Initial eVRF-based DKG

Cargo.lock

@@ -2132,7 +2132,10 @@ dependencies = [
  "chacha20",
  "ciphersuite",
  "dleq",
+ "ec-divisors",
+ "evrf",
  "flexible-transcript",
+ "generalized-bulletproofs",
  "multiexp",
  "rand_core",
  "schnorr-signatures",

crypto/dkg/Cargo.toml

@@ -36,6 +36,10 @@ multiexp = { path = "../multiexp", version = "0.4", default-features = false }
 schnorr = { package = "schnorr-signatures", path = "../schnorr", version = "^0.5.1", default-features = false }
 dleq = { path = "../dleq", version = "^0.4.1", default-features = false }
 
+generalized-bulletproofs = { path = "../evrf/generalized-bulletproofs", default-features = false, optional = true }
+ec-divisors = { path = "../evrf/divisors", default-features = false, optional = true }
+evrf = { path = "../evrf", default-features = false, optional = true }
+
 [dev-dependencies]
 rand_core = { version = "0.6", default-features = false, features = ["getrandom"] }
 ciphersuite = { path = "../ciphersuite", default-features = false, features = ["ristretto"] }
@@ -62,5 +66,6 @@ std = [
   "dleq/serialize"
 ]
 borsh = ["dep:borsh"]
+evrf = ["std", "dep:ec-divisors", "dep:generalized-bulletproofs", "dep:evrf"]
 tests = ["rand_core/getrandom"]
 default = ["std"]

crypto/dkg/src/encryption.rs

@@ -345,29 +345,27 @@ pub(crate) struct Decryption<C: Ciphersuite> {
 }
 
 impl<C: Ciphersuite> Decryption<C> {
-pub(crate) fn new(context: [u8; 32]) -> Self { Self { context, enc_keys: HashMap::new()} }
-pub(crate) fn register(
-  &mut self,
-  participant: Participant,
-  key: C::G,
-) {
+  pub(crate) fn new(context: [u8; 32]) -> Self {
+    Self { context, enc_keys: HashMap::new() }
+  }
+  pub(crate) fn register(&mut self, participant: Participant, key: C::G) {
     assert!(
       !self.enc_keys.contains_key(&participant),
       "Re-registering encryption key for a participant"
     );
     self.enc_keys.insert(participant, key);
-}
+  }
 
-// Given a message, and the intended decryptor, and a proof for its key, decrypt the message.
-// Returns None if the key was wrong.
-pub(crate) fn decrypt_with_proof<E: Encryptable>(
+  // Given a message, and the intended decryptor, and a proof for its key, decrypt the message.
+  // Returns None if the key was wrong.
+  pub(crate) fn decrypt_with_proof<E: Encryptable>(
     &self,
     from: Participant,
     decryptor: Participant,
     mut msg: EncryptedMessage<C, E>,
     // There's no encryption key proof if the accusation is of an invalid signature
     proof: Option<EncryptionKeyProof<C>>,
-) -> Result<Zeroizing<E>, DecryptionError> {
+  ) -> Result<Zeroizing<E>, DecryptionError> {
     if !msg.pop.verify(
       msg.key,
       pop_challenge::<C>(self.context, msg.pop.R, msg.key, from, msg.msg.deref().as_ref()),
@@ -391,7 +389,7 @@ pub(crate) fn decrypt_with_proof<E: Encryptable>(
     } else {
       Err(DecryptionError::InvalidProof)
     }
-}
+  }
 }
 
 // A simple box for managing encryption.
@@ -427,11 +425,7 @@ impl<C: Ciphersuite> Zeroize for Encryption<C> {
 }
 
 impl<C: Ciphersuite> Encryption<C> {
-  pub(crate) fn new(
-    context: [u8; 32],
-    i: Participant,
-    enc_key: Zeroizing<C::F>,
-  ) -> Self {
+  pub(crate) fn new(context: [u8; 32], i: Participant, enc_key: Zeroizing<C::F>) -> Self {
     Self {
       context,
       i,
@@ -445,11 +439,7 @@ impl<C: Ciphersuite> Encryption<C> {
     EncryptionKeyMessage { msg, enc_key: self.enc_pub_key }
   }
 
