+ to SSL so it will never negotiate the use of SSLv2.</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">Establishing a Session</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. Although 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. This
+ avoids the performance penalty of repeating the many steps needed
+ to start a session. To do this, the server assigns each SSL session a
+ unique session identifier which is cached in the server and which the
+ client can use in future connections to reduce the handshake time
+ (until the session identifier expires from 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 supported 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
+ RSA key exchange (when certificates are used), and Diffie-Hellman key
+ exchange (for exchanging keys without certificates, or 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 protection
+ against a man-in-the-middle-attack during the information exchange
+ used to 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 conventional symmetric cryptography, as described earlier,
+ for encrypting messages in a session.
+ There are nine choices of how to encrypt, including the option not to
+ encrypt:</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>"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 currently one of
+ the best and cryptographically strongest algorithms available,
+ and "RC2" is a proprietary algorithm from RSA DSI [<a href="#AC96">AC96</a>, ch13].</p>
+
+
+<h3><a name="digestfunction" id="digestfunction">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 verify integrity and to
+ protect 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 was no previous session,
+ the Null cipher suite is used, which means there will be no encryption and
+ messages will 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, where necessary 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, no major SSL implementations include 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 does not preclude the use of
+ non-secured HTTP - the secure version (called HTTPS) is the same as
+ plain HTTP over SSL, but uses the URL scheme <code>https</code>
+ rather than <code>http</code>, and a different server port (by default,
+ port 443). This functionality is a large part of what <code class="module"><a href="../mod/mod_ssl.html">mod_ssl</a></code> provides 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="ASN1" name="ASN1">[ASN1]</a></dt>
+<dd>ITU-T Recommendation X.208, <q>Specification of Abstract Syntax Notation
+One (ASN.1)</q>, last updated 2008. See <a href="http://www.itu.int/ITU-T/asn1/">http://www.itu.int/ITU-T/asn1/</a>.
+</dd>
+
+<dt><a id="X509" name="X509">[X509]</a></dt>
+<dd>ITU-T Recommendation X.509, <q>The Directory - Authentication
+Framework</q>. For references, see <a href="http://en.wikipedia.org/wiki/X.509">http://en.wikipedia.org/wiki/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://tools.ietf.org/html/rfc2045">http://tools.ietf.org/html/rfc2045</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>
+
+<dt><a id="TLS11" name="TLS11">[TLS11]</a></dt>
+<dd><q>The TLS Protocol Version 1.1</q>,
+2006. See <a href="http://tools.ietf.org/html/rfc4346">http://tools.ietf.org/html/rfc4346</a>.</dd>
+
+<dt><a id="TLS12" name="TLS12">[TLS12]</a></dt>
+<dd><q>The TLS Protocol Version 1.2</q>,
+2008. See <a href="http://tools.ietf.org/html/rfc5246">http://tools.ietf.org/html/rfc5246</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="../fr/ssl/ssl_intro.html" hreflang="fr" rel="alternate" title="Français">&nbsp;fr&nbsp;</a> |
+<a href="../ja/ssl/ssl_intro.html" hreflang="ja" rel="alternate" title="Japanese">&nbsp;ja&nbsp;</a></p>
+</div><div class="top"><a href="#page-header"><img src="../images/up.gif" alt="top" /></a></div><div class="section"><h2><a id="comments_section" name="comments_section">Comments</a></h2><div class="warning"><strong>Notice:</strong><br />This is not a Q&amp;A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Libera.chat, or sent to our <a href="https://httpd.apache.org/lists.html">mailing lists</a>.</div>
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