Add ProtecingKeymanager interface and calls (#4982)

* Add ProtectedKeymanager interface and calls

* Rename interface

Co-authored-by: Raul Jordan <raul@prysmaticlabs.com>

validator/keymanager/BUILD.bazel

@@ -19,6 +19,7 @@ go_library(
         "//shared/bytesutil:go_default_library",
         "//shared/interop:go_default_library",
         "//validator/accounts:go_default_library",
+        "@com_github_prysmaticlabs_ethereumapis//eth/v1alpha1:go_default_library",
         "@com_github_sirupsen_logrus//:go_default_library",
         "@com_github_wealdtech_go_eth2_wallet//:go_default_library",
         "@com_github_wealdtech_go_eth2_wallet_store_filesystem//:go_default_library",

validator/keymanager/keymanager.go

@@ -3,6 +3,7 @@ package keymanager
 import (
 	"errors"
 
+	ethpb "github.com/prysmaticlabs/ethereumapis/eth/v1alpha1"
 	"github.com/prysmaticlabs/prysm/shared/bls"
 )
 
@@ -12,6 +13,9 @@ var ErrNoSuchKey = errors.New("no such key")
 // ErrCannotSign is returned whenever a signing attempt fails.
 var ErrCannotSign = errors.New("cannot sign")
 
+// ErrCouldSlash is returned whenever a signing attempt is refused due to a potential slashing event.
+var ErrCouldSlash = errors.New("could result in a slashing event")
+
 // KeyManager controls access to private keys by the validator.
 type KeyManager interface {
 	// FetchValidatingKeys fetches the list of public keys that should be used to validate with.
@@ -19,3 +23,12 @@ type KeyManager interface {
 	// Sign signs a message for the validator to broadcast.
 	Sign(pubKey [48]byte, root [32]byte, domain uint64) (*bls.Signature, error)
 }
+
+// ProtectingKeyManager provides access to a keymanager that protects its clients from slashing events.
+type ProtectingKeyManager interface {
+	// SignProposal signs a block proposal for the validator to broadcast.
+	SignProposal(pubKey [48]byte, domain uint64, data *ethpb.BeaconBlock) (*bls.Signature, error)
+
+	// SignAttestation signs an attestation for the validator to broadcast.
+	SignAttestation(pubKey [48]byte, domain uint64, data *ethpb.AttestationData) (*bls.Signature, error)
+}