-  pub(crate) fn register(
-    &mut self,
-    participant: Participant,
-    key: C::G,
-  ) {
+  pub(crate) fn register(&mut self, participant: Participant, key: C::G) {
     self.decryption.register(participant, key)
   }
 
@@ -496,5 +486,7 @@ impl<C: Ciphersuite> Encryption<C> {
     )
   }
 
-  pub(crate) fn into_decryption(self) -> Decryption<C> { self.decryption }
+  pub(crate) fn into_decryption(self) -> Decryption<C> {
+    self.decryption
+  }
 }

crypto/dkg/src/evrf.rs

@@ -0,0 +1,384 @@
+use core::ops::Deref;
+use std::{
+  io::{self, Read, Write},
+  collections::HashMap,
+};
+
+use rand_core::{RngCore, CryptoRng};
+
+use zeroize::{Zeroize, Zeroizing};
+
+use ciphersuite::{
+  group::ff::{Field, PrimeField},
+  Ciphersuite,
+};
+use multiexp::multiexp_vartime;
+
+use generalized_bulletproofs::{Generators, BatchVerifier, arithmetic_circuit_proof::*};
+use ec_divisors::DivisorCurve;
+use evrf::*;
+
+use crate::{
+  Participant, DkgError, ThresholdParams, ThresholdCore,
+  encryption::{ReadWrite, EncryptedMessage, Encryption, EncryptionKeyProof},
+  pedpop::SecretShare,
+};
+
+type EvrfError<C> = DkgError<EncryptionKeyProof<C>>;
+
+/// The commitments message, intended to be broadcast to all other parties.
+///
+/// Every participant should only provide one set of commitments to all parties. If any
+/// participant sends multiple sets of commitments, they are faulty and should be presumed
+/// malicious. As this library does not handle networking, it is unable to detect if any
+/// participant is so faulty. That responsibility lies with the caller.
+#[derive(Clone, PartialEq, Eq, Debug, Zeroize)]
+pub struct Commitments {
+  proof: Vec<u8>,
+}
+
+impl ReadWrite for Commitments {
+  fn read<R: Read>(reader: &mut R, _params: ThresholdParams) -> io::Result<Self> {
+    // TODO: Replace `len` with some calculcation deterministic to the params
+    let mut len = [0; 4];
+    reader.read_exact(&mut len)?;
+    let len = usize::try_from(u32::from_le_bytes(len)).expect("<32-bit platform?");
+
+    // Don't allocate a buffer for the claimed length
+    // Read chunks until we reach the claimed length
+    // This means if we were told to read GB, we must actually be sent GB before allocating as such
+    const CHUNK_SIZE: usize = 1024;
+    let mut proof = Vec::with_capacity(len.min(CHUNK_SIZE));
+    while proof.len() < len {
+      let next_chunk = (len - proof.len()).min(CHUNK_SIZE);
+      let old_proof_len = proof.len();
+      proof.resize(old_proof_len + next_chunk, 0);
+      reader.read_exact(&mut proof[old_proof_len ..])?;
+    }
+
+    Ok(Commitments { proof })
+  }
+
+  fn write<W: Write>(&self, writer: &mut W) -> io::Result<()> {
+    writer.write_all(&u32::try_from(self.proof.len()).unwrap().to_le_bytes())?;
+    writer.write_all(&self.proof)?;
+    Ok(())
+  }
+}
+
+fn polynomial<F: PrimeField + Zeroize>(
+  coefficients: &[Zeroizing<F>],
+  l: Participant,
+) -> Zeroizing<F> {
+  let l = F::from(u64::from(u16::from(l)));
+  // This should never be reached since Participant is explicitly non-zero
+  assert!(l != F::ZERO, "zero participant passed to polynomial");
+  let mut share = Zeroizing::new(F::ZERO);
+  for (idx, coefficient) in coefficients.iter().rev().enumerate() {
+    *share += coefficient.deref();
+    if idx != (coefficients.len() - 1) {
+      *share *= l;
+    }
+  }
+  share
+}
+
+/// Struct to perform/verify the DKG with.
+#[derive(Debug, Zeroize)]
+pub struct EvrfDkg;
+
+enum AccumulationStrategy<C: EvrfCurve> {
+  #[rustfmt::skip]
+  WaitingForThreshold {
+    pending_verification: HashMap<Participant, (Commitments, Zeroizing<C::F>)>,
+  },
+  Incremental {
+    accumulated: HashMap<Participant, (Vec<C::G>, Zeroizing<C::F>)>,
+  },
+}
+
+struct EvrfAccumulatorCore<'a, C: EvrfCurve> {
+  generators: &'a Generators<C>,
