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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) 2016 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 (see the file COPYING included with this
* distribution); if not, write to the Free Software Foundation, Inc.,
* 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
*/
/**
* @defgroup tls_crypt Control channel encryption (--tls-crypt)
* @ingroup control_tls
* @{
*
* @par
* Control channel encryption uses a pre-shared static key (like the --tls-auth
* key) to encrypt control channel packets.
*
* @par
* Encrypting control channel packets has three main advantages:
* - It provides more privacy by hiding the certificate used for the TLS
* connection.
* - It is harder to identify OpenVPN traffic as such.
* - It provides "poor-man's" post-quantum security, against attackers who
* will never know the pre-shared key (i.e. no forward secrecy).
*
* @par Specification
* Control channel encryption is based on the SIV construction [0], to achieve
* nonce misuse-resistant authenticated encryption:
*
* @par
* \code{.unparsed}
* msg = control channel plaintext
* header = opcode (1 byte) || session_id (8 bytes) || packet_id (8 bytes)
* Ka = authentication key (256 bits)
* Ke = encryption key (256 bits)
* (Ka and Ke are pre-shared keys, like with --tls-auth)
*
* auth_tag = HMAC-SHA256(Ka, header || msg)
* IV = 128 most-significant bits of auth_tag
* ciph = AES256-CTR(Ke, IV, msg)
*
* output = Header || Tag || Ciph
* \endcode
*
* @par
* This boils down to the following on-the-wire packet format:
*
* @par
* \code{.unparsed}
* - opcode - || - session_id - || - packet_id - || auth_tag || * payload *
* \endcode
*
* @par
* Where
* <tt>- XXX -</tt> means authenticated, and
* <tt>* XXX *</tt> means authenticated and encrypted.
*/
#ifndef TLSCRYPT_H
#define TLSCRYPT_H
#include "buffer.h"
#include "crypto.h"
#include "session_id.h"
#define TLS_CRYPT_TAG_SIZE (256/8)
#define TLS_CRYPT_PID_SIZE (sizeof (packet_id_type) + sizeof (net_time_t))
#define TLS_CRYPT_BLOCK_SIZE (128/8)
#define TLS_CRYPT_OFF_PID (1 + SID_SIZE)
#define TLS_CRYPT_OFF_TAG (TLS_CRYPT_OFF_PID + TLS_CRYPT_PID_SIZE)
#define TLS_CRYPT_OFF_CT (TLS_CRYPT_OFF_TAG + TLS_CRYPT_TAG_SIZE)
/**
* Initialize a key_ctx_bi structure for use with --tls-crypt.
*
* @param key The key context to initialize
* @param key_file The file to read the key from (or the inline tag to
* indicate and inline key).
* @param key_inline Array containing (zero-terminated) inline key, or NULL
* if not used.
* @param tls_server Must be set to true is this is a TLS server instance.
*/
void tls_crypt_init_key (struct key_ctx_bi *key, const char *key_file,
const char *key_inline, bool tls_server);
/**
* Returns the maximum overhead (in bytes) added to the destination buffer by
* tls_crypt_wrap().
*/
int tls_crypt_buf_overhead(void);
/**
* Adjust frame parameters for --tls-crypt overhead.
*/
void tls_crypt_adjust_frame_parameters(struct frame *frame);
/**
* Wrap a control channel packet (both authenticates and encrypts the data).
*
* @param src Data to authenticate and encrypt.
* @param dst Any data present in this buffer is first authenticated, then
* the wrapped packet id and data from the src buffer are appended.
* Must have at least tls_crypt_buf_overhead()+BLEN(src) headroom.
* @param opt The crypto state for this --tls-crypt instance.
*
* @returns true iff wrapping succeeded.
*/
bool tls_crypt_wrap (const struct buffer *src, struct buffer *dst,
struct crypto_options *opt);
/**
* Unwrap a control channel packet (decrypts, authenticates and performs
* replay checks).
*
* @param src Data to decrypt and authenticate.
* @param dst Returns the decrypted data, if unwrapping was successful.
* @param opt The crypto state for this --tls-crypt instance.
*
* @returns true iff unwrapping succeeded (data authenticated correctly and was
* no replay).
*/
bool tls_crypt_unwrap (const struct buffer *src, struct buffer *dst,
struct crypto_options *opt);
/** @} */
#endif /* TLSCRYPT_H */
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