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<chapter>
  <header>
    <copyright>
      <year>2003</year><year>2015</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
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      Licensed under the Apache License, Version 2.0 (the "License");
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    <title>TLS and its Predecessor, SSL</title>
    <prepared></prepared>
    <responsible></responsible>
    <docno></docno>
    <approved></approved>
    <checked></checked>
    <date></date>
    <rev></rev>
    <file>ssl_protocol.xml</file>
  </header>
  
  <p>The Erlang SSL application implements the SSL/TLS protocol
  for the currently supported versions,  see the
  <seealso marker="ssl">ssl(3)</seealso> manual page.
  </p>

  <p>By default SSL/TLS is run over the TCP/IP protocol even
  though you can plug in any other reliable transport protocol
  with the same Application Programming Interface (API) as the
  <c>gen_tcp</c> module in Kernel.</p>
  
  <p>If a client and a server wants to use an upgrade mechanism, such as
  defined by RFC 2817, to upgrade a regular TCP/IP connection to an SSL
  connection, this is supported by the Erlang SSL application API. This can be 
  useful for, for example, supporting HTTP and HTTPS on the same port and
  implementing virtual hosting.
  </p>

  <section>
    <title>Security Overview</title>
      
   <p>To achieve authentication and privacy, the client and server
    perform a TLS handshake procedure before transmitting or receiving
    any data. During the handshake, they agree on a protocol version and
    cryptographic algorithms, generate shared secrets using public
    key cryptographies, and optionally authenticate each other with
    digital certificates.</p>
  </section>
  
  <section>
    <title>Data Privacy and Integrity</title>
    
    <p>A <em>symmetric key</em> algorithm has one key only. The key is
    used for both encryption and decryption. These algorithms are fast,
    compared to public key algorithms (using two keys, one public and one
    private) and are therefore typically used for encrypting bulk
    data.
    </p>
    
    <p>The keys for the symmetric encryption are generated uniquely
    for each connection and are based on a secret negotiated
    in the TLS handshake.</p>
    
    <p>The TLS handshake protocol and data transfer is run on top of
    the TLS Record Protocol, which uses a keyed-hash Message
    Authenticity Code (MAC), or a Hash-based MAC (HMAC),
    to protect the message data
    integrity. From the TLS RFC: "A Message Authentication Code is a
    one-way hash computed from a message and some secret data. It is
    difficult to forge without knowing the secret data. Its purpose is
    to detect if the message has been altered."
    </p>
    
  </section>

   <section>
     <title>Digital Certificates</title>
     <p>A certificate is similar to a driver's license, or a
     passport. The holder of the certificate is called the
     <em>subject</em>. The certificate is signed 
     with the private key of the issuer of the certificate. A chain
     of trust is built by having the issuer in its turn being
     certified by another certificate, and so on, until you reach the
     so called root certificate, which is self-signed, that is, issued
     by itself.</p>
     
     <p>Certificates are issued by Certification Authorities (CAs) only. 
     A handful of top CAs in the world issue root certificates. You can
     examine several of these certificates by clicking
     through the menus of your web browser.
     </p>
   </section>
	   
   <section>
     <title>Peer Authentication</title>
      
     <p>Authentication of the peer is done by public key path
     validation as defined in RFC 3280. This means basically 
     the following:</p>
     <list type="bulleted">
       <item>Each certificate in the certificate chain is issued by the 
       previous one.</item>
       <item>The certificates attributes are valid.</item>
       <item>The root certificate is a trusted certificate that is present 
       in the trusted certificate database kept by the peer.</item>
     </list>
     
     <p>The server always sends a certificate chain as part of
     the TLS handshake, but the client only sends one if requested
     by the server. If the client does not have
     an appropriate certificate, it can send an "empty" certificate
     to the server.</p>
     
     <p>The client can choose to accept some path evaluation errors,
     for example, a web browser can ask the user whether to
     accept an unknown CA root certificate. The server, if it requests
     a certificate, does however not accept any path validation
     errors. It is configurable if the server is to accept
     or reject an "empty" certificate as response to
     a certificate request.</p>
   </section>
  
   <section>
     <title>TLS Sessions</title>
     
     <p>From the TLS RFC: "A TLS session is an association between a
     client and a server. Sessions are created by the handshake
     protocol. Sessions define a set of cryptographic security
     parameters, which can be shared among multiple
     connections. Sessions are used to avoid the expensive negotiation
     of new security parameters for each connection."</p>

     <p>Session data is by default kept by the SSL application in a
     memory storage, hence session data is lost at application
     restart or takeover. Users can define their own callback module
     to handle session data storage if persistent data storage is
     required. Session data is also invalidated after 24 hours
     from it was saved, for security reasons. The amount of time the 
     session data is to be saved can be configured.</p>

     <p>By default the SSL clients try to reuse an available session and 
     by default the SSL servers agree to reuse sessions when clients
     ask for it.</p>

   </section>
 </chapter>


