fix: pad function calculating minQuestionSize (#25)

fix problem with calculating closest divisible by 64
closes #24

util.go

@@ -42,7 +42,7 @@ import (
 // <56-bytes-query> 0x80 (0x00 * 199)
 func pad(packet []byte) []byte {
 	// get closest divisible by 64 to <packet-len> + 1 byte for 0x80
-	minQuestionSize := (len(packet)+1+63)/64 + 64
+	minQuestionSize := len(packet) + 1 + (64 - (len(packet)+1)%64)
 
 	// padded size can't be less than minUDPQuestionSize
 	minQuestionSize = max(minUDPQuestionSize, minQuestionSize)

util_test.go

@@ -0,0 +1,31 @@
+package dnscrypt
+
+import (
+	"crypto/rand"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestPadUnpad(t *testing.T) {
+	longBuf := make([]byte, 272)
+	_, err := rand.Read(longBuf)
+	require.NoError(t, err)
+
+	tests := []struct {
+		packet       []byte
+		expPaddedLen int
+	}{
+		{[]byte("Example Test DNS packet"), 256},
+		{longBuf, 320},
+	}
+	for i, test := range tests {
+		padded := pad(test.packet)
+		assert.Equal(t, test.expPaddedLen, len(padded), "test %d", i)
+
+		unpadded, err := unpad(padded)
+		assert.Nil(t, err, "test %d", i)
+		assert.Equal(t, test.packet, unpadded, "test %d", i)
+	}
+}