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_timestamp.go

@@ -0,0 +1,31 @@
+package device
+
+import (
+	"encoding/binary"
+	"time"
+)
+
+func newTimestampObf(_ string) (obf, error) {
+	return &timestampObf{}, nil
+}
+
+type timestampObf struct{}
+
+func (o *timestampObf) Obfuscate(dst, src []byte) {
+	t := uint32(time.Now().Unix())
+	binary.BigEndian.PutUint32(dst, t)
+}
+
+func (o *timestampObf) Deobfuscate(dst, src []byte) bool {
+	// replay attack check?
+	// requires time to be always synchronized
+	return true
+}
+
+func (o *timestampObf) ObfuscatedLen(n int) int {
+	return 4
+}
+
+func (o *timestampObf) DeobfuscatedLen(n int) int {
+	return 0
+}