[#140] sdk/object: Implement object format control functions

Implement function for calculating, setting and checking object verification
fields.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/object/fmt_test.go

@@ -0,0 +1,79 @@
+package object
+
+import (
+	"crypto/rand"
+	"testing"
+
+	"github.com/nspcc-dev/neofs-crypto/test"
+	"github.com/stretchr/testify/require"
+)
+
+func TestVerificationFields(t *testing.T) {
+	obj := NewRaw()
+
+	payload := make([]byte, 10)
+	_, _ = rand.Read(payload)
+
+	obj.SetPayload(payload)
+	obj.SetPayloadSize(uint64(len(payload)))
+
+	require.NoError(t, SetVerificationFields(test.DecodeKey(-1), obj))
+
+	require.NoError(t, CheckVerificationFields(obj.Object()))
+
+	items := []struct {
+		corrupt func()
+		restore func()
+	}{
+		{
+			corrupt: func() {
+				payload[0]++
+			},
+			restore: func() {
+				payload[0]--
+			},
+		},
+		{
+			corrupt: func() {
+				obj.SetPayloadSize(obj.GetPayloadSize() + 1)
+			},
+			restore: func() {
+				obj.SetPayloadSize(obj.GetPayloadSize() - 1)
+			},
+		},
+		{
+			corrupt: func() {
+				obj.GetID().ToV2().GetValue()[0]++
+			},
+			restore: func() {
+				obj.GetID().ToV2().GetValue()[0]--
+			},
+		},
+		{
+			corrupt: func() {
+				obj.GetSignature().GetKey()[0]++
+			},
+			restore: func() {
+				obj.GetSignature().GetKey()[0]--
+			},
+		},
+		{
+			corrupt: func() {
+				obj.GetSignature().GetSign()[0]++
+			},
+			restore: func() {
+				obj.GetSignature().GetSign()[0]--
+			},
+		},
+	}
+
+	for _, item := range items {
+		item.corrupt()
+
+		require.Error(t, CheckVerificationFields(obj.Object()))
+
+		item.restore()
+
+		require.NoError(t, CheckVerificationFields(obj.Object()))
+	}
+}