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<manualpage metafile="ssl_intro.xml.meta">
<parentdocument href="./">SSL/TLS</parentdocument>

  <title>SSL/TLS Strong Encryption: An Introduction</title>

<summary>
<blockquote>
<p>The nice thing about standards is that there are so many to choose
from. And if you really don't like all the standards you just have to
wait another year until the one arises you are looking for.</p>

<p class="cite">-- <cite>A. Tanenbaum</cite>, "Introduction to
Computer Networks"</p>
</blockquote>

<p>As an introduction this chapter is aimed at readers who are familiar
with the Web, HTTP, and Apache, but are not security experts. It is not
intended to be a definitive guide to the SSL protocol, nor does it discuss
specific techniques for managing certificates in an organization, or the
important legal issues of patents and import and export restrictions.
Rather, it is intended to provide a common background to mod_ssl users by
pulling together various concepts, definitions, and examples as a starting
point for further exploration.</p>

<p>The presented content is mainly derived, with permission by the author,
from the article <a
href="http://home.earthlink.net/~fjhirsch/Papers/wwwj/article.html">Introducing
SSL and Certificates using SSLeay</a> from <a
href="http://home.earthlink.net/~fjhirsch/">Frederick J. Hirsch</a>, of The
Open Group Research Institute, which was published in <a
href="http://www.ora.com/catalog/wjsum97/">Web Security: A Matter of
Trust</a>, World Wide Web Journal, Volume 2, Issue 3, Summer 1997.
Please send any positive feedback to <a
href="mailto:hirsch@fjhirsch.com">Frederick Hirsch</a> (the original
article author) and all negative feedback to <a
href="mailto:rse@engelschall.com">Ralf S. Engelschall</a> (the
<module>mod_ssl</module> author).</p>
</summary>

<section id="cryptographictech">
<title>Cryptographic Techniques</title>
<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>

<section id="cryptographicalgo">
<title>Cryptographic Algorithms</title>
    <p>Suppose Alice wants to send a message to her bank to transfer some
    money. Alice would like the message to be private, since it will
    include information such as her account number and transfer amount. One
    solution is to use a cryptographic algorithm, a technique that would
    transform her message into an encrypted form, unreadable except by
    those it is intended for. Once in this form, the message may only be
    interpreted through the use of a secret key. Without the key the
    message is useless: good cryptographic algorithms make it so difficult
    for intruders to decode the original text that it isn't worth their
    effort.</p>

    <p>There are two categories of cryptographic algorithms: conventional
    and public key.</p>

    <dl>
    <dt>Conventional cryptography</dt>
    <dd>also known as symmetric cryptography, requires the sender and
    receiver to share a key: a secret piece of information that may be
    used to encrypt or decrypt a message. If this key is secret, then
    nobody other than the sender or receiver may read the message. If
    Alice and the bank know a secret key, then they may send each other
    private messages. The task of privately choosing a key before
    communicating, however, can be problematic.</dd>

    <dt>Public key cryptography</dt>
    <dd>also known as asymmetric cryptography, solves the key exchange
    problem by defining an algorithm which uses two keys, each of which
    may be used to encrypt a message. If one key is used to encrypt a
    message then the other must be used to decrypt it. This makes it
    possible to receive secure messages by simply publishing one key
    (the public key) and keeping the other secret (the private key).</dd>
    </dl>

    <p>Anyone may encrypt a message using the public key, but only the
    owner of the private key will be able to read it. In this way, Alice
    may send private messages to the owner of a key-pair (the bank), by
    encrypting it using their public key. Only the bank will be able to
    decrypt it.</p>
</section>

<section id="messagedigests">
<title>Message Digests</title>
    <p>Although Alice may encrypt her message to make it private, there
    is still a concern that someone might modify her original message or
    substitute it with a different one, in order to transfer the money
    to themselves, for instance. One way of guaranteeing the integrity
    of Alice's message is to create a concise summary of her message and
    send this to the bank as well. Upon receipt of the message, the bank
    creates its own summary and compares it with the one Alice sent. If
    they agree then the message was received intact.</p>

