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+/*
+ *  OpenVPN -- An application to securely tunnel IP networks
+ *             over a single TCP/UDP port, with support for SSL/TLS-based
+ *             session authentication and key exchange,
+ *             packet encryption, packet authentication, and
+ *             packet compression.
+ *
+ *  Copyright (C) 2010-2018 Fox Crypto B.V. <openvpn@fox-it.com>
+ *
+ *
+ *  This program is free software; you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License version 2
+ *  as published by the Free Software Foundation.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License along
+ *  with this program; if not, write to the Free Software Foundation, Inc.,
+ *  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+/**
+ * @file
+ * Key generation documentation file.
+ */
+
+/**
+ * @page key_generation Data channel %key generation
+ *
+ * This section describes how OpenVPN peers generate and exchange %key
+ * material necessary for the security operations performed on data
+ * channel packets.
+ *
+ * The %key generation and exchange process between OpenVPN client and
+ * server occurs every time data channel security parameters are
+ * negotiated, for example during the initial setup of a VPN tunnel or
+ * when the active security parameters expire.  In source code terms, this
+ * is when a new key_state structure is initialized.
+ *
+ * @section key_generation_method Key methods
+ *
+ * OpenVPN supports two different ways of generating and exchanging %key
+ * material between client and server.  These are known as %key method 1
+ * and %key method 2.  %Key method 2 is the recommended method. Both are
+ * explained below.
+ *
+ * @subsection key_generation_method_1 Key method 1
+ *
+ * -# Each host generates its own random material.
+ * -# Each host uses its locally generated random material as %key data
+ *    for encrypting and signing packets sent to the remote peer.
+ * -# Each host then sends its random material to the remote peer, so that
+ *    the remote peer can use that %key data for authenticating and
+ *    decrypting received packets.
+ *
+ * @subsection key_generation_method_2 Key method 2
+ *
+ * -# The client generates random material in the following amounts:
+ *    - Pre-master secret: 48 bytes
+ *    - Client's PRF seed for master secret: 32 bytes
+ *    - Client's PRF seed for %key expansion: 32 bytes
+ * -# The client sends its share of random material to the server.
+ * -# The server generates random material in the following amounts:
+ *    - Server's PRF seed for master secret: 32 bytes
+ *    - Server's PRF seed for %key expansion: 32 bytes
+ * -# The server computes the %key expansion using its own and the
+ *    client's random material.
+ * -# The server sends its share of random material to the client.
+ * -# The client computes the %key expansion using its own and the
+ *    server's random material.
+ *
+ * %Key method 2 %key expansion is performed by the \c
+ * generate_key_expansion() function.  Please refer to its source code for
+ * details of the %key expansion process.
+ *
+ * @subsection key_generation_random Source of random material
+ *
+ * OpenVPN uses the either the OpenSSL library or the mbed TLS library as its
+ * source of random material.
+ *
+ * In OpenSSL, the \c RAND_bytes() function is called
+ * to supply cryptographically strong pseudo-random data.  The following links
+ * contain more information on this subject:
+ * - For OpenSSL's \c RAND_bytes() function:
+ *   http://www.openssl.org/docs/crypto/RAND_bytes.html
+ * - For OpenSSL's pseudo-random number generating system:
+ *   http://www.openssl.org/docs/crypto/rand.html
+ * - For OpenSSL's support for external crypto modules:
+ *   http://www.openssl.org/docs/crypto/engine.html
+ *
+ * In mbed TLS, the Havege random number generator is used. For details, see
+ * the mbed TLS documentation.
+ *
+ * @section key_generation_exchange Key exchange:
+ *
+ * The %key exchange process is initiated by the OpenVPN process running
+ * in client mode.  After the initial three-way handshake has successfully
+ * completed, the client sends its share of random material to the server,
+ * after which the server responds with its part.  This process is
+ * depicted below:
+ *
+@verbatim
+  Client           Client                           Server          Server
+  State            Action                           Action          State
+----------  --------------------            --------------------  ----------
+
+             ... waiting until three-way handshake complete ...
+S_START                                                              S_START
+            key_method_?_write()
+            send to server  --> --> --> -->  receive from client
+S_SENT_KEY                                   key_method_?_read()
+                                                                   S_GOT_KEY
+                                            key_method_?_write()
+            receive from server  <-- <-- <-- <--  send to client
+            key_method_?_read()                                   S_SENT_KEY
+S_GOT_KEY
+          ... waiting until control channel fully synchronized ...
+S_ACTIVE                                                            S_ACTIVE
+@endverbatim
+ *
+ * For more information about the client and server state values, see the
+ * \link control_processor Control Channel Processor module\endlink.
+ *
+ * Depending on which %key method is used, the \c ? in the function names
+ * of the diagram above is a \c 1 or a \c 2.  For example, if %key method
+ * 2 is used, that %key exchange would be started by the client calling \c
+ * key_method_2_write().  These functions are called from the \link
+ * control_processor Control Channel Processor module's\endlink \c
+ * tls_process() function and control the %key generation and exchange
+ * process as follows:
+ * - %Key method 1 has been removed in OpenVPN 2.5
+ * - %Key method 2:
+ *   - \c key_method_2_write(): generate random material locally, and if
+ *     in server mode generate %key expansion.
+ *   - \c key_method_2_read(): read random material received from remote
+ *     peer, and if in client mode generate %key expansion.
+ *
+ * @subsection key_generation_encapsulation Transmission of key material
+ *
+ * The OpenVPN client and server communicate with each other through their
+ * control channel.  This means that all of the data transmitted over the
+ * network, such as random material for %key generation, is encapsulated
+ * in a TLS layer.  For more details, see the \link control_tls Control
+ * Channel TLS module\endlink documentation.
+ */