Implement and tested go-tpm direct function TPM2_GetRandom (google#277)

* Implement and tested go-tpm direct function TPM2_GetRandom

* Resolved all nits from PR

Co-authored-by: Matthew Tsai <matthewtsai@google.com>

direct/tpm2/get_random_test.go

@@ -0,0 +1,23 @@
+package tpm2
+
+import (
+	"testing"
+
+	"github.com/google/go-tpm/direct/transport/simulator"
+)
+
+func TestGetRandom(t *testing.T) {
+	thetpm, err := simulator.OpenSimulator()
+	if err != nil {
+		t.Fatalf("could not connect to TPM simulator: %v", err)
+	}
+	defer thetpm.Close()
+
+	grc := GetRandom{
+		BytesRequested: 16,
+	}
+
+	if _, err := grc.Execute(thetpm); err != nil {
+		t.Fatalf("GetRandom failed: %v", err)
+	}
+}

direct/tpm2/tpm2.go

@@ -387,6 +387,34 @@ type CreateLoadedResponse struct {
 // Response implements the Response interface.
 func (*CreateLoadedResponse) Response() tpm.CC { return tpm.CCCreateLoaded }
 
+// GetRandom is the input to TPM2_GetRandom.
+// See definition in Part 3, Commands, section 16.1
+type GetRandom struct {
+	// number of octets to return
+	BytesRequested uint16
+}
+
+// Command implements the Command interface.
+func (*GetRandom) Command() tpm.CC { return tpm.CCGetRandom }
+
+// Execute executes the command and returns the response.
+func (cmd *GetRandom) Execute(t transport.TPM, s ...Session) (*GetRandomResponse, error) {
+	var rsp GetRandomResponse
+	if err := execute(t, cmd, &rsp, s...); err != nil {
+		return nil, err
+	}
+	return &rsp, nil
+}
+
+// GetRandomReponse is the reponse from TPM2_GetRandom.
+type GetRandomResponse struct {
+	// the random octets
+	RandomBytes tpm2b.Digest
+}
+
+// Reponse implements the Response interface.
+func (*GetRandomResponse) Response() tpm.CC { return tpm.CCGetRandom }
+
 // Quote is the input to TPM2_Quote.
 // See definition in Part 3, Commands, section 18.4
 type Quote struct {