One Time Auth

One-time authentication (shortened as OTA) is a new experimental feature designed to improve the security against CCA. You should understand the protocol before reading this document.

By default, the server that supports OTA should run in the compatible mode. OTA is only applied if the client's request header has a flag set. However, if the server switch on OTA explicitly, all clients must switch on OTA, otherwise connections will be denied.

The authentication method is HMAC-SHA1 which has wide supports among all major platforms and fairly good speed.


The structure of an OTA-enabled request (unencrypted) is shown below:

| ATYP | Destination Address | Destination Port | HMAC-SHA1 |
|  1   |       Variable      |         2        |    10     |

ATYP is a 8-bit char where the rightmost four bits, 0b00001111 (0xf), are reserved for address types, the flag bit of OTA is 0b00010000 (0x10). In C/C++, simply check if ATYP & 0x10 == 0x10, then OTA is enabled.

The key of HMAC-SHA1 is (IV + KEY), and the input is the whole header (not including HMAC-SHA1). The output of HMAC-SHA1 is truncated to leftmost 80 bits (10 bytes) according to RFC 2104.

Chunk Authentication

The structure of an OTA-enabled chunk (decrypted) of shadowsocks TCP relay is shown below:

| DATA.LEN | HMAC-SHA1 |   DATA   | ...
|     2    |     10    | Variable | ...

DATA.LEN is a 16-bit big-endian integer indicating the length of DATA.

The input of HMAC-SHA1 is DATA. And the key of HMAC-SHA1 is (IV + Chunk ID) where Chunk ID is an unsigned integer counted per connection from 0. In order to achieve this, both server side and client side need to keep a counter for each TCP connection. Chunk ID must be converted to big-endian before constructing the key of HMAC-SHA1.

For a client, after constructing an OTA-enabled request, the whole chunk is encrypted as a payload then sent to server-side.


  • The server must check the completeness of a shadowsocks TCP request before verifying HMAC-SHA1 and forwarding.
  • The chunk authentication is only applied for the packets sent from client-side shadowsocks.


There is no session in UDP relay, each UDP packet contains both header and data. Therefore, for an OTA-enabled UDP packet, the datagram structure (unencrypted) is slightly different:

| ATYP | Destination Address | Destination Port |   DATA   | HMAC-SHA1 |
|  1   |       Variable      |         2        | Variable |     10    |

The key of HMAC-SHA1 is (IV + KEY), and the input is the header plus data.