invisible wire protocol:

as of version 0 dtls is used, future versions will use this encrypted udp
transport protocol.

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].

cryptography:

see crypto_v0.txt


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(initiater, recipiant, 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
32 bytes encrypted alice's transport public encryption key e
variadic bytes padding, w0

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

w0 = "[insert variable length random padding here]"
n = RAND(32)
e = SE(a.k, HS(b.k + n), n[0:24])
S = TKE(a.k, b.k, n)
h = MDS(n + e + w0, S)

Bob recieves ( s + n + e + 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

w1 = "[insert variable length random padding here]"
token = RAND(32)
S = TKE(a.k, b.k, n)
x = SE(k, token, n[0:24])
h = MDS(n + x + w1, S)

Bob transmits ( s + n + x + w1 ), r is ignored and discarded
Alice recieves ( s + n + x + w1 ) and verifies the signature silently
dropping if it does not match.

2) session start

Alice uses the token from the previous message to start the wire session

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

w2 = "[insert variable length random padding here]"
e_K = TKE(a.k, b.k, n)
x = SE(e_K, token, n[0:24])
h = MDS(n + x + w2, e_K)
T = HS(token + n)
K = TKE(a.k, b.k, T)

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

the session is now established with session key K,
Bob replies by transmitting a LIM

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

Encryption:

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

Decryption:

verify h == MDS(n + x, S)
D = SD(x, S, n)
H = D[0:4]
P = D[4:4+H.s]

message types:

ALIV = 0x00 

keepalive message

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
16 bits unsigned int fragment size bytes, S
16 bits size of last fragment in bytes, L
16 bits reserved for future
8 bits unsigned int nonzero number of fragments, n
8 bits reserved flags, f
if f LSB is set then last fragment is included and is l bytes long

f's LSB MUST be set as of protocol version 0.

msg_bytes is S * (n - 1) + L bytes long

FRAG payload:

transmit a link layer message fragment

64 bits message id
8 bits unsigned int fragment number
S bytes of payload fragment data
remaining bytes discarded

ACKS payload:

indicates we which chunks we have recieved

64 bits message id
32 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.