lokinet/doc/proto_v0.txt

LLARP v0

LLARP (Low Latency Anon Routing Protocol) is a protocol for anonymizing senders and
recipiants of encrypted messages sent over the internet without a centralied
trusted party.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].

basic structures:

all structures are key, value dictionaries encoded with bittorrent encoding
notation:

a + b is a concatanated with b

a ^ b is a bitwise XOR b

x[a:b] is a memory slice of x from index a to b

BE(x) is bittorrent encode x

BD(x) is bittorrent decode x

{ a: b, y: z } is a dictionary with two keys a and y
    who's values are b and z respectively

[ a, b, c ... ] is a list containing a b c and more items in that order

"<description>" is a bytestring who's contents and length is described by the
    quoted value <description>

"<value>" * N is a bytestring containing the <value> concatenated N times.

cryptography:

H(x) is 512 bit blake2b digest of x
HS(x) is 256 bit blake2b digest of x
MD(x, k) is 512 bit blake2b hmac of x with secret value k
MDS(x, k) is 256 bit blake2b hmac of x with secret value k
NE(k, x) is sntrup4591761 encrypt data x to public key k
ND(k, x) is sntrup4591761 decrypt data x with private key k
SE(k, n, x) is chacha20 encrypt data x using symettric key k and nounce n
SD(k, n, x) is chacha20 dectypt data x using symettric key k and nounce n
S(k, x) is sign x with ed25519 using seed k
ECKG() is generate ec keypair (p, s) public key p, seed s, both 32 bytes
V(k, x, sig) is verify x data using signature sig using public key k
DH(x, y) is a ecdh key exchange using ed25519 scalarmult between public keys x
  and y
KE(x, y) is a ecdh key exchange using H(x + y + DH(x, y))
PKE(x, y, n) is a path key exchange using MDS(n, KE(x, y))
TKE(x, y, n) is a transport key exchange using MD(n, KE(x, y))
RAND(n) is n random bytes

---


wire protocol:

as of version 0 plaintext sctp is used, future versions will use an encrypted
udp transport (IWP).


wire decryption:

the first 32 bytes are message authentication bytes, h
the next 32 bytes are nounce for shared secret, n
the remaining bytes are interpreted as ciphertext, x

a shared secret s is generated via TKE(us, them, n)
next the integrity of the ciphertext is done by checking MDS(n + x, s) == h
if the ciphertext is valid then the frame is decrypted via SD(s, n, x)

wire encryption:

given variadic sized payload p, 32 byte nounce n and public encryption keys A
and B

s = TKE(A, B, n)
x = SE(s, n, p)
h = MDS(n + x, s)

the resulting data is:

h + n + x


handshake:

0) intro

32 bytes hmac, h
32 bytes nounce, n
64 bytes elligator sqaured encoded alice's transport public encryption key, a.k
variadic bytes padding, w0

Alice transmits ( h + n + a.k + w0 ) to Bob from the transport address matching
his public transport encryption key.

Bob recieves ( h + n + a.k + w0 ) 

1) intro ack

sent in reply to an intro, bob sends an intro ack encrypted to Alice using

32 bytes hmac, h
32 bytes nounce, n
32 bytes ciphertext, x
variadic bytes padding, w1

token = RAND(32)
k = TKE(a.k, b.k, n)
x = SE(k, token, n[0:24])
h = MDS(n + x, k)

Bob transmits ( h + n + x + w1 )
Alice recieves ( h + n + x + w1 ) and verifies that h == MDS(n + x, k) silently
dropping if it does not match.

