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diff --git a/docs/manual/ssl/ssl_intro.html.en b/docs/manual/ssl/ssl_intro.html.en new file mode 100644 index 0000000..a8c5d0b --- /dev/null +++ b/docs/manual/ssl/ssl_intro.html.en @@ -0,0 +1,672 @@ +<?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> +<meta content="text/html; charset=ISO-8859-1" http-equiv="Content-Type" /> +<!-- + XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX + This file is generated from xml source: DO NOT EDIT + XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX + --> +<title>SSL/TLS Strong Encryption: An Introduction - Apache HTTP Server Version 2.4</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 rel="stylesheet" type="text/css" href="../style/css/prettify.css" /> +<script src="../style/scripts/prettify.min.js" type="text/javascript"> +</script> + +<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="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="<-" 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 2.4</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="../fr/ssl/ssl_intro.html" hreflang="fr" rel="alternate" title="Français"> fr </a> | +<a href="../ja/ssl/ssl_intro.html" hreflang="ja" rel="alternate" title="Japanese"> ja </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----- +MIIC7jCCAlegAwIBAgIBATANBgkqhkiG9w0BAQQFADCBqTELMAkGA1UEBhMCWFkx +FTATBgNVBAgTDFNuYWtlIERlc2VydDETMBEGA1UEBxMKU25ha2UgVG93bjEXMBUG +A1UEChMOU25ha2UgT2lsLCBMdGQxHjAcBgNVBAsTFUNlcnRpZmljYXRlIEF1dGhv +cml0eTEVMBMGA1UEAxMMU25ha2UgT2lsIENBMR4wHAYJKoZIhvcNAQkBFg9jYUBz +bmFrZW9pbC5kb20wHhcNOTgxMDIxMDg1ODM2WhcNOTkxMDIxMDg1ODM2WjCBpzEL +MAkGA1UEBhMCWFkxFTATBgNVBAgTDFNuYWtlIERlc2VydDETMBEGA1UEBxMKU25h +a2UgVG93bjEXMBUGA1UEChMOU25ha2UgT2lsLCBMdGQxFzAVBgNVBAsTDldlYnNl +cnZlciBUZWFtMRkwFwYDVQQDExB3d3cuc25ha2VvaWwuZG9tMR8wHQYJKoZIhvcN +AQkBFhB3d3dAc25ha2VvaWwuZG9tMIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKB +gQDH9Ge/s2zcH+da+rPTx/DPRp3xGjHZ4GG6pCmvADIEtBtKBFAcZ64n+Dy7Np8b +vKR+yy5DGQiijsH1D/j8HlGE+q4TZ8OFk7BNBFazHxFbYI4OKMiCxdKzdif1yfaa +lWoANFlAzlSdbxeGVHoT0K+gT5w3UxwZKv2DLbCTzLZyPwIDAQABoyYwJDAPBgNV +HRMECDAGAQH/AgEAMBEGCWCGSAGG+EIBAQQEAwIAQDANBgkqhkiG9w0BAQQFAAOB +gQAZUIHAL4D09oE6Lv2k56Gp38OBDuILvwLg1v1KL8mQR+KFjghCrtpqaztZqcDt +2q2QoyulCgSzHbEGmi0EsdkPfg6mp0penssIFePYNI+/8u9HT4LuKMJX15hxBam7 +dUHzICxBVC1lnHyYGjDuAMhe396lYAn8bCld1/L4NMGBCQ== +-----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"> en </a> | +<a href="../fr/ssl/ssl_intro.html" hreflang="fr" rel="alternate" title="Français"> fr </a> | +<a href="../ja/ssl/ssl_intro.html" hreflang="ja" rel="alternate" title="Japanese"> ja </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&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed again by our moderators if they are either implemented or considered invalid/off-topic. 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