Update transformations

    <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 digests for each

    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 

    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

    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>

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

<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

<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

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

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

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

    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

    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>

    <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

    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>

    <p>A Certificate Authority may define a policy specifying which

    distinguished field names are optional, and which are required. It

    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 

    <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

    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

    <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

        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 they keep lists of

        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

        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 

        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. 

        When examining certificates for validity, therefore, 

        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>

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

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

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>

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>

        <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>First SSL protocol for which implementations exist</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>

        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>

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