+  evrf_public_keys: Vec<<C::EmbeddedCurve as Ciphersuite>::G>,
+  context: [u8; 32],
+  params: ThresholdParams,
+}
+
+pub struct EvrfAccumulator<'a, C: EvrfCurve> {
+  core: EvrfAccumulatorCore<'a, C>,
+
+  encryption: Encryption<C::EmbeddedCurve>,
+
+  our_commitments: Vec<C::G>,
+  accumulation: AccumulationStrategy<C>,
+  resulting_share: Zeroizing<C::F>,
+}
+
+pub struct EvrfShare<C: EvrfCurve> {
+  commitments: Commitments,
+  shares: HashMap<Participant, EncryptedMessage<C::EmbeddedCurve, SecretShare<C::F>>>,
+}
+
+impl EvrfDkg {
+  /// Participate in performing the DKG for the specified parameters.
+  ///
+  /// The context MUST be unique across invocations. Reuse of context will lead to sharing
+  /// prior-shared secrets.
+  // TODO: Have this return an accumulator
+  pub fn share<'a, C: EvrfCurve>(
+    rng: &mut (impl RngCore + CryptoRng),
+    generators: &'a Generators<C>,
+    evrf_public_keys: Vec<<C::EmbeddedCurve as Ciphersuite>::G>,
+    context: [u8; 32],
+    params: ThresholdParams,
+    evrf_private_key: Zeroizing<<C::EmbeddedCurve as Ciphersuite>::F>,
+  ) -> Result<(EvrfAccumulator<'a, C>, EvrfShare<C>), AcError>
+  where
+    <<C as EvrfCurve>::EmbeddedCurve as Ciphersuite>::G:
+      DivisorCurve<FieldElement = <C as Ciphersuite>::F>,
+  {
+    // TODO: Confirm `n` == the amount of evrf_public_keys
+    // TODO: Confirm evrf_public_keys[i] == evrf_private_key * G
+    // TODO: Hash context to include the list of public keys
+
+    let EvrfProveResult { scalars, proof } =
+      Evrf::prove(rng, generators, evrf_private_key.clone(), context, usize::from(params.t()))?;
+
+    /*
+      We reuse the eVRF key for receiving encrypted messages.
+
+      For encrypting to other parties, we use a randomly generated ephemeral key, so there's no
+      risk there.
+
+      When decrypting, we calculcate the ECDH of our private key with the ephemeral public key. If
+      the decryption fails, we publish the ECDH with a proof. If the ephemeral public key is one
+      of the eVRF points, this would leak a secret. Since ephemeral public keys must be associated
+      with PoKs for their discrete logarithms, and the eVRF points have unknown discrete
+      logarithms, this is still secure.
+    */
+    let mut encryption = Encryption::new(context, params.i(), evrf_private_key);
+    for (i, evrf_public_key) in evrf_public_keys.iter().enumerate() {
+      encryption
+        .register(Participant::new(u16::try_from(i + 1).unwrap()).unwrap(), *evrf_public_key);
+    }
+
+    let mut resulting_share = None;
+    let mut shares = HashMap::new();
+    for l in (1 ..= params.n()).map(Participant) {
+      let share = polynomial::<C::F>(&scalars, l);
+
+      // Don't insert our own share as we don't need to send out our own share
+      if l == params.i() {
+        resulting_share = Some(share);
+        continue;
+      }
+
+      let share_bytes = Zeroizing::new(SecretShare::<C::F>(share.to_repr()));
+      shares.insert(l, encryption.encrypt(rng, l, share_bytes));
+    }
+
+    let accumulator = EvrfAccumulator {
+      core: EvrfAccumulatorCore { generators, evrf_public_keys, context, params },
+
+      encryption,
+
+      our_commitments: scalars.iter().map(|scalar| C::generator() * **scalar).collect(),
+      accumulation: AccumulationStrategy::WaitingForThreshold {
+        pending_verification: HashMap::new(),
+      },
+      resulting_share: resulting_share.unwrap(),
+    };
+    Ok((accumulator, EvrfShare { commitments: Commitments { proof }, shares }))
+  }
+}
+
+fn exponential<C: Ciphersuite>(i: Participant, values: &[C::G]) -> C::G {
+  let i = C::F::from(u16::from(i).into());
+  let mut res = Vec::with_capacity(values.len());
+  (0 .. values.len()).fold(C::F::ONE, |exp, l| {
+    res.push((exp, values[l]));
+    exp * i
