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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"><html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"><head><!-- 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" /><link href="../style/css/manual-print.css" rel="stylesheet" media="print" type="text/css" /><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 2.2</p><img alt="" src="../images/feather.gif" /></div><div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div><div id="path"><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 2.2</a> > <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"> en </a> |<a href="../ja/ssl/ssl_intro.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a></p></div><blockquote><p>The nice thing about standards is that there are so many to choosefrom. And if you really don't like all the standards you just have towait another year until the one arises you are looking for.</p><p class="cite">-- <cite>A. Tanenbaum</cite>, "Introduction toComputer Networks"</p></blockquote><p>As an introduction this chapter is aimed at readers who are familiarwith the Web, HTTP, and Apache, but are not security experts. It is notintended to be a definitive guide to the SSL protocol, nor does it discussspecific techniques for managing certificates in an organization, or theimportant 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><p>The presented content is mainly derived, with the author's permission,from the article <a href="http://home.comcast.net/~fjhirsch/Papers/wwwj/">IntroducingSSL and Certificates using SSLeay</a> by <a href="http://home.comcast.net/~fjhirsch/">Frederick J. Hirsch</a>, of TheOpen Group Research Institute, which was published in <a href="http://www.ora.com/catalog/wjsum97/">Web Security: A Matter ofTrust</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 originalarticle 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 cryptographicalgorithms, message digest functions (aka. one-way or hash functions), anddigital signatures. These techniques are the subject of entire books (seefor 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 themessage is really from her, so an intruder cannot request a transactioninvolving her account. A <em>digital signature</em>, created by Alice andincluded with the message, serves this purpose.</p><p>Digital signatures are created by encrypting a digest of the message andother 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 thesender knows the private key. This means that only the sender can have signedthe message. Including the digest in the signature means the signature is onlygood for that message; it also ensures the integrity of the message since no onecan change the digest and still sign it.</p><p>To guard against interception and reuse of the signature by an intruder at alater date, the signature contains a unique sequence number. This protectsthe 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, signedit and ensured the integrity of the message, she still needs to be surethat she is really communicating with the bank. This means that she needsto 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 messagesignature 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 bothcan be assured that they are communicating with whom they think they are.Such a trusted agency is called a <em>Certificate Authority</em> andcertificates 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>⌨️ 快捷键说明
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