diff --git a/padding/ansi_x923.go b/padding/ansi_x923.go
new file mode 100644
index 0000000..68cf570
--- /dev/null
+++ b/padding/ansi_x923.go
@@ -0,0 +1,41 @@
+// https://www.ibm.com/docs/en/linux-on-systems?topic=processes-ansi-x923-cipher-block-chaining
+package padding
+
+import (
+	"errors"
+
+	"github.com/emmansun/gmsm/internal/subtle"
+)
+
+type ansiX923Padding uint
+
+func (pad ansiX923Padding) BlockSize() int {
+	return int(pad)
+}
+
+func (pad ansiX923Padding) Pad(src []byte) []byte {
+	overhead := pad.BlockSize() - len(src)%pad.BlockSize()
+	ret, out := subtle.SliceForAppend(src, overhead)
+	out[overhead-1] = byte(overhead)
+	for i := 0; i < overhead-1; i++ {
+		out[i] = 0
+	}
+	return ret
+}
+
+func (pad ansiX923Padding) Unpad(src []byte) ([]byte, error) {
+	srcLen := len(src)
+	if srcLen == 0 || srcLen%pad.BlockSize() != 0 {
+		return nil, errors.New("ansi x9.23: invalid src length")
+	}
+	paddedLen := src[srcLen-1]
+	if paddedLen == 0 || int(paddedLen) > pad.BlockSize() {
+		return nil, errors.New("ansi x9.23: invalid padding length")
+	}
+	for _, b := range src[srcLen-int(paddedLen) : srcLen-1] {
+		if b != 0 {
+			return nil, errors.New("ansi x9.23: invalid padding bytes")
+		}
+	}
+	return src[:srcLen-int(paddedLen)], nil
+}
diff --git a/padding/ansi_x923_test.go b/padding/ansi_x923_test.go
new file mode 100644
index 0000000..e08ef69
--- /dev/null
+++ b/padding/ansi_x923_test.go
@@ -0,0 +1,83 @@
+// https://www.ibm.com/docs/en/linux-on-systems?topic=processes-ansi-x923-cipher-block-chaining
+
+package padding
+
+import (
+	"reflect"
+	"testing"
+)
+
+func Test_ansiX923Padding_Pad(t *testing.T) {
+	x923 := NewANSIX923Padding(16)
+	tests := []struct {
+		name string
+		src  []byte
+		want []byte
+	}{
+		{"16 bytes", []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15}, []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 16}},
+		{"15 bytes", []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14}, []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 1}},
+		{"14 bytes", []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13}, []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 0, 2}},
+		{"13 bytes", []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12}, []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 0, 0, 3}},
+		{"12 bytes", []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11}, []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 0, 0, 0, 4}},
+		{"11 bytes", []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10}, []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 0, 0, 0, 0, 5}},
+		{"10 bytes", []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}, []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 0, 0, 0, 0, 6}},
+		{"9 bytes", []byte{0, 1, 2, 3, 4, 5, 6, 7, 8}, []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 0, 0, 0, 0, 0, 0, 7}},
+		{"8 bytes", []byte{0, 1, 2, 3, 4, 5, 6, 7}, []byte{0, 1, 2, 3, 4, 5, 6, 7, 0, 0, 0, 0, 0, 0, 0, 8}},
+		{"7 bytes", []byte{0, 1, 2, 3, 4, 5, 6}, []byte{0, 1, 2, 3, 4, 5, 6, 0, 0, 0, 0, 0, 0, 0, 0, 9}},
+		{"6 bytes", []byte{0, 1, 2, 3, 4, 5}, []byte{0, 1, 2, 3, 4, 5, 0, 0, 0, 0, 0, 0, 0, 0, 0, 10}},
+		{"5 bytes", []byte{0, 1, 2, 3, 4}, []byte{0, 1, 2, 3, 4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 11}},
+		{"4 bytes", []byte{0, 1, 2, 3}, []byte{0, 1, 2, 3, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 12}},
+		{"3 bytes", []byte{0, 1, 2}, []byte{0, 1, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 13}},
+		{"2 bytes", []byte{0, 1}, []byte{0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 14}},
+		{"1 bytes", []byte{0}, []byte{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 15}},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := x923.Pad(tt.src); !reflect.DeepEqual(got, tt.want) {
+				t.Errorf("ansiX923Padding.Pad() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func Test_ansiX923Padding_Unpad(t *testing.T) {
+	x923 := NewANSIX923Padding(16)
+	tests := []struct {
+		name    string
+		want    []byte
+		src     []byte
+		wantErr bool
+	}{
+		{"16 bytes", []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15}, []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 16}, false},
+		{"15 bytes", []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14}, []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 1}, false},
+		{"14 bytes", []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13}, []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 0, 2}, false},
+		{"13 bytes", []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12}, []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 0, 0, 3}, false},
+		{"12 bytes", []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11}, []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 0, 0, 0, 4}, false},
+		{"11 bytes", []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10}, []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 0, 0, 0, 0, 5}, false},
+		{"10 bytes", []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}, []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 0, 0, 0, 0, 6}, false},
+		{"9 bytes", []byte{0, 1, 2, 3, 4, 5, 6, 7, 8}, []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 0, 0, 0, 0, 0, 0, 7}, false},
+		{"8 bytes", []byte{0, 1, 2, 3, 4, 5, 6, 7}, []byte{0, 1, 2, 3, 4, 5, 6, 7, 0, 0, 0, 0, 0, 0, 0, 8}, false},
+		{"7 bytes", []byte{0, 1, 2, 3, 4, 5, 6}, []byte{0, 1, 2, 3, 4, 5, 6, 0, 0, 0, 0, 0, 0, 0, 0, 9}, false},
+		{"6 bytes", []byte{0, 1, 2, 3, 4, 5}, []byte{0, 1, 2, 3, 4, 5, 0, 0, 0, 0, 0, 0, 0, 0, 0, 10}, false},
+		{"5 bytes", []byte{0, 1, 2, 3, 4}, []byte{0, 1, 2, 3, 4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 11}, false},
+		{"4 bytes", []byte{0, 1, 2, 3}, []byte{0, 1, 2, 3, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 12}, false},
+		{"3 bytes", []byte{0, 1, 2}, []byte{0, 1, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 13}, false},
+		{"2 bytes", []byte{0, 1}, []byte{0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 14}, false},
+		{"1 bytes", []byte{0}, []byte{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 15}, false},
+		{"invalid src length", nil, []byte{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 15}, true},
+		{"invalid padding length", nil, []byte{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 17}, true},
+		{"invalid padding bytes", nil, []byte{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 14, 15}, true},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := x923.Unpad(tt.src)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("ansiX923Padding.Unpad() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+			if !reflect.DeepEqual(got, tt.want) {
+				t.Errorf("ansiX923Padding.Unpad() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
diff --git a/padding/pkcs7.go b/padding/pkcs7.go
index c070519..7b275c5 100644
--- a/padding/pkcs7.go
+++ b/padding/pkcs7.go
@@ -2,7 +2,6 @@
 package padding
 
