Link

A link is an ephemeral, forward-secret session between two destinations, established by an ECDH handshake. This section is proven by [VEC-LINK].

Link request

The initiator sends a LINKREQUEST packet (packet_type = 0x02) whose data is (Link.py:308-317):

ephemeral_X25519_public(32) || ephemeral_Ed25519_public(32) || signalling(3)

ECPUBSIZE = 64 (the two public keys). The 3-byte signalling field encodes the proposed link MTU (21 bits) and mode (3 bits): (mtu & 0x1FFFFF) | ((mode<<5 & 0xE0)<<16), packed big-endian, low 3 bytes (Link.signalling_bytes, Link.py:148-151). The only enabled mode is MODE_AES256_CBC = 0x01.

Link id

hashable = packet.get_hashable_part()        # masked-flags byte || addressing || data
if len(packet.data) > ECPUBSIZE:             # strip trailing signalling bytes
    hashable = hashable[:-(len(packet.data) - ECPUBSIZE)]
link_id  = truncated_hash(hashable)          # 16 bytes

(Link.link_id_from_lr_packet, Link.py:340-347): the hashable part is trimmed by the number of bytes the request data exceeds the 64-byte key block (i.e. the signalling bytes) before hashing. [VEC-LINK] link id 4725ac1375601d182afec3610f019b25. The link id replaces the destination hash in the addressing of all subsequent link packets, and is also the link's salt (below).

Proof and handshake

The responder replies with a PROOF packet, context LRPROOF (0xFF), data (Link.py:371-377):

signature(64) || ephemeral_X25519_public(32) || signalling(3)

where signature = sign( link_id || responder_eph_X25519_pub || responder_eph_Ed25519_pub || signalling ) (Link.py:373). The initiator validates it against the destination's known identity (Link.py:417-420).

Both sides then derive the session key (Link.handshake, Link.py:353-366):

shared      = own_ephemeral_private.exchange(peer_ephemeral_public)   # X25519 ECDH
session_key = hkdf(length=64, derive_from=shared, salt=link_id, context=None)

64 bytes for MODE_AES256_CBC (32 key + 32 HMAC). get_salt() returns the link id and get_context() returns None (Link.py:643,646). [VEC-LINK] proves the ECDH is symmetric (ecdh_agreement = true, both sides compute the same shared secret) and records the resulting 64-byte session key 569ac51a07fb242f…. An implementation MUST derive the link id and session key identically.

Link operations (context bytes)

Once active, link packets are DATA packets addressed by link id, encrypted with the session key, distinguished by context byte:

After proof, the initiator measures RTT and sends an LRRTT packet; either side MAY identify (prove an identity over the link) by sending a LINKIDENTIFY packet. Keepalive cadence, the stale/close watchdog, and MTU discovery are informative.