feat: ethtest.UNSAFEDeterministicPrivateKey() (#244)

## Why this should be merged

The ability to deterministically generate private keys (and
corresponding EOAs) is useful in testing.

## How this works

A `[]byte` seed is written to a Keccak state, which is rejection-sampled
to find a scalar `<` the order of `S256` to use as the private key.

## How this was tested

Unit test that demonstrates (a) determinism of generation; and (b)
correct computation of the public key as proven by ECDSA recovery using
the standard tx-sender functionality.

libevm/ethtest/devkey.go

@@ -0,0 +1,66 @@
+// Copyright 2025 the libevm authors.
+//
+// The libevm additions to go-ethereum are free software: you can redistribute
+// them and/or modify them 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 libevm additions are distributed in the hope that they 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 ethtest
+
+import (
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"math/big"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/ava-labs/libevm/crypto"
+)
+
+// UNSAFEDeterministicPrivateKey returns a new [crypto.S256] private key,
+// deterministically generated from the `seed`.
+func UNSAFEDeterministicPrivateKey(tb testing.TB, seed []byte) *ecdsa.PrivateKey {
+	tb.Helper()
+
+	curve := crypto.S256()
+	d := privateKeyScalar(tb, seed, curve)
+
+	x, y := curve.ScalarBaseMult(d.Bytes())
+	return &ecdsa.PrivateKey{
+		D: d,
+		PublicKey: ecdsa.PublicKey{
+			X:     x,
+			Y:     y,
+			Curve: curve,
+		},
+	}
+}
+
+func privateKeyScalar(tb testing.TB, seed []byte, curve elliptic.Curve) *big.Int {
+	tb.Helper()
+
+	s := crypto.NewKeccakState()
+	_, err := s.Write(seed)
+	require.NoError(tb, err, "%T.Write()", s)
+
+	buf := make([]byte, 32)
+	for {
+		_, err := s.Read(buf)
+		require.NoError(tb, err, "%T.Read()", s)
+		d := new(big.Int).SetBytes(buf)
+
+		if isZero := d.Sign() == 0; !isZero && d.Cmp(curve.Params().N) == -1 {
+			return d
+		}
+	}
+}

libevm/ethtest/devkey_test.go

@@ -0,0 +1,72 @@
+// Copyright 2025 the libevm authors.
+//
+// The libevm additions to go-ethereum are free software: you can redistribute
+// them and/or modify them 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 libevm additions are distributed in the hope that they 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 ethtest
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/ava-labs/libevm/common"
+	"github.com/ava-labs/libevm/core/types"
+	"github.com/ava-labs/libevm/crypto"
+	"github.com/ava-labs/libevm/params"
+)
+
+func TestDeterministicPrivateKey(t *testing.T) {
+	tests := []struct {
+		seed []byte
+		// Specific values are random, but we lock them in to ensure
+		// deterministic generation.
+		want common.Address
+	}{
+		{
+			seed: nil,
+			want: common.HexToAddress("0x9cce34F7aB185c7ABA1b7C8140d620B4BDA941d6"),
+		},
+		{
+			seed: []byte{0},
+			want: common.HexToAddress("0xa385D2E939787Af0B304512b2b6d56364F1722FA"),
+		},
+		{
+			seed: []byte{1},
+			want: common.HexToAddress("0x3Eea25034397B249a3eD8614BB4d0533e5b03594"),
+		},
+	}
+
+	signer := types.LatestSigner(params.MergedTestChainConfig)
+
+	for _, tt := range tests {
+		t.Run("", func(t *testing.T) {
+			key := UNSAFEDeterministicPrivateKey(t, tt.seed)
+
+			t.Run("address_from_pubkey", func(t *testing.T) {
+				got := crypto.PubkeyToAddress(key.PublicKey)
+				require.Equal(t, tt.want, got, "crypto.PubKeyToAddress(UNSAFEDeterministicPrivateKey())")
+			})
+
+			t.Run("address_via_sender_recovery", func(t *testing.T) {
+				got, err := types.Sender(
+					signer,
+					types.MustSignNewTx(key, signer, &types.LegacyTx{}),
+				)
+				require.NoError(t, err, "types.Sender(...)")
+				require.Equal(t, tt.want, got, "types.Sender(..., types.MustSignNewTx(UNSAFEDeterministicPrivateKey(), ....))")
+			})
+		})
+	}
+}