Function perform_multi_challenge_handshake 

pub async fn perform_multi_challenge_handshake(
    send: SendStream,
    recv: RecvStream,
    server_keypair: &KeyPair,
) -> Result<HashSet<VerifyingKey>>

Performs a multi-challenge handshake with a client

This function implements the server side of the new flexible challenge protocol:

1. Sends multiple challenges sequentially (ML-DSA, proof-of-work, etc.)
2. Receives and verifies each challenge response
3. Returns the set of successfully verified ML-DSA public keys

The server can send multiple different challenge types, and the client must respond to each one. The handshake continues until all challenges are completed or a challenge fails.

Arguments

• send - Stream for sending data to the client
• recv - Stream for receiving data from the client
• server_keypair - Server’s keypair for signing the challenge nonce

Returns

A BTreeSet of successfully verified public keys (as encoded bytes)

Errors

Returns an error if:

• Network I/O fails
• Serialization/deserialization fails
• Any challenge fails verification
• Client response is malformed or too large

Example

use zoe_relay::challenge::perform_multi_challenge_handshake;

let verified_keys = perform_multi_challenge_handshake(
    send_stream,
    recv_stream,
    &server_keypair
).await?;

debug!("Verified {} keys after multi-challenge handshake", verified_keys.len());