diff --git a/internal/saltfilter.go b/internal/saltfilter.go
index 8913da54..edb7e2d1 100644
--- a/internal/saltfilter.go
+++ b/internal/saltfilter.go
@@ -27,7 +27,7 @@ var initSaltfilterOnce sync.Once
 
 // GetSaltFilterSingleton returns the BloomRing singleton,
 // initializing it on first call.
-func GetSaltFilterSingleton() *BloomRing {
+func getSaltFilterSingleton() *BloomRing {
 	initSaltfilterOnce.Do(func() {
 		var (
 			finalCapacity = DefaultSFCapacity
@@ -69,3 +69,13 @@ func GetSaltFilterSingleton() *BloomRing {
 	})
 	return saltfilter
 }
+
+// TestSalt returns true if salt is repeated
+func TestSalt(b []byte) bool {
+	return getSaltFilterSingleton().Test(b)
+}
+
+// AddSalt salt to filter
+func AddSalt(b []byte) {
+	getSaltFilterSingleton().Add(b)
+}
diff --git a/shadowaead/packet.go b/shadowaead/packet.go
index 7329f2b0..6f48f14c 100644
--- a/shadowaead/packet.go
+++ b/shadowaead/packet.go
@@ -29,7 +29,7 @@ func Pack(dst, plaintext []byte, ciph Cipher) ([]byte, error) {
 	if err != nil {
 		return nil, err
 	}
-	internal.GetSaltFilterSingleton().Add(salt)
+	internal.AddSalt(salt)
 
 	if len(dst) < saltSize+len(plaintext)+aead.Overhead() {
 		return nil, io.ErrShortBuffer
@@ -45,16 +45,15 @@ func Unpack(dst, pkt []byte, ciph Cipher) ([]byte, error) {
 	if len(pkt) < saltSize {
 		return nil, ErrShortPacket
 	}
-	saltfilter := internal.GetSaltFilterSingleton()
 	salt := pkt[:saltSize]
-	if saltfilter.Test(salt) {
+	if internal.TestSalt(salt) {
 		return nil, ErrRepeatedSalt
 	}
 	aead, err := ciph.Decrypter(salt)
 	if err != nil {
 		return nil, err
 	}
-	saltfilter.Add(salt)
+	internal.AddSalt(salt)
 	if len(pkt) < saltSize+aead.Overhead() {
 		return nil, ErrShortPacket
 	}
diff --git a/shadowaead/stream.go b/shadowaead/stream.go
index 1af2ea3f..a41e14ea 100644
--- a/shadowaead/stream.go
+++ b/shadowaead/stream.go
@@ -205,15 +205,14 @@ func (c *streamConn) initReader() error {
 	if _, err := io.ReadFull(c.Conn, salt); err != nil {
 		return err
 	}
-	saltfilter := internal.GetSaltFilterSingleton()
-	if saltfilter.Test(salt) {
+	if internal.TestSalt(salt) {
 		return ErrRepeatedSalt
 	}
 	aead, err := c.Decrypter(salt)
 	if err != nil {
 		return err
 	}
-	saltfilter.Add(salt)
+	internal.AddSalt(salt)
 
 	c.r = newReader(c.Conn, aead)
 	return nil
@@ -250,7 +249,7 @@ func (c *streamConn) initWriter() error {
 	if err != nil {
 		return err
 	}
-	internal.GetSaltFilterSingleton().Add(salt)
+	internal.AddSalt(salt)
 	c.w = newWriter(c.Conn, aead)
 	return nil
 }