diff --git a/cipher/gcm_sm4_test.go b/cipher/gcm_sm4_test.go
index 7bfca9c..be09fa2 100644
--- a/cipher/gcm_sm4_test.go
+++ b/cipher/gcm_sm4_test.go
@@ -397,7 +397,7 @@ func TestGCMCounterWrap(t *testing.T) {
 func TestSM4GCMRandom(t *testing.T) {
 	key := []byte("0123456789ABCDEF")
 	nonce := []byte("0123456789AB")
-	plaintext := make([]byte, 464)
+	plaintext := make([]byte, 0x198)
 	
 	io.ReadFull(rand.Reader, plaintext)
 	c, err := sm4.NewCipher(key)
@@ -410,11 +410,11 @@ func TestSM4GCMRandom(t *testing.T) {
 	}
 	got := aead.Seal(nil, nonce, plaintext, nil)
 
-	result, err := aead.Open(nil, nonce, got, nil)
+	result, err := aead.Open(got[:0], nonce, got, nil)
 	if err != nil {
 		t.Fatal(err)
 	}
 	if !bytes.Equal(result, plaintext) {
-		t.Error("gcm seal/open 464 bytes fail")
+		t.Error("gcm seal/open 408 bytes fail")
 	}
 }
diff --git a/cipher/hctr.go b/cipher/hctr.go
new file mode 100644
index 0000000..f8ac95d
--- /dev/null
+++ b/cipher/hctr.go
@@ -0,0 +1,212 @@
+package cipher
+
+import (
+	_cipher "crypto/cipher"
+	"encoding/binary"
+	"errors"
+
+	"github.com/emmansun/gmsm/internal/alias"
+	"github.com/emmansun/gmsm/internal/subtle"
+)
+
+// A LengthPreservingMode represents a block cipher running in a length preserving mode (HCTR,
+// HCTR2 etc).
+type LengthPreservingMode interface {
+	// Encrypt encrypts a number of plaintext bytes. The length of
+	// src must be NOT smaller than block size. Dst and src must overlap
+	// entirely or not at all.
+	//
+	// If len(dst) < len(src), Encrypt should panic. It is acceptable
+	// to pass a dst bigger than src, and in that case, Encrypt will
+	// only update dst[:len(src)] and will not touch the rest of dst.
+	//
+	// Multiple calls to Encrypt behave NOT same as if the concatenation of
+	// the src buffers was passed in a single run.
+	Encrypt(dst, src []byte)
+
+	// Decrypt decrypts a number of ciphertext bytes. The length of
+	// src must be NOT smaller than block size. Dst and src must overlap
+	// entirely or not at all.
+	//
+	// If len(dst) < len(src), Decrypt should panic. It is acceptable
+	// to pass a dst bigger than src, and in that case, Decrypt will
+	// only update dst[:len(src)] and will not touch the rest of dst.
+	//
+	// Multiple calls to Decrypt behave NOT same as if the concatenation of
+	// the src buffers was passed in a single run.
+	Decrypt(dst, src []byte)
+}
+
+// hctr represents a Varaible-Input-Length enciphering mode with a specific block cipher,
+// and specific tweak and a hash key. See
+// https://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.470.5288
+// GB/T 17964-2021 第11章 带泛杂凑函数的计数器工作模式
+type hctr struct {
+	cipher _cipher.Block
+	tweak  [blockSize]byte
+	hkey   [blockSize]byte
+}
+
+// NewHCTR returns a [LengthPreservingMode] which encrypts/decrypts useing the given [Block]
+// in HCTR mode. The lenght of tweak and hash key must be the same as the [Block]'s block size.
+func NewHCTR(cipher _cipher.Block, tweak, hkey []byte) (LengthPreservingMode, error) {
+	if len(tweak) != blockSize || len(hkey) != blockSize {
+		return nil, errors.New("hctr: invalid tweak and/or hash key length")
+	}
+	c := &hctr{}
+	c.cipher = cipher
+	copy(c.hkey[:], hkey)
+	copy(c.tweak[:], tweak)
+	return c, nil
+}
+
+func _mul2(v *[blockSize]byte) {
+	var carryIn byte
+	for j := range v {
+		carryOut := (v[j] << 7) & 0x80
+		v[j] = (v[j] >> 1) + carryIn
+		carryIn = carryOut
+	}
+	if carryIn != 0 {
+		v[0] ^= 0xE1 //  1<<7 | 1<<6 | 1<<5 | 1
+	}
+}
+
+// mul sets y to y*hkey.
