/*
* rdbx.c
*
* a replay database with extended range, using a rollover counter
*
* David A. McGrew
* Cisco Systems, Inc.
*/
/*
*
* Copyright (c) 2001-2005, Cisco Systems, Inc.
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
*
* Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
*
* Redistributions in binary form must reproduce the above
* copyright notice, this list of conditions and the following
* disclaimer in the documentation and/or other materials provided
* with the distribution.
*
* Neither the name of the Cisco Systems, Inc. nor the names of its
* contributors may be used to endorse or promote products derived
* from this software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
* FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
* COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT,
* INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
* (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
* SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
* STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
* OF THE POSSIBILITY OF SUCH DAMAGE.
*
*/
#include "rdbx.h"
#define rdbx_high_bit_in_bitmask 127
/*
* from draft-ietf-avt-srtp-00.txt:
*
* A receiver reconstructs the index i of a packet with sequence
* number s using the estimate
*
* i = 65,536 * t + s,
*
* where t is chosen from the set { r-1, r, r+1 } such that i is
* closest to the value 65,536 * r + s_l. If the value r+1 is used,
* then the rollover counter r in the cryptographic context is
* incremented by one (if the packet containing s is authentic).
*/
/*
* rdbx implementation notes
*
* A xtd_seq_num_t is essentially a sequence number for which some of
* the data on the wire are implicit. It logically consists of a
* rollover counter and a sequence number; the sequence number is the
* explicit part, and the rollover counter is the implicit part.
*
* Upon receiving a sequence_number (e.g. in a newly received SRTP
* packet), the complete xtd_seq_num_t can be estimated by using a
* local xtd_seq_num_t as a basis. This is done using the function
* index_guess(&local, &guess, seq_from_packet). This function
* returns the difference of the guess and the local value. The local
* xtd_seq_num_t can be moved forward to the guess using the function
* index_advance(&guess, delta), where delta is the difference.
*
*
* A rdbx_t consists of a xtd_seq_num_t and a bitmask. The index is highest
* sequence number that has been received, and the bitmask indicates
* which of the recent indicies have been received as well. The
* highest bit in the bitmask corresponds to the index in the bitmask.
*/
void
index_init(xtd_seq_num_t *pi) {
#ifdef NO_64BIT_MATH
*pi = make64(0,0);
#else
*pi = 0;
#endif
}
void
index_advance(xtd_seq_num_t *pi, sequence_number_t s) {
#ifdef NO_64BIT_MATH
/* a > ~b means a+b will generate a carry */
/* s is uint16 here */
*pi = make64(high32(*pi) + (s > ~low32(*pi) ? 1 : 0),low32(*pi) + s);
#else
*pi += s;
#endif
}
/*
* index_guess(local, guess, s)
*
* given a xtd_seq_num_t local (which represents the last
* known-to-be-good received xtd_seq_num_t) and a sequence number s
* (from a newly arrived packet), sets the contents of *guess to
* contain the best guess of the packet index to which s corresponds,
* and returns the difference between *guess and *local
*
* nota bene - the output is a signed integer, DON'T cast it to a
* unsigned integer!
