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_randchars.go

@@ -0,0 +1,48 @@
+package device
+
+import (
+	"crypto/rand"
+	"strconv"
+	"unicode"
+)
+
+const chars52 = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
+
+func newRandCharObf(val string) (obf, error) {
+	length, err := strconv.Atoi(val)
+	if err != nil {
+		return nil, err
+	}
+
+	return &randCharObf{
+		length: length,
+	}, nil
+}
+
+type randCharObf struct {
+	length int
+}
+
+func (o *randCharObf) Obfuscate(dst, src []byte) {
+	rand.Read(dst[:o.length])
+	for i := range dst[:o.length] {
+		dst[i] = chars52[dst[i]%52]
+	}
+}
+
+func (o *randCharObf) Deobfuscate(dst, src []byte) bool {
+	for _, b := range src[:o.length] {
+		if !unicode.IsLetter(rune(b)) {
+			return false
+		}
+	}
+	return true
+}
+
+func (o *randCharObf) ObfuscatedLen(n int) int {
+	return o.length
+}
+
+func (o *randCharObf) DeobfuscatedLen(n int) int {
+	return 0
+}