diff --git a/cipher_test.go b/cipher_test.go
new file mode 100644
index 0000000..b0152a7
--- /dev/null
+++ b/cipher_test.go
@@ -0,0 +1,136 @@
+// cipher_test.go
+package fastchacha20
+
+import (
+	"bytes"
+	"crypto/rand"
+	"testing"
+)
+
+func generateRandomKey(t *testing.T, size int) []byte {
+	key := make([]byte, size)
+	if _, err := rand.Read(key); err != nil {
+		t.Fatalf("Failed to generate random key: %v", err)
+	}
+	return key
+}
+
+func generateRandomNonce(t *testing.T, size int) []byte {
+	nonce := make([]byte, size)
+	if _, err := rand.Read(nonce); err != nil {
+		t.Fatalf("Failed to generate random nonce: %v", err)
+	}
+	return nonce
+}
+
+func TestEncryptDecrypt(t *testing.T) {
+	// Generate random key and nonce
+	key := generateRandomKey(t, 32)     // 256-bit key
+	nonce := generateRandomNonce(t, 24) // 192-bit nonce (for ChaCha20-Poly1305)
+
+	cipher, err := NewCipher(key)
+	if err != nil {
+		t.Fatalf("Failed to create cipher: %v", err)
+	}
+
+	plaintext := []byte("This is a test plaintext message.")
+	aad := []byte("Additional data for authentication")
+
+	// Encrypt the plaintext
+	ciphertext, err := cipher.Encrypt(nonce, plaintext, aad)
+	if err != nil {
+		t.Fatalf("Encryption failed: %v", err)
+	}
+
+	// Decrypt the ciphertext
+	decryptedPlaintext, err := cipher.Decrypt(nonce, ciphertext, aad)
+	if err != nil {
+		t.Fatalf("Decryption failed: %v", err)
+	}
+
+	// Verify that the decrypted plaintext matches the original plaintext
+	if !bytes.Equal(plaintext, decryptedPlaintext) {
+		t.Errorf("Decrypted plaintext does not match the original.\nOriginal: %s\nDecrypted: %s", plaintext, decryptedPlaintext)
+	}
+}
+
+func TestEncryptDecryptWithEmptyPlaintext(t *testing.T) {
+	// Generate random key and nonce
+	key := generateRandomKey(t, 32)
+	nonce := generateRandomNonce(t, 24)
+
+	cipher, err := NewCipher(key)
+	if err != nil {
+		t.Fatalf("Failed to create cipher: %v", err)
+	}
+
+	plaintext := []byte("") // Empty plaintext
+	aad := []byte("Additional data")
+
+	// Encrypt the empty plaintext
+	ciphertext, err := cipher.Encrypt(nonce, plaintext, aad)
+	if err != nil {
+		t.Fatalf("Encryption failed: %v", err)
+	}
+
+	// Decrypt the ciphertext
+	decryptedPlaintext, err := cipher.Decrypt(nonce, ciphertext, aad)
+	if err != nil {
+		t.Fatalf("Decryption failed: %v", err)
+	}
+
+	// Verify that the decrypted plaintext is also empty
+	if len(decryptedPlaintext) != 0 {
+		t.Errorf("Expected empty decrypted plaintext, got length %d", len(decryptedPlaintext))
+	}
+}
+
+func TestInvalidNonceLength(t *testing.T) {
+	// Generate a valid key
+	key := generateRandomKey(t, 32)
+
+	cipher, err := NewCipher(key)
+	if err != nil {
+		t.Fatalf("Failed to create cipher: %v", err)
+	}
+
+	// Generate invalid nonce
+	invalidNonce := generateRandomNonce(t, 16) // Should be 24 bytes for ChaCha20-Poly1305
+	plaintext := []byte("Test plaintext")
+	aad := []byte("AAD data")
+
+	// Try to encrypt with the invalid nonce
+	_, err = cipher.Encrypt(invalidNonce, plaintext, aad)
+	if err == nil {
+		t.Errorf("Expected error due to invalid nonce length, but encryption succeeded")
+	}
+}
+
+func TestDecryptWithInvalidNonce(t *testing.T) {
+	// Generate random key and nonce
+	key := generateRandomKey(t, 32)
+	nonce := generateRandomNonce(t, 24)
+
+	cipher, err := NewCipher(key)
+	if err != nil {
+		t.Fatalf("Failed to create cipher: %v", err)
+	}
+
+	plaintext := []byte("Valid plaintext message.")
+	aad := []byte("Authenticated data")
+
+	// Encrypt the plaintext
+	ciphertext, err := cipher.Encrypt(nonce, plaintext, aad)
+	if err != nil {
+		t.Fatalf("Encryption failed: %v", err)
+	}
+
+	// Modify the nonce (introduce an error)
+	invalidNonce := generateRandomNonce(t, 24)
+
+	// Attempt to decrypt with an invalid nonce
+	_, err = cipher.Decrypt(invalidNonce, ciphertext, aad)
+	if err == nil {
+		t.Error("Decryption should have failed due to invalid nonce, but it succeeded")
+	}
+}
diff --git a/parallel.go b/parallel.go
index 9286536..29eca00 100644
--- a/parallel.go
+++ b/parallel.go
@@ -1,11 +1,10 @@
-// fastchacha20_parallel.go
+// parallel.go
 package fastchacha20
 
 import (
 	"bytes"
 	"crypto/rand"
 	"encoding/binary"
-	"errors"
 	"sync"
 )
 
@@ -40,9 +39,16 @@ func (c *Cipher) EncryptChunks(plaintext []byte) ([][]byte, error) {
 			aad := make([]byte, 8)
 			binary.BigEndian.PutUint64(aad, uint64(i))
 
+			// Encrypt the chunk using the Encrypt method from cipher.go
+			ciphertext, e := c.Encrypt(nonce, chunk, aad)
+			if e != nil {
+				errOnce.Do(func() { err = e })
+				return
+			}
+
 			// Prepend nonce to the ciphertext for storage
-			ciphertext := c.aead.Seal(nonce, nonce, chunk, aad)
-			encryptedChunks[i] = ciphertext
+			encryptedChunk := append(nonce, ciphertext...)
+			encryptedChunks[i] = encryptedChunk
 		}(i)
 	}
 	wg.Wait()
@@ -68,7 +74,7 @@ func (c *Cipher) DecryptChunks(encryptedChunks [][]byte) ([]byte, error) {
 			defer wg.Done()
 			nonceSize := c.aead.NonceSize()
 			if len(chunk) < nonceSize+16 { // 16 bytes for Poly1305 tag
-				errOnce.Do(func() { err = errors.New("ciphertext too short") })
+				errOnce.Do(func() { err = ErrInvalidCiphertext })
 				return
 			}
 
@@ -79,7 +85,8 @@ func (c *Cipher) DecryptChunks(encryptedChunks [][]byte) ([]byte, error) {
 			aad := make([]byte, 8)
 			binary.BigEndian.PutUint64(aad, uint64(i))
 
-			plaintext, e := c.aead.Open(nil, nonce, ciphertext, aad)
+			// Decrypt the chunk using the Decrypt method from cipher.go
+			plaintext, e := c.Decrypt(nonce, ciphertext, aad)
 			if e != nil {
 				errOnce.Do(func() { err = e })
 				return