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diff --git a/third_party/srtp/crypto/replay/rdbx.c b/third_party/srtp/crypto/replay/rdbx.c
new file mode 100644
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+++ b/third_party/srtp/crypto/replay/rdbx.c
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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"
+
+#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
+}