 import (
-	goSubtle "crypto/subtle"
 	"errors"
 
 	"github.com/emmansun/gmsm/internal/subtle"
@@ -24,21 +23,18 @@ func (pad pkcs7Padding) Pad(src []byte) []byte {
 }
 
 func (pad pkcs7Padding) Unpad(src []byte) ([]byte, error) {
-	if len(src)%pad.BlockSize() != 0 {
-		return nil, errors.New("pkcs7: invalid src size")
+	srcLen := len(src)
+	if srcLen == 0 || srcLen%pad.BlockSize() != 0 {
+		return nil, errors.New("pkcs7: invalid src length")
 	}
-	overhead := src[len(src)-1]
-	if overhead == 0 || int(overhead) > pad.BlockSize() {
+	paddedLen := src[srcLen-1]
+	if paddedLen == 0 || int(paddedLen) > pad.BlockSize() {
 		return nil, errors.New("pkcs7: invalid padding byte/length")
 	}
-	tag := make([]byte, pad.BlockSize())
-	copy(tag, src[len(src)-pad.BlockSize():])
-	for i := pad.BlockSize() - int(overhead); i < pad.BlockSize(); i++ {
-		tag[i] = byte(overhead)
+	for _, b := range src[srcLen-int(paddedLen) : srcLen-1] {
+		if b != paddedLen {
+			return nil, errors.New("pkcs7: inconsistent padding bytes")
+		}
 	}
-	if goSubtle.ConstantTimeCompare(tag, src[len(src)-pad.BlockSize():]) != 1 {
-		return nil, errors.New("pkcs7: inconsistent padding bytes")
-	}
-
-	return src[:len(src)-int(overhead)], nil
+	return src[:srcLen-int(paddedLen)], nil
 }
diff --git a/padding/pkcs7_test.go b/padding/pkcs7_test.go
index 0d359fd..97a7775 100644
--- a/padding/pkcs7_test.go
+++ b/padding/pkcs7_test.go
@@ -8,14 +8,13 @@ import (
 )
 