    <p>A summary such as this is called a <dfn>message digest</dfn>, <em>one-way
function</em> or <em>hash function</em>. Message digests are used to create
short, fixed-length representations of longer, variable-length messages.
Digest algorithms are designed to produce unique digests for different
messages. Message digests are designed to make it too difficult to determine
the message from the digest, and also impossible to find two different
messages which create the same digest -- thus eliminating the possibility of
substituting one message for another while maintaining the same digest.</p>
<p>Another challenge that Alice faces is finding a way to send the digest to the
bank securely; when this is achieved, the integrity of the associated message
is assured. One way to do this is to include the digest in a digital
signature.</p>
</section>

<section id="digitalsignatures"><title>Digital Signatures</title>
<p>When Alice sends a message to the bank, the bank needs to ensure that the
message is really from her, so an intruder does not request a transaction
involving her account. A <em>digital signature</em>, created by Alice and
included with the message, serves this purpose.</p>

<p>Digital signatures are created by encrypting a digest of the message,
and other information (such as a sequence number) with the sender's
private key. Though anyone may <em>decrypt</em> the signature using the public
key, only the signer knows the private key. This means that only they may
have signed it. Including the digest in the signature means the signature is
only good for that message; it also ensures the integrity of the message since
no one can change the digest and still sign it.</p>
<p>To guard against interception and reuse of the signature by an intruder at a
later date, the signature contains a unique sequence number. This protects
the bank from a fraudulent claim from Alice that she did not send the message
-- only she could have signed it (non-repudiation).</p>
</section>
</section>
<!-- /cryptographictech -->

<section id="certificates">
<title>Certificates</title>
<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>

<section id="certificatecontents">
<title>Certificate Contents</title>
    <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>

    <section id="table1">
    <title>Table 1: Certificate Information</title>
    <table>
    <columnspec><column width=".35"/><column width=".35"/>
    </columnspec>
    <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>
    </section>

    <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>

    <section id="table2">
    <title>Table 2: Distinguished Name Information</title>
    <table border="1">
    <columnspec><column width=".25"/><column width=".15"/>
      <column width=".3"/><column width=".25"/></columnspec>
    <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>
    </section>

    <p>A Certificate Authority may define a policy specifying which
    distinguished field names are optional, and which are required. It
    may also place requirements upon the field contents, as may users of
    certificates. As an example, a Netscape browser requires that the
    Common Name for a certificate representing a server has a name which
    matches a wildcard pattern for the domain name of that server, such
    as <code>*.snakeoil.com</code>.</p>

    <p>The binary format of a certificate is defined using the ASN.1
    notation [<a href="#X208">X208</a>] [<a href="#PKCS">PKCS</a>]. This
    notation defines how to specify the contents, and encoding rules
    define how this information is translated into binary form. The binary
    encoding of the certificate is defined using Distinguished Encoding
    Rules (DER), which are based on the more general Basic Encoding Rules
    (BER). For those transmissions which cannot handle binary, the binary
    form may be translated into an ASCII form by using Base64 encoding
    [<a href="#MIME">MIME</a>]. This encoded version is called PEM encoded
    (the name comes from "Privacy Enhanced Mail"), when placed between
    begin and end delimiter lines as illustrated in the following
    example.</p>

    <example>
    <title>Example of a PEM-encoded certificate (snakeoil.crt)</title>
    <pre>-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----</pre>
    </example>
</section>

<section id="certificateauthorities">
<title>Certificate Authorities</title>
    <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>

    <section id="certificatechains">
    <title>Certificate Chains</title>
        <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>
    </section>

    <section id="rootlevelca">
    <title>Creating a Root-Level CA</title>
        <p>As noted earlier, each certificate requires an issuer to assert
        the validity of the identity of the certificate subject, up to
        the top-level Certificate Authority (CA). This presents a problem:
        Since this is who vouches for the certificate of the top-level
        authority, which has no issuer? In this unique case, the
        certificate is "self-signed", so the issuer of the certificate is
        the same as the subject. As a result, one must exercise extra care
        in trusting a self-signed certificate. The wide publication of a
        public key by the root authority reduces the risk in trusting this
        key -- it would be obvious if someone else publicized a key
        claiming to be the authority. Browsers are preconfigured to trust
        well-known certificate authorities.</p>

        <p>A number of companies, such as <a href="http://www.thawte.com/"
        >Thawte</a> and <a href="http://www.verisign.com/">VeriSign</a>
        have established themselves as Certificate Authorities. These
        companies provide the following services:</p>

