common/math: optimized modexp (+ fuzzer)

common/math/modexp.go

@@ -0,0 +1,82 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package math
+
+import (
+	"math/big"
+	"math/bits"
+
+	"github.com/ethereum/go-ethereum/common"
+)
+
+// FastExp is semantically equivalent to x.Exp(x,y, m), but is faster for even
+// modulus.
+func FastExp(x, y, m *big.Int) *big.Int {
+	// Split m = m1 × m2 where m1 = 2ⁿ
+	n := m.TrailingZeroBits()
+	m1 := new(big.Int).Lsh(common.Big1, n)
+	mask := new(big.Int).Sub(m1, common.Big1)
+	m2 := new(big.Int).Rsh(m, n)
+
+	// We want z = x**y mod m.
+	// z1 = x**y mod m1 = (x**y mod m) mod m1 = z mod m1
+	// z2 = x**y mod m2 = (x**y mod m) mod m2 = z mod m2
+	z1 := fastExpPow2(x, y, mask)
+	z2 := new(big.Int).Exp(x, y, m2)
+
+	// Reconstruct z from z1, z2 using CRT, using algorithm from paper,
+	// which uses only a single modInverse.
+	//	p = (z1 - z2) * m2⁻¹ (mod m1)
+	//	z = z2 + p * m2
+	z := new(big.Int).Set(z2)
+
+	// Compute (z1 - z2) mod m1 [m1 == 2**n] into z1.
+	z1 = z1.And(z1, mask)
+	z2 = z2.And(z2, mask)
+	z1 = z1.Sub(z1, z2)
+	if z1.Sign() < 0 {
+		z1 = z1.Add(z1, m1)
+	}
+
+	// Reuse z2 for p = z1 * m2inv.
+	m2inv := new(big.Int).ModInverse(m2, m1)
+	z2 = z2.Mul(z1, m2inv)
+	z2 = z2.And(z2, mask)
+
+	// Reuse z1 for m2 * p.
+	z = z.Add(z, z1.Mul(z2, m2))
+	z = z.Rem(z, m)
+
+	return z
+}
+
+func fastExpPow2(x, y *big.Int, mask *big.Int) *big.Int {
+	z := big.NewInt(1)
+	if y.Sign() == 0 {
+		return z
+	}
+	p := new(big.Int).Set(x)
+	p = p.And(p, mask)
+	if p.Cmp(z) <= 0 { // p <= 1
+		return p
+	}
+	if y.Cmp(mask) > 0 {
+		y = new(big.Int).And(y, mask)
+	}
+	t := new(big.Int)
+
+	for _, b := range y.Bits() {
+		for i := 0; i < bits.UintSize; i++ {
+			if b&1 != 0 {
+				z, t = t.Mul(z, p), z
+				z = z.And(z, mask)
+			}
+			p, t = t.Mul(p, p), p
+			p = p.And(p, mask)
+			b >>= 1
+		}
+	}
+	return z
+}

core/vm/contracts.go

@@ -380,12 +380,23 @@ func (c *bigModExp) Run(input []byte) ([]byte, error) {
 		base = new(big.Int).SetBytes(getData(input, 0, baseLen))
 		exp  = new(big.Int).SetBytes(getData(input, baseLen, expLen))
 		mod  = new(big.Int).SetBytes(getData(input, baseLen+expLen, modLen))
+		v    []byte
 	)
-	if mod.BitLen() == 0 {
+	switch {
+	case mod.BitLen() == 0:
 		// Modulo 0 is undefined, return zero
 		return common.LeftPadBytes([]byte{}, int(modLen)), nil
+	case base.Cmp(common.Big1) == 0:
+		//If base == 1, then we can just return base % mod (if mod >= 1, which it is)
+		v = base.Mod(base, mod).Bytes()
+	//case mod.Bit(0) == 0:
+	//	// Modulo is even
+	//	v = math.FastExp(base, exp, mod).Bytes()
+	default:
+		// Modulo is odd
+		v = base.Exp(base, exp, mod).Bytes()
 	}
-	return common.LeftPadBytes(base.Exp(base, exp, mod).Bytes(), int(modLen)), nil
+	return common.LeftPadBytes(v, int(modLen)), nil
 }
 
 // newCurvePoint unmarshals a binary blob into a bn256 elliptic curve point,

oss-fuzz.sh

@@ -125,5 +125,7 @@ compile_fuzzer tests/fuzzers/snap  FuzzSRange fuzz_storage_range
 compile_fuzzer tests/fuzzers/snap  FuzzByteCodes fuzz_byte_codes
 compile_fuzzer tests/fuzzers/snap  FuzzTrieNodes fuzz_trie_nodes
 
+compile_fuzzer tests/fuzzers/modexp  Fuzz fuzzModexp
+
 #TODO: move this to tests/fuzzers, if possible
 compile_fuzzer crypto/blake2b  Fuzz      fuzzBlake2b

tests/fuzzers/modexp/modexp-fuzzer.go

@@ -0,0 +1,84 @@
+// Copyright 2022 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package modexp
+
+import (
+	"fmt"
+	"math/big"
+
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/common/math"
+	"github.com/ethereum/go-ethereum/core/vm"
+)
+
+// The function must return
+// 1 if the fuzzer should increase priority of the
+//    given input during subsequent fuzzing (for example, the input is lexically
+//    correct and was parsed successfully);
+// -1 if the input must not be added to corpus even if gives new coverage; and
+// 0  otherwise
+// other values are reserved for future use.
+func Fuzz(input []byte) int {
+	if len(input) <= 96 {
+		return -1
+	}
+	// Abort on too expensive inputs
+	precomp := vm.PrecompiledContractsBerlin[common.BytesToAddress([]byte{5})]
+	if gas := precomp.RequiredGas(input); gas > 40_000_000 {
+		return 0
+	}
+	var (
+		baseLen = new(big.Int).SetBytes(getData(input, 0, 32)).Uint64()
+		expLen  = new(big.Int).SetBytes(getData(input, 32, 32)).Uint64()
+		modLen  = new(big.Int).SetBytes(getData(input, 64, 32)).Uint64()
+	)
+	// Handle a special case when both the base and mod length is zero
+	if baseLen == 0 && modLen == 0 {
+		return -1
+	}
+	input = input[96:]
+	// Retrieve the operands and execute the exponentiation
+	var (
+		base = new(big.Int).SetBytes(getData(input, 0, baseLen))
+		exp  = new(big.Int).SetBytes(getData(input, baseLen, expLen))
+		mod  = new(big.Int).SetBytes(getData(input, baseLen+expLen, modLen))
+	)
+	if mod.BitLen() == 0 {
+		// Modulo 0 is undefined, return zero
+		return -1
+	}
+	var a = math.FastExp(new(big.Int).Set(base), new(big.Int).Set(exp), new(big.Int).Set(mod))
+	var b = base.Exp(base, exp, mod)
+	if a.Cmp(b) != 0 {
+		panic(fmt.Sprintf("Inequality %x != %x", a, b))
+	}
+	return 1
+}
+
+// getData returns a slice from the data based on the start and size and pads
+// up to size with zero's. This function is overflow safe.
+func getData(data []byte, start uint64, size uint64) []byte {
+	length := uint64(len(data))
+	if start > length {
+		start = length
+	}
+	end := start + size
+	if end > length {
+		end = length
+	}
+	return common.RightPadBytes(data[start:end], int(size))
+}