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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) 2016-2021 Fox Crypto B.V. <openvpn@foxcrypto.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.
*/
/**
* @defgroup tls_crypt Control channel encryption (--tls-crypt, --tls-crypt-v2)
* @ingroup control_tls
* @{
*
* Control channel encryption uses a pre-shared static key (like the --tls-auth
* key) to encrypt control channel packets.
*
* 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).
*
* --tls-crypt uses a tls-auth-style group key, where all servers and clients
* share the same group key. --tls-crypt-v2 adds support for client-specific
* keys, where all servers share the same client-key encryption key, and each
* clients receives a unique client key, both in plaintext and in encrypted
* form. When connecting to a server, the client sends the encrypted key to
* the server in the first packet (P_CONTROL_HARD_RESET_CLIENT_V3). The server
* then decrypts that key, and both parties can use the same client-specific
* key for tls-crypt packets. See doc/tls-crypt-v2.txt for more details.
*
* @par On-the-wire tls-crypt packet specification
* @parblock
* Control channel encryption is based on the SIV construction [0], to achieve
* nonce misuse-resistant authenticated encryption:
*
* \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
*
* This boils down to the following on-the-wire packet format:
*
* \code{.unparsed}
* - opcode - || - session_id - || - packet_id - || auth_tag || * payload *
* \endcode
*
* Where
* <tt>- XXX -</tt> means authenticated, and
* <tt>* XXX *</tt> means authenticated and encrypted.
*
* @endparblock
*/
#ifndef TLSCRYPT_H
#define TLSCRYPT_H
#include "base64.h"
#include "buffer.h"
#include "crypto.h"
#include "session_id.h"
#include "ssl_common.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)
#define TLS_CRYPT_V2_MAX_WKC_LEN (1024)
#define TLS_CRYPT_V2_CLIENT_KEY_LEN (2048 / 8)
#define TLS_CRYPT_V2_SERVER_KEY_LEN (sizeof(struct key))
#define TLS_CRYPT_V2_TAG_SIZE (TLS_CRYPT_TAG_SIZE)
#define TLS_CRYPT_V2_MAX_METADATA_LEN (unsigned)(TLS_CRYPT_V2_MAX_WKC_LEN \
- (TLS_CRYPT_V2_CLIENT_KEY_LEN + TLS_CRYPT_V2_TAG_SIZE \
+ sizeof(uint16_t)))
#define TLS_CRYPT_V2_MAX_B64_METADATA_LEN \
OPENVPN_BASE64_LENGTH(TLS_CRYPT_V2_MAX_METADATA_LEN - 1)
/**
* 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 key itself if
* key_inline is true.
* @param key_inline True if key_file contains an inline key, False
* otherwise.
* @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,
bool 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);
/**
* Initialize a tls-crypt-v2 server key (used to encrypt/decrypt client keys).
*
* @param key Key structure to be initialized. Must be non-NULL.
* @parem encrypt If true, initialize the key structure for encryption,
* otherwise for decryption.
* @param key_file File path of the key file to load or the key itself if
* key_inline is true.
* @param key_inline True if key_file contains an inline key, False
* otherwise.
*
*/
void tls_crypt_v2_init_server_key(struct key_ctx *key_ctx, bool encrypt,
const char *key_file, bool key_inline);
/**
* Initialize a tls-crypt-v2 client key.
*
* @param key Key structure to be initialized with the client
* key.
* @param wrapped_key_buf Returns buffer containing the wrapped key that will
* be sent to the server when connecting. Caller must
* free this buffer when no longer needed.
* @param key_file File path of the key file to load or the key itself
* if key_inline is true.
* @param key_inline True if key_file contains an inline key, False
* otherwise.
*/
void tls_crypt_v2_init_client_key(struct key_ctx_bi *key,
struct buffer *wrapped_key_buf,
const char *key_file, bool key_inline);
/**
* Extract a tls-crypt-v2 client key from a P_CONTROL_HARD_RESET_CLIENT_V3
* message, and load the key into the supplied tls wrap context.
*
* @param buf Buffer containing a received P_CONTROL_HARD_RESET_CLIENT_V3
* message.
* @param ctx tls-wrap context to be initialized with the client key.
*
* @returns true if a key was successfully extracted.
*/
bool tls_crypt_v2_extract_client_key(struct buffer *buf,
struct tls_wrap_ctx *ctx,
const struct tls_options *opt);
/**
* Generate a tls-crypt-v2 server key, and write to file.
*
* @param filename Filename of the server key file to create.
*/
void tls_crypt_v2_write_server_key_file(const char *filename);
/**
* Generate a tls-crypt-v2 client key, and write to file.
*
* @param filename Filename of the client key file to create.
* @param b64_metadata Base64 metadata to be included in the client key.
* @param key_file File path of the server key to use for wrapping the
* client key or the key itself if key_inline is true.
* @param key_inline True if key_file contains an inline key, False
* otherwise.
*/
void tls_crypt_v2_write_client_key_file(const char *filename,
const char *b64_metadata,
const char *key_file, bool key_inline);
/** @} */
#endif /* TLSCRYPT_H */
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