diff --git a/api/stun1.txt b/api/stun1.txt
index 2f05693..8bd0929 100644
--- a/api/stun1.txt
+++ b/api/stun1.txt
@@ -134,7 +134,6 @@ pkg github.com/gortc/stun, method (*Client) Do(*Message, time.Time, func(Event))
 pkg github.com/gortc/stun, method (*Client) Indicate(*Message) error
 pkg github.com/gortc/stun, method (*Client) Start(*Message, time.Time, Handler) error
 pkg github.com/gortc/stun, method (*ErrorCodeAttribute) GetFrom(*Message) error
-pkg github.com/gortc/stun, method (*IntegrityErr) Error() string
 pkg github.com/gortc/stun, method (*MappedAddress) AddTo(*Message) error
 pkg github.com/gortc/stun, method (*MappedAddress) GetFrom(*Message) error
 pkg github.com/gortc/stun, method (*Message) Add(AttrType, []uint8)
@@ -274,9 +273,6 @@ pkg github.com/gortc/stun, type Getter interface, GetFrom(*Message) error
 pkg github.com/gortc/stun, type Handler interface { HandleEvent }
 pkg github.com/gortc/stun, type Handler interface, HandleEvent(Event)
 pkg github.com/gortc/stun, type HandlerFunc func(Event)
-pkg github.com/gortc/stun, type IntegrityErr struct
-pkg github.com/gortc/stun, type IntegrityErr struct, Actual []uint8
-pkg github.com/gortc/stun, type IntegrityErr struct, Expected []uint8
 pkg github.com/gortc/stun, type MappedAddress struct
 pkg github.com/gortc/stun, type MappedAddress struct, IP net.IP
 pkg github.com/gortc/stun, type MappedAddress struct, Port int
@@ -322,6 +318,7 @@ pkg github.com/gortc/stun, var ErrClientClosed error
 pkg github.com/gortc/stun, var ErrClientNotInitialized error
 pkg github.com/gortc/stun, var ErrDecodeToNil error
 pkg github.com/gortc/stun, var ErrFingerprintBeforeIntegrity error
+pkg github.com/gortc/stun, var ErrIntegrityMismatch error
 pkg github.com/gortc/stun, var ErrNoConnection error
 pkg github.com/gortc/stun, var ErrNoDefaultReason error
 pkg github.com/gortc/stun, var ErrTransactionExists error
diff --git a/checksize.go b/checks.go
similarity index 56%
rename from checksize.go
rename to checks.go
index 0f376f5..10e604d 100644
--- a/checksize.go
+++ b/checks.go
@@ -2,6 +2,8 @@
 
 package stun
 
+import "github.com/gortc/stun/internal/hmac"
+
 // CheckSize returns ErrAttrSizeInvalid if got is not equal to expected.
 func CheckSize(_ AttrType, got, expected int) error {
 	if got == expected {
@@ -9,3 +11,10 @@ func CheckSize(_ AttrType, got, expected int) error {
 	}
 	return ErrAttrSizeInvalid
 }
+
+func checkHMAC(got, expected []byte) error {
+	if hmac.Equal(got, expected) {
+		return nil
+	}
+	return ErrIntegrityMismatch
+}
diff --git a/checksize_debug.go b/checks_debug.go
similarity index 57%
rename from checksize_debug.go
rename to checks_debug.go
index ee7328a..4a1a73f 100644
--- a/checksize_debug.go
+++ b/checks_debug.go
@@ -2,6 +2,8 @@
 
 package stun
 
+import "github.com/gortc/stun/internal/hmac"
+
 // CheckSize returns *AttrLengthError if got is not equal to expected.
 func CheckSize(a AttrType, got, expected int) error {
 	if got == expected {
@@ -13,3 +15,13 @@ func CheckSize(a AttrType, got, expected int) error {
 		Attr:     a,
 	}
 }
+
+func checkHMAC(got, expected []byte) error {
+	if hmac.Equal(got, expected) {
+		return nil
+	}
+	return &IntegrityErr{
+		Expected: expected,
+		Actual:   got,
+	}
+}
diff --git a/go.test.sh b/go.test.sh
index 66c0ea4..8c3b012 100755
--- a/go.test.sh
+++ b/go.test.sh
@@ -9,6 +9,9 @@ go test -tags gofuzz -run TestFuzz -v .
 # quick-test without -race
 go test ./...
 
