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policy.go
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policy.go
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// Copyright (c) 2013-2016 The btcsuite developers
// Use of this source code is governed by an ISC
// license that can be found in the LICENSE file.
package mempool
import (
"fmt"
"time"
"github.com/btcsuite/btcd/blockchain"
"github.com/btcsuite/btcd/txscript"
"github.com/btcsuite/btcd/wire"
"github.com/btcsuite/btcutil"
)
const (
// maxStandardP2SHSigOps is the maximum number of signature operations
// that are considered standard in a pay-to-script-hash script.
maxStandardP2SHSigOps = 15
// maxStandardTxCost is the max weight permitted by any transaction
// according to the current default policy.
maxStandardTxWeight = 400000
// maxStandardSigScriptSize is the maximum size allowed for a
// transaction input signature script to be considered standard. This
// value allows for a 15-of-15 CHECKMULTISIG pay-to-script-hash with
// compressed keys.
//
// The form of the overall script is: OP_0 <15 signatures> OP_PUSHDATA2
// <2 bytes len> [OP_15 <15 pubkeys> OP_15 OP_CHECKMULTISIG]
//
// For the p2sh script portion, each of the 15 compressed pubkeys are
// 33 bytes (plus one for the OP_DATA_33 opcode), and the thus it totals
// to (15*34)+3 = 513 bytes. Next, each of the 15 signatures is a max
// of 73 bytes (plus one for the OP_DATA_73 opcode). Also, there is one
// extra byte for the initial extra OP_0 push and 3 bytes for the
// OP_PUSHDATA2 needed to specify the 513 bytes for the script push.
// That brings the total to 1+(15*74)+3+513 = 1627. This value also
// adds a few extra bytes to provide a little buffer.
// (1 + 15*74 + 3) + (15*34 + 3) + 23 = 1650
maxStandardSigScriptSize = 1650
// DefaultMinRelayTxFee is the minimum fee in satoshi that is required
// for a transaction to be treated as free for relay and mining
// purposes. It is also used to help determine if a transaction is
// considered dust and as a base for calculating minimum required fees
// for larger transactions. This value is in Satoshi/1000 bytes.
DefaultMinRelayTxFee = btcutil.Amount(1000)
// maxStandardMultiSigKeys is the maximum number of public keys allowed
// in a multi-signature transaction output script for it to be
// considered standard.
maxStandardMultiSigKeys = 3
)
// calcMinRequiredTxRelayFee returns the minimum transaction fee required for a
// transaction with the passed serialized size to be accepted into the memory
// pool and relayed.
func calcMinRequiredTxRelayFee(serializedSize int64, minRelayTxFee btcutil.Amount) int64 {
// Calculate the minimum fee for a transaction to be allowed into the
// mempool and relayed by scaling the base fee (which is the minimum
// free transaction relay fee). minRelayTxFee is in Satoshi/kB so
// multiply by serializedSize (which is in bytes) and divide by 1000 to
// get minimum Satoshis.
minFee := (serializedSize * int64(minRelayTxFee)) / 1000
if minFee == 0 && minRelayTxFee > 0 {
minFee = int64(minRelayTxFee)
}
// Set the minimum fee to the maximum possible value if the calculated
// fee is not in the valid range for monetary amounts.
if minFee < 0 || minFee > btcutil.MaxSatoshi {
minFee = btcutil.MaxSatoshi
}
return minFee
}
// checkInputsStandard performs a series of checks on a transaction's inputs
// to ensure they are "standard". A standard transaction input within the
// context of this function is one whose referenced public key script is of a
// standard form and, for pay-to-script-hash, does not have more than
// maxStandardP2SHSigOps signature operations. However, it should also be noted
// that standard inputs also are those which have a clean stack after execution
// and only contain pushed data in their signature scripts. This function does
// not perform those checks because the script engine already does this more
// accurately and concisely via the txscript.ScriptVerifyCleanStack and
// txscript.ScriptVerifySigPushOnly flags.
