AB#1943 Extract KMS package (#56)

* Extract kmsapi from coordinator

* Add kmsapi cmd server

kms/server/kmsapi/kmsapi.go

@@ -0,0 +1,46 @@
+// Package kmsapi implements an API to manage encryption keys.
+package kmsapi
+
+import (
+	"context"
+
+	"github.com/edgelesssys/constellation/kms/kms"
+	"github.com/edgelesssys/constellation/kms/server/kmsapi/kmsproto"
+	"go.uber.org/zap"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/status"
+)
+
+// API resembles an encryption key management api server through logger, CloudKMS and proto-unimplemented server.
+type API struct {
+	logger *zap.Logger
+	conKMS kms.CloudKMS
+	kmsproto.UnimplementedAPIServer
+}
+
+// New creates a new API.
+func New(logger *zap.Logger, conKMS kms.CloudKMS) *API {
+	return &API{
+		logger: logger,
+		conKMS: conKMS,
+	}
+}
+
+// GetDataKey returns a data key.
+func (a *API) GetDataKey(ctx context.Context, in *kmsproto.GetDataKeyRequest) (*kmsproto.GetDataKeyResponse, error) {
+	// Error on 0 key length
+	if in.Length == 0 {
+		return nil, status.Error(codes.InvalidArgument, "can't derive key with length zero")
+	}
+
+	// Error on empty DataKeyId
+	if in.DataKeyId == "" {
+		return nil, status.Error(codes.InvalidArgument, "no data key id specified")
+	}
+
+	key, err := a.conKMS.GetDEK(ctx, "Constellation", "key-"+in.DataKeyId, int(in.Length))
+	if err != nil {
+		return nil, status.Errorf(codes.Internal, "%v", err)
+	}
+	return &kmsproto.GetDataKeyResponse{DataKey: key}, nil
+}