2) token offer

Alice sends the token from the intro ack back to Bob

32 bytes hmac, h
32 bytes nounce, n
32 bytes ciphertext, x
variadic byttes padding, w2

k = TKE(a.k, b.k, n)
x = SE(k, token, n[0:24])
h = MDS(n + x, k)

Alice transmits ( h + n + x + w2 )
Bob recieves ( h + n + x + w2) and verifies that h == MDS(n + x, k) silently
drops if not matching

4) token ack

Bob acks the token that he got from Alice

32 bytes hmac, h
32 bytes nounce, n
32 bytes ciphertext, x
variadic byttes padding, w3

S = TKE(a.k, b.k, token)
x = SE(S, token, n[0:24])
h = MDS(n + x, S)

Alice transmits ( h + n + x + w3 ) to Bob and the session is now established
using shared secret S

Bob receves ( h + n + x + w2 ) and verifies that h == MDS(n + x, S)

IWP payload format:

ciphertext:
32 bytes hmac, h
32 bytes nounce, n
N bytes of ciphertext, x

plaintext header, H
8 bits protocol version, v (currently 0)
8 bits message type, t
12 bits payload size, s
4 bits flags, f

plaintext payload: P
s bytes of data
N bytes remaining data is discarded

D = H + P
x = SE(D, S, n)
h = MDS(n + x, S)

Alice transmits h + n + x

Bob recieves recieve h + n + x

Bob checks hmac by verifying h == MDS(n + x, S)

if the hmac fails the data is silently dropped

message types:

XMIT = 0x01

begin link layer message transmission

ACKS = 0x02

acknolege link layer message fragment

FRAG = 0x03

transmit link layer message fragment

flags:

SESSION_INVALIDATED = 1 << 0

this session is now invalidated and a new session is required

HIGH_PACKET_DROP    = 1 << 1

high packet drop detected

HIGH_MTU_DETECTED   = 1 << 2

the network uses an mtu greater than 1488 bytes

PROTOCOL_UPGRADE    = 1 << 3

indicates we want to do protocol upgrade (future use)

XMIT payload:

start transmiting a link layer message 

msg_bytes = BE(msg)

32 bytes hash of message computed by HS(msg_bytes)
64 bits unsigned int message id
12 bits unsigned int fragment size bytes, s
4 bits unsigned int nonzero number of fragments, n
8 bits size of last fragment in bytes, l

msg_bytes is s * (n - 1) + l bytes long

FRAG payload:

transmit a link layer message fragment

64 bits message id
4 bits ignored
4 bits unsigned int fragment number
remaining bytes of payload are fragment data

ACKS payload:

indicates we which chunks we have recieved

64 bits message id
16 bits bitmask of chunks we have received
remaining bytes discarded


control flow:

To transmit link message over an established session the transmitter sends an
XMIT frame.
In reply to an XMIT frame the recipiant MUST send an ACKS frame with an emtpy
bitmask.
After the transmitter recieves the first ACKS frame it is allowed to start
sending FRAG messages.
When all fragmenets are obtained by the recipiant, the recipiant sends an ACKS
frame with a full bitfield (0xFFFF), to indicate the link message was recieved.
In the event of packet drop the sender decides when to retransmit FRAG frames
with expontential backoff.

In the event of packet loss greater than 50% over 10 second the session is
invalidated and must be renegotiated with a new handshake.

---

datastructures:

all datastructures are assumed version 0 if they lack a v value
otherwise version is provided by the v value

address info (AI)

An address info (AI) defines a publically reachable ipv6 endpoint

{
  c: transport_rank_uint16,
  e: "<32 bytes public encryption key>",
  d: "<transport dialect name>",
  i: "<16 bytes big endian public ipv6 address>",
  p: port_uint16,
  v: 0
}

Exit Info (XI)

{
  a: "<16 bytes big endian ipv6 address>",
  b: "<16 bytes big endian ipv6 netmask>",
  v: 0
}

router contact (RC)

{
  a: [ one, or, many, AI, here ... ],
  k: "<32 bytes public signing/encryption identity key>",
  x: [ Exit, Infos ],
  v: 0,
  z: "<64 bytes signature using identity key>"
}

service info (SI)

{
  n: "<optional claimed name>",
  s: "<32 bytes public signing key>",
  v: 0,
  x: "<optional nounce for vanity>"
}

service address (SA)

H(BE(SI))

introducer (I)

{
  i: "<32 bytes public key of router>",
  p: path_id_uint64,
  v: 0,
  x: time_expires_seconds_since_epoch_uint64
}

introducer set (IS)

{
  a: "<64 bytes SA>",
  e: "<1218 bytes ntru public encryption key>",
  i: [ I, I, I, ... ],
  v: 0,
  z: "<64 bytes signature using service info signing key>"
}


---

Encrypted frames:

Encrypted frames are encrypted containers for link message records like LRCR.