        <ul>
        <li>Verifying certificate requests</li>
        <li>Processing certificate requests</li>
        <li>Issuing and managing certificates</li>
        </ul>

        <p>It is also possible to create your own Certificate Authority.
        Although risky in the Internet environment, it may be useful
        within an Intranet where the organization can easily verify the
        identities of individuals and servers.</p>
    </section>

    <section id="certificatemanagement">
    <title>Certificate Management</title>
        <p>Establishing a Certificate Authority is a responsibility which
        requires a solid administrative, technical, and management
        framework. Certificate Authorities not only issue certificates,
        they also manage them -- that is, they determine how long
        certificates are valid, they renew them, and they keep lists of
        certificates that have already been issued but are no longer valid
        (Certificate Revocation Lists, or CRLs). Say Alice is entitled to
        a certificate as an employee of a company. Say too, that the
        certificate needs to be revoked when Alice leaves the company. Since
        certificates are objects that get passed around, it is impossible
        to tell from the certificate alone that it has been revoked. When
        examining certificates for validity, therefore, it is necessary to
        contact the issuing Certificate Authority to check CRLs -- this
        is not usually an automated part of the process.</p>

        <note><title>Note</title>
        <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>
        </note>
    </section>
</section>
<!-- /certificateauthorities -->
</section>
<!-- /certificates -->

<section id="ssl">
<title>Secure Sockets Layer (SSL)</title>
<p>The Secure Sockets Layer protocol is a protocol layer which may be
placed between a reliable connection-oriented network layer protocol
(e.g. TCP/IP) and the application protocol layer (e.g. HTTP). SSL provides
for secure communication between client and server by allowing mutual
authentication, the use of digital signatures for integrity, and encryption
for privacy.</p>

<p>The protocol is designed to support a range of choices for specific
algorithms used for cryptography, digests, and signatures. This allows
algorithm selection for specific servers to be made based on legal, export
or other concerns, and also enables the protocol to take advantage of new
algorithms. Choices are negotiated between client and server at the start
of establishing a protocol session.</p>

<section id="table4">
<title>Table 4: Versions of the SSL protocol</title>
    <table border="1">
    <columnspec><column width=".15"/><column width=".2"/>
     <column width=".30"/><column width=".25"/></columnspec>
    <tr><th>Version</th>
        <th>Source</th>
        <th>Description</th>
        <th>Browser Support</th></tr>
    <tr><td>SSL v2.0</td>
        <td>Vendor Standard (from Netscape Corp.) [<a href="#SSL2"
        >SSL2</a>]</td>
        <td>First SSL protocol for which implementations exists</td>
        <td>- NS Navigator 1.x/2.x<br />
        - MS IE 3.x<br />
        - Lynx/2.8+OpenSSL</td></tr>
    <tr><td>SSL v3.0</td>
        <td>Expired Internet Draft (from Netscape Corp.) [<a href="#SSL3"
        >SSL3</a>]</td>
        <td>Revisions to prevent specific security attacks, add non-RSA
        ciphers, and support for certificate chains</td>
        <td>- NS Navigator 2.x/3.x/4.x<br />
        - MS IE 3.x/4.x<br />
        - Lynx/2.8+OpenSSL</td></tr>
    <tr><td>TLS v1.0</td>
        <td>Proposed Internet Standard (from IETF) [<a href="#TLS1"
        >TLS1</a>]</td>
        <td>Revision of SSL 3.0 to update the MAC layer to HMAC, add block
        padding for block ciphers, message order standardization and more
        alert messages.</td>
        <td>- Lynx/2.8+OpenSSL</td></tr>
    </table>
</section>

<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>

<section id="session">
<title>Session Establishment</title>
    <p>The SSL session is established by following a handshake sequence
    between client and server, as shown in <a href="#figure1"
    >Figure 1</a>. This sequence may vary, depending on whether the server
    is configured to provide a server certificate or request a client
    certificate. Though cases exist where additional handshake steps
    are required for management of cipher information, this article
    summarizes one common scenario: see the SSL specification for the full
    range of possibilities.</p>

