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. It runs under GNU/Linux and FreeBSD.
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)
41 * Higher performance in some cases
45 B -- bad or timeouted UDP packet (maybe network is inactive)
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
59 Let's assume that there is some insecure link between your computer and
60 WiFi-reachable gateway. You have got preconfigured wlan0 network
61 interface with 192.168.0/24 network. You want to create virtual
62 encrypted and authenticated 172.16.0/24 network and use it as a default
63 transport. MTU for that wlan0 is 1500 bytes. GoVPN will say that maximum
64 MTU for the link is 1476, however it does not take in account TAP's
65 Ethernet frame header length, that in my case is 14 bytes long (1476 - 14).
67 gateway% ip addr add 192.168.0.1/24 dev wlan0
68 gateway% tunctl -t tap10
69 gateway% ip link set mtu 1462 dev tap10
70 gateway% ip addr add 172.16.0.1/24 dev tap10
71 gateway% ip link set up dev tap10
72 gateway% govpn -key KEY -iface tap10 -bind 192.168.0.1:1194
74 pc% ip addr add 192.168.0.2/24 dev wlan0
76 pc% ip link set mtu 1462 dev tap10
77 pc% ip addr add 172.16.0.2/24 dev tap10
78 pc% ip link set up dev tap10
79 pc% ip route add default via 172.16.0.1
80 pc% while :; do govpn -key KEY -iface tap10 -remote 192.168.0.1:1194; done
82 If client won't finish handshake during -timeout, then it will exit.
83 If no packets are received from remote side during timeout, then daemon
84 will stop sending packets to the client and client will exit. In every
85 cases you have to rehandshake again.
90 Message authentication: Poly1305
91 Password authenticated key agreement: Curve25519 based DH-EKE
92 Packet overhead: 24 bytes per packet
93 Handshake overhead: 4 UDP (2 from client, 2 from server) packets,
94 232 bytes total payload
98 SERIAL + ENC(KEY, SERIAL, DATA) + AUTH(SERIAL + ENC_DATA)
100 where SERIAL is message serial number. Odds are reserved for
101 client->server, evens are for server->client. SERIAL is used as a nonce
102 for DATA encryption: encryption key is different during each handshake,
103 so (key, nonce) pair is always used once.
105 We generate Salsa20's output using this key and nonce for each message:
106 * first 256 bits are used as a one-time key for Poly1305 authentication
107 * next 256 bits of output are ignored
108 * and all remaining ones XORed with the data, encrypting it
115 │ │ R=rand(64bit); CPrivKey=rand(256bit)
118 │ R, enc(PSK, R, CPubKey) │
119 │ ────────────────────────────────────────>
122 │ │ │ SPrivKey=rand(256bit)
126 │ │ │ K=DH(SPrivKey, CPubKey)
130 │ │ │ RS=rand(64bit); SS=rand(256bit)
133 │ enc(PSK, R+1, SPubKey); enc(K, R, RS+SS)│
134 │ <────────────────────────────────────────
137 │ │ K=DH(CPrivKey, SPubKey) │
141 │ │ RC=rand(64bit); SC=rand(256bit) │
144 │ enc(K, R+1, RS+RC+SC) │
145 │ ────────────────────────────────────────>
152 │ │ │ MasterKey=SS XOR SC
156 │ <────────────────────────────────────────
163 │ │ MasterKey=SS XOR SC │
169 * client generates CPubKey, random 64bit R that is used as a nonce
171 * R + enc(PSK, R, CPubKey) + NULLs -> Server [56 bytes]
172 * server remembers clients address, decrypt CPubKey, generates
173 SPrivKey/SPubKey, computes common shared key K (based on
174 CPubKey and SPrivKey), generates 64bit random number RS and
175 256bit random SS. PSK-encryption uses incremented R (from previous
177 * enc(PSK, SPubKey) + enc(K, RS + SS) + NULLs -> Client [88 bytes]
178 * client decrypt SPubKey, computes K, decrypts RS, SS with key K,
179 remembers SS, generates 64bit random number RC and 256bit random SC,
180 * enc(K, RS + RC + SC) + NULLs -> Server [64 bytes]
181 * server decrypt RS, RC, SC with key K, compares RS with it's own one
182 send before, computes final main encryption key S = SS XOR SC
183 * ENC(K, RC) + NULLs -> Client [24 bytes]
184 * server switches to the new client
185 * client decrypts RC and compares with it's own generated one, computes
186 final main encryption key S
188 Where PSK is 256bit pre-shared key, NULLs are 16 null-bytes. R* are
189 required for handshake randomization and two-way authentication. K key
190 is used only during handshake. NULLs are required to differentiate
191 common transport protocol messages from handshake ones. DH public keys
192 can be trivially derived from private ones.
197 * http://cr.yp.to/ecdh.html
198 * http://cr.yp.to/snuffle.html
199 * http://cr.yp.to/mac.html
200 * http://grouper.ieee.org/groups/1363/passwdPK/contributions/jablon.pdf
201 * Applied Cryptography (C) 1996 Bruce Schneier
205 * Move decryption and encryption processes into goroutines
206 * Add identity management (client can send it's identification, server has
207 on-disk id↔key plaintext database)
208 * Implement alternative Secure Remote Password protocol (it is much slower,
209 technically has more code, but human memorized passwords can be used
214 This program is free software: you can redistribute it and/or modify
215 it under the terms of the GNU General Public License as published by
216 the Free Software Foundation, either version 3 of the License, or
219 This program is distributed in the hope that it will be useful,
220 but WITHOUT ANY WARRANTY; without even the implied warranty of
221 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
222 GNU General Public License for more details.