This section, unfortunately, is not yet finished, although the protocol is stable (until bugs in the
cryptography are found, which will likely completely change the following description). Nevertheless, it
should give you some overview over the protocol.
AnatomyofaVPNpacket
The exact layout and field lengths of a VPN packet is determined at compile time and doesn't change. The
same structure is used for all transport protocols, be it RAWIP or TCP.
+------+------+--------+------+
| HMAC | TYPE | SRCDST | DATA |
+------+------+--------+------+
The HMAC field is present in all packets, even if not used (e.g. in auth request packets), in which case
it is set to all zeroes. The MAC itself is calculated over the TYPE, SRCDST and DATA fields in all cases.
The TYPE field is a single byte and determines the purpose of the packet (e.g. RESET,
COMPRESSED/UNCOMPRESSED DATA, PING, AUTH REQUEST/RESPONSE, CONNECT REQUEST/INFO etc.).
SRCDST is a three byte field which contains the source and destination node IDs (12 bits each).
The DATA portion differs between each packet type, naturally, and is the only part that can be encrypted.
Data packets contain more fields, as shown:
+------+------+--------+-------+------+
| HMAC | TYPE | SRCDST | SEQNO | DATA |
+------+------+--------+-------+------+
SEQNO is a 32-bit sequence number. It is negotiated at every connection initialization and starts at some
random 31 bit value. GVPE currently uses a sliding window of 512 packets/sequence numbers to detect
reordering, duplication and replay attacks.
The encryption is done on SEQNO+DATA in CTR mode with IV generated from the seqno (for AES: seqno ||
seqno || seqno || (u32)0), which ensures uniqueness for a given key.
Theauthentication/keyexchangeprotocol
Before nodes can exchange packets, they need to establish authenticity of the other side and a key. Every
node has a private RSA key and the public RSA keys of all other nodes.
When a node wants to establish a connection to another node, it sends an RSA-OEAP-encrypted challenge and
an ECDH (curve25519) key. The other node replies with its own ECDH key and a HKDF of the challenge and
both ECDH keys to prove its identity.
The remote node enganges in exactly the same protocol. When both nodes have exchanged their challenge and
verified the response, they calculate a cipher key and a HMAC key and start exchanging data packets.
In detail, the challenge consist of:
RSA-OAEP (SEQNO MAC CIPHER SALT EXTRA-AUTH) ECDH1
That is, it encrypts (with the public key of the remote node) an initial sequence number for data
packets, key material for the HMAC key, key material for the cipher key, a salt used by the HKDF (as
shown later) and some extra random bytes that are unused except for authentication. It also sends the
public key of a curve25519 exchange.
The remote node decrypts the RSA data, generates its own ECDH key (ECDH2), and replies with:
HKDF-Expand (HKDF-Extract (ECDH2, RSA), ECDH1, AUTH_DIGEST_SIZE) ECDH2
That is, it extracts from the decrypted RSA challenge, using its ECDH key as salt, and then expands using
the requesting node's ECDH1 key. The resulting hash is returned as a proof that the node could decrypt
the RSA challenge data, together with the ECDH key.
After both nodes have done this to each other, they calculate the shared ECDH secret, cipher and HMAC
keys for the session (each node generates two cipher and HMAC keys, one for sending and one for
receiving).
The HMAC key for sending is generated as follow:
HMAC_KEY = HKDF-Expand (HKDF-Extract (REMOTE_SALT, MAC ECDH_SECRET), info, HMAC_MD_SIZE)
It extracts from MAC and ECDH_SECRET using the remote SALT, then expands using a static info string.
The cipher key is generated in the same way, except using the CIPHER part of the original challenge.
The result of this process is to authenticate each node to the other node, while exchanging keys using
both RSA and ECDH, the latter providing perfect forward secrecy.
The protocol has been overdesigned where this was possible without increasing implementation complexity,
in an attempt to protect against implementation or protocol failures. For example, if the ECDH challenge
was found to be flawed, perfect forward secrecy would be lost, but the data would likely still be
protected. Likewise, standard algorithms and implementations are used where possible.
Retrying
When there is no response to an auth request, the node will send auth requests in bursts with an
exponential back-off. After some time it will resort to PING packets, which are very small (8 bytes +
protocol header) and lightweight (no RSA operations required). A node that receives ping requests from an
unconnected peer will respond by trying to create a connection.
In addition to the exponential back-off, there is a global rate-limit on a per-IP base. It allows long
bursts but will limit total packet rate to something like one control packet every ten seconds, to avoid
accidental floods due to protocol problems (like a RSA key file mismatch between two nodes).
The intervals between retries are limited by the max-retry configuration value. A node with connect =
always will always retry, a node with connect = ondemand will only try (and re-try) to connect as long as
there are packets in the queue, usually this limits the retry period to max-ttl seconds.
Sending packets over the VPN will reset the retry intervals as well, which means as long as somebody is
trying to send packets to a given node, GVPE will try to connect every few seconds.
RoutingandProtocoltranslation
The GVPE routing algorithm is easy: there isn't much routing to speak of: When routing packets to another
node, GVPE tries the following options, in order:
If the two nodes should be able to reach each other directly (common protocol, port known), then GVPE
will send the packet directly to the other node.
If this isn't possible (e.g. because the node doesn't have a hostname or known port), but the nodes speak
a common protocol and a router is available, then GVPE will ask a router to "mediate" between both nodes
(see below).
If a direct connection isn't possible (no common protocols) or forbidden (deny-direct) and there are any
routers, then GVPE will try to send packets to the router with the highest priority that is connected
already and is able (as specified by the config file) to connect directly to the target node.
If no such router exists, then GVPE will simply send the packet to the node with the highest priority
available.
Failing all that, the packet will be dropped.
A host can usually declare itself unreachable directly by setting its port number(s) to zero. It can
declare other hosts as unreachable by using a config-file that disables all protocols for these other
hosts. Another option is to disable all protocols on that host in the other config files.
If two hosts cannot connect to each other because their IP address(es) are not known (such as dial-up
hosts), one side will send a mediated connection request to a router (routers must be configured to act
as routers!), which will send both the originating and the destination host a connection info request
with protocol information and IP address of the other host (if known). Both hosts will then try to
establish a direct connection to the other peer, which is usually possible even when both hosts are
behind a NAT gateway.
Routing via other nodes works because the SRCDST field is not encrypted, so the router can just forward
the packet to the destination host. Since each host uses its own private key, the router will not be able
to decrypt or encrypt packets, it will just act as a simple router and protocol translator.
2.25 2015-10-31 GVPE.PROTOCOL(7)
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