Tickets

A ticket is a 16-byte shared secret (TICKET_LENGTH, LXMessage.py:41) 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:297, validated at :274):

stamp = truncated_hash(ticket || message_id)

with value COST_TICKET = 256 (LXMessage.py:52,298). On the receiving side, validate_stamp accepts the message if stamp equals truncated_hash(ticket || message_id) for any held inbound ticket (LXMessage.py:271-277). An implementation MUST use truncated_hash (16 bytes), matching the stamp width expectation of this path.

Issuing

generate_ticket(destination_hash, expiry) (LXMRouter.py:1025-1052) returns [expires, ticket] where:

• ticket = os.urandom(16) (LXMRouter.py:1048);
• expires = now + TICKET_EXPIRY (LXMRouter.py:1047).

An existing inbound ticket with more than TICKET_RENEW validity left is reused rather than reissued (LXMRouter.py:1039-1041), and a new ticket is not issued to a destination more often than TICKET_INTERVAL (LXMRouter.py:1028-1033).

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:1054-1057) and uses it for subsequent stamps until it expires (get_outbound_ticket, LXMRouter.py:1059-1065).

Timing constants

The validity windows are part of the interoperable behaviour (a peer expects a ticket to remain valid for TICKET_EXPIRY plus TICKET_GRACE); the exact reuse and reissue scheduling around them is informative.