Add optional aes128xor layer

https://github.com/XTLS/Xray-core/pull/4952#issuecomment-3172703168

proxy/vless/encryption/xor.go

@@ -0,0 +1,63 @@
+package encryption
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/rand"
+	"io"
+	"net"
+)
+
+type XorConn struct {
+	net.Conn
+	key     []byte
+	ctr     cipher.Stream
+	peerCtr cipher.Stream
+}
+
+func NewXorConn(conn net.Conn, key []byte) *XorConn {
+	return &XorConn{Conn: conn, key: key[:16]}
+}
+
+func (c *XorConn) Write(b []byte) (int, error) {
+	if len(b) == 0 {
+		return 0, nil
+	}
+	var iv []byte
+	if c.ctr == nil {
+		block, _ := aes.NewCipher(c.key)
+		iv = make([]byte, 16)
+		rand.Read(iv)
+		c.ctr = cipher.NewCTR(block, iv)
+	}
+	c.ctr.XORKeyStream(b, b) // caller MUST discard b
+	if iv != nil {
+		b = append(iv, b...)
+	}
+	if _, err := c.Conn.Write(b); err != nil {
+		return 0, err
+	}
+	if iv != nil {
+		b = b[16:]
+	}
+	return len(b), nil
+}
+
+func (c *XorConn) Read(b []byte) (int, error) {
+	if len(b) == 0 {
+		return 0, nil
+	}
+	if c.peerCtr == nil {
+		peerIv := make([]byte, 16)
+		if _, err := io.ReadFull(c.Conn, peerIv); err != nil {
+			return 0, err
+		}
+		block, _ := aes.NewCipher(c.key)
+		c.peerCtr = cipher.NewCTR(block, peerIv)
+	}
+	n, err := c.Conn.Read(b)
+	if n > 0 {
+		c.peerCtr.XORKeyStream(b[:n], b[:n])
+	}
+	return n, err
+}