the Elixir version uses an ASN.1 based format, but the support in other
languages (besides C) for ASN.1 is surprisingly bad.
So I'll resort to a hard-coded format. Again.

----

InitMessage ::=
    capabilities:ShortList<u32(big-endian)>

(* the InitMessage is the first thing sent on a newly established connection.
 * both sides are expected to send the capabilities they support
 * then wait for the other side to send theirs (meaning they don't
 * block each other during this), and then use just the capabilities
 * which both peers support *)

ProtoMessage ::=
    algorithm:SigAlgo
    pubkeylen:u16(big-endian)
    length:u16(big-endian)
    data:[length]u8=ProtoMsgKind<pubkeylen>

ProtoMsgKind<pubkeylen> ::=
    (* xfer           *) [0x00] XferSlice<pubkeylen>
  | (* summary-pull   *) [0x01] SummaryList<pubkeylen>
  | (* summary-ihave  *) [0x02] SummaryList<pubkeylen>

(* after the initial capabilities are sent, both sides are expected to send
 * summaries for all public keys they know + all available slices for these
 * pubkeys                                                                  *)

ShortList<T> ::= length:u16(big-endian) data:[length]T
XSList<T> ::= length:u8 data:[length]T

(* "raw" summary header overhead = 41 bytes, per 65535 slices *)
SummaryList<pubkeylen> ::=
    data:[*]Slice<pubkeylen> (* uses all remaining space in the packet *)

Slice<pubkeylen> ::=
    pubkey:[pubkeylen]u8
    start:u64
    length:u16

XferSlice<pubkeylen> ::=
    pubkey:[pubkeylen]u8
    siglen:u16(big-endian)
    start:u64
    data:[*]XferBlob<siglen> (* uses all remaining space in the packet *)

XferBlob<siglen> ::=
    signature:[siglen]u8 (* signature gets calculated over `blid ++ data` *)
    ttl:u8
    data:XferInner

XferInner ::=
    (* normal *) [0x00] XferNormal
  | (* prune  *) [0x01] (* empty; NOTE: this packet indicates that
     * all packets before it should be deleted on all servers;
     * used for data protection/privacy compliance;
     * although it is possible to delete such packets after a grace period
     * (e.g. 1 year) during which every peer should've had a chance to retrieve
     * it, doing that is discouraged *)

XferNormal ::=
    attrs:ShortList<KeyValuePair>
    data:ShortList<u8>

KeyValuePair ::=
    key:XSList<u8>
    value:ShortList<u8>

SigAlgo ::=
    (* ed25519   *) [0x65, 0x64, 0xff, 0x13]