* 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
Encryption used to be inlined into FROST. When writing the documentation, I
realized it was decently hard to review. It also was antagonistic to other
hosted DKG algorithms by not allowing code re-use.
Encryption is now a standalone module, providing clear boundaries and
reusability.
Additionally, the DKG protocol itself used to use the ciphersuite's specified
hash function (with an HKDF to prevent length extension attacks). Now,
RecommendedTranscript is used to achieve much more robust transcripting and
remove the HKDF dependency. This does add Blake2 into all consumers yet is
preferred for its security properties and ease of review.
