From 7dc1a24bced36d043ff62fceca8746ec30a46cd4 Mon Sep 17 00:00:00 2001 From: Luke Parker Date: Sun, 15 Oct 2023 01:39:56 -0400 Subject: [PATCH] Move DkgConfirmer to its own file, document --- coordinator/src/tributary/dkg_confirmer.rs | 194 +++++++++++++++++++++ coordinator/src/tributary/handle.rs | 132 +------------- coordinator/src/tributary/mod.rs | 2 + 3 files changed, 200 insertions(+), 128 deletions(-) create mode 100644 coordinator/src/tributary/dkg_confirmer.rs diff --git a/coordinator/src/tributary/dkg_confirmer.rs b/coordinator/src/tributary/dkg_confirmer.rs new file mode 100644 index 00000000..dc2fdecd --- /dev/null +++ b/coordinator/src/tributary/dkg_confirmer.rs @@ -0,0 +1,194 @@ +use std::collections::HashMap; + +use zeroize::Zeroizing; + +use rand_core::SeedableRng; +use rand_chacha::ChaCha20Rng; + +use transcript::{Transcript, RecommendedTranscript}; +use ciphersuite::{Ciphersuite, Ristretto}; +use frost::{ + FrostError, + dkg::{Participant, musig::musig}, + sign::*, +}; +use frost_schnorrkel::Schnorrkel; + +use serai_client::validator_sets::primitives::{KeyPair, musig_context, set_keys_message}; + +use crate::tributary::TributarySpec; + +/* + The following confirms the results of the DKG performed by the Processors onto Substrate. + + This is done by a signature over the generated key pair by the validators' MuSig-aggregated + public key. The MuSig-aggregation achieves on-chain efficiency and prevents on-chain censorship + of individual validator's DKG results by the Serai validator set. + + Since we're using the validators public keys, as needed for their being the root of trust, the + coordinator must perform the signing. This is distinct from all other group-signing operations + which are generally done by the processor. + + Instead of maintaining state, the following rebuilds the full state on every call. This is deemed + acceptable re: performance as: + + 1) The DKG confirmation is only done upon the start of the Tributary. + 2) This is an O(n) algorithm. + 3) The size of the validator set is bounded by MAX_KEY_SHARES_PER_SET. + + Accordingly, this should be infrequently ran and of tolerable algorithmic complexity. + + As for safety, it is explicitly unsafe to reuse nonces across signing sessions. This is in + contradiction with our rebuilding which is dependent on deterministic nonces. Safety is derived + from the deterministic nonces being context-bound under a BFT protocol. The flow is as follows: + + 1) Derive a deterministic nonce by hashing the private key, Tributary parameters, and attempt. + 2) Publish the nonces' commitments, receiving everyone elses *and the DKG shares determining the + message to be signed*. + 3) Sign and publish the signature share. + + In order for nonce re-use to occur, the received nonce commitments, or the received DKG shares, + would have to be distinct and sign would have to be called again. + + Before we act on any received messages, they're ordered and finalized by a BFT algorithm. The + only way to operate on distinct received messages would be if: + + 1) A logical flaw exists, letting new messages over write prior messages + 2) A reorganization occured from chain A to chain B, and with it, different messages + + Reorganizations are not supported, as BFT is assumed by the presence of a BFT algorithm. While + a significant amount of processes may be byzantine, leading to BFT being broken, that still will + not trigger a reorganization. The only way to move to a distinct chain, with distinct messages, + would be by rebuilding the local process entirely (this time following