Refresh shares with DKG (#663)

frost-core/src/keys/refresh.rs

@@ -7,14 +7,18 @@
 use std::collections::BTreeMap;
 
 use crate::{
+    keys::dkg::{compute_proof_of_knowledge, round1, round2},
     keys::{
-        generate_coefficients, generate_secret_shares, validate_num_of_signers,
-        CoefficientCommitment, PublicKeyPackage, SigningKey, SigningShare, VerifyingShare,
+        evaluate_polynomial, generate_coefficients, generate_secret_polynomial,
+        generate_secret_shares, validate_num_of_signers, CoefficientCommitment, PublicKeyPackage,
+        SigningKey, SigningShare, VerifyingShare,
     },
-    Ciphersuite, CryptoRng, Error, Field, Group, Identifier, RngCore,
+    Ciphersuite, CryptoRng, Error, Field, Group, Header, Identifier, RngCore,
 };
 
-use super::{KeyPackage, SecretShare, VerifiableSecretSharingCommitment};
+use core::iter;
+
+use super::{dkg::round1::Package, KeyPackage, SecretShare, VerifiableSecretSharingCommitment};
 
 /// Refreshes shares using a trusted dealer
 pub fn calculate_zero_key<C: Ciphersuite, R: RngCore + CryptoRng>(
@@ -52,6 +56,9 @@ pub fn calculate_zero_key<C: Ciphersuite, R: RngCore + CryptoRng>(
     let mut zero_shares_minus_identity: Vec<SecretShare<C>> = Vec::new();
 
     for share in zero_shares.clone() {
+        if old_pub_key_package.verifying_shares.get(&share.identifier) == None {
+            return Err(Error::UnknownIdentifier);
+        }
         let signer_public = SigningShare::into(share.signing_share);
         verifying_shares.insert(share.identifier, signer_public);
         let mut coefficients = share.commitment.0;
@@ -109,3 +116,241 @@ pub fn refresh_share<C: Ciphersuite>(
 
