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core/vm, protocol_params: implement eip-2565 modexp repricing #21607

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Nov 13, 2020
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core/vm, protocol_params: implement eip-2565 modexp repricing #21607

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4188dcb
core/vm, protocol_params: implement eip-2565 modexp repricing
holiman Sep 8, 2020
2c99023
core/vm: fix review concerns
holiman Oct 20, 2020
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80 changes: 62 additions & 18 deletions core/vm/contracts.go
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Original file line number Diff line number Diff line change
Expand Up @@ -58,7 +58,7 @@ var PrecompiledContractsByzantium = map[common.Address]PrecompiledContract{
common.BytesToAddress([]byte{2}): &sha256hash{},
common.BytesToAddress([]byte{3}): &ripemd160hash{},
common.BytesToAddress([]byte{4}): &dataCopy{},
common.BytesToAddress([]byte{5}): &bigModExp{},
common.BytesToAddress([]byte{5}): &bigModExp{eip2565: false},
common.BytesToAddress([]byte{6}): &bn256AddByzantium{},
common.BytesToAddress([]byte{7}): &bn256ScalarMulByzantium{},
common.BytesToAddress([]byte{8}): &bn256PairingByzantium{},
Expand All @@ -71,7 +71,7 @@ var PrecompiledContractsIstanbul = map[common.Address]PrecompiledContract{
common.BytesToAddress([]byte{2}): &sha256hash{},
common.BytesToAddress([]byte{3}): &ripemd160hash{},
common.BytesToAddress([]byte{4}): &dataCopy{},
common.BytesToAddress([]byte{5}): &bigModExp{},
common.BytesToAddress([]byte{5}): &bigModExp{eip2565: false},
common.BytesToAddress([]byte{6}): &bn256AddIstanbul{},
common.BytesToAddress([]byte{7}): &bn256ScalarMulIstanbul{},
common.BytesToAddress([]byte{8}): &bn256PairingIstanbul{},
Expand All @@ -85,7 +85,7 @@ var PrecompiledContractsYoloV2 = map[common.Address]PrecompiledContract{
common.BytesToAddress([]byte{2}): &sha256hash{},
common.BytesToAddress([]byte{3}): &ripemd160hash{},
common.BytesToAddress([]byte{4}): &dataCopy{},
common.BytesToAddress([]byte{5}): &bigModExp{},
common.BytesToAddress([]byte{5}): &bigModExp{eip2565: false},
common.BytesToAddress([]byte{6}): &bn256AddIstanbul{},
common.BytesToAddress([]byte{7}): &bn256ScalarMulIstanbul{},
common.BytesToAddress([]byte{8}): &bn256PairingIstanbul{},
Expand Down Expand Up @@ -222,14 +222,19 @@ func (c *dataCopy) Run(in []byte) ([]byte, error) {
}

// bigModExp implements a native big integer exponential modular operation.
type bigModExp struct{}
type bigModExp struct {
eip2565 bool
}

var (
big0 = big.NewInt(0)
big1 = big.NewInt(1)
big3 = big.NewInt(3)
big4 = big.NewInt(4)
big7 = big.NewInt(7)
big8 = big.NewInt(8)
big16 = big.NewInt(16)
big20 = big.NewInt(20)
big32 = big.NewInt(32)
big64 = big.NewInt(64)
big96 = big.NewInt(96)
Expand All @@ -239,6 +244,34 @@ var (
big199680 = big.NewInt(199680)
)

// modexpMultComplexity implements bigModexp multComplexity formula, as defined in EIP-198
//
// def mult_complexity(x):
// if x <= 64: return x ** 2
// elif x <= 1024: return x ** 2 // 4 + 96 * x - 3072
// else: return x ** 2 // 16 + 480 * x - 199680
//
// where is x is max(length_of_MODULUS, length_of_BASE)
func modexpMultComplexity(x *big.Int) *big.Int {
switch {
case x.Cmp(big64) <= 0:
x.Mul(x, x) // x ** 2
case x.Cmp(big1024) <= 0:
// (x ** 2 // 4 ) + ( 96 * x - 3072)
x = new(big.Int).Add(
new(big.Int).Div(new(big.Int).Mul(x, x), big4),
new(big.Int).Sub(new(big.Int).Mul(big96, x), big3072),
)
default:
// (x ** 2 // 16) + (480 * x - 199680)
x = new(big.Int).Add(
new(big.Int).Div(new(big.Int).Mul(x, x), big16),
new(big.Int).Sub(new(big.Int).Mul(big480, x), big199680),
)
}
return x
}

// RequiredGas returns the gas required to execute the pre-compiled contract.
func (c *bigModExp) RequiredGas(input []byte) uint64 {
var (
Expand Down Expand Up @@ -273,25 +306,36 @@ func (c *bigModExp) RequiredGas(input []byte) uint64 {
adjExpLen.Mul(big8, adjExpLen)
}
adjExpLen.Add(adjExpLen, big.NewInt(int64(msb)))
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The new EIP at least has some funky part:

def calculate_iteration_count(exponent_length, exponent):
    iteration_count = 0
    if exponent_length <= 32 and exponent == 0: iteration_count = 0
    elif exponent_length <= 32: iteration_count = exponent.bit_length() - 1
    elif exponent_length > 32: iteration_count = (8 * (exponent_length - 32)) + ((exponent & (2**256 - 1)).bit_length() - 1)
    return max(iteration_count, 1)

Namely, in the 3rd clause, it has ((exponent & (2**256 - 1)).bit_length() vs exponent.bit_length() in the second. The code however treats both cases as msb. Is this correct?