func checkInputsStandard(tx *btcutil.Tx, utxoView *blockchain.UtxoViewpoint) error {
// NOTE: The reference implementation also does a coinbase check here,
// but coinbases have already been rejected prior to calling this
// function so no need to recheck.
for i, txIn := range tx.MsgTx().TxIn {
// It is safe to elide existence and index checks here since
// they have already been checked prior to calling this
// function.
entry := utxoView.LookupEntry(txIn.PreviousOutPoint)
originPkScript := entry.PkScript()
switch txscript.GetScriptClass(originPkScript) {
case txscript.ScriptHashTy:
numSigOps := txscript.GetPreciseSigOpCount(
txIn.SignatureScript, originPkScript, true)
if numSigOps > maxStandardP2SHSigOps {
str := fmt.Sprintf("transaction input #%d has "+
"%d signature operations which is more "+
"than the allowed max amount of %d",
i, numSigOps, maxStandardP2SHSigOps)
return txRuleError(wire.RejectNonstandard, str)
}
case txscript.NonStandardTy:
str := fmt.Sprintf("transaction input #%d has a "+
"non-standard script form", i)
return txRuleError(wire.RejectNonstandard, str)
}
}
return nil
}
// checkPkScriptStandard performs a series of checks on a transaction output
// script (public key script) to ensure it is a "standard" public key script.
// A standard public key script is one that is a recognized form, and for
// multi-signature scripts, only contains from 1 to maxStandardMultiSigKeys
// public keys.
func checkPkScriptStandard(pkScript []byte, scriptClass txscript.ScriptClass) error {
switch scriptClass {
case txscript.MultiSigTy:
numPubKeys, numSigs, err := txscript.CalcMultiSigStats(pkScript)
if err != nil {
str := fmt.Sprintf("multi-signature script parse "+
"failure: %v", err)
return txRuleError(wire.RejectNonstandard, str)
}
// A standard multi-signature public key script must contain
// from 1 to maxStandardMultiSigKeys public keys.
if numPubKeys < 1 {
str := "multi-signature script with no pubkeys"
return txRuleError(wire.RejectNonstandard, str)
}
if numPubKeys > maxStandardMultiSigKeys {
str := fmt.Sprintf("multi-signature script with %d "+
"public keys which is more than the allowed "+
"max of %d", numPubKeys, maxStandardMultiSigKeys)
return txRuleError(wire.RejectNonstandard, str)
}
// A standard multi-signature public key script must have at
// least 1 signature and no more signatures than available
// public keys.
if numSigs < 1 {
return txRuleError(wire.RejectNonstandard,
"multi-signature script with no signatures")
}
if numSigs > numPubKeys {
str := fmt.Sprintf("multi-signature script with %d "+
"signatures which is more than the available "+
"%d public keys", numSigs, numPubKeys)
return txRuleError(wire.RejectNonstandard, str)
}
case txscript.NonStandardTy:
return txRuleError(wire.RejectNonstandard,
"non-standard script form")
}
return nil
}
// isDust returns whether or not the passed transaction output amount is
// considered dust or not based on the passed minimum transaction relay fee.
// Dust is defined in terms of the minimum transaction relay fee. In
// particular, if the cost to the network to spend coins is more than 1/3 of the
// minimum transaction relay fee, it is considered dust.
func isDust(txOut *wire.TxOut, minRelayTxFee btcutil.Amount) bool {
// Unspendable outputs are considered dust.
if txscript.IsUnspendable(txOut.PkScript) {
return true
}
// The total serialized size consists of the output and the associated
// input script to redeem it. Since there is no input script
// to redeem it yet, use the minimum size of a typical input script.