chain B). + + Accordingly, safety follows if: + + 1) The local view of received messages is static + 2) The local process doesn't rebuild after a byzantine fault produces multiple blockchains + + We assume the former. The latter is deemed acceptable but sub-optimal. + + The benefit for this behavior is that on a validator's infrastructure collapsing, they can + successfully rebuild on a new system. + + TODO: Replace this with entropy. If a validator happens to have their infrastructure fail at this + exact moment, they should just be kicked out and accept the loss. The risk of losing a private + key on rebuild, by a feature meant to enable rebuild, can't be successfully argued for. + + Not only do we need to use randomly selected entropy, we need to confirm our local preprocess + matches the on-chain preprocess before actually publishing our shares. + + We also need to review how we're handling Processor preprocesses and likely implement the same + on-chain-preprocess-matches-presumed-preprocess check before publishing shares (though a delay of + the re-attempt protocol's trigger length would also be sufficient). +*/ +pub(crate) struct DkgConfirmer; +impl DkgConfirmer { + fn preprocess_internal( + spec: &TributarySpec, + key: &Zeroizing<::F>, + attempt: u32, + ) -> (AlgorithmSignMachine, [u8; 64]) { + // TODO: Does Substrate already have a validator-uniqueness check? + let validators = spec.validators().iter().map(|val| val.0).collect::>(); + + let context = musig_context(spec.set()); + let mut chacha = ChaCha20Rng::from_seed({ + let mut entropy_transcript = RecommendedTranscript::new(b"DkgConfirmer Entropy"); + entropy_transcript.append_message(b"spec", spec.serialize()); + entropy_transcript.append_message(b"key", Zeroizing::new(key.to_bytes())); + entropy_transcript.append_message(b"attempt", attempt.to_le_bytes()); + Zeroizing::new(entropy_transcript).rng_seed(b"preprocess") + }); + let (machine, preprocess) = AlgorithmMachine::new( + Schnorrkel::new(b"substrate"), + musig(&context, key, &validators) + .expect("confirming the DKG for a set we aren't in/validator present multiple times") + .into(), + ) + .preprocess(&mut chacha); + + (machine, preprocess.serialize().try_into().unwrap()) + } + // Get the preprocess for this confirmation. + pub(crate) fn preprocess( + spec: &TributarySpec, + key: &Zeroizing<::F>, + attempt: u32, + ) -> [u8; 64] { + Self::preprocess_internal(spec, key, attempt).1 + } + + fn share_internal( + spec: &TributarySpec, + key: &Zeroizing<::F>, + attempt: u32, + preprocesses: HashMap>, + key_pair: &KeyPair, + ) -> Result<(AlgorithmSignatureMachine, [u8; 32]), Participant> { + let machine = Self::preprocess_internal(spec, key, attempt).0; + let preprocesses = preprocesses + .into_iter() + .map(|(p, preprocess)| { + machine + .read_preprocess(&mut preprocess.as_slice()) + .map(|preprocess| (p, preprocess)) + .map_err(|_| p) + }) + .collect::, _>>()?; + let (machine, share) = machine + .sign(preprocesses, &set_keys_message(&spec.set(), key_pair)) + .map_err(|e| match e { + FrostError::InternalError(e) => unreachable!("FrostError::InternalError {e}"), + FrostError::InvalidParticipant(_, _) | + FrostError::InvalidSigningSet(_) | + FrostError::InvalidParticipantQuantity(_, _) | + FrostError::DuplicatedParticipant(_) | + FrostError::MissingParticipant(_) => unreachable!("{e:?