+func (h *hctr) mul(y *[blockSize]byte) {
+	var z [blockSize]byte
+	for _, i := range h.hkey {
+		for k := 0; k < 8; k++ {
+			if (i>>(7-k))&1 == 1 {
+				subtle.XORBytes(z[:], z[:], y[:])
+			}
+			_mul2(y)
+		}
+	}
+	copy(y[:], z[:])
+}
+
+// Universal Hash Function.
+// Chapter 3.3 in https://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.470.5288.
+func (h *hctr) uhash(m []byte, dst *[blockSize]byte) {
+	for k := 0; k < blockSize; k++ {
+		dst[k] = 0
+	}
+	msg := m
+	for len(msg) >= blockSize {
+		subtle.XORBytes(dst[:], dst[:], msg[:blockSize])
+		h.mul(dst)
+		msg = msg[blockSize:]
+	}
+	var v [blockSize]byte
+	if len(msg) > 0 {
+		copy(v[:], msg)
+		copy(v[len(msg):], h.tweak[:])
+		subtle.XORBytes(dst[:], dst[:], v[:])
+		h.mul(dst)
+		copy(v[:], h.tweak[len(msg):])
+		for i := len(msg); i < blockSize; i++ {
+			v[i] = 0
+		}
+		subtle.XORBytes(dst[:], dst[:], v[:])
+		h.mul(dst)
+		for i := 0; i < len(msg); i++ {
+			v[i] = 0
+		}
+	} else {
+		subtle.XORBytes(dst[:], dst[:], h.tweak[:])
+		h.mul(dst)
+	}
+	// (|M|)₂
+	binary.BigEndian.PutUint64(v[8:], uint64(len(m)+blockSize)<<3)
+	subtle.XORBytes(dst[:], dst[:], v[:])
+	h.mul(dst)
+}
+
+func (h *hctr) Encrypt(ciphertext, plaintext []byte) {
+	if len(ciphertext) < len(plaintext) {
+		panic("hctr: ciphertext is smaller than plaintext")
+	}
+	if len(plaintext) < blockSize {
+		panic("hctr: plaintext length is smaller than the block size")
+	}
+	if alias.InexactOverlap(ciphertext[:len(plaintext)], plaintext) {
+		panic("hctr: invalid buffer overlap")
+	}
+
+	var z1, z2 [blockSize]byte
+
+	// a) z1 generation
+	h.uhash(plaintext[blockSize:], &z1)
+	subtle.XORBytes(z1[:], z1[:], plaintext[:blockSize])
+	// b) z2 generation
+	h.cipher.Encrypt(z2[:], z1[:])
+	// c) CTR
+	subtle.XORBytes(z1[:], z1[:], z2[:])
+	h.ctr(ciphertext[blockSize:], plaintext[blockSize:], &z1)
+	// d) first ciphertext block generation
+	h.uhash(ciphertext[blockSize:], &z1)
+	subtle.XORBytes(ciphertext, z2[:], z1[:])
+}
+
+func (h *hctr) Decrypt(plaintext, ciphertext []byte) {
+	if len(plaintext) < len(ciphertext) {
+		panic("hctr: plaintext is smaller than cihpertext")
+	}
+	if len(ciphertext) < blockSize {
+		panic("hctr: ciphertext length is smaller than the block size")
+	}
+	if alias.InexactOverlap(plaintext[:len(ciphertext)], ciphertext) {
+		panic("hctr: invalid buffer overlap")
+	}
+
+	var z1, z2 [blockSize]byte
+
+	// a) z2 generation
+	h.uhash(ciphertext[blockSize:], &z2)
+	subtle.XORBytes(z2[:], z2[:], ciphertext[:blockSize])
+	// b) z1 generation
+	h.cipher.Decrypt(z1[:], z2[:])
+	// c) CTR
+	subtle.XORBytes(z2[:], z2[:], z1[:])
+	h.ctr(plaintext[blockSize:], ciphertext[blockSize:], &z2)
+	// d) first plaintext block generation
+	h.uhash(plaintext[blockSize:], &z2)
+	subtle.XORBytes(plaintext, z2[:], z1[:])
+}
+
+func (h *hctr) ctr(dst, src []byte, baseCtr *[blockSize]byte) {
+	ctr := make([]byte, blockSize)
+	num := make([]byte, blockSize)
