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 /*

  * 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"

 /*

  * from RFC 3711:

  *

  * A receiver reconstructs the index i of a packet with sequence

  *  number SEQ using the estimate

  *

  * i = 2^16 * v + SEQ,

  *

  * where v is chosen from the set { ROC-1, ROC, ROC+1 } such that i is

  * closest to the value 2^16 * ROC + 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, ws) initializes the rdbx_t pointed to by r with window size ws

  */

 err_status_t

 rdbx_init(rdbx_t *rdbx, unsigned long ws) {

   if (ws == 0)

     return err_status_bad_param;

   if (bitvector_alloc(&rdbx->bitmask, ws) != 0)

     return err_status_alloc_fail;

   index_init(&rdbx->index);

   return err_status_ok;

 }

 /*

  *  rdbx_dealloc(&r) frees memory for the rdbx_t pointed to by r

  */

 err_status_t

 rdbx_dealloc(rdbx_t *rdbx) {

   bitvector_dealloc(&rdbx->bitmask);

   return err_status_ok;

 }

 /*

  * rdbx_set_roc(rdbx, roc) initalizes the rdbx_t at the location rdbx

  * to have the rollover counter value roc.  If that value is less than

  * the current rollover counter value, then the function returns

  * err_status_replay_old; otherwise, err_status_ok is returned.

  * 

  */

 err_status_t

 rdbx_set_roc(rdbx_t *rdbx, uint32_t roc) {

   bitvector_set_to_zero(&rdbx->bitmask);

 #ifdef NO_64BIT_MATH

   #error not yet implemented

 #else

   /* make sure that we're not moving backwards */

   if (roc < (rdbx->index >> 16))

     return err_status_replay_old;

   rdbx->index &= 0xffff;   /* retain lowest 16 bits */

   rdbx->index |= ((uint64_t)roc) << 16;  /* set ROC */

 #endif

   return err_status_ok;

 }

 /*

  * rdbx_get_packet_index(rdbx) returns the value of the packet index

  * for the rdbx_t pointed to by rdbx

  * 

  */

 xtd_seq_num_t

 rdbx_get_packet_index(const rdbx_t *rdbx) {

   return rdbx->index;   

 }

 /*

  * rdbx_get_window_size(rdbx) returns the value of the window size

  * for the rdbx_t pointed to by rdbx

  * 

  */

 unsigned long

 rdbx_get_window_size(const rdbx_t *rdbx) {

   return bitvector_get_length(&rdbx->bitmask);

 }

 /*

  * 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 ((int)(bitvector_get_length(&rdbx->bitmask) - 1) + delta < 0) {   

                          /* if delta is lower than the bitmask, it's bad */

     return err_status_replay_old; 

   } else if (bitvector_get_bit(&rdbx->bitmask, 

 			       (int)(bitvector_get_length(&rdbx->bitmask) - 1) + 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);

     bitvector_left_shift(&rdbx->bitmask, delta);

     bitvector_set_bit(&rdbx->bitmask, bitvector_get_length(&rdbx->bitmask) - 1);

   } else {

     /* delta is in window */

     bitvector_set_bit(&rdbx->bitmask, bitvector_get_length(&rdbx->bitmask) -1 + 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

 }