diff --git a/base58/base58_test.go b/base58/base58_test.go
index 59f3304..8524643 100644
--- a/base58/base58_test.go
+++ b/base58/base58_test.go
@@ -7,42 +7,58 @@ import (
 	"time"
 
 	"github.com/osamingo/indigo/base58"
-	"github.com/stretchr/testify/assert"
-	"github.com/stretchr/testify/require"
 )
 
 func TestStdSource(t *testing.T) {
-	require.Equal(t, base58.StdSource(), "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz")
+	const expect = "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz"
+	if base58.StdSource() != expect {
+		t.Error("should be", expect)
+	}
 }
 
 func TestMustNewEncoder(t *testing.T) {
 
-	var enc *base58.Encoder
-	require.NotPanics(t, func() {
-		enc = base58.MustNewEncoder("rpshnaf39wBUDNEGHJKLM4PQRST7VWXYZ2bcdeCg65jkm8oFqi1tuvAxyz")
-	})
-	require.NotNil(t, enc)
+	enc := base58.MustNewEncoder("rpshnaf39wBUDNEGHJKLM4PQRST7VWXYZ2bcdeCg65jkm8oFqi1tuvAxyz")
+	if enc == nil {
+		t.Error("should not be nil")
+	}
 
-	require.Panics(t, func() {
+	func() {
+		defer func() {
+			recover()
+		}()
 		base58.MustNewEncoder("")
-	})
+		t.Error("should be panic")
+	}()
 
-	require.Panics(t, func() {
+	func() {
+		defer func() {
+			recover()
+		}()
 		base58.MustNewEncoder("test")
-	})
+		t.Error("should be panic")
+	}()
 }
 
 func TestNewEncoder(t *testing.T) {
 
 	enc, err := base58.NewEncoder("rpshnaf39wBUDNEGHJKLM4PQRST7VWXYZ2bcdeCg65jkm8oFqi1tuvAxyz")
-	require.NoError(t, err)
-	require.NotNil(t, enc)
+	if err != nil {
+		t.Error("should be nil")
+	}
+	if enc == nil {
+		t.Error("should not be nil")
+	}
 
 	_, err = base58.NewEncoder("")
-	require.Error(t, err)
+	if err == nil {
+		t.Error("should not be nil")
+	}
 
 	_, err = base58.NewEncoder("test")
-	require.Error(t, err)
+	if err == nil {
+		t.Error("should not be nil")
+	}
 }
 
 func TestEncoder_Encode(t *testing.T) {
@@ -58,10 +74,14 @@ func TestEncoder_Encode(t *testing.T) {
 
 	enc := base58.MustNewEncoder(base58.StdSource())
 	id := enc.Encode(0)
-	assert.Equal(t, "1", id)
+	if id != "1" {
+		t.Error("should be", "1")
+	}
 
 	for k, v := range bc {
-		assert.Equal(t, v, enc.Encode(k))
+		if enc.Encode(k) != v {
+			t.Error("should be", v)
+		}
 	}
 }
 
@@ -78,15 +98,23 @@ func TestEncoder_Decode(t *testing.T) {
 
 	enc := base58.MustNewEncoder(base58.StdSource())
 	_, err := enc.Decode("")
-	require.Error(t, err)
+	if err == nil {
+		t.Error("should not be nil")
+	}
 
 	_, err = enc.Decode("0")
-	require.Error(t, err)
+	if err == nil {
+		t.Error("should not be nil")
+	}
 
 	for k, v := range bc {
 		r, err := enc.Decode(v)
-		require.NoError(t, err)
-		assert.Equal(t, k, r)
+		if err != nil {
+			t.Error("should be nil")
+		}
+		if r != k {
+			t.Error("should be", k)
+		}
 	}
 }
 
diff --git a/go.mod b/go.mod
index f9fb4d2..2b4b105 100644
--- a/go.mod
+++ b/go.mod
@@ -1,11 +1,5 @@
 module github.com/osamingo/indigo
 
-go 1.13
+go 1.15
 
-require (
-	github.com/davecgh/go-spew v1.1.1 // indirect
-	github.com/kr/pretty v0.1.0 // indirect
-	github.com/sony/sonyflake v1.0.1-0.20200827011719-848d664ceea4
-	github.com/stretchr/testify v1.6.1
-	gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127 // indirect
-)
+require github.com/sony/sonyflake v1.0.1-0.20200827011719-848d664ceea4
diff --git a/go.sum b/go.sum
index dcc4132..684abda 100644
--- a/go.sum
+++ b/go.sum
@@ -1,22 +1,2 @@
-github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
-github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
-github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
-github.com/kr/pretty v0.1.0 h1:L/CwN0zerZDmRFUapSPitk6f+Q3+0za1rQkzVuMiMFI=
-github.com/kr/pretty v0.1.0/go.mod h1:dAy3ld7l9f0ibDNOQOHHMYYIIbhfbHSm3C4ZsoJORNo=
-github.com/kr/pty v1.1.1/go.mod h1:pFQYn66WHrOpPYNljwOMqo10TkYh1fy3cYio2l3bCsQ=
-github.com/kr/text v0.1.0 h1:45sCR5RtlFHMR4UwH9sdQ5TC8v0qDQCHnXt+kaKSTVE=
-github.com/kr/text v0.1.0/go.mod h1:4Jbv+DJW3UT/LiOwJeYQe1efqtUx/iVham/4vfdArNI=
-github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
-github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/sony/sonyflake v1.0.1-0.20200827011719-848d664ceea4 h1:KPf4aWX4JGEdMICunm9A0YTFbD6OL23G14BR7cL/0wg=
 github.com/sony/sonyflake v1.0.1-0.20200827011719-848d664ceea4/go.mod h1:LORtCywH/cq10ZbyfhKrHYgAUGH7mOBa76enV9txy/Y=
-github.com/stretchr/objx v0.1.0 h1:4G4v2dO3VZwixGIRoQ5Lfboy6nUhCyYzaqnIAPPhYs4=
-github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
-github.com/stretchr/testify v1.6.1 h1:hDPOHmpOpP40lSULcqw7IrRb/u7w6RpDC9399XyoNd0=
-github.com/stretchr/testify v1.6.1/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
-gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
-gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
-gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127 h1:qIbj1fsPNlZgppZ+VLlY7N33q108Sa+fhmuc+sWQYwY=
-gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
-gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c h1:dUUwHk2QECo/6vqA44rthZ8ie2QXMNeKRTHCNY2nXvo=
-gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
diff --git a/indigo_test.go b/indigo_test.go
index 17fd172..f2ee5d6 100644
--- a/indigo_test.go
+++ b/indigo_test.go
@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"math"
 	"math/rand"
+	"reflect"
 	"sort"
 	"sync"
 	"testing"
@@ -11,8 +12,6 @@ import (
 