+  });
+  multiexp_vartime(&res)
+}
+
+struct Blame;
+
+impl<'a, C: EvrfCurve> EvrfAccumulatorCore<'a, C>
+where
+  <<C as EvrfCurve>::EmbeddedCurve as Ciphersuite>::G:
+    DivisorCurve<FieldElement = <C as Ciphersuite>::F>,
+{
+  fn verify_evrf(
+    &mut self,
+    rng: &mut (impl RngCore + CryptoRng),
+    verifier: &mut BatchVerifier<C>,
+    from: Participant,
+    commitments: &Commitments,
+  ) -> Result<Vec<C::G>, ()> {
+    // TODO: Verify from is in-range and distinct from params.i()
+    let from_public_key = self.evrf_public_keys[usize::from(u16::from(from) - 1)];
+    Evrf::verify(
+      rng,
+      self.generators,
+      verifier,
+      from_public_key,
+      self.context,
+      usize::from(self.params.t()),
+      &commitments.proof,
+    )
+  }
+}
+
+impl<'a, C: EvrfCurve> EvrfAccumulator<'a, C>
+where
+  <<C as EvrfCurve>::EmbeddedCurve as Ciphersuite>::G:
+    DivisorCurve<FieldElement = <C as Ciphersuite>::F>,
+{
+  /// Verify a secret sharing.
+  pub fn accumulate(
+    &mut self,
+    rng: &mut (impl RngCore + CryptoRng),
+    from: Participant,
+    commitments: Commitments,
+    share: EncryptedMessage<C::EmbeddedCurve, SecretShare<C::F>>,
+  ) -> Vec<Blame> {
+    // TODO: Confirm `n` == the amount of evrf_public_keys
+    // TODO: Confirm evrf_public_keys[i] == evrf_private_key * G
+    // TODO: Hash context to include the list of public keys
+    // TODO: Check not prior accumulated
+
+    // This uses an ephemeral BatchVerifier as if we verify an invalid proof, it'll corrupt the
+    // BatchVerifier. If we tried to form a BatchVerifier, it'd need reconstruction on such error,
+    // increasing complexity and opening potential DoS vectors
+    let mut ephemeral_verifier = self.core.generators.batch_verifier();
+    let Ok(actual_commitments) =
+      self.core.verify_evrf(rng, &mut ephemeral_verifier, from, &commitments)
+    else {
+      return vec![Blame];
+    };
+
+    // Decrypt the share
+    let mut batch = multiexp::BatchVerifier::new(1);
+    let (mut share_bytes, blame) = self.encryption.decrypt(rng, &mut batch, (), from, share);
+    let Some(share) = Option::<C::F>::from(C::F::from_repr(share_bytes.0)) else {
+      return vec![Blame];
+    };
+    let share = Zeroizing::new(share);
+    share_bytes.zeroize();
+
+    if exponential::<C>(self.core.params.i(), &actual_commitments) !=
+      (self.core.generators.g() * *share)
+    {
+      return vec![Blame];
+    }
+
+    match &mut self.accumulation {
+      AccumulationStrategy::WaitingForThreshold { ref mut pending_verification } => {
+        pending_verification.insert(from, (commitments, share));
+
+        // If we now have the necessary threshold to consider this DKG as having succeeded, verify
+        // the proofs with a batch verification
+        if pending_verification.len() == usize::from(self.core.params.t()) {
+          let mut batch_verifier = self.core.generators.batch_verifier();
+          let mut all_pending_verification = HashMap::new();
+          for (participant, (commitments, share)) in &mut *pending_verification {
+            let actual_commitments = self
+              .core
+              .verify_evrf(rng, &mut batch_verifier, *participant, commitments)
+              .expect("prior verified evrf proof now errors upon verification");
+            all_pending_verification.insert(*participant, (actual_commitments, share.clone()));
+          }
+
+          if self.core.generators.verify(batch_verifier) {
+            // If the verification succeeded, marked the proofs pending verification as accumulated
+            self.accumulation =
+              AccumulationStrategy::Incremental { accumulated: all_pending_verification };
+          } else {
+            // Find the faulty proof(s)
+            let mut accumulated = HashMap::new();