32 bytes hmac, h
32 bytes nounce, n
32 bytes ephmeral sender's public encryption key, k
remaining bytes ciphertext, x

decryption:

0) verify hmac

S = PKE(n, k, our_RC.K)
verify h == MDS(n + k + x, S)

If the hmac verification fails the entire parent message is discarded

1) decrypt and decode

new_x = SD(S, n[0:24], x)
msg = BD(new_x)

If the decoding fails the entire parent message is discarded

encryption:

to encrypt a frame to a router with public key B.k

0) prepare nounce n, ephemeral keypair (A.k, s) and derive shared secret S

A.k, s = ECKG()
n = RAND(32)
S = PKE(p, A.k, B.k)

1) encode and encrypt

x = BE(msg)
new_x = SE(S, n[0:24], x)

2) generate hmac

h = MDS(n + A.k + x, S)

resulting frame is h + n + A.k + new_x


---

link layer messages:

the link layer is responsible for anonymising the source and destination of
routing layer messages.

any link layer message without a key v is assumed to be version 0 otherwise
indicates the protocol version in use.

link relay commit message (LRCM)


{
  a: "c",
  b: [ list, of, encrypted, frames ],
  v: 0,
}


link relay commit record (LRCR)

record requesting path with id p relay messages for x seconds to router
on network who's i is equal to RC.k and decrypt data any messages using
PKE(n, rc.K, c) as symettric key for encryption and decryption.

{
  c: "<32 byte public signing/encryption key used for further communication>",
  i: "<32 byte RC.k of next hop>",
  n: "<32 bytes nounce for key exchange>",
  p: path_id_uint64,
  v: 0,
  x: seconds_lifetime_uint64
}

if i is equal to RC.k then any LRDM.z values are decrypted and interpreted as
routing layer messages.

if i is not equal to RC.k then forward the LRCM with first element removed
and the last element holding our hop's reply. this ensures that the first entry
in the forwarded LRCM is for the next hop in the requested path.

if i is equal to RC.k unconditionally send a LRDM with encrypted payload
holding a LRSM with our record at the end and the previous ones in the front.

link relay reject record (LRRR)

sent in reply to a LRCM indicating we have rejected the request to relay data
for path with id p, the recipiant of this message MUST backoff sending LRCM for
b milliseconds or recipiant MAY get banned by recipiant router for an undefined
amount of time. r contains a bytestring of 7 bit clean ascii metadata indicating
why the commit was rejected. if included r MUST be logged or collected for later
review by node operator. inclusion of r is OPTIONAL. review of collected events
is RECOMMENDED.

{
  b: miliseconds_backoff_uint64,
  c: "r",
  p: path_id_uint64,
  r: "<optional reason metadata here>",
  v: 0,
  x: "<N bytes arbirary padding>"
}

link relay accept record (LRAR)

sent in reply to a LRCM indicating we have accepted the request to relay data
for path with id p.

{
  c: "a",
  p: path_id_uint64,
  v: 0,
  x: "<N bytes arbitrary padding>"
}


link relay status message (LRSM)

sent inside a LRDM after build has reached the end of the path to finish the
path build and send the result of the build.

{
  a: "s",
  p: [list, of, encrypted, replies],
  v: 0,
}


link relay upstream message (LRUM)

sent to relay data via upstream direction of a previously created path.
decrypt z using previously derived key and nounce y. Relay with new_y and new_z
in upstream direction as a LRUM.

new_z = SD(k, y, z)
new_y = y ^ new_z[0:24]

{
  a: "u",
  p: path_id_uint64,
  v: 0,
  y: "<insert 24 bytes nounce here>",
  z: "<insert N bytes payload here>"
}

link relay downstream message (LRDM)

sent to relay data via downstream direction of a previously created path.
encrypt z using previously derived key and nonce new_y and relay in downstream
direction as a LRDM.

new_y = y ^ z[0:24]
new_z = SE(k, new_y, z)

{
  a: "d",
  p: path_id_uint64,
  v: 0,
  y: "<insert 24 bytes nounce here>",
  z: "<insert N bytes payload here>"
}

link relay exit message (LRXM)

sent to exit a previously commited path before it expires.
verify signature using cancel key c in relay commit message.

