Support caching preprocesses in FROST (#190)

* Remove the explicit included participants from FROST

Now, whoever submits preprocesses becomes the signing set. Better separates
preprocess from sign, at the cost of slightly more annoying integrations
(Monero needs to now independently lagrange/offset its key images).

* Support caching preprocesses

Closes https://github.com/serai-dex/serai/issues/40.

I *could* have added a serialization trait to Algorithm and written a ton of
data to disk, while requiring Algorithm implementors also accept such work.
Instead, I moved preprocess to a seeded RNG (Chacha20) which should be as
secure as the regular RNG. Rebuilding from cache simply loads the previously
used Chacha seed, making the Algorithm oblivious to the fact it's being
rebuilt from a cache. This removes any requirements for it to be modified
while guaranteeing equivalency.

This builds on the last commit which delayed determining the signing set till
post-preprocess acquisition. Unfortunately, that commit did force preprocess
from ThresholdView to ThresholdKeys which had visible effects on Monero.

Serai will actually need delayed set determination for #163, and overall,
it remains better, hence it's inclusion.

* Document FROST preprocess caching

* Update ethereum to new FROST

* Fix bug in Monero offset calculation and update processor

processor/src/coin/mod.rs

@@ -75,7 +75,6 @@ pub trait Coin {
   async fn attempt_send(
     &self,
     transaction: Self::SignableTransaction,
-    included: &[u16],
   ) -> Result<Self::TransactionMachine, CoinError>;
 
   async fn publish_transaction(

processor/src/coin/monero.rs

@@ -188,7 +188,6 @@ impl Coin for Monero {
   async fn attempt_send(
     &self,
     transaction: SignableTransaction,
-    included: &[u16],
   ) -> Result<Self::TransactionMachine, CoinError> {
     transaction
       .actual
@@ -198,7 +197,6 @@ impl Coin for Monero {
         transaction.keys.clone(),
         transaction.transcript.clone(),
         transaction.height,
-        included.to_vec(),
       )
       .await
       .map_err(|_| CoinError::ConnectionError)
@@ -209,7 +207,6 @@ impl Coin for Monero {
     tx: &Self::Transaction,
   ) -> Result<(Vec<u8>, Vec<<Self::Output as OutputTrait>::Id>), CoinError> {
     self.rpc.publish_transaction(tx).await.map_err(|_| CoinError::ConnectionError)?;
-
     Ok((tx.hash().to_vec(), tx.prefix.outputs.iter().map(|output| output.key.to_bytes()).collect()))
   }
 

processor/src/tests/mod.rs

@@ -98,11 +98,7 @@ async fn test_send<C: Coin + Clone>(coin: C, fee: C::Fee) {
       .unwrap()
       .1
       .swap_remove(0);
-    futures.push(wallet.attempt_send(
-      network,
-      signable,
-      (1 ..= threshold).into_iter().collect::<Vec<_>>(),
-    ));
+    futures.push(wallet.attempt_send(network, signable));
   }
 
   println!("{:?}", hex::encode(futures::future::join_all(futures).await.swap_remove(0).unwrap().0));

processor/src/wallet.rs

@@ -339,10 +339,8 @@ impl<D: CoinDb, C: Coin> Wallet<D, C> {
     &mut self,
     network: &mut N,
     prepared: C::SignableTransaction,
-    included: Vec<u16>,
   ) -> Result<(Vec<u8>, Vec<<C::Output as Output>::Id>), SignError> {
-    let attempt =
-      self.coin.attempt_send(prepared, &included).await.map_err(SignError::CoinError)?;
+    let attempt = self.coin.attempt_send(prepared).await.map_err(SignError::CoinError)?;
 
     let (attempt, commitments) = attempt.preprocess(&mut OsRng);
     let commitments = network