5 govpn is simple high-performance secure virtual private network daemon.
6 It uses DH-EKE for mutual zero-knowledge authentication and
7 authenticated encrypted transport.
11 All packets captured on network interface are encrypted, authenticated
12 and sent to remote server, that writes them to his interface, and vice
13 versa. Client and server use pre-shared authentication key (PSK).
14 Because of stateless UDP nature, after some timeout of inactivity peers
15 forget about each other and have to retry handshake process again. As a
16 rule, there are enough time-to-time traffic in ordinary Ethernet
17 networks to heartbeat connection.
19 Handshake is used to mutually authenticate peers, exchange common secret
20 per-session encryption key and checks UDP transport availability.
22 Because of UDP and authentication overhead: each packet grows in size
23 during transmission, so you have to lower you maximum transmission unit
24 (MTU) on network interface.
26 High security and high performance are the goals for that daemon. It
27 uses fast cryptography algorithms with 128bit security margin, strong
28 mutual zero-knowledge authentication and perfect-forward secrecy
29 property. An attacker can not know anything from captured traffic, even
30 if pre-shared key is compromised.
35 * Perfect-forward secrecy (if long-term pre-shared keys are compromised,
36 no captured traffic can be decrypted anyway)
37 * Mutual two-side authentication (noone will send real network interface
38 data unless the other side is authenticated)
39 * Zero-knowledge authentication (pre-shared key is not transmitted in
40 any form between the peers, not even it's hash value)
45 B -- bad UDP packet (some system error)
46 T -- bad tag on packet (MiTM, unordered packet)
47 R -- invalid sequence number (MiTM, unordered packet)
48 [HS?] -- unknown handshake message
49 w -- successful write to remote peer
50 r -- successful read from remote peer
51 [HS1], [HS2], [HS3], [HS4] -- handshake packet stage
52 [rS?] -- invalid server's random authentication number received (MiTM, bad PSK)
53 [rC?] -- invalid client's random authentication number received (MiTM, bad PSK)
54 [S?] -- invalid handshake stage is trying to perform (MiTM, duplicate packet)
55 [OK] -- handshake's stage passed
60 Message authentication: Poly1305
61 Password authenticated key agreement: Curve25519 based DH-EKE
62 Packet overhead: 24 bytes per packet
63 Handshake overhead: 4 UDP (2 from client, 2 from server) packets,
64 232 bytes total payload
68 SERIAL + ENC(KEY, SERIAL, DATA) + AUTH(SERIAL + ENC_DATA)
70 where SERIAL is message serial number. Odds are reserved for
71 client->server, evens are for server->client. SERIAL is used as a nonce
72 for DATA encryption: encryption key is different during each handshake,
73 so (key, nonce) pair is always used once.
75 We generate Salsa20's output using this key and nonce for each message:
76 * first 256 bits are used as a one-time key for Poly1305 authentication
77 * next 256 bits of output are ignored
78 * and all remaining ones XORed with the data, encrypting it
85 │ │ R=rand(64bit); CPrivKey=rand(256bit)
88 │ R, enc(PSK, R, CPubKey) │
89 │ ────────────────────────────────────────>
92 │ │ │ SPrivKey=rand(256bit)
96 │ │ │ K=DH(SPrivKey, CPubKey)
100 │ │ │ RS=rand(64bit); SS=rand(256bit)
103 │ enc(PSK, R+1, SPubKey); enc(K, R, RS+SS)│
104 │ <────────────────────────────────────────
107 │ │ K=DH(CPrivKey, SPubKey) │
111 │ │ RC=rand(64bit); SC=rand(256bit) │
114 │ enc(K, R+1, RS+RC+SC) │
115 │ ────────────────────────────────────────>
122 │ │ │ MasterKey=SS XOR SC
126 │ <────────────────────────────────────────
133 │ │ MasterKey=SS XOR SC │
139 * client generates CPubKey, random 64bit R that is used as a nonce
141 * R + enc(PSK, R, CPubKey) + NULLs -> Server [56 bytes]
142 * server remembers clients address, decrypt CPubKey, generates
143 SPrivKey/SPubKey, computes common shared key K (based on
144 CPubKey and SPrivKey), generates 64bit random number RS and
145 256bit random SS. PSK-encryption uses incremented R (from previous
147 * enc(PSK, SPubKey) + enc(K, RS + SS) + NULLs -> Client [88 bytes]
148 * client decrypt SPubKey, computes K, decrypts RS, SS with key K,
149 remembers SS, generates 64bit random number RC and 256bit random SC,
150 * enc(K, RS + RC + SC) + NULLs -> Server [64 bytes]
151 * server decrypt RS, RC, SC with key K, compares RS with it's own one
152 send before, computes final main encryption key S = SS XOR SC
153 * ENC(K, RC) + NULLs -> Client [24 bytes]
154 * server switches to the new client
155 * client decrypts RC and compares with it's own generated one, computes
156 final main encryption key S
158 Where PSK is 256bit pre-shared key, NULLs are 16 null-bytes. R* are
159 required for handshake randomization and two-way authentication. K key
160 is used only during handshake. NULLs are required to differentiate
161 common transport protocol messages from handshake ones. DH public keys
162 can be trivially derived from private ones.
167 * http://cr.yp.to/ecdh.html
168 * http://cr.yp.to/snuffle.html
169 * http://cr.yp.to/mac.html
170 * http://grouper.ieee.org/groups/1363/passwdPK/contributions/jablon.pdf
171 * Applied Cryptography (C) 1996 Bruce Schneier
175 * Move decryption and encryption processes into goroutines
176 * Add identity management (client can send it's identification, server has
177 on-disk id↔key plaintext database)
178 * Implement alternative Secure Remote Password protocol (it is much slower,
179 technically has more code, but human memorized passwords can be used
184 This program is free software: you can redistribute it and/or modify
185 it under the terms of the GNU General Public License as published by
186 the Free Software Foundation, either version 3 of the License, or
189 This program is distributed in the hope that it will be useful,
190 but WITHOUT ANY WARRANTY; without even the implied warranty of
191 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
192 GNU General Public License for more details.