diff --git a/pam/gdm/conversation.go b/pam/gdm/conversation.go
new file mode 100644
index 000000000..faec34fd5
--- /dev/null
+++ b/pam/gdm/conversation.go
@@ -0,0 +1,28 @@
+package gdm
+
+import "C"
+
+import (
+	"errors"
+
+	"github.com/msteinert/pam"
+)
+
+func sendToGdm(pamMTx pam.ModuleTransaction, data []byte) ([]byte, error) {
+	binReq, err := NewBinaryJSONProtoRequest(data)
+	if err != nil {
+		return nil, err
+	}
+	defer binReq.Release()
+	res, err := pamMTx.StartConv(binReq)
+	if err != nil {
+		return nil, err
+	}
+
+	binRes, ok := res.(pam.BinaryConvResponse)
+	if !ok {
+		return nil, errors.New("returned value is not in binary form")
+	}
+	defer binRes.Release()
+	return binRes.Decode(decodeJSONProtoMessage)
+}
diff --git a/pam/gdm/conversation_test.go b/pam/gdm/conversation_test.go
new file mode 100644
index 000000000..ed278b503
--- /dev/null
+++ b/pam/gdm/conversation_test.go
@@ -0,0 +1,107 @@
+package gdm
+
+import (
+	"testing"
+
+	"github.com/msteinert/pam"
+	"github.com/stretchr/testify/require"
+	"github.com/ubuntu/authd/pam/pam_test"
+)
+
+func TestSendToGdm(t *testing.T) {
+	t.Parallel()
+	t.Cleanup(pam_test.MaybeDoLeakCheck)
+
+	testCases := map[string]struct {
+		value []byte
+
+		wantReturn []byte
+		wantError  error
+	}{
+		"JSON null data can be sent and received": {
+			value: []byte(`null`),
+		},
+
+		"JSON number can be sent and received": {
+			value: []byte(`1.5`),
+		},
+
+		"Single char is sent and received as string": {
+			value: []byte(`"m"`),
+		},
+
+		"JSON null is returned": {
+			value:      []byte(`"give me 🚫"`),
+			wantReturn: []byte("null"),
+		},
+
+		"Utf-8 data is sent and returned": {
+			value:      []byte(`"give me 🍕"`),
+			wantReturn: []byte(`"😋"`),
+		},
+
+		// Error cases
+		"Error on empty data": {
+			value:     []byte{},
+			wantError: ErrInvalidJSON,
+		},
+
+		"Error on nil data": {
+			value:     nil,
+			wantError: ErrInvalidJSON,
+		},
+
+		"Error with empty data returned": {
+			value:      []byte(`"give me 🗑‼"`),
+			wantReturn: []byte{},
+			wantError:  ErrInvalidJSON,
+		},
+
+		"Error with nil data returned": {
+			value:      []byte(`"give me 🚫"`),
+			wantReturn: []byte(nil),
+			wantError:  ErrInvalidJSON,
+		},
+	}
+
+	for name, tc := range testCases {
+		tc := tc
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+			t.Cleanup(pam_test.MaybeDoLeakCheck)
+
+			convFuncCalled := true
+			mt := pam_test.NewModuleTransactionDummy(pam.BinaryPointerConversationFunc(
+				func(ptr pam.BinaryPointer) (pam.BinaryPointer, error) {
+					convFuncCalled = true
+					require.NotNil(t, ptr)
+					req, err := decodeJSONProtoMessage(ptr)
+					require.NoError(t, err)
+					require.Equal(t, tc.value, req)
+					if tc.wantReturn != nil {
+						msg, err := newJSONProtoMessage(tc.wantReturn)
+						return pam.BinaryPointer(msg), err
+					}
+					msg, err := newJSONProtoMessage(req)
+					return pam.BinaryPointer(msg), err
+				}))
+
+			data, err := sendToGdm(mt, tc.value)
+			require.True(t, convFuncCalled)
+
+			if tc.wantError != nil {
+				require.Error(t, tc.wantError, err)
+				return
+			}
+
+			require.NoError(t, err)
+
+			if tc.wantReturn != nil {
+				require.Equal(t, tc.wantReturn, data)
+				return
+			}
+
+			require.Equal(t, tc.value, data)
+		})
+	}
+}