diff --git a/sm4/cbc_cipher_asm.go b/sm4/cbc_cipher_asm.go
index 7ec8155..5a65b02 100644
--- a/sm4/cbc_cipher_asm.go
+++ b/sm4/cbc_cipher_asm.go
@@ -10,8 +10,8 @@ import (
 	"github.com/emmansun/gmsm/internal/xor"
 )
 
-// Assert that sm4CipherAsm implements the cbcDecAble interfaces.
-var _ cbcDecAble = (*sm4CipherAsm)(nil)
+// Assert that sm4CipherAsm implements the cbcEncAble and cbcDecAble interfaces.
+var _ cbcEncAble = (*sm4CipherAsm)(nil)
 var _ cbcDecAble = (*sm4CipherAsm)(nil)
 
 const cbcEncrypt = 1
diff --git a/sm4/modes.go b/sm4/modes.go
index 93bfdd7..61e58e2 100644
--- a/sm4/modes.go
+++ b/sm4/modes.go
@@ -2,6 +2,13 @@ package sm4
 
 import "crypto/cipher"
 
+// cbcEncAble is implemented by cipher.Blocks that can provide an optimized
+// implementation of CBC encryption through the cipher.BlockMode interface.
+// See crypto/cipher/cbc.go.
+type cbcEncAble interface {
+	NewCBCEncrypter(iv []byte) cipher.BlockMode
+}
+
 // cbcDecAble is implemented by cipher.Blocks that can provide an optimized
 // implementation of CBC decryption through the cipher.BlockMode interface.
 // See crypto/cipher/cbc.go.