    <note><title>Note</title>
    <p>Once an SSL session has been established it may be reused, thus
    avoiding the performance penalty of repeating the many steps needed
    to start a session. For this the server assigns each SSL session a
    unique session identifier which is cached in the server and which the
    client can use on forthcoming connections to reduce the handshake
    (until the session identifer expires in the cache of the server).</p>
    </note>

    <p class="figure">
    <img src="ssl_intro_fig1.gif" alt="" width="423" height="327" /><br />
    <a id="figure1" name="figure1"><dfn>Figure 1</dfn></a>: Simplified SSL
    Handshake Sequence</p>

    <p>The elements of the handshake sequence, as used by the client and
    server, are listed below:</p>

    <ol>
    <li>Negotiate the Cipher Suite to be used during data transfer</li>
    <li>Establish and share a session key between client and server</li>
    <li>Optionally authenticate the server to the client</li>
    <li>Optionally authenticate the client to the server</li>
    </ol>

    <p>The first step, Cipher Suite Negotiation, allows the client and
    server to choose a Cipher Suite supportable by both of them. The SSL3.0
    protocol specification defines 31 Cipher Suites. A Cipher Suite is
    defined by the following components:</p>

    <ul>
    <li>Key Exchange Method</li>
    <li>Cipher for Data Transfer</li>
    <li>Message Digest for creating the Message Authentication Code (MAC)</li>
    </ul>

    <p>These three elements are described in the sections that follow.</p>
</section>

<section id="keyexchange">
<title>Key Exchange Method</title>
    <p>The key exchange method defines how the shared secret symmetric
    cryptography key used for application data transfer will be agreed
    upon by client and server. SSL 2.0 uses RSA key exchange only, while
    SSL 3.0 supports a choice of key exchange algorithms including the
    RSA key exchange when certificates are used, and Diffie-Hellman key
    exchange for exchanging keys without certificates and without prior
    communication between client and server.</p>

    <p>One variable in the choice of key exchange methods is digital
    signatures -- whether or not to use them, and if so, what kind of
    signatures to use. Signing with a private key provides assurance
    against a man-in-the-middle-attack during the information exchange
    used in generating the shared key [<a href="#AC96">AC96</a>, p516].</p>
</section>

<section id="ciphertransfer">
<title>Cipher for Data Transfer</title>
    <p>SSL uses the conventional cryptography algorithm (symmetric
    cryptography) described earlier for encrypting messages in a session.
    There are nine choices, including the choice to perform no
    encryption:</p>

    <ul>
    <li>No encryption</li>
    <li>Stream Ciphers
        <ul>
        <li>RC4 with 40-bit keys</li>
        <li>RC4 with 128-bit keys</li>
        </ul></li>
    <li>CBC Block Ciphers
        <ul><li>RC2 with 40 bit key</li>
        <li>DES with 40 bit key</li>
        <li>DES with 56 bit key</li>
        <li>Triple-DES with 168 bit key</li>
        <li>Idea (128 bit key)</li>
        <li>Fortezza (96 bit key)</li>
        </ul></li>
    </ul>

    <p>Here "CBC" refers to Cipher Block Chaining, which means that a
    portion of the previously encrypted cipher text is used in the
    encryption of the current block. "DES" refers to the Data Encryption
    Standard [<a href="#AC96">AC96</a>, ch12], which has a number of
    variants (including DES40 and 3DES_EDE). "Idea" is one of the best
    and cryptographically strongest available algorithms, and "RC2" is
    a proprietary algorithm from RSA DSI [<a href="#AC96">AC96</a>,
    ch13].</p>
</section>

<section id="digestfuntion">
<title>Digest Function</title>
    <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>
</section>

<section id="handshake">
<title>Handshake Sequence Protocol</title>
    <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="ssl_intro_fig2.gif" alt="" width="428" height="217" /><br />
    <a id="figure2" name="figure2"><dfn>Figure 2</dfn></a>: SSL Protocol Stack
    </p>

    <p>The encapsulation of SSL control protocols by the record protocol
    means that if an active session is renegotiated the control protocols
    will be transmitted securely. If there were no session before, then
    the Null cipher suite is used, which means there is no encryption and
    messages have no integrity digests until the session has been
    established.</p>
</section>

