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authorGregory Maxwell <greg@xiph.org>2013-02-15 15:27:57 -0800
committerGregory Maxwell <greg@xiph.org>2013-02-18 14:13:39 -0800
commit907a2aa4c78833ce93455567ae10ff2f506e752e (patch)
treeac144e428ed415231c36e6580c2cf9b69d1bd821 /src/wallet.cpp
parent2f0fa79db290d5139c27409055b2035099afa6fd (diff)
Internal RNG for approximateBestSubset to prevent degenerate behavior.
This fixes test_bitcoin failures on openbsd reported by dhill on IRC. On some systems rand() is a simple LCG over 2^31 and so it produces an even-odd sequence. ApproximateBestSubset was only using the least significant bit and so every run of the iterative solver would be the same for some inputs, resulting in some pretty dumb decisions. Using something other than the least significant bit would paper over the issue but who knows what other way a system's rand() might get us here. Instead we use an internal RNG with a period of something like 2^60 which is well behaved. This also makes it possible to make the selection deterministic for the tests, if we wanted to implement that.
Diffstat (limited to 'src/wallet.cpp')
-rw-r--r--src/wallet.cpp10
1 files changed, 9 insertions, 1 deletions
diff --git a/src/wallet.cpp b/src/wallet.cpp
index 2317ac31ac..067473d087 100644
--- a/src/wallet.cpp
+++ b/src/wallet.cpp
@@ -973,6 +973,8 @@ static void ApproximateBestSubset(vector<pair<int64, pair<const CWalletTx*,unsig
vfBest.assign(vValue.size(), true);
nBest = nTotalLower;
+ seed_insecure_rand();
+
for (int nRep = 0; nRep < iterations && nBest != nTargetValue; nRep++)
{
vfIncluded.assign(vValue.size(), false);
@@ -982,7 +984,13 @@ static void ApproximateBestSubset(vector<pair<int64, pair<const CWalletTx*,unsig
{
for (unsigned int i = 0; i < vValue.size(); i++)
{
- if (nPass == 0 ? rand() % 2 : !vfIncluded[i])
+ //The solver here uses a randomized algorithm,
+ //the randomness serves no real security purpose but is just
+ //needed to prevent degenerate behavior and it is important
+ //that the rng fast. We do not use a constant random sequence,
+ //because there may be some privacy improvement by making
+ //the selection random.
+ if (nPass == 0 ? insecure_rand()&1 : !vfIncluded[i])
{
nTotal += vValue[i].first;
vfIncluded[i] = true;