*/
int
index_guess(const xtd_seq_num_t *local,
xtd_seq_num_t *guess,
sequence_number_t s) {
#ifdef NO_64BIT_MATH
uint32_t local_roc = ((high32(*local) << 16) |
(low32(*local) >> 16));
uint16_t local_seq = (uint16_t) (low32(*local));
#else
uint32_t local_roc = (uint32_t)(*local >> 16);
uint16_t local_seq = (uint16_t) *local;
#endif
#ifdef NO_64BIT_MATH
uint32_t guess_roc = ((high32(*guess) << 16) |
(low32(*guess) >> 16));
uint16_t guess_seq = (uint16_t) (low32(*guess));
#else
uint32_t guess_roc = (uint32_t)(*guess >> 16);
uint16_t guess_seq = (uint16_t) *guess;
#endif
int difference;
if (local_seq < seq_num_median) {
if (s - local_seq > seq_num_median) {
guess_roc = local_roc - 1;
difference = seq_num_max - s + local_seq;
} else {
guess_roc = local_roc;
difference = s - local_seq;
}
} else {
if (local_seq - seq_num_median > s) {
guess_roc = local_roc+1;
difference = seq_num_max - local_seq + s;
} else {
difference = s - local_seq;
guess_roc = local_roc;
}
}
guess_seq = s;
/* Note: guess_roc is 32 bits, so this generates a 48-bit result! */
#ifdef NO_64BIT_MATH
*guess = make64(guess_roc >> 16,
(guess_roc << 16) | guess_seq);
#else
*guess = (((uint64_t) guess_roc) << 16) | guess_seq;
#endif
return difference;
}
/*
* rdbx
*
*/
/*
* rdbx_init(&r) initalizes the rdbx_t pointed to by r
*/
err_status_t
rdbx_init(rdbx_t *rdbx) {
v128_set_to_zero(&rdbx->bitmask);
index_init(&rdbx->index);
return err_status_ok;
}
/*
* rdbx_check(&r, delta) checks to see if the xtd_seq_num_t
* which is at rdbx->index + delta is in the rdb
*/
err_status_t
rdbx_check(const rdbx_t *rdbx, int delta) {
if (delta > 0) { /* if delta is positive, it's good */
return err_status_ok;
} else if (rdbx_high_bit_in_bitmask + delta < 0) {
/* if delta is lower than the bitmask, it's bad */
return err_status_replay_old;
} else if (v128_get_bit(&rdbx->bitmask,
rdbx_high_bit_in_bitmask + delta) == 1) {
/* delta is within the window, so check the bitmask */
return err_status_replay_fail;
}
/* otherwise, the index is okay */
return err_status_ok;
}
/*
* rdbx_add_index adds the xtd_seq_num_t at rdbx->window_start + d to
* replay_db (and does *not* check if that xtd_seq_num_t appears in db)
*
* this function should be called only after replay_check has
* indicated that the index does not appear in the rdbx, e.g., a mutex
* should protect the rdbx between these calls if need be
*/
err_status_t
rdbx_add_index(rdbx_t *rdbx, int delta) {
if (delta > 0) {
/* shift forward by delta */
index_advance(&rdbx->index, (sequence_number_t)delta);
v128_left_shift(&rdbx->bitmask, delta);
v128_set_bit(&rdbx->bitmask, 127);
} else {
/* delta is in window, so flip bit in bitmask */
v128_set_bit(&rdbx->bitmask, -delta);
}
/* note that we need not consider the case that delta == 0 */
return err_status_ok;
}
/*
* rdbx_estimate_index(rdbx, guess, s)
*
* given an rdbx and a sequence number s (from a newly arrived packet),
* sets the contents of *guess to contain the best guess of the packet
* index to which s corresponds, and returns the difference between
* *guess and the locally stored synch info
*/
int
rdbx_estimate_index(const rdbx_t *rdbx,
xtd_seq_num_t *guess,
sequence_number_t s) {
/*
* if the sequence number and rollover counter in the rdbx are
* non-zero, then use the index_guess(...) function, otherwise, just
* set the rollover counter to zero (since the index_guess(...)
* function might incorrectly guess that the rollover counter is
* 0xffffffff)
*/
#ifdef NO_64BIT_MATH
/* seq_num_median = 0x8000 */
if (high32(rdbx->index) > 0 ||
low32(rdbx->index) > seq_num_median)
#else
if (rdbx->index > seq_num_median)
#endif
return index_guess(&rdbx->index, guess, s);
#ifdef NO_64BIT_MATH
*guess = make64(0,(uint32_t) s);
#else
*guess = s;
#endif
#ifdef NO_64BIT_MATH
return s - (uint16_t) low32(rdbx->index);
#else
return s - (uint16_t) rdbx->index;
#endif
}