diff --git a/common/storage.go b/common/storage.go
index 3b9e494..afa4e7b 100644
--- a/common/storage.go
+++ b/common/storage.go
@@ -1,6 +1,7 @@
 package common
 
 import (
+	"github.com/nspcc-dev/neo-go/pkg/interop/convert"
 	"github.com/nspcc-dev/neo-go/pkg/interop/native/std"
 	"github.com/nspcc-dev/neo-go/pkg/interop/storage"
 )
@@ -10,3 +11,18 @@ func SetSerialized(ctx storage.Context, key interface{}, value interface{}) {
 	data := std.Serialize(value)
 	storage.Put(ctx, key, data)
 }
+
+// ToFixedWidth64 converts x to bytes such that numbers <= math.MaxUint64
+// have constant with of 9.
+func ToFixedWidth64(x int) []byte {
+	data := convert.ToBytes(x)
+	if x < 0 || len(data) >= 9 {
+		return data
+	}
+	return append(data, make([]byte, 9-len(data))...)
+}
+
+// FromFixedWidth64 is a reverse function for ToFixedWidth64.
+func FromFixedWidth64(x []byte) int {
+	return convert.ToInteger(x)
+}
diff --git a/tests/common_test.go b/tests/common_test.go
new file mode 100644
index 0000000..0085d2c
--- /dev/null
+++ b/tests/common_test.go
@@ -0,0 +1,61 @@
+package tests
+
+import (
+	"math"
+	"math/big"
+	"path"
+	"testing"
+
+	"github.com/nspcc-dev/neo-go/pkg/neotest"
+	"github.com/stretchr/testify/require"
+)
+
+const testdataPath = "./testdata"
+
+func newTestdataInvoker(t *testing.T) *neotest.ContractInvoker {
+	e := newExecutor(t)
+	ctr := neotest.CompileFile(t, e.CommitteeHash, testdataPath, path.Join(testdataPath, "config.yml"))
+	e.DeployContract(t, ctr, nil)
+	return e.CommitteeInvoker(ctr.Hash)
+}
+
+func TestEncodeU64(t *testing.T) {
+	// Let's check boundary values for all bit sizes:
+	var nums []uint64
+	for i := 0; i < 64; i++ {
+		if i != 0 {
+			nums = append(nums, (1<<i)-1)
+		}
+		nums = append(nums, 1<<i)
+		if i != 63 {
+			nums = append(nums, (1<<i)+1)
+		}
+	}
+
+	c := newTestdataInvoker(t)
+	for _, n := range nums {
+		v, err := c.TestInvoke(t, "encode", n)
+		require.NoError(t, err)
+		require.Equal(t, 1, v.Len())
+
+		r := v.Pop().Bytes()
+		require.Equal(t, 9, len(r), "got: %x", r)
+
+		v, err = c.TestInvoke(t, "encodeDecode", n)
+		require.NoError(t, err)
+		require.Equal(t, 1, v.Len())
+		require.Equal(t, n, v.Pop().BigInt().Uint64())
+	}
+
+	t.Run("bad cases should be handled", func(t *testing.T) {
+		x := new(big.Int).SetUint64(math.MaxUint64)
+		x.Add(x, big.NewInt(1))
+
+		nums := []*big.Int{x, big.NewInt(-1), big.NewInt(-128)}
+		for _, n := range nums {
+			v, err := c.TestInvoke(t, "encodeDecode", n)
+			require.NoError(t, err)
+			require.Equal(t, 0, v.Pop().BigInt().Cmp(n))
+		}
+	})
+}
diff --git a/tests/testdata/config.yml b/tests/testdata/config.yml
new file mode 100644
index 0000000..6294558
--- /dev/null
+++ b/tests/testdata/config.yml
@@ -0,0 +1,2 @@
+name: "TestContract"
+safemethods: ["encodeDecode"]
diff --git a/tests/testdata/encode.go b/tests/testdata/encode.go
new file mode 100644
index 0000000..02d9b4e
--- /dev/null
+++ b/tests/testdata/encode.go
@@ -0,0 +1,17 @@
+package testdata
+
+import (
+	"git.frostfs.info/TrueCloudLab/frostfs-contract/common"
+)
+
+// EncodeDecode encodes x in fixed-width little-endian representation
+// and deserializes it back.
+func EncodeDecode(x int) int {
+	y := common.ToFixedWidth64(x)
+	return common.FromFixedWidth64(y)
+}
+
+// Encode encodes x in fixed-width little-endian representation.
+func Encode(x int) []byte {
+	return common.ToFixedWidth64(x)
+}