+# test with "debug" tag
+go test -tags debug ./...
+
 for d in $(go list ./... | grep -v vendor); do
     go test -race -coverprofile=profile.out -covermode=atomic "$d"
     if [ -f profile.out ]; then
diff --git a/integrity.go b/integrity.go
index a03032c..d83535b 100644
--- a/integrity.go
+++ b/integrity.go
@@ -85,18 +85,8 @@ func (i MessageIntegrity) AddTo(m *Message) error {
 	return nil
 }
 
-// IntegrityErr occurs when computed HMAC differs from expected.
-type IntegrityErr struct {
-	Expected []byte
-	Actual   []byte
-}
-
-func (i *IntegrityErr) Error() string {
-	return fmt.Sprintf(
-		"Integrity check failed: 0x%x (expected) !- 0x%x (actual)",
-		i.Expected, i.Actual,
-	)
-}
+// ErrIntegrityMismatch means that computed HMAC differs from expected.
+var ErrIntegrityMismatch = errors.New("integrity check failed")
 
 func newHMAC(key, message, buf []byte) []byte {
 	mac := hmac.AcquireSHA1(key)
@@ -137,11 +127,8 @@ func (i MessageIntegrity) Check(m *Message) error {
 	expected := newHMAC(i, b, m.Raw[len(m.Raw):])
 	m.Length = length
 	m.WriteLength() // writing length back
-	if !hmac.Equal(v, expected) {
-		return &IntegrityErr{
-			Expected: expected,
-			Actual:   v,
-		}
+	if err = checkHMAC(v, expected); err != nil {
+		return err
 	}
 	return nil
 }
diff --git a/integrity_debug.go b/integrity_debug.go
new file mode 100644
index 0000000..6b8a303
--- /dev/null
+++ b/integrity_debug.go
@@ -0,0 +1,18 @@
+// +build debug
+
+package stun
+
+import "fmt"
+
+// IntegrityErr occurs when computed HMAC differs from expected.
+type IntegrityErr struct {
+	Expected []byte
+	Actual   []byte
+}
+
+func (i *IntegrityErr) Error() string {
+	return fmt.Sprintf(
+		"Integrity check failed: 0x%x (expected) !- 0x%x (actual)",
+		i.Expected, i.Actual,
+	)
+}
diff --git a/integrity_test.go b/integrity_test.go
index ba5f23f..1bd64db 100644
--- a/integrity_test.go
+++ b/integrity_test.go
@@ -3,7 +3,6 @@ package stun
 import (
 	"bytes"
 	"encoding/hex"
-	"fmt"
 	"testing"
 )
 
@@ -14,10 +13,7 @@ func TestMessageIntegrity_AddTo_Simple(t *testing.T) {
 		t.Fatal(err)
 	}
 	if !bytes.Equal(expected, i) {
-		t.Error(&IntegrityErr{
-			Expected: expected,
-			Actual:   i,
-		})
+		t.Error(ErrIntegrityMismatch)
 	}
 	t.Run("Check", func(t *testing.T) {
 		m := new(Message)
@@ -36,8 +32,8 @@ func TestMessageIntegrity_AddTo_Simple(t *testing.T) {
 			t.Error(err)
 		}
 		dM.Raw[24] += 12 // HMAC now invalid
-		if err, ok := i.Check(dM).(*IntegrityErr); !ok {
-			t.Error(err, "should be *IntegrityErr")
+		if i.Check(dM) == nil {
+			t.Error("should be invalid")
 		}
 	})
 }
@@ -64,12 +60,8 @@ func TestMessageIntegrityWithFingerprint(t *testing.T) {
 		t.Fatal(err)
 	}
 	m.Raw[24] = 33
-	errStr := fmt.Sprintf("Integrity check failed: 0x%s (expected) !- 0x%s (actual)",
-		"19985afb819c098acfe1c2771881227f14c70eaf",
-		"ef9da0e0caf0b0e4ff321e7b56f1e114c802cb7e",
-	)
-	if err := i.Check(m); err.Error() != errStr {
-		t.Fatal(err, "!=", errStr)
+	if err := i.Check(m); err == nil {
+		t.Fatal("mismatch expected")
 	}
 }
 
diff --git a/message_test.go b/message_test.go
index f329ceb..7e16edc 100644
--- a/message_test.go
+++ b/message_test.go
@@ -707,14 +707,6 @@ func ExampleMessage() {
 	fmt.Println("for corrupted message:")
 	decoded.Raw[22] = 33
 	fmt.Println("fingerprint:", Fingerprint.Check(decoded))
-	iErr, ok := i.Check(decoded).(*IntegrityErr)
-	if ok {
-		fmt.Println("integrity check failed")
-		fmt.Printf("got:  %x\n", iErr.Actual)
-		fmt.Printf("want: %x\n", iErr.Expected)
-	} else {
-		fmt.Println("assertion failed")
-	}
 
 	// Output:
 	// binding request l=48 attrs=3 id=AQIDBAUGBwgJAAEA buff length: 68
@@ -726,9 +718,6 @@ func ExampleMessage() {
 	// integrity ok
 	// for corrupted message:
 	// fingerprint: CRC mismatch: b36d2c38 (expected) != 8ef13141 (actual)
-	// integrity check failed
-	// got:  06f0692c159f4256c14b9442927889e341256ac2
-	// want: c1105962efee5c96f4f194cc91b4eb8ab7667c7a
 }
 
 func TestAllocations(t *testing.T) {