path: root/src/crypto/tree-hash.c

// Copyright (c) 2014-2019, The Monero Project
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// Parts of this file are originally copyright (c) 2012-2013 The Cryptonote developers

#include <assert.h>
#include <stddef.h>
#include <stdlib.h>
#include <string.h>

#include "hash-ops.h"

/*** 
* Round to power of two, for count>=3 and for count being not too large (as reasonable for tree hash calculations)
*/
size_t tree_hash_cnt(size_t count) {
	// This algo has some bad history but all we are doing is 1 << floor(log2(count))
	// There are _many_ ways to do log2, for some reason the one selected was the most obscure one,
	// and fixing it made it even more obscure.
	//
	// Iterative method implemented below aims for clarity over speed, if performance is needed
	// then my advice is to use the BSR instruction on x86
	//
	// All the paranoid asserts have been removed since it is trivial to mathematically prove that
	// the return will always be a power of 2.
	// Problem space has been defined as 3 <= count <= 2^28. Of course quarter of a billion transactions
	// is not a sane upper limit for a block, so there will be tighter limits in other parts of the code

	assert( count >= 3 ); // cases for 0,1,2 are handled elsewhere
	assert( count <= 0x10000000 ); // sanity limit to 2^28, MSB=1 will cause an inf loop

	size_t pow = 2;
	while(pow < count) pow <<= 1;
	return pow >> 1;
}

void tree_hash(const char (*hashes)[HASH_SIZE], size_t count, char *root_hash) {
// The blockchain block at height 202612 https://moneroblocks.info/block/202612
// contained 514 transactions, that triggered bad calculation of variable "cnt" in the original version of this function
// as from CryptoNote code.
//
// This bug applies to all CN altcoins.
//
// Mathematical bug here was first published on 14:45:34 (GMT+2) 2014-09-04 by Rafal Freeman <rfree>
// https://github.com/rfree2monero/bitmonero/commit/b417abfb7a297d09f1bbb6de29030f8de9952ac8
// and soon also applied to CryptoNote (15:10 GMT+2), and BoolBerry used not fully correct work around:
// the work around of sizeof(size_t)*8 or <<3 as used before in 2 coins and in BBL later was blocking
// exploitation on normal platforms, how ever we strongly recommend the following fix because it removes
// mistake in mathematical formula.

  assert(count > 0);
  if (count == 1) {
    memcpy(root_hash, hashes, HASH_SIZE);
  } else if (count == 2) {
    cn_fast_hash(hashes, 2 * HASH_SIZE, root_hash);
  } else {
    size_t i, j;

    size_t cnt = tree_hash_cnt( count );

    char *ints = calloc(cnt, HASH_SIZE);  // zero out as extra protection for using uninitialized mem
    assert(ints);

    memcpy(ints, hashes, (2 * cnt - count) * HASH_SIZE);

    for (i = 2 * cnt - count, j = 2 * cnt - count; j < cnt; i += 2, ++j) {
      cn_fast_hash(hashes[i], 64, ints + j * HASH_SIZE);
    }
    assert(i == count);

    while (cnt > 2) {
      cnt >>= 1;
      for (i = 0, j = 0; j < cnt; i += 2, ++j) {
        cn_fast_hash(ints + i * HASH_SIZE, 64, ints + j * HASH_SIZE);
      }
    }

    cn_fast_hash(ints, 64, root_hash);
    free(ints);
  }
}