 	"github.com/osamingo/indigo"
 	"github.com/osamingo/indigo/base58"
-	"github.com/stretchr/testify/assert"
-	"github.com/stretchr/testify/require"
 )
 
 func TestNew(t *testing.T) {
@@ -22,7 +21,9 @@ func TestNew(t *testing.T) {
 		indigo.MachineID(func() (uint16, error) { return math.MaxUint16, nil }),
 		indigo.CheckMachineID(nil),
 	)
-	require.NotNil(t, g)
+	if g == nil {
+		t.Error("should not be nil")
+	}
 }
 
 func TestGenerator_NextID(t *testing.T) {
@@ -35,13 +36,25 @@ func TestGenerator_NextID(t *testing.T) {
 	)
 
 	id1, err := g.NextID()
-	require.NoError(t, err)
-	assert.NotEmpty(t, id1)
+	if err != nil {
+		t.Error("should be nil")
+	}
+	if id1 == "" {
+		t.Error("should not be empty")
+	}
 
 	id2, err := g.NextID()
-	require.NoError(t, err)
-	assert.NotEmpty(t, id2)
-	assert.NotEqual(t, id1, id2)
+	if err != nil {
+		t.Error("should be nil")
+	}
+
+	if id2 == "" {
+		t.Error("should not be empty")
+	}
+
+	if id1 == id2 {
+		t.Error("should not be equal")
+	}
 }
 
 func TestGenerator_Decompose(t *testing.T) {
@@ -54,12 +67,20 @@ func TestGenerator_Decompose(t *testing.T) {
 	)
 
 	m, err := g.Decompose("KGuFE14P")
-	require.NoError(t, err)
-	require.NotEmpty(t, m)
-	assert.NotEmpty(t, m["id"])
+	if err != nil {
+		t.Error("should be nil")
+	}
+	if len(m) == 0 {
+		t.Error("should not be empty")
+	}
+	if _, ok := m["id"]; !ok {
+		t.Error("should not be empty")
+	}
 
 	_, err = g.Decompose("")
-	require.Error(t, err)
+	if err == nil {
+		t.Error("should not be nil")
+	}
 }
 
 func TestGenerator_NextID_Race(t *testing.T) {
@@ -79,8 +100,12 @@ func TestGenerator_NextID_Race(t *testing.T) {
 		go func() {
 			defer wg.Done()
 			id, err := g.NextID()
-			require.NoError(t, err)
-			require.NotEmpty(t, id)
+			if err != nil {
+				t.Error("should be nil")
+			}
+			if id == "" {
+				t.Error("should not be empty")
+			}
 		}()
 	}
 
@@ -112,7 +137,9 @@ func TestGenerator_NextID_SortIDs(t *testing.T) {
 			for j := 0; j < th; j++ {
 				time.Sleep(10*time.Millisecond + time.Duration(r.Intn(1e9)))
 				id, err := g.NextID()
-				require.NoError(t, err)
+				if err != nil {
+					t.Error("should be nil")
+				}
 				s = append(s, id)
 			}
 
@@ -126,10 +153,14 @@ func TestGenerator_NextID_SortIDs(t *testing.T) {
 
 	old := make([]string, 100)
 	copy(old, ids)
-	require.Equal(t, old, ids)
+	if !reflect.DeepEqual(ids, old) {
+		t.Error("should be equal")
+	}
 
 	sort.Strings(ids)
-	require.NotEqual(t, old, ids)
+	if reflect.DeepEqual(ids, old) {
+		t.Error("should not be equal")
+	}
 
 	g := indigo.New(
 		nil,
@@ -140,8 +171,12 @@ func TestGenerator_NextID_SortIDs(t *testing.T) {
 	var prev uint64
 	for i := range ids {
 		m, err := g.Decompose(ids[i])
-		require.NoError(t, err)
-		require.True(t, prev <= m["time"])
+		if err != nil {
+			t.Error("should be nil")
+		}
+		if !(prev <= m["time"]) {
+			t.Error("should be true")
+		}
 		prev = m["time"]
 	}
 }