+	i := uint64(1)
+
+	if concCipher, ok := h.cipher.(concurrentBlocks); ok {
+		batchSize := concCipher.Concurrency() * blockSize
+		if len(src) >= batchSize {
+			var ctrs []byte = make([]byte, batchSize)
+			for len(src) >= batchSize {
+				for j := 0; j < concCipher.Concurrency(); j++ {
+					// (i)₂
+					binary.BigEndian.PutUint64(num[blockSize-8:], i)
+					subtle.XORBytes(ctrs[j*blockSize:], baseCtr[:], num)
+					i++
+				}
+				concCipher.EncryptBlocks(ctrs, ctrs)
+				subtle.XORBytes(dst, src, ctrs)
+				src = src[batchSize:]
+				dst = dst[batchSize:]
+			}
+		}
+	}
+
+	for len(src) > 0 {
+		// (i)₂
+		binary.BigEndian.PutUint64(num[blockSize-8:], i)
+		subtle.XORBytes(ctr, baseCtr[:], num)
+		h.cipher.Encrypt(ctr, ctr)
+		n := subtle.XORBytes(dst, src, ctr)
+		src = src[n:]
+		dst = dst[n:]
+		i++
+	}
+}
diff --git a/cipher/hctr_test.go b/cipher/hctr_test.go
new file mode 100644
index 0000000..61d9ca5
--- /dev/null
+++ b/cipher/hctr_test.go
@@ -0,0 +1,75 @@
+package cipher_test
+
+import (
+	"bytes"
+	"encoding/hex"
+	"testing"
+
+	"github.com/emmansun/gmsm/cipher"
+	"github.com/emmansun/gmsm/sm4"
+)
+
+var hctrSM4TestVectors = []struct {
+	key        string
+	hashKey    string
+	tweak      string
+	plaintext  string
+	ciphertext string
+}{
+	{
+		"2B7E151628AED2A6ABF7158809CF4F3C",
+		"000102030405060708090A0B0C0D0E0F",
+		"F0F1F2F3F4F5F6F7F8F9FAFBFCFDFEFF",
+		"6bc1bee22e409f96e93d7e117393172aae2d8a571e03ac9c9eb76fac45af8e5130c81c46a35ce411e5fbc1191a0a52eff69f2445df4f9b17ad2b417be66c37106bc1bee22e409f96e93d7e117393172aae2d8a571e03ac9c9eb76fac45af8e5130c81c46a35ce411e5fbc1191a0a52eff69f2445df4f9b17ad2b417be66c37106bc1bee22e409f96e93d7e117393172aae2d8a571e03ac9c9eb76fac45af8e5130c81c46a35ce411e5fbc1191a0a52eff69f2445df4f9b17ad2b417be66c3710",
+		"8858dda3034233e377936b76ce7edeb6a245075a37800b0b996e8e974c9032ac8de40d90ee4ee5fb58bc10cbc95779485ab38ffb0b4f961d85f086db705ff723edbeaec649b3b406b11b96a418a9c2c51ef41cdd24e472c18336e9efcd07b7e264a1e2d46615198eb74938d72104fa89294a6360cdb6b032a704cf07a087bb2283598552701b2f710d6528d9c3f4dab529afef4413f25169b6cbf8168ccbfa02a2f507513d0cb3802da34dbd928b67e6afc30ca91011070cfd40c2ef3d4ac041",
+	},
+	{
+		"2B7E151628AED2A6ABF7158809CF4F3C",
+		"000102030405060708090A0B0C0D0E0F",
+		"F0F1F2F3F4F5F6F7F8F9FAFBFCFDFEFF",
+		"6bc1bee22e409f96e93d7e117393172aae2d8a571e03ac9c9eb76fac45af8e5130c81c46a35ce411e5fbc1191a0a52eff69f2445df4f9b17ad2b417be66c3710",
+		"9cd7481d3b7ca904b14b4084d9d4c83ed39eac8e16747895fc2ae1eecd220276af3d0d2f21cb3807561347c81ad138117dd85c652afe16a47dc68eb884068ae3",
+	},
+	{
+		"2B7E151628AED2A6ABF7158809CF4F3C",
+		"000102030405060708090A0B0C0D0E0F",
+		"F0F1F2F3F4F5F6F7F8F9FAFBFCFDFEFF",
+		"6bc1bee22e409f96e93d7e117393172aae2d8a571e03ac9c9eb76fac45af8e5130c81c46a35ce411e5fbc1191a0a52eff69f2445df4f9b17ad2b417b",
+		"f7505aff357ac13107cdb2848c6bb2dcdda473f7a6ea939d44f52c986c11ca9341042f2b0091a1ca5c8f708cae8ca6a5c59e2228b3616c4455627722",
