ServerChallengeEvent

Arrives because you sent ClientChallengeInitRequest, and must be answered with ClientChallengeProofRequest. THE HANDSHAKE DOES NOT COMPLETE UNTIL YOU DO.

WHEN IT ARRIVES

Shortly after your init request, and only then - the server does not send it on its own schedule. But it is delivered as an ordinary event: the client matches it BY TYPE, not by the envelope id it allocated for the init. There is only ever one of these exchanges in flight, so nothing needs correlating, and you should not sit waiting for a uid to come back.

It is the third of four steps. ClientIdRequest and ClientChallengeInitRequest publish the two values this challenge is checked against; if you have not sent both, there is nothing here you can usefully answer.

READ THE PAYLOAD - THIS IS THE ONE STEP WHERE IT MATTERS

PARSE IT AS AN ARBITRARY-PRECISION INTEGER. The value is a decimal string and the real client parses it as a big integer, multiplies it by its private exponent and reduces the result. It is not bounded by anything you can hold in 64 bits, and truncating or overflowing it produces a proof that is silently wrong rather than an error you can see.

ABSENT IS NOT EMPTY, AND ABSENT DOES NOT MEAN SKIP IT. The field has explicit presence and the client tests for it. When the value is missing the client does NOT stay silent and does not fail - it computes a challenge of its own by hashing its handshake values together with a device fingerprint, and answers with that. Treating a missing value as "nothing to do here" leaves the handshake open, which is exactly the state the exchange exists to detect. ClientChallengeProofRequest gives the full recipe for both branches.

WHERE THE ANSWER GOES

ON THE MAIN GAME CONNECTION, even when a second one is open. While a fight-server transfer is active the client holds two sockets (see IdentificationRequest) and routes outbound traffic between them by message type. All three verification requests are on the short list pinned to the FIRST connection - ordinary gameplay traffic is what goes to the fight server. Answering on the wrong socket is a mistake the real client cannot make.

THE COMPLETE ALGORITHM IS ON ONE PAGE. Constants, both exponents, both branches, what the server checks and the ways an implementation goes wrong are written out in full under ClientChallengeProofRequest. Read that before implementing any part of this exchange.