Tickets

A ticket is a 16-byte shared secret (TICKET_LENGTH, LXMessage.py:42) that lets a known correspondent skip proof-of-work. The recipient issues a ticket to a sender; the sender then derives stamps from it cheaply.

Derivation

A ticketed stamp is (LXMessage.py:300, validated at :274):

stamp = truncated_hash(ticket || message_id)

with value COST_TICKET = 256 (LXMessage.py:53,301). On the receiving side, validate_stamp accepts the message if stamp equals truncated_hash(ticket || message_id) for any held inbound ticket (LXMessage.py:274-280). An implementation MUST use truncated_hash (16 bytes), matching the stamp width expectation of this path.

Issuing

generate_ticket(destination_hash, expiry) (LXMRouter.py:1073-1100) returns [expires, ticket] where:

• ticket = os.urandom(16) (LXMRouter.py:1096);
• expires = now + TICKET_EXPIRY (LXMRouter.py:1095).

An existing inbound ticket with more than TICKET_RENEW validity left is reused rather than reissued (LXMRouter.py:1083-1089), and a new ticket is not issued to a destination more often than TICKET_INTERVAL (LXMRouter.py:1076-1081).

Exchange

A ticket is delivered to a correspondent inside a message via FIELD_TICKET (0x0C, LXMF.py:19), carrying the [expires, ticket] pair. The receiver remembers it as an outbound ticket (remember_ticket, LXMRouter.py:1102-1105) and uses it for subsequent stamps until it expires (get_outbound_ticket, LXMRouter.py:1107-1113).

The Rust router follows the same default: enqueue() automatically derives and attaches the 16-byte delivery stamp whenever TicketStore holds a valid outbound ticket for the destination. This happens before propagated recipient encryption, but the ticket stamp remains distinct from the required 32-byte outer propagation-node stamp. To grant a reply ticket, issue_ticket_field() returns the bounded/persisted FIELD_TICKET value that must be passed to Message::create() so it is covered by the signature.

Timing constants

The validity windows are part of the interoperable behaviour: a ticket can stamp messages until its encoded expiry, while the issuer retains its record for the additional TICKET_GRACE cleanup window. The exact reuse and reissue scheduling around them is informative.