Alternatively, & (2**256 - 1) seems to mean "truncate to 32 bytes". But when can it be larger than 32 bytes? Can it? If not, I think we need to clear up the spec as this is super weird.

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I think it means truncate. I'm not sure -- that is old code..

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I hadn't seen that final version of the EIP -- however, the EIP consists of three separate changes to the previous version. That's what's implemented.

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ethereum/EIPs#2892 (comment)


// Calculate the gas cost of the operation
gas := new(big.Int).Set(math.BigMax(modLen, baseLen))
switch {
case gas.Cmp(big64) <= 0:
if c.eip2565 {
// EIP-2565 has three changes
// 1. Different multComplexity (inlined here)
// in EIP-2565 (https://eips.ethereum.org/EIPS/eip-2565):
//
// def mult_complexity(x):
// ceiling(x/8)^2
//
//where is x is max(length_of_MODULUS, length_of_BASE)
gas = gas.Add(gas, big7)
gas = gas.Div(gas, big8)
gas.Mul(gas, gas)
case gas.Cmp(big1024) <= 0:
gas = new(big.Int).Add(
new(big.Int).Div(new(big.Int).Mul(gas, gas), big4),
new(big.Int).Sub(new(big.Int).Mul(big96, gas), big3072),
)
default:
gas = new(big.Int).Add(
new(big.Int).Div(new(big.Int).Mul(gas, gas), big16),
new(big.Int).Sub(new(big.Int).Mul(big480, gas), big199680),
)

gas.Mul(gas, math.BigMax(adjExpLen, big1))
// 2. Different divisor (`GQUADDIVISOR`) (3)
gas.Div(gas, big3)
if gas.BitLen() > 64 {
return math.MaxUint64
}
// 3. Minimum price of 200 gas
if gas.Uint64() < 200 {
return 200
}
return gas.Uint64()
}
gas = modexpMultComplexity(gas)
gas.Mul(gas, math.BigMax(adjExpLen, big1))
gas.Div(gas, new(big.Int).SetUint64(params.ModExpQuadCoeffDiv))
gas.Div(gas, big20)

if gas.BitLen() > 64 {
return math.MaxUint64
Expand Down
27 changes: 26 additions & 1 deletion core/vm/contracts_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -43,7 +43,29 @@ type precompiledFailureTest struct {
Name string
}

var allPrecompiles = PrecompiledContractsYoloV2
// allPrecompiles does not map to the actual set of precompiles, as it also contains
// repriced versions of precompiles at certain slots
var allPrecompiles = map[common.Address]PrecompiledContract{
common.BytesToAddress([]byte{1}): &ecrecover{},
common.BytesToAddress([]byte{2}): &sha256hash{},
common.BytesToAddress([]byte{3}): &ripemd160hash{},
common.BytesToAddress([]byte{4}): &dataCopy{},
common.BytesToAddress([]byte{5}): &bigModExp{eip2565: false},
common.BytesToAddress([]byte{0xf5}): &bigModExp{eip2565: true},
common.BytesToAddress([]byte{6}): &bn256AddIstanbul{},
common.BytesToAddress([]byte{7}): &bn256ScalarMulIstanbul{},
common.BytesToAddress([]byte{8}): &bn256PairingIstanbul{},
common.BytesToAddress([]byte{9}): &blake2F{},
common.BytesToAddress([]byte{10}): &bls12381G1Add{},
common.BytesToAddress([]byte{11}): &bls12381G1Mul{},
common.BytesToAddress([]byte{12}): &bls12381G1MultiExp{},
common.BytesToAddress([]byte{13}): &bls12381G2Add{},
common.BytesToAddress([]byte{14}): &bls12381G2Mul{},
common.BytesToAddress([]byte{15}): &bls12381G2MultiExp{},
common.BytesToAddress([]byte{16}): &bls12381Pairing{},
common.BytesToAddress([]byte{17}): &bls12381MapG1{},
common.BytesToAddress([]byte{18}): &bls12381MapG2{},
}

// EIP-152 test vectors
var blake2FMalformedInputTests = []precompiledFailureTest{
Expand Down Expand Up @@ -213,6 +235,9 @@ func BenchmarkPrecompiledIdentity(bench *testing.B) {
func TestPrecompiledModExp(t *testing.T) { testJson("modexp", "05", t) }
func BenchmarkPrecompiledModExp(b *testing.B) { benchJson("modexp", "05", b) }

func TestPrecompiledModExpEip2565(t *testing.T) { testJson("modexp_eip2565", "f5", t) }
func BenchmarkPrecompiledModExpEip2565(b *testing.B) { benchJson("modexp_eip2565", "f5", b) }

// Tests the sample inputs from the elliptic curve addition EIP 213.
func TestPrecompiledBn256Add(t *testing.T) { testJson("bn256Add", "06", t) }
func BenchmarkPrecompiledBn256Add(b *testing.B) { benchJson("bn256Add", "06", b) }
Expand Down
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