//
// Pay-to-pubkey-hash bytes breakdown:
//
// Output to hash (34 bytes):
// 8 value, 1 script len, 25 script [1 OP_DUP, 1 OP_HASH_160,
// 1 OP_DATA_20, 20 hash, 1 OP_EQUALVERIFY, 1 OP_CHECKSIG]
//
// Input with compressed pubkey (148 bytes):
// 36 prev outpoint, 1 script len, 107 script [1 OP_DATA_72, 72 sig,
// 1 OP_DATA_33, 33 compressed pubkey], 4 sequence
//
// Input with uncompressed pubkey (180 bytes):
// 36 prev outpoint, 1 script len, 139 script [1 OP_DATA_72, 72 sig,
// 1 OP_DATA_65, 65 compressed pubkey], 4 sequence
//
// Pay-to-pubkey bytes breakdown:
//
// Output to compressed pubkey (44 bytes):
// 8 value, 1 script len, 35 script [1 OP_DATA_33,
// 33 compressed pubkey, 1 OP_CHECKSIG]
//
// Output to uncompressed pubkey (76 bytes):
// 8 value, 1 script len, 67 script [1 OP_DATA_65, 65 pubkey,
// 1 OP_CHECKSIG]
//
// Input (114 bytes):
// 36 prev outpoint, 1 script len, 73 script [1 OP_DATA_72,
// 72 sig], 4 sequence
//
// Pay-to-witness-pubkey-hash bytes breakdown:
//
// Output to witness key hash (31 bytes);
// 8 value, 1 script len, 22 script [1 OP_0, 1 OP_DATA_20,
// 20 bytes hash160]
//
// Input (67 bytes as the 107 witness stack is discounted):
// 36 prev outpoint, 1 script len, 0 script (not sigScript), 107
// witness stack bytes [1 element length, 33 compressed pubkey,
// element length 72 sig], 4 sequence
//
//
// Theoretically this could examine the script type of the output script
// and use a different size for the typical input script size for
// pay-to-pubkey vs pay-to-pubkey-hash inputs per the above breakdowns,
// but the only combination which is less than the value chosen is
// a pay-to-pubkey script with a compressed pubkey, which is not very
// common.
//
// The most common scripts are pay-to-pubkey-hash, and as per the above
// breakdown, the minimum size of a p2pkh input script is 148 bytes. So
// that figure is used. If the output being spent is a witness program,
// then we apply the witness discount to the size of the signature.
//
// The segwit analogue to p2pkh is a p2wkh output. This is the smallest
// output possible using the new segwit features. The 107 bytes of
// witness data is discounted by a factor of 4, leading to a computed
// value of 67 bytes of witness data.
//
// Both cases share a 41 byte preamble required to reference the input
// being spent and the sequence number of the input.
totalSize := txOut.SerializeSize() + 41
if txscript.IsWitnessProgram(txOut.PkScript) {
totalSize += (107 / blockchain.WitnessScaleFactor)
} else {
totalSize += 107
}
// The output is considered dust if the cost to the network to spend the
// coins is more than 1/3 of the minimum free transaction relay fee.
// minFreeTxRelayFee is in Satoshi/KB, so multiply by 1000 to
// convert to bytes.
//
// Using the typical values for a pay-to-pubkey-hash transaction from
// the breakdown above and the default minimum free transaction relay
// fee of 1000, this equates to values less than 546 satoshi being
// considered dust.
//
// The following is equivalent to (value/totalSize) * (1/3) * 1000
// without needing to do floating point math.
return txOut.Value*1000/(3*int64(totalSize)) < int64(minRelayTxFee)
}
// checkTransactionStandard performs a series of checks on a transaction to
// ensure it is a "standard" transaction. A standard transaction is one that
// conforms to several additional limiting cases over what is considered a
// "sane" transaction such as having a version in the supported range, being
// finalized, conforming to more stringent size constraints, having scripts
// of recognized forms, and not containing "dust" outputs (those that are
// so small it costs more to process them than they are worth).
func checkTransactionStandard(tx *btcutil.Tx, height int32,
medianTimePast time.Time, minRelayTxFee btcutil.Amount,
maxTxVersion int32) error {
// The transaction must be a currently supported version.