{
  a: "x",
  b: [ list, of, exit, records, as, encrpyted, frames ],
  v: 0,
}

link relay exit record (LRXR)

{
  c: "x",
  p: path_id_uint64,
  v: 0,
  x: "<N bytes padding>",
  z: "<64 bytes signature>"
}

---

direct paths:

a direct path is a "0 hop" path built by Alice to communicate directly to Bob
for point to point transmission of routing layer messages.

these are built by sending a LRCM where B has 1 entry

---

routing layer:

the routing layer provides inter network communication between the LLARP link
layer and ip (internet protocol) for exit traffic or ap (anonymous protocol) for
hidden services. replies to messages are sent back via the path they
originated from inside a LRDM.

for direct communication between routers a direct path MUST be used, these
messages MUST NOT be sent on the link leyer.

obtain exit address message (OXAM)

sent to an exit router to obtain a NAT ip address for ip exit traffic.
replies are sent down the path that messages originate from.

{
  A: "X",
  I: "<32 bytes signing public key for future communication>",
  V: 0,
  X: lifetime_of_address_mapping_in_seconds_uint64,
}

grant exit address messsage (GXAM)

sent in response to a OXAM to grant an ip for exit traffic from an external
ip address used for exit traffic.

{
  A: "G",
  E: "<16 byte big endian externally reachable ipv6 address>",
  I: "<32 bytes signing public key of requester>",
  V: 0,
  Z: "<64 bytes signature using exit's signing key>"
}

reject exit address message (RXAM)

{
  A: "R",
  B: backoff_milliseconds_uint64,
  I: "<32 bytes signing public key of requester>",
  R: "<optional reject metadata>",
  V: 0,
  Z: "<64 bytes signature signed by exit>"
}



transfer data fragment message (TDFM)

variant 1 (with path id):

transfer data to another path with id P on the local router place Y and X values
into y and z values in LRDM message respectively.

{
  A: "T",
  P: path_id_uint64,
  V: 0,
  X: "<N bytes payload>", 
  Y: "<24 bytes nounce>",
  Z: "<64 bytes signature of entire message where Z is set to NUL>",
}

variant 2 (no path id):

transfer ip traffic for exit

{
  A: "T",
  V: 0,
  X: "<N bytes ipv6 packet>",
  Z: "<64 bytes signature of previously provided signing key>"
}

find introduction message (FIM)

{
  A: "F",
  S: "<64 bytes dht key>",
  V: 0,
  T: transaction_id_uint64
}

got introduction message (GIM)

{
  A: "G",
  T: transaction_id_uint64,
  V: 0,
  X: [ IS, IS, IS, ... ]
}

publish introduction message (PIM)

publish one or many IM into the dht at once.
each IS will be placed in the dht

version 0 uses the SA of each IS as the keyspace location.

in the future the location will be determined by the dht kdf
which uses a shared random source to obfuscate keyspace location.


{
  A: "P",
  T: transaction_id_uint64,
  V: 0,
  X: [ IS, IS, IS, ... ],
}

acknoleged introduction message (AIM)

acknolege the publishing of a previous PIM, X contains the backoff values in ms
for the previously provided IS, if backoff is 0 the operation was successfull

{
  A: "A",
  T: transaction_id_uint64,
  V: 0,
  X: [ 0, 0, backoff, ...],
}

find router contact message (FRCM)

find a router by public key

{
  A: "F",
  K: "<32 byte public key of router>",
  T: transaction_id_uint64
  V: 0
}

got router contact message (GRCM)

R is a list containing a single RC if found or is an empty list if not found
sent in reply to FRCM only
{
  A: "G",
  R: [RC],
  T: transaction_id_uint64,
  V: 0
}