+            let mut blames = vec![];
+            for (participant, (commitments, share)) in &mut *pending_verification {
+              let mut verifier = self.core.generators.batch_verifier();
+              let actual_commitments = self
+                .core
+                .verify_evrf(rng, &mut verifier, *participant, commitments)
+                .expect("prior verified evrf proof now errors upon verification");
+              if self.core.generators.verify(verifier) {
+                accumulated.insert(*participant, (actual_commitments, share.clone()));
+              } else {
+                blames.push(Blame);
+              }
+            }
+            self.accumulation = AccumulationStrategy::Incremental { accumulated };
+
+            // Now that we've marked all proofs as accumulated/faulty, return the blame
+            return blames;
+          }
+        }
+      }
+      AccumulationStrategy::Incremental { ref mut accumulated } => {
+        if self.core.generators.verify(ephemeral_verifier) {
+          accumulated.insert(from, (actual_commitments, share));
+        } else {
+          return vec![Blame];
+        }
+      }
+    }
+
+    vec![]
+  }
+
+  #[allow(clippy::needless_pass_by_value)]
+  pub fn process_blame(&mut self, blame: Blame) {
+    todo!("TODO");
+  }
+
+  pub fn introspect_group_key(&self) -> Result<C::G, ()> {
+    let AccumulationStrategy::Incremental { accumulated } = &self.accumulation else { Err(())? };
+    if (1 + accumulated.len()) < usize::from(self.core.params.t()) {
+      Err(())?
+    }
+    Ok(
+      accumulated.values().map(|(commitments, _)| commitments[0]).sum::<C::G>() +
+        self.our_commitments[0],
+    )
+  }
+
+  /// Finish accumulation.
+  pub fn complete(mut self) -> Result<ThresholdCore<C>, ()> {
+    let AccumulationStrategy::Incremental { accumulated } = self.accumulation else { Err(())? };
+
+    if (1 + accumulated.len()) < usize::from(self.core.params.t()) {
+      Err(())?
+    }
+
+    let commitments = accumulated
+      .values()
+      .map(|(commitments, _)| commitments)
+      .chain(core::iter::once(&self.our_commitments));
+    // Stripe commitments per t and sum them in advance
+    // Calculating verification shares relies on these sums so preprocessing them is a massive
+    // speedup
+    let mut stripes = Vec::with_capacity(usize::from(self.core.params.t()));
+    for t in 0 .. usize::from(self.core.params.t()) {
+      stripes.push(commitments.clone().map(|commitments| commitments[t]).sum());
+    }
+
+    // Calculate each user's verification share
+    let mut verification_shares = HashMap::new();
+    for i in (1 ..= self.core.params.n()).map(Participant) {
+      verification_shares.insert(i, exponential::<C>(i, &stripes));
+    }
+
+    for (_, share) in accumulated.values() {
+      *self.resulting_share += **share;
+    }
+    Ok(ThresholdCore {
+      params: self.core.params,
+      secret_share: self.resulting_share,
+      group_key: stripes[0],
+      verification_shares,
+    })
+  }
+}

crypto/dkg/src/lib.rs

@@ -21,6 +21,10 @@ pub mod encryption;
 #[cfg(feature = "std")]
 pub mod pedpop;
 
+/// The one-round DKG described in the [eVRF paper](https://eprint.iacr.org/2024/397).
+#[cfg(all(feature = "std", feature = "evrf"))]
+pub mod evrf;
+
 /// Promote keys between ciphersuites.
 #[cfg(feature = "std")]
 pub mod promote;

tests/processor/src/networks.rs

@@ -452,7 +452,7 @@ impl Wallet {
           );
         }
 
-        let to_spend_key = decompress_point(<[u8; 32]>::try_from(to.as_ref()).unwrap()).unwrap();
+        let to_spend_key = decompress_point(<[u8; 32]>::try_from(to.as_slice()).unwrap()).unwrap();
         let to_view_key = additional_key::<Monero>(0);
         let to_addr = Address::new(
           Network::Mainnet,