crypto: add in secp256k1 support (#5500)

Secp256k1 was removed in the protobuf migration, this pr adds it back in order to provide this functionality for users (band)

Closes: #5495

crypto/encoding/codec.go

@@ -5,6 +5,7 @@ import (
 
 	"github.com/tendermint/tendermint/crypto"
 	"github.com/tendermint/tendermint/crypto/ed25519"
+	"github.com/tendermint/tendermint/crypto/secp256k1"
 	"github.com/tendermint/tendermint/libs/json"
 	pc "github.com/tendermint/tendermint/proto/tendermint/crypto"
 )
@@ -12,6 +13,7 @@ import (
 func init() {
 	json.RegisterType((*pc.PublicKey)(nil), "tendermint.crypto.PublicKey")
 	json.RegisterType((*pc.PublicKey_Ed25519)(nil), "tendermint.crypto.PublicKey_Ed25519")
+	json.RegisterType((*pc.PublicKey_Secp256K1)(nil), "tendermint.crypto.PublicKey_Secp256K1")
 }
 
 // PubKeyToProto takes crypto.PubKey and transforms it to a protobuf Pubkey
@@ -24,6 +26,12 @@ func PubKeyToProto(k crypto.PubKey) (pc.PublicKey, error) {
 				Ed25519: k,
 			},
 		}
+	case secp256k1.PubKey:
+		kp = pc.PublicKey{
+			Sum: &pc.PublicKey_Secp256K1{
+				Secp256K1: k,
+			},
+		}
 	default:
 		return kp, fmt.Errorf("toproto: key type %v is not supported", k)
 	}
@@ -41,6 +49,14 @@ func PubKeyFromProto(k pc.PublicKey) (crypto.PubKey, error) {
 		pk := make(ed25519.PubKey, ed25519.PubKeySize)
 		copy(pk, k.Ed25519)
 		return pk, nil
+	case *pc.PublicKey_Secp256K1:
+		if len(k.Secp256K1) != secp256k1.PubKeySize {
+			return nil, fmt.Errorf("invalid size for PubKeyEd25519. Got %d, expected %d",
+				len(k.Secp256K1), secp256k1.PubKeySize)
+		}
+		pk := make(secp256k1.PubKey, secp256k1.PubKeySize)
+		copy(pk, k.Secp256K1)
+		return pk, nil
 	default:
 		return nil, fmt.Errorf("fromproto: key type %v is not supported", k)
 	}