}"), + FrostError::InvalidPreprocess(p) | FrostError::InvalidShare(p) => p, + })?; + + Ok((machine, share.serialize().try_into().unwrap())) + } + // Get the share for this confirmation, if the preprocesses are valid. + pub(crate) fn share( + spec: &TributarySpec, + key: &Zeroizing<::F>, + attempt: u32, + preprocesses: HashMap>, + key_pair: &KeyPair, + ) -> Result<[u8; 32], Participant> { + Self::share_internal(spec, key, attempt, preprocesses, key_pair).map(|(_, share)| share) + } + + pub(crate) fn complete( + spec: &TributarySpec, + key: &Zeroizing<::F>, + attempt: u32, + preprocesses: HashMap>, + key_pair: &KeyPair, + shares: HashMap>, + ) -> Result<[u8; 64], Participant> { + let machine = Self::share_internal(spec, key, attempt, preprocesses, key_pair) + .expect("trying to complete a machine which failed to preprocess") + .0; + + let shares = shares + .into_iter() + .map(|(p, share)| { + machine.read_share(&mut share.as_slice()).map(|share| (p, share)).map_err(|_| p) + }) + .collect::, _>>()?; + let signature = machine.complete(shares).map_err(|e| match e { + FrostError::InternalError(e) => unreachable!("FrostError::InternalError {e}"), + FrostError::InvalidParticipant(_, _) | + FrostError::InvalidSigningSet(_) | + FrostError::InvalidParticipantQuantity(_, _) | + FrostError::DuplicatedParticipant(_) | + FrostError::MissingParticipant(_) => unreachable!("{e:?}"), + FrostError::InvalidPreprocess(p) | FrostError::InvalidShare(p) => p, + })?; + + Ok(signature.to_bytes()) + } +} diff --git a/coordinator/src/tributary/handle.rs b/coordinator/src/tributary/handle.rs index a1fe2939..cde71d3c 100644 --- a/coordinator/src/tributary/handle.rs +++ b/coordinator/src/tributary/handle.rs @@ -3,21 +3,12 @@ use std::collections::HashMap; use zeroize::Zeroizing; -use rand_core::SeedableRng; -use rand_chacha::ChaCha20Rng; - -use transcript::{Transcript, RecommendedTranscript}; use ciphersuite::{group::GroupEncoding, Ciphersuite, Ristretto}; -use frost::{ - FrostError, - dkg::{Participant, musig::musig}, - sign::*, -}; -use frost_schnorrkel::Schnorrkel; +use frost::dkg::Participant; use serai_client::{ Signature, - validator_sets::primitives::{ValidatorSet, KeyPair, musig_context, set_keys_message}, + validator_sets::primitives::{ValidatorSet, KeyPair}, subxt::utils::Encoded, SeraiValidatorSets, }; @@ -37,6 +28,7 @@ use crate::{ tributary::{ Transaction, TributarySpec, Topic, DataSpecification, TributaryDb, nonce_decider::NonceDecider, + dkg_confirmer::DkgConfirmer, scanner::{RecognizedIdType, RIDTrait}, }, }; @@ -54,122 +46,6 @@ const BATCH_SHARE: &str = "b_share"; const SIGN_PREPROCESS: &str = "s_preprocess"; const SIGN_SHARE: &str = "s_share"; -// Instead of maintaing state, this simply re-creates the machine(s) in-full on every call (which -// should only be once per tributary). -// This simplifies data flow and prevents requiring multiple paths. -// While more expensive, this only runs an O(n) algorithm, which is tolerable to run multiple -// times. -struct DkgConfirmer; -impl DkgConfirmer { - fn preprocess_internal( - spec: &TributarySpec, - key: &Zeroizing<::F>, - attempt: u32, - ) -> (AlgorithmSignMachine, [u8; 64]) { - // TODO: Does Substrate already have a validator-uniqueness check? - let validators = spec.validators().iter().map(|val| val.0).collect::>(); - - let context = musig_context(spec.set()); - let mut chacha = ChaCha20Rng::from_seed({ - let mut entropy_transcript = RecommendedTranscript::new(b"DkgConfirmer Entropy"); - entropy_transcript.append_message(b"spec", spec.serialize()); - entropy_transcript.append_message(b"key", Zeroizing::new(key.to_bytes())); - entropy_transcript.append_message(b"attempt", attempt.to_le_bytes()); - Zeroizing::new(entropy_transcript).rng_seed(b"preprocess") - }); - let (machine, preprocess) = AlgorithmMachine::new( - Schnorrkel::new(b"substrate"), - musig(&context, key, &validators) - .expect("confirming