Add Better Zero Key Checks (#9022)

* add better checks

* preston's review

shared/bls/blst/secret_key.go

@@ -4,6 +4,7 @@
 package blst
 
 import (
+	"crypto/subtle"
 	"fmt"
 
 	"github.com/prysmaticlabs/prysm/shared/bls/common"
@@ -28,7 +29,7 @@ func RandKey() (common.SecretKey, error) {
 	}
 	// Defensive check, that we have not generated a secret key,
 	secKey := &bls12SecretKey{blst.KeyGen(ikm[:])}
-	if secKey.IsZero() {
+	if IsZero(secKey.Marshal()) {
 		return nil, common.ErrZeroKey
 	}
 	return secKey, nil
@@ -44,7 +45,7 @@ func SecretKeyFromBytes(privKey []byte) (common.SecretKey, error) {
 		return nil, common.ErrSecretUnmarshal
 	}
 	wrappedKey := &bls12SecretKey{p: secKey}
-	if wrappedKey.IsZero() {
+	if IsZero(privKey) {
 		return nil, common.ErrZeroKey
 	}
 	return wrappedKey, nil
@@ -56,9 +57,12 @@ func (s *bls12SecretKey) PublicKey() common.PublicKey {
 }
 
 // IsZero checks if the secret key is a zero key.
-func (s *bls12SecretKey) IsZero() bool {
-	zeroKey := new(blst.SecretKey)
-	return s.p.Equals(zeroKey)
+func IsZero(sKey []byte) bool {
+	b := byte(0)
+	for _, s := range sKey {
+		b |= s
+	}
+	return subtle.ConstantTimeByteEq(b, 0) == 1
 }
 
 // Sign a message using a secret key - in a beacon/validator client.
@@ -80,9 +84,5 @@ func (s *bls12SecretKey) Sign(msg []byte) common.Signature {
 // Marshal a secret key into a LittleEndian byte slice.
 func (s *bls12SecretKey) Marshal() []byte {
 	keyBytes := s.p.Serialize()
-	if len(keyBytes) < params.BeaconConfig().BLSSecretKeyLength {
-		emptyBytes := make([]byte, params.BeaconConfig().BLSSecretKeyLength-len(keyBytes))
-		keyBytes = append(emptyBytes, keyBytes...)
-	}
 	return keyBytes
 }

shared/bls/blst/secret_key_test.go

@@ -5,6 +5,7 @@ package blst_test
 
 import (
 	"bytes"
+	"crypto/rand"
 	"errors"
 	"testing"
 
@@ -85,3 +86,14 @@ func TestSerialize(t *testing.T) {
 	_, err = blst.SecretKeyFromBytes(b)
 	assert.NoError(t, err)
 }
+
+func TestZeroKey(t *testing.T) {
+	// Is Zero
+	zKey := [32]byte{}
+	assert.Equal(t, true, blst.IsZero(zKey[:]))
+
+	// Is Not Zero
+	_, err := rand.Read(zKey[:])
+	assert.NoError(t, err)
+	assert.Equal(t, false, blst.IsZero(zKey[:]))
+}

shared/bls/common/interface.go

@@ -10,7 +10,6 @@ type SecretKey interface {
 	PublicKey() PublicKey
 	Sign(msg []byte) Signature
 	Marshal() []byte
-	IsZero() bool
 }
 
 // PublicKey represents a BLS public key.