p2p/discover: add NodeID.Pubkey

p2p/discover/node.go

@@ -7,6 +7,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"math/big"
 	"math/rand"
 	"net"
 	"net/url"
@@ -14,6 +15,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/ethereum/go-ethereum/crypto"
 	"github.com/ethereum/go-ethereum/crypto/secp256k1"
 	"github.com/ethereum/go-ethereum/rlp"
 )
@@ -187,6 +189,19 @@ func PubkeyID(pub *ecdsa.PublicKey) NodeID {
 	return id
 }
 
+// Pubkey returns the public key represented by the node ID.
+// It returns an error if the ID is not a point on the curve.
+func (id NodeID) Pubkey() (*ecdsa.PublicKey, error) {
+	p := &ecdsa.PublicKey{Curve: crypto.S256(), X: new(big.Int), Y: new(big.Int)}
+	half := len(id) / 2
+	p.X.SetBytes(id[:half])
+	p.Y.SetBytes(id[half:])
+	if !p.Curve.IsOnCurve(p.X, p.Y) {
+		return nil, errors.New("not a point on the S256 curve")
+	}
+	return p, nil
+}
+
 // recoverNodeID computes the public key used to sign the
 // given hash from the signature.
 func recoverNodeID(hash, sig []byte) (id NodeID, err error) {

p2p/discover/node_test.go

@@ -133,6 +133,24 @@ func TestNodeID_recover(t *testing.T) {
 	if pub != recpub {
 		t.Errorf("recovered wrong pubkey:\ngot:  %v\nwant: %v", recpub, pub)
 	}
+
+	ecdsa, err := pub.Pubkey()
+	if err != nil {
+		t.Errorf("Pubkey error: %v", err)
+	}
+	if !reflect.DeepEqual(ecdsa, &prv.PublicKey) {
+		t.Errorf("Pubkey mismatch:\n  got:  %#v\n  want: %#v", ecdsa, &prv.PublicKey)
+	}
+}
+
+func TestNodeID_pubkeyBad(t *testing.T) {
+	ecdsa, err := NodeID{}.Pubkey()
+	if err == nil {
+		t.Error("expected error for zero ID")
+	}
+	if ecdsa != nil {
+		t.Error("expected nil result")
+	}
 }
 
 func TestNodeID_distcmp(t *testing.T) {