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Fix tree-hash cnt n^2. Asserts, comment. Squash2

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rfree2monero authored and fluffypony committed Sep 4, 2014
1 parent c054899 commit 2ef0aee81d20c002ed50d6dec4baceee1ac40b44
Showing with 48 additions and 6 deletions.
  1. +48 −6 src/crypto/tree-hash.c
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@@ -35,32 +35,74 @@
#include "hash-ops.h"
/// Quick check if this is power of two (use on unsigned types; in this case for size_t only)
bool ispowerof2_size_t(size_t x) {
return x && !(x & (x - 1));
}
/***
* 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) {
assert( count >= 3); // cases for 0,1,2 are handled elsewhere
// Round down the count size: fun(2**n)= 2**(n-1) to round down to power of two
size_t tmp = count - 1;
size_t jj = 1;
for (jj=1 ; tmp != 0 ; ++jj) {
tmp /= 2; // dividing by 2 until to get how many powers of 2 fits size_to tmp
}
size_t cnt = 1 << (jj-2); // cnt is the count, but rounded down to power of two
// printf("count=%zu cnt=%zu jj=%zu tmp=%zu \n" , count,cnt,jj,tmp);
assert( cnt > 0 ); assert( cnt >= count/2 ); assert( cnt <= count );
assert( ispowerof2_size_t( cnt ));
return cnt;
}
void tree_hash(const char (*hashes)[HASH_SIZE], size_t count, char *root_hash) {
// The blockchain block at height 202612 http://monerochain.info/block/bbd604d2ba11ba27935e006ed39c9bfdd99b76bf4a50654bc1e1e61217962698
// 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 = count - 1;
size_t cnt = tree_hash_cnt( count );
size_t max_size_t = (size_t) -1; // max allowed value of size_t
assert( cnt < max_size_t/2 ); // reasonable size to avoid any overflows. /2 is extra; Anyway should be limited much stronger by logical code
// as we have sane limits on transactions counts in blockchain rules
char (*ints)[HASH_SIZE];
for (i = 1; i < sizeof(size_t); i <<= 1) {
cnt |= cnt >> i;
}
cnt &= ~(cnt >> 1);
ints = alloca(cnt * HASH_SIZE);
size_t ints_size = cnt * HASH_SIZE;
ints = alloca(ints_size); memset( ints , 0 , ints_size); // allocate, and zero out as extra protection for using uninitialized mem
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]);
}
assert(i == count);
while (cnt > 2) {
cnt >>= 1;
for (i = 0, j = 0; j < cnt; i += 2, ++j) {
cn_fast_hash(ints[i], 64, ints[j]);
}
}
cn_fast_hash(ints[0], 64, root_hash);
}
}

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