msgTx := tx.MsgTx()
if msgTx.Version > maxTxVersion || msgTx.Version < 1 {
str := fmt.Sprintf("transaction version %d is not in the "+
"valid range of %d-%d", msgTx.Version, 1,
maxTxVersion)
return txRuleError(wire.RejectNonstandard, str)
}
// The transaction must be finalized to be standard and therefore
// considered for inclusion in a block.
if !blockchain.IsFinalizedTransaction(tx, height, medianTimePast) {
return txRuleError(wire.RejectNonstandard,
"transaction is not finalized")
}
// Since extremely large transactions with a lot of inputs can cost
// almost as much to process as the sender fees, limit the maximum
// size of a transaction. This also helps mitigate CPU exhaustion
// attacks.
txWeight := blockchain.GetTransactionWeight(tx)
if txWeight > maxStandardTxWeight {
str := fmt.Sprintf("weight of transaction %v is larger than max "+
"allowed weight of %v", txWeight, maxStandardTxWeight)
return txRuleError(wire.RejectNonstandard, str)
}
for i, txIn := range msgTx.TxIn {
// Each transaction input signature script must not exceed the
// maximum size allowed for a standard transaction. See
// the comment on maxStandardSigScriptSize for more details.
sigScriptLen := len(txIn.SignatureScript)
if sigScriptLen > maxStandardSigScriptSize {
str := fmt.Sprintf("transaction input %d: signature "+
"script size of %d bytes is large than max "+
"allowed size of %d bytes", i, sigScriptLen,
maxStandardSigScriptSize)
return txRuleError(wire.RejectNonstandard, str)
}
// Each transaction input signature script must only contain
// opcodes which push data onto the stack.
if !txscript.IsPushOnlyScript(txIn.SignatureScript) {
str := fmt.Sprintf("transaction input %d: signature "+
"script is not push only", i)
return txRuleError(wire.RejectNonstandard, str)
}
}
// None of the output public key scripts can be a non-standard script or
// be "dust" (except when the script is a null data script).
numNullDataOutputs := 0
for i, txOut := range msgTx.TxOut {
scriptClass := txscript.GetScriptClass(txOut.PkScript)
err := checkPkScriptStandard(txOut.PkScript, scriptClass)
if err != nil {
// Attempt to extract a reject code from the error so
// it can be retained. When not possible, fall back to
// a non standard error.
rejectCode := wire.RejectNonstandard
if rejCode, found := extractRejectCode(err); found {
rejectCode = rejCode
}
str := fmt.Sprintf("transaction output %d: %v", i, err)
return txRuleError(rejectCode, str)
}
// Accumulate the number of outputs which only carry data. For
// all other script types, ensure the output value is not
// "dust".
if scriptClass == txscript.NullDataTy {
numNullDataOutputs++
} else if isDust(txOut, minRelayTxFee) {
str := fmt.Sprintf("transaction output %d: payment "+
"of %d is dust", i, txOut.Value)
return txRuleError(wire.RejectDust, str)
}
}
// A standard transaction must not have more than one output script that
// only carries data.
if numNullDataOutputs > 1 {
str := "more than one transaction output in a nulldata script"
return txRuleError(wire.RejectNonstandard, str)
}
return nil
}
// GetTxVirtualSize computes the virtual size of a given transaction. A
// transaction's virtual size is based off its weight, creating a discount for
// any witness data it contains, proportional to the current
// blockchain.WitnessScaleFactor value.
func GetTxVirtualSize(tx *btcutil.Tx) int64 {
// vSize := (weight(tx) + 3) / 4
// := (((baseSize * 3) + totalSize) + 3) / 4
// We add 3 here as a way to compute the ceiling of the prior arithmetic
// to 4. The division by 4 creates a discount for wit witness data.
return (blockchain.GetTransactionWeight(tx) + (blockchain.WitnessScaleFactor - 1)) /
blockchain.WitnessScaleFactor
}