+	},
+	{
+		"2B7E151628AED2A6ABF7158809CF4F3C",
+		"000102030405060708090A0B0C0D0E0F",
+		"F0F1F2F3F4F5F6F7F8F9FAFBFCFDFEFF",
+		"6bc1bee22e409f96e93d7e117393172a",
+		"b7b1dd75f608012dc69621d4ea720a60",
+	},
+}
+
+func TestHCTR(t *testing.T) {
+	for i, test := range hctrSM4TestVectors {
+		key1, _ := hex.DecodeString(test.key)
+		key2, _ := hex.DecodeString(test.hashKey)
+		tw, _ := hex.DecodeString(test.tweak)
+		plaintext, _ := hex.DecodeString(test.plaintext)
+		ciphertext, _ := hex.DecodeString(test.ciphertext)
+		got := make([]byte, len(plaintext))
+		c, err := sm4.NewCipher(key1)
+		if err != nil {
+			t.Fatal(err)
+		}
+		hctr, err := cipher.NewHCTR(c, tw, key2)
+		if err != nil {
+			t.Fatal(err)
+		}
+		hctr.Encrypt(got, plaintext)
+		if !bytes.Equal(got, ciphertext) {
+			t.Fatalf("%v case encrypt failed, got %x\n", i+1, got)
+		}
+
+		hctr.Decrypt(got, ciphertext)
+		if !bytes.Equal(got, plaintext) {
+			t.Fatalf("%v case decrypt failed, got %x\n", i+1, got)
+		}
+	}
+}
diff --git a/cipher/xts.go b/cipher/xts.go
index 50fbeb7..22a9611 100644
--- a/cipher/xts.go
+++ b/cipher/xts.go
@@ -317,8 +317,10 @@ func (c *xtsDecrypter) CryptBlocks(plaintext, ciphertext []byte) {
 func mul2Generic(tweak *[blockSize]byte, isGB bool) {
 	var carryIn byte
 	if !isGB {
-		// tweak[0] represents the coefficients of {x^7, x^6, ..., x^0}
-		// tweak[15] represents the coefficients of {x^127, x^126, ..., x^120}
+		// the coefficient of x⁰ can be obtained by tweak[0] & 1
+		// the coefficient of x⁷ can be obtained by tweak[0] >> 7
+		// the coefficient of x¹²⁰ can be obtained by tweak[15] & 1
+		// the coefficient of x¹²⁷ can be obtained by tweak[15] >> 7
 		for j := range tweak {
 			carryOut := tweak[j] >> 7
 			tweak[j] = (tweak[j] << 1) + carryIn
@@ -334,9 +336,11 @@ func mul2Generic(tweak *[blockSize]byte, isGB bool) {
 			tweak[0] ^= GF128_FDBK // 1<<7 | 1<<2 | 1<<1 | 1
 		}
 	} else {
-		// GB/T 17964-2021, because of the bit-ordering, doubling is actually a right shift.
-		// tweak[0] represents the coefficients of {x^0, x^1, ..., x^7}
-		// tweak[15] represents the coefficients of {x^120, x^121, ..., x^127}
+		// GB/T 17964-2021, 
+		// the coefficient of x⁰ can be obtained by tweak[0] >> 7
+		// the coefficient of x⁷ can be obtained by tweak[0] & 1
+		// the coefficient of x¹²⁰ can be obtained by tweak[15] >> 7
+		// the coefficient of x¹²⁷ can be obtained by tweak[15] & 1
 		for j := range tweak {
 			carryOut := (tweak[j] << 7) & 0x80
 			tweak[j] = (tweak[j] >> 1) + carryIn
diff --git a/internal/subtle/xor_amd64.s b/internal/subtle/xor_amd64.s
index 9e57e81..f2dfa15 100644
--- a/internal/subtle/xor_amd64.s
+++ b/internal/subtle/xor_amd64.s
@@ -62,7 +62,7 @@ ret:
 	RET
 
 avx2:
-	TESTQ $31, DX            // AND 31 & len, if not zero jump to not_aligned.
+	TESTQ $31, DX            // AND 31 & len, if not zero jump to avx2_not_aligned.
 	JNZ   avx2_not_aligned
 
 avx2_aligned: