/* * rdbx.c * * a replay database with extended range, using a rollover counter * * David A. McGrew * Cisco Systems, Inc. */ /* * * Copyright (c) 2001-2006, 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, 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 }

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