Protocol

Protocol

The *** protocol is very similar to SOCKS5 but encrypted and simpler.

Below is the structure of a *** request (sent from client-side), which is identical for both TCP and UDP connections before encrypted (or after decrypted).

+--------------+---------------------+------------------+----------+
| Address Type | Destination Address | Destination Port |   Data   |
+--------------+---------------------+------------------+----------+
|      1       |       Variable      |         2        | Variable |
+--------------+---------------------+------------------+----------+

Possible values of address type are 1 (IPv4), 4 (IPv6), 3 (hostname). For IPv4 address, it's packed as a 32-bit (4-byte) big-endian integer. For IPv6 address, a compact representation (16-byte array) is used. For hostname, the first byte of destination address indicates the length, which limits the length of hostname to 255. The destination port is also a big-endian integer.

The request is encrypted using the specified cipher with a random IV and the pre-shared key, it then becomes so-called payload.

TCP

The first packet of a *** TCP connection sent either from server-side or client-side must contains the randomly generated IV that used for the encryption.

+-------+----------+
|  IV   | Payload  |
+-------+----------+
| Fixed | Variable |
+-------+----------+

Once this packet is received, payload is decrypted using the specified cipher with the IV in the packet and the pre-shared key. For the server-side, the data is then forwarded to the destination. For client-side, the data is forwarded to the application. And this *** TCP relay goes into stream stage, in which the data is being encrypted with the same IV and transmitted directly without IV prepended.

+----------+
| Payload  |
+----------+
| Variable |
+----------+

UDP

When the client-side receives a UDP request from other applications, RSV and FRAG are dropped and a *** UDP request is made out from it. A random IV is always generated and used for the encryption of *** UDP request and response. Therefore, all UDP requests and responses have the same structure, no matter whether it's the first packet or not.

+-------+----------+
|  IV   | Payload  |
+-------+----------+
| Fixed | Variable |
+-------+----------+