 func Test_pkcs7Padding_Pad(t *testing.T) {
-	pad := NewPKCS7Padding(16)
+	pkcs7 := NewPKCS7Padding(16)
 
 	tests := []struct {
 		name string
 		src  []byte
 		want []byte
 	}{
-		// TODO: Add test cases.
 		{"16 bytes", []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15}, []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16}},
 		{"15 bytes", []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14}, []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 1}},
 		{"14 bytes", []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13}, []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 2, 2}},
@@ -35,7 +34,7 @@ func Test_pkcs7Padding_Pad(t *testing.T) {
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			if got := pad.Pad(tt.src); !reflect.DeepEqual(got, tt.want) {
+			if got := pkcs7.Pad(tt.src); !reflect.DeepEqual(got, tt.want) {
 				t.Errorf("pkcs7Padding.Pad() = %v, want %v", got, tt.want)
 			}
 		})
@@ -43,17 +42,13 @@ func Test_pkcs7Padding_Pad(t *testing.T) {
 }
 
 func Test_pkcs7Padding_Unpad(t *testing.T) {
-	pad := NewPKCS7Padding(16)
-	type args struct {
-		src []byte
-	}
+	pkcs7 := NewPKCS7Padding(16)
 	tests := []struct {
 		name    string
 		want    []byte
 		src     []byte
 		wantErr bool
 	}{
-		// TODO: Add test cases.
 		{"16 bytes", []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15}, []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16}, false},
 		{"15 bytes", []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14}, []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 1}, false},
 		{"14 bytes", []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13}, []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 2, 2}, false},
@@ -76,7 +71,7 @@ func Test_pkcs7Padding_Unpad(t *testing.T) {
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			got, err := pad.Unpad(tt.src)
+			got, err := pkcs7.Unpad(tt.src)
 			if (err != nil) != tt.wantErr {
 				t.Errorf("pkcs7Padding.Unpad() error = %v, wantErr %v", err, tt.wantErr)
 				return
diff --git a/padding/pads.go b/padding/schemes.go
similarity index 66%
rename from padding/pads.go
rename to padding/schemes.go
index 7728e34..8e762e3 100644
--- a/padding/pads.go
+++ b/padding/schemes.go
@@ -13,3 +13,10 @@ func NewPKCS7Padding(blockSize uint) Padding {
 	}
 	return pkcs7Padding(blockSize)
 }
+
+func NewANSIX923Padding(blockSize uint) Padding {
+	if blockSize == 0 || blockSize > 255 {
+		panic("padding: invalid block size")
+	}
+	return ansiX923Padding(blockSize)
+}
diff --git a/sm4_test/cbc_sm4_test.go b/sm4_test/cbc_sm4_test.go
index bba1655..f478d00 100644
--- a/sm4_test/cbc_sm4_test.go
+++ b/sm4_test/cbc_sm4_test.go
@@ -118,7 +118,7 @@ var cbcSM4Tests = []struct {
 }
 
 func TestCBCEncrypterSM4(t *testing.T) {
-	pad := padding.NewPKCS7Padding(sm4.BlockSize)
+	pkcs7 := padding.NewPKCS7Padding(sm4.BlockSize)
 	for _, test := range cbcSM4Tests {
 		c, err := sm4.NewCipher(test.key)
 		if err != nil {
@@ -128,7 +128,7 @@ func TestCBCEncrypterSM4(t *testing.T) {
 
 		encrypter := cipher.NewCBCEncrypter(c, test.iv)
 
-		plainText := pad.Pad(test.in)
+		plainText := pkcs7.Pad(test.in)
 		data := make([]byte, len(plainText))
 		copy(data, plainText)
 
@@ -146,7 +146,7 @@ func TestCBCEncrypterSM4(t *testing.T) {
 }
 
 func TestCBCDecrypterSM4(t *testing.T) {
-	pad := padding.NewPKCS7Padding(sm4.BlockSize)
+	pkcs7 := padding.NewPKCS7Padding(sm4.BlockSize)
 	for _, test := range cbcSM4Tests {
 		c, err := sm4.NewCipher(test.key)
 		if err != nil {
@@ -160,7 +160,7 @@ func TestCBCDecrypterSM4(t *testing.T) {
 		copy(data, test.out)
 
 		decrypter.CryptBlocks(data, data)
-		data, err = pad.Unpad(data)
+		data, err = pkcs7.Unpad(data)
 		if err != nil {
 			t.Fatal(err)
 		}