fix: refactor processing of junk packets (#103)

- fix the bug that transport packet interprets as init/resp/cookie with the same size
- cleanup error responses
- reduce buffer allocations

device/obf_datasize.go

@@ -0,0 +1,38 @@
+package device
+
+import "strconv"
+
+func newDataSizeObf(val string) (obf, error) {
+	length, err := strconv.Atoi(val)
+	if err != nil {
+		return nil, err
+	}
+
+	return &dataSizeObf{
+		length: length,
+	}, nil
+}
+
+type dataSizeObf struct {
+	length int
+}
+
+func (o *dataSizeObf) Obfuscate(dst, src []byte) {
+	srcLen := len(src)
+	for i := o.length - 1; i >= 0; i-- {
+		dst[i] = byte(srcLen & 0xFF)
+		srcLen >>= 8
+	}
+}
+
+func (o *dataSizeObf) Deobfuscate(dst, src []byte) bool {
+	return true
+}
+
+func (o *dataSizeObf) ObfuscatedLen(srcLen int) int {
+	return o.length
+}
+
+func (o *dataSizeObf) DeobfuscatedLen(srcLen int) int {
+	return 0
+}