package shared

import "encoding/binary"

const PROTO_VERSION uint16 = 1

// pubKey        - public key generated by the client
// privKey       - private key generated by the client
// serverPubKey  - pre-shared public key used for authenticating the server
// serverPrivKey - long-term private key used for authenticating the server
// sessionKey    - per-peer symmetrical key derived via HPKE for encrypting unicast packets
// multicastKey  - key shared between everyone for encrypting multicast packets
// authSecret    - secret assigned to a client for authenticating subsequent requests

// SYM  = XChaCha20‑Poly1305 with nonce prepended
// HMAC = HMAC-SHA-256
// hpke = HPKE(MLKEM768X25519, ChaCha20Poly1305, HKDF-SHA-256)
// ds   = ML-DSA

// packet format
// [ version - 2 bytes ] [ type - 2 bytes ] [ payload - see below ]
const (
	_ uint16 = iota
	// (client -> server) requests a challenge to prove the ownership of the privKey
	// [ pubKey - 1216 bytes ]
	REQ_GET_CHALLENGE
	// (server -> client) provides an encrypted authSecret
	// [ ds.Sign(serverPrivKey, rest) - 2420 bytes ] [ HPKE(pubKey, authSecret) - 1168 bytes ]
	RESP_GET_CHALLENGE
	// (client -> server) requests an IP
	// [ HMAC(authSecret, pubKey) - 32 bytes ] [ pubKey - 1216 bytes ]
	REQ_REGISTER
	// (server -> client) returns the assigned IP and encrypted multicastKey
	// [ ds.Sign(serverPrivKey, rest) - 2420 bytes ] [ ip - 16 bytes ] [ hpke.Seal(pubKey, multicastKey) - 1168 bytes ]
	RESP_REGISTER
	// (client -> server) requests peer's pubKey from the server for encapsulation
	// [ HMAC(authSecret, rest) - 32 bytes ] [ ip - 16 bytes ]
	REQ_GET_PUBKEY
	// (server -> client) provides requested pubKey
	// [ ds.Sign(serverPrivKey, rest) - 2420 bytes ] [ ip - 16 bytes ] [ pubKey - 1216 bytes ]
	RESP_GET_PUBKEY
	// (client -> server -> client2) establishes a sessionKey with another peer
	// [ HMAC(authSecret, rest) - 32 bytes ] [ destIP - 16 bytes ] [ srcIP - 16 bytes ] [ hpke.Encap(pubKey) - 1120 bytes ]
	REQ_ESTABLISH
	// (client2 -> server -> client) acknowledges the sessionKey was established
	// [ HMAC(authSecret, rest) - 32 bytes ] [ destIP - 16 bytes ] [ srcIP - 16 bytes ]
	RESP_ESTABLISH
	// (client -> server -> client2) relays an encrypted packet to a specified peer
	// [ HMAC(authSecret, rest) - 32 bytes ] [ destIP - 16 bytes ] [ srcIP - 16 bytes ] [ SYM(sessionKey, pkt) ]
	UNICAST_PKT
	// (client -> server -> *) broadcasts an encrypted packet
	// [ HMAC(authSecret, rest) - 32 bytes ] [ SYM(multicastKey, pkt) ]
	BROADCAST_PKT
)

func BuildPkt(pktType uint16, parts ...[]byte) []byte {
	var total int
	for _, p := range parts {
		total += len(p)
	}

	out := make([]byte, 4, 4+total)
	binary.LittleEndian.PutUint16(out[0:], PROTO_VERSION)
	binary.LittleEndian.PutUint16(out[2:], pktType)

	for _, p := range parts {
		out = append(out, p...)
	}
	return out
}