<section id="datatransfer">
<title>Data Transfer</title>
    <p>The SSL Record Protocol, shown in <a href="#figure3">Figure 3</a>,
    is used to transfer application and SSL Control data between the
    client and server, possibly fragmenting this data into smaller units,
    or combining multiple higher level protocol data messages into single
    units. It may compress, attach digest signatures, and encrypt these
    units before transmitting them using the underlying reliable transport
    protocol (Note: currently all major SSL implementations lack support
    for compression).</p>

    <p class="figure">
    <img src="ssl_intro_fig3.gif" alt="" width="423" height="323" /><br />
    <a id="figure3" name="figure3"><dfn>Figure 3</dfn></a>: SSL Record Protocol
    </p>
</section>

<section id="securehttp">
<title>Securing HTTP Communication</title>
    <p>One common use of SSL is to secure Web HTTP communication between
    a browser and a webserver. This case does not preclude the use of
    non-secured HTTP. The secure version is mainly plain HTTP over SSL
    (named HTTPS), but with one major difference: it uses the URL scheme
    <code>https</code> rather than <code>http</code> and a different
    server port (by default 443). This mainly is what <module
    >mod_ssl</module> provides to you for the Apache webserver...</p>
</section>
</section>
<!-- /ssl -->

<section id="references">
<title>References</title>
<dl>
<dt><a id="AC96" name="AC96">[AC96]</a></dt>
<dd>Bruce Schneier, <q>Applied Cryptography</q>, 2nd Edition, Wiley,
1996. See <a href="http://www.counterpane.com/"
>http://www.counterpane.com/</a> for various other materials by Bruce
Schneier.</dd>

<dt><a id="X208" name="X208">[X208]</a></dt>
<dd>ITU-T Recommendation X.208, <q>Specification of Abstract Syntax Notation
One (ASN.1)</q>, 1988. See for instance <a
href="http://www.itu.int/rec/recommendation.asp?type=items&amp;lang=e&amp;parent=T-REC-X.208-198811-I"
>http://www.itu.int/rec/recommendation.asp?type=items&amp;lang=e&amp;parent=T-REC-X.208-198811-I</a>.
</dd>

<dt><a id="X509" name="X509">[X509]</a></dt>
<dd>ITU-T Recommendation X.509, <q>The Directory - Authentication
Framework</q>. See for instance <a
href="http://www.itu.int/rec/recommendation.asp?type=folders&amp;lang=e&amp;parent=T-REC-X.509"
>http://www.itu.int/rec/recommendation.asp?type=folders&amp;lang=e&amp;parent=T-REC-X.509</a>.
</dd>

<dt><a id="PKCS" name="PKCS">[PKCS]</a></dt>
<dd><q>Public Key Cryptography Standards (PKCS)</q>, 
RSA Laboratories Technical Notes, See <a
href="http://www.rsasecurity.com/rsalabs/pkcs/"
>http://www.rsasecurity.com/rsalabs/pkcs/</a>.</dd>

<dt><a id="MIME" name="MIME">[MIME]</a></dt>
<dd>N. Freed, N. Borenstein, <q>Multipurpose Internet Mail Extensions
(MIME) Part One: Format of Internet Message Bodies</q>, RFC2045.
See for instance <a href="http://ietf.org/rfc/rfc2045.txt"
>http://ietf.org/rfc/rfc2045.txt</a>.</dd>

<dt><a id="SSL2" name="SSL2">[SSL2]</a></dt>
<dd>Kipp E.B. Hickman, <q>The SSL Protocol</q>, 1995. See <a
href="http://www.netscape.com/eng/security/SSL_2.html"
>http://www.netscape.com/eng/security/SSL_2.html</a>.</dd>

<dt><a id="SSL3" name="SSL3">[SSL3]</a></dt>
<dd>Alan O. Freier, Philip Karlton, Paul C. Kocher, <q>The SSL Protocol
Version 3.0</q>, 1996. See <a
href="http://www.netscape.com/eng/ssl3/draft302.txt"
>http://www.netscape.com/eng/ssl3/draft302.txt</a>.</dd>

<dt><a id="TLS1" name="TLS1">[TLS1]</a></dt>
<dd>Tim Dierks, Christopher Allen, <q>The TLS Protocol Version 1.0</q>,
1999. See <a href="http://ietf.org/rfc/rfc2246.txt"
>http://ietf.org/rfc/rfc2246.txt</a>.</dd>
</dl>
</section>
<!-- /references -->

</manualpage>
