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Merge #13666: Always create signatures with Low R values

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e306be7429 Use 72 byte dummy signatures when watching only inputs may be used (Andrew Chow)
48b1473c89 Use 71 byte signature for DUMMY_SIGNATURE_CREATOR (Andrew Chow)
18dfea0dd0 Always create 70 byte signatures with low R values (Andrew Chow)

Pull request description:

  When creating signatures for transactions, always make one which has a 32 byte or smaller R and 32 byte or smaller S value. This results in signatures that are always less than 71 bytes (32 byte R + 32 byte S + 6 bytes DER + 1 byte sighash) with low R values. In most cases, the signature will be 71 bytes.

  Because R is not mutable in the same way that S is, a low R value can only be found by trying different nonces. RFC 6979 for deterministic nonce generation has the option to specify additional entropy, so we simply use that and add a uin32_t counter which we increment in order to try different nonces. Nonces are sill deterministically generated as the nonce used will the be the first one where the counter results in a nonce that results in a low R value. Because different nonces need to be tried, time to produce a signature does increase. On average, it takes twice as long to make a signature as two signatures need to be created, on average, to find one with a low R.

  Having a fixed size signature makes size calculations easier and also saves half a byte of transaction size, on average.

  DUMMY_SIGNATURE_CREATOR has been modified to produce 71 byte dummy signatures instead of 72 byte signatures.

Tree-SHA512: 3cd791505126ce92da7c631856a97ba0b59e87d9c132feff6e0eef1dc47768e81fbb38bfbe970371bedf9714b7f61a13a5fe9f30f962c81734092a4d19a4ef33
This commit is contained in:
Wladimir J. van der Laan
2018-08-13 11:54:07 +02:00
parent 13d51a2b61 e306be7429
commit 2115cba9c6
12 changed files with 118 additions and 62 deletions
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33
src/wallet/wallet.cpp
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@@ -1542,30 +1542,29 @@ int64_t CWalletTx::GetTxTime() const
return n ? n : nTimeReceived;
}
// Helper for producing a max-sized low-S signature (eg 72 bytes)
bool CWallet::DummySignInput(CTxIn &tx_in, const CTxOut &txout) const
// Helper for producing a max-sized low-S low-R signature (eg 71 bytes)
// or a max-sized low-S signature (e.g. 72 bytes) if use_max_sig is true
bool CWallet::DummySignInput(CTxIn &tx_in, const CTxOut &txout, bool use_max_sig) const
{
// Fill in dummy signatures for fee calculation.
const CScript& scriptPubKey = txout.scriptPubKey;
SignatureData sigdata;
if (!ProduceSignature(*this, DUMMY_SIGNATURE_CREATOR, scriptPubKey, sigdata))
{
if (!ProduceSignature(*this, use_max_sig ? DUMMY_MAXIMUM_SIGNATURE_CREATOR : DUMMY_SIGNATURE_CREATOR, scriptPubKey, sigdata)) {
return false;
} else {
UpdateInput(tx_in, sigdata);
}
UpdateInput(tx_in, sigdata);
return true;
}
// Helper for producing a bunch of max-sized low-S signatures (eg 72 bytes)
bool CWallet::DummySignTx(CMutableTransaction &txNew, const std::vector<CTxOut> &txouts) const
// Helper for producing a bunch of max-sized low-S low-R signatures (eg 71 bytes)
bool CWallet::DummySignTx(CMutableTransaction &txNew, const std::vector<CTxOut> &txouts, bool use_max_sig) const
{
// Fill in dummy signatures for fee calculation.
int nIn = 0;
for (const auto& txout : txouts)
{
if (!DummySignInput(txNew.vin[nIn], txout)) {
if (!DummySignInput(txNew.vin[nIn], txout, use_max_sig)) {
return false;
}
@@ -1574,7 +1573,7 @@ bool CWallet::DummySignTx(CMutableTransaction &txNew, const std::vector<CTxOut>
return true;
}
int64_t CalculateMaximumSignedTxSize(const CTransaction &tx, const CWallet *wallet)
int64_t CalculateMaximumSignedTxSize(const CTransaction &tx, const CWallet *wallet, bool use_max_sig)
{
std::vector<CTxOut> txouts;
// Look up the inputs. We should have already checked that this transaction
@@ -1588,14 +1587,14 @@ int64_t CalculateMaximumSignedTxSize(const CTransaction &tx, const CWallet *wall
assert(input.prevout.n < mi->second.tx->vout.size());
txouts.emplace_back(mi->second.tx->vout[input.prevout.n]);
}
return CalculateMaximumSignedTxSize(tx, wallet, txouts);
return CalculateMaximumSignedTxSize(tx, wallet, txouts, use_max_sig);
}
// txouts needs to be in the order of tx.vin
int64_t CalculateMaximumSignedTxSize(const CTransaction &tx, const CWallet *wallet, const std::vector<CTxOut>& txouts)
int64_t CalculateMaximumSignedTxSize(const CTransaction &tx, const CWallet *wallet, const std::vector<CTxOut>& txouts, bool use_max_sig)
{
CMutableTransaction txNew(tx);
if (!wallet->DummySignTx(txNew, txouts)) {
if (!wallet->DummySignTx(txNew, txouts, use_max_sig)) {
// This should never happen, because IsAllFromMe(ISMINE_SPENDABLE)
// implies that we can sign for every input.
return -1;
@@ -1603,11 +1602,11 @@ int64_t CalculateMaximumSignedTxSize(const CTransaction &tx, const CWallet *wall
return GetVirtualTransactionSize(txNew);
}
int CalculateMaximumSignedInputSize(const CTxOut& txout, const CWallet* wallet)
int CalculateMaximumSignedInputSize(const CTxOut& txout, const CWallet* wallet, bool use_max_sig)
{
CMutableTransaction txn;
txn.vin.push_back(CTxIn(COutPoint()));
if (!wallet->DummySignInput(txn.vin[0], txout)) {
if (!wallet->DummySignInput(txn.vin[0], txout, use_max_sig)) {
// This should never happen, because IsAllFromMe(ISMINE_SPENDABLE)
// implies that we can sign for every input.
return -1;
@@ -2334,7 +2333,7 @@ void CWallet::AvailableCoins(std::vector<COutput> &vCoins, bool fOnlySafe, const
bool solvable = IsSolvable(*this, pcoin->tx->vout[i].scriptPubKey);
bool spendable = ((mine & ISMINE_SPENDABLE) != ISMINE_NO) || (((mine & ISMINE_WATCH_ONLY) != ISMINE_NO) && (coinControl && coinControl->fAllowWatchOnly && solvable));
vCoins.push_back(COutput(pcoin, i, nDepth, spendable, solvable, safeTx));
vCoins.push_back(COutput(pcoin, i, nDepth, spendable, solvable, safeTx, (coinControl && coinControl->fAllowWatchOnly)));
// Checks the sum amount of all UTXO's.
if (nMinimumSumAmount != MAX_MONEY) {
@@ -2889,7 +2888,7 @@ bool CWallet::CreateTransaction(const std::vector<CRecipient>& vecSend, CTransac
txNew.vin.push_back(CTxIn(coin.outpoint,CScript()));
}
nBytes = CalculateMaximumSignedTxSize(txNew, this);
nBytes = CalculateMaximumSignedTxSize(txNew, this, coin_control.fAllowWatchOnly);
if (nBytes < 0) {
strFailReason = _("Signing transaction failed");
return false;