the DKG for a set we aren't in/validator present multiple times") - .into(), - ) - .preprocess(&mut chacha); - - (machine, preprocess.serialize().try_into().unwrap()) - } - // Get the preprocess for this confirmation. - fn preprocess( - spec: &TributarySpec, - key: &Zeroizing<::F>, - attempt: u32, - ) -> [u8; 64] { - Self::preprocess_internal(spec, key, attempt).1 - } - - fn share_internal( - spec: &TributarySpec, - key: &Zeroizing<::F>, - attempt: u32, - preprocesses: HashMap>, - key_pair: &KeyPair, - ) -> Result<(AlgorithmSignatureMachine, [u8; 32]), Participant> { - let machine = Self::preprocess_internal(spec, key, attempt).0; - let preprocesses = preprocesses - .into_iter() - .map(|(p, preprocess)| { - machine - .read_preprocess(&mut preprocess.as_slice()) - .map(|preprocess| (p, preprocess)) - .map_err(|_| p) - }) - .collect::, _>>()?; - let (machine, share) = machine - .sign(preprocesses, &set_keys_message(&spec.set(), key_pair)) - .map_err(|e| match e { - FrostError::InternalError(e) => unreachable!("FrostError::InternalError {e}"), - FrostError::InvalidParticipant(_, _) | - FrostError::InvalidSigningSet(_) | - FrostError::InvalidParticipantQuantity(_, _) | - FrostError::DuplicatedParticipant(_) | - FrostError::MissingParticipant(_) => unreachable!("{e:?}"), - FrostError::InvalidPreprocess(p) | FrostError::InvalidShare(p) => p, - })?; - - Ok((machine, share.serialize().try_into().unwrap())) - } - // Get the share for this confirmation, if the preprocesses are valid. - fn share( - spec: &TributarySpec, - key: &Zeroizing<::F>, - attempt: u32, - preprocesses: HashMap>, - key_pair: &KeyPair, - ) -> Result<[u8; 32], Participant> { - Self::share_internal(spec, key, attempt, preprocesses, key_pair).map(|(_, share)| share) - } - - fn complete( - spec: &TributarySpec, - key: &Zeroizing<::F>, - attempt: u32, - preprocesses: HashMap>, - key_pair: &KeyPair, - shares: HashMap>, - ) -> Result<[u8; 64], Participant> { - let machine = Self::share_internal(spec, key, attempt, preprocesses, key_pair) - .expect("trying to complete a machine which failed to preprocess") - .0; - - let shares = shares - .into_iter() - .map(|(p, share)| { - machine.read_share(&mut share.as_slice()).map(|share| (p, share)).map_err(|_| p) - }) - .collect::, _>>()?; - let signature = machine.complete(shares).map_err(|e| match e { - FrostError::InternalError(e) => unreachable!("FrostError::InternalError {e}"), - FrostError::InvalidParticipant(_, _) | - FrostError::InvalidSigningSet(_) | - FrostError::InvalidParticipantQuantity(_, _) | - FrostError::DuplicatedParticipant(_) | - FrostError::MissingParticipant(_) => unreachable!("{e:?}"), - FrostError::InvalidPreprocess(p) | FrostError::InvalidShare(p) => p, - })?; - - Ok(signature.to_bytes()) - } -} - fn read_known_to_exist_data( getter: &G, spec: &TributarySpec, @@ -280,7 +156,7 @@ pub(crate) async fn handle_application_tx< }; // If they've already published a TX for this attempt, slash - if let Some(_) = TributaryDb::::data(txn, genesis, data_spec, signed.signer) { + if TributaryDb::::data(txn, genesis, data_spec, signed.signer).is_some() { fatal_slash::(txn, genesis, signed.signer.to_bytes(), "published data multiple times"); return None; } diff --git a/coordinator/src/tributary/mod.rs b/coordinator/src/tributary/mod.rs index e0533792..8f4bdf2d 100644 --- a/coordinator/src/tributary/mod.rs +++ b/coordinator/src/tributary/mod.rs @@ -33,6 +33,8 @@ pub use db::*; mod nonce_decider; pub use nonce_decider::*; +mod dkg_confirmer; + mod handle; pub use handle::*;