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            <h1>Secure Socket Layer (SSL) Overview</h1>
            <p>GoAhead supports the Secure Sockets Layer (SSL) protocol for authenticating systems and encrypting data.
            Use of this protocol enables secure data transmission to and from clients in a standards-based manner. 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>This following document explains SSL and how to configure SSL in GoAhead. It has heavily borrowed from
            the Apache Documentation. Many thanks to the fine folks at Apache for this excellent base material and for
            Frederick Hirsch who authored quite a bit of the original material.</p>
            <p>If you have a good understanding of SSL and now you want to configure SSL for GoAhead, you may wish 
            to skip the background and read immediately how to setup 

            <a href="ssl.html">SSL for GoAhead</a>.</p>
            <a id="cryptographicTechniques"></a>
            <h2 >Cryptographic Techniques</h2>
            <p>Understanding SSL requires an understanding of cryptographic algorithms, message digest functions (aka.
            one-way or hash functions), and digital signatures. The following section provides a brief introduction to
            the cryptographic foundations of SSL. If you are already familiar with this material, please skip forward
            to the <a href="#sslOverview">SSL Overview</a> section.</p>

            <h3>Cryptographic Algorithms</h3>
            <p>Consider the example of Alice who wants to send a message to her bank to transfer some money. She 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>

            <h4><a id="symmetricCryptography"></a>Symmetric Cryptography</h4>
            <p>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.</p>

            <h4><a id="publicKeyCryptography"></a>Public Key Cryptography</h4>
            <p>Public key cryptography 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 may 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).</p>
            <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, you 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 id="messageDigests"></a>Message Digests</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 <b>message digest</b>, <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 &mdash; 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 id="digitalSignatures"></a>Digital Signatures</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 &mdash; only she could have signed it (non-repudiation).</p><a id="certificates"></a>

            <h2 >Certificates</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>Certificate Contents</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 id="table1"></a>Table 1: Certificate Information</h4>
            <table title="cert" class="ui table segment">
                <tbody>
                    <tr>
                        <td class="four wide">Subject</td>
                        <td>Distinguished Name, Public Key</td>
                    </tr>
                    <tr>
                        <td>Issuer</td>
                        <td>Distinguished Name, Signature</td>
                    </tr>
                    <tr>
                        <td>Period of Validity</td>
                        <td>Not Before Date, Not After Date</td>
                    </tr>
                    <tr>
                        <td class="pivot class">Administrative Information</td>
                        <td>Version, Serial Number</td>
                    </tr>
                    <tr>
                        <td>Extended Information</td>
                        <td>Basic Constraints, Netscape Flags, etc.</td>
                    </tr>
                </tbody>
            </table>
            <p>A distinguished name is used to provide an identity in a specific context &mdash; 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, which defines the fields, field names, and
            abbreviations used to refer to the fields (see <a href="#table2">Table 2</a>).</p>
            <h4><a id="table2"></a>Table 2: Distinguished Name Information</h4>
            <table title="info" class="ui table segment">
                <thead>
                    <tr>
                        <th>DN Field</th>
                        <th>Abbrev.</th>
                        <th>Description</th>
                        <th>Example</th>
                    </tr>
                </thead>
                <tbody>
                    <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 organization</td>
                        <td>O=Snake Oil, Ltd.</td>
                    </tr>
                    <tr>
                        <td>Organizational Unit</td>
                        <td>OU</td>
                        <td>Name is associated with this 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>
                </tbody>
            </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. 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. 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>
            <h3>Example of a PEM-encoded certificate (snakeoil.crt)</h3>
            <pre class="ui code segment">
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
</pre>
            <h3><a id="certificateAuthorities"></a>Certificate Authorities</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>Certificate Chains</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>Creating a Root-Level CA</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 &mdash; 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>Certificate Management</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 &mdash; 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 &mdash;
            this is not usually an automated part of the process.</p>
            <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>
        
        <a id="sslOverview"></a>
        <h2 >Secure Sockets Layer (SSL) Overview</h2>
        <p>The Secure Sockets Layer protocols 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>Table 4: Versions of the SSL protocol</h3>
        <a id="table4"></a>
        <table title="protocols" class="ui table segment">
            <thead>
                <tr>
                    <th>Version</th>
                    <th>Source</th>
                    <th>Description</th>
                </tr>
            </thead>
            <tbody>
                <tr>
                    <td>SSL v2.0</td>
                    <td>Vendor Standard (from Netscape Corp.)</td>
                    <td>First SSL protocol for which implementations exists</td>
                </tr>
                <tr>
                    <td>SSL v3.0</td>
                    <td>Expired Internet Draft (from Netscape Corp.)</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)</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>
            </tbody>
        </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 protocol standard, currently in development by the Internet Engineering Task Force
        (IETF).</p>
        <h3>Session Establishment</h3>
        <p>The SSL session is established by following a handshake sequence between client and server. 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>
        <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 identifier expires in the cache of the server).</p>
        <p class="figure">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>Key Exchange Method</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 &mdash; 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.</p>
        <h3>Cipher for Data Transfer</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, 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.</p>
        <h3>Digest Function</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>Handshake Sequence Protocol</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>. 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>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>Data Transfer</h3>
        <p>The SSL Record Protocol 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>
        <h3 class="figure">Securing HTTP Communication</h3>
        <p>One common use of SSL is to secure Web HTTP communication between a browser and a web server. 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 <i>https</i> rather than <i>http</i> and a
        different server port (by default 443).</p>

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