     Ok(new_key_package)
 }
+
+/// Part 1 of refresh share with DKG. A refreshing_key is generated and a new package and secret_package are generated.
+/// The identity commitment is removed from the packages.
+pub fn refresh_dkg_part_1<C: Ciphersuite, R: RngCore + CryptoRng>(
+    identifier: Identifier<C>,
+    max_signers: u16,
+    min_signers: u16,
+    mut rng: R,
+) -> Result<(round1::SecretPackage<C>, round1::Package<C>), Error<C>> {
+    validate_num_of_signers::<C>(min_signers, max_signers)?;
+
+    // Build refreshing shares
+    let refreshing_key = SigningKey {
+        scalar: <<C::Group as Group>::Field>::zero(),
+    };
+
+    // Round 1, Step 1
+    let coefficients = generate_coefficients::<C, R>(min_signers as usize - 1, &mut rng);
+
+    let (coefficients, commitment) =
+        generate_secret_polynomial(&refreshing_key, max_signers, min_signers, coefficients)?;
+
+    // Remove identity element from coefficients
+    let coeff_comms = commitment.0[1..].to_vec();
+    let commitment = VerifiableSecretSharingCommitment::new(coeff_comms.clone());
+
+    let proof_of_knowledge =
+        compute_proof_of_knowledge(identifier, &coefficients, &commitment, &mut rng)?;
+
+    let secret_package = round1::SecretPackage {
+        identifier,
+        coefficients: coefficients.clone(),
+        commitment: commitment.clone(),
+        min_signers,
+        max_signers,
+    };
+    let package = round1::Package {
+        header: Header::default(),
+        commitment,
+        proof_of_knowledge,
+    };
+
+    Ok((secret_package, package))
+}
+
+/// Part 2 of refresh share with DKG. The identity commitment needs to be added back into the secret package.
+pub fn refresh_dkg_part2<C: Ciphersuite>(
+    mut secret_package: round1::SecretPackage<C>,
+    round1_packages: &BTreeMap<Identifier<C>, round1::Package<C>>,
+) -> Result<
+    (
+        round2::SecretPackage<C>,
+        BTreeMap<Identifier<C>, round2::Package<C>>,
+    ),
+    Error<C>,
+> {
+    if round1_packages.len() != (secret_package.max_signers - 1) as usize {
+        return Err(Error::IncorrectNumberOfPackages);
+    }
+
+    // The identity commitment needs to be added to the VSS commitment for secret package
+    let identity_commitment: Vec<CoefficientCommitment<C>> =
+        vec![CoefficientCommitment::new(C::Group::identity())];
+
+    let refreshing_secret_share_commitments: Vec<CoefficientCommitment<C>> = identity_commitment
+        .into_iter()
+        .chain(secret_package.commitment.0.clone())
+        .collect();
+
+    secret_package.commitment =
+        VerifiableSecretSharingCommitment::<C>::new(refreshing_secret_share_commitments);
+
+    let mut round2_packages = BTreeMap::new();
+
+    for (sender_identifier, round1_package) in round1_packages {
+        // The identity commitment needs to be added to the VSS commitment for every round 1 package
+        let identity_commitment: Vec<CoefficientCommitment<C>> =
+            vec![CoefficientCommitment::new(C::Group::identity())];
+
+        let refreshing_share_commitments: Vec<CoefficientCommitment<C>> = identity_commitment
+            .into_iter()
+            .chain(round1_package.commitment.0.clone())
+            .collect();
+
+        if refreshing_share_commitments.clone().len() != secret_package.min_signers as usize {
+            return Err(Error::IncorrectNumberOfCommitments);
+        }
+
+        let ell = *sender_identifier;
+
+        // Round 1, Step 5
+        // We don't need to verify the proof of knowledge
+
+        // Round 2, Step 1
+        //
+        // > Each P_i securely sends to each other participant P_ℓ a secret share (ℓ, f_i(ℓ)),
+        // > deleting f_i and each share afterward except for (i, f_i(i)),
+        // > which they keep for themselves.
+        let signing_share = SigningShare::from_coefficients(&secret_package.coefficients, ell);
+
+        round2_packages.insert(
+            ell,
+            round2::Package {
+                header: Header::default(),
+                signing_share,
+            },
+        );
+    }
+    let fii = evaluate_polynomial(secret_package.identifier, &secret_package.coefficients);
+
+    Ok((
+        round2::SecretPackage {
+            identifier: secret_package.identifier,
+            commitment: secret_package.commitment,
+            secret_share: fii,
+            min_signers: secret_package.min_signers,
+            max_signers: secret_package.max_signers,
+        },
+        round2_packages,
+    ))
+}
+
+/// This is the step that actually refreshes the shares. New public key packages
+/// and key packages are created.
+pub fn refresh_dkg_shares<C: Ciphersuite>(
+    round2_secret_package: &round2::SecretPackage<C>,
+    round1_packages: &BTreeMap<Identifier<C>, round1::Package<C>>,
+    round2_packages: &BTreeMap<Identifier<C>, round2::Package<C>>,
+    old_pub_key_package: PublicKeyPackage<C>,
+    old_key_package: KeyPackage<C>,
+) -> Result<(KeyPackage<C>, PublicKeyPackage<C>), Error<C>> {
+    // Add identity commitment back into round1_packages
+    let mut new_round_1_packages = BTreeMap::new();
+    for (sender_identifier, round1_package) in round1_packages {
+        // The identity commitment needs to be added to the VSS commitment for every round 1 package
+        let identity_commitment: Vec<CoefficientCommitment<C>> =
+            vec![CoefficientCommitment::new(C::Group::identity())];
+
+        let refreshing_share_commitments: Vec<CoefficientCommitment<C>> = identity_commitment
+            .into_iter()
+            .chain(round1_package.commitment.0.clone())
+            .collect();
+
+        let new_commitments =
+            VerifiableSecretSharingCommitment::<C>::new(refreshing_share_commitments);
+
+        let new_round_1_package = Package {
+            header: round1_package.header,
+            commitment: new_commitments,
+            proof_of_knowledge: round1_package.proof_of_knowledge,
+        };
+
+        new_round_1_packages.insert(*sender_identifier, new_round_1_package);
+    }
+
+    if new_round_1_packages.len() != (round2_secret_package.max_signers - 1) as usize {
+        return Err(Error::IncorrectNumberOfPackages);
+    }
+    if new_round_1_packages.len() != round2_packages.len() {
+        return Err(Error::IncorrectNumberOfPackages);
+    }
+    if new_round_1_packages
+        .keys()
+        .any(|id| !round2_packages.contains_key(id))
+    {
+        return Err(Error::IncorrectPackage);
+    }
+
+    let mut signing_share = <<C::Group as Group>::Field>::zero();
+
+    for (sender_identifier, round2_package) in round2_packages {
+        // Round 2, Step 2
+        //
+        // > Each P_i verifies their shares by calculating:
+        // > g^{f_ℓ(i)} ≟ ∏^{t−1}_{k=0} φ^{i^k mod q}_{ℓk}, aborting if the
+        // > check fails.
+        let ell = *sender_identifier;
+        let f_ell_i = round2_package.signing_share;
+
+        let commitment = &round1_packages
+            .get(&ell)
+            .ok_or(Error::PackageNotFound)?
+            .commitment;
+
+        // Round 2, Step 3
+        //
+        // > Each P_i calculates their long-lived private signing share by computing
+        // > s_i = ∑^n_{ℓ=1} f_ℓ(i), stores s_i securely, and deletes each f_ℓ(i).
+        signing_share = signing_share + f_ell_i.to_scalar();
+    }
+
+    signing_share = signing_share + round2_secret_package.secret_share;
+
+    // Build new signing share
+    let old_signing_share = old_key_package.signing_share.to_scalar();
+    signing_share = signing_share + old_signing_share;
+    let signing_share = SigningShare::new(signing_share);
+
+    // Round 2, Step 4
+    //
+    // > Each P_i calculates their public verification share Y_i = g^{s_i}.
+    let verifying_share = signing_share.into();
+
+    let commitments: BTreeMap<_, _> = new_round_1_packages
+        .iter()
+        .map(|(id, package)| (*id, &package.commitment))
+        .chain(iter::once((
+            round2_secret_package.identifier,
+            &round2_secret_package.commitment,
+        )))
+        .collect();
+
+    let zero_shares_public_key_package = PublicKeyPackage::from_dkg_commitments(&commitments)?;
+
+    let mut new_verifying_shares = BTreeMap::new();
+
+    for (identifier, verifying_share) in zero_shares_public_key_package.verifying_shares {
+        let new_verifying_share = verifying_share.to_element() + old_pub_key_package.verifying_shares[&identifier].to_element();
+        new_verifying_shares.insert(identifier, VerifyingShare::new(new_verifying_share));
+    }
+
+    let public_key_package = PublicKeyPackage {
+        header: old_pub_key_package.header,
+        verifying_shares: new_verifying_shares,
+        verifying_key: old_pub_key_package.verifying_key,
+    };
+
+    let key_package = KeyPackage {
+        header: Header::default(),
+        identifier: round2_secret_package.identifier,
+        signing_share,
+        verifying_share,
+        verifying_key: public_key_package.verifying_key,
+        min_signers: round2_secret_package.min_signers,
+    };
+
+    Ok((key_package, public_key_package))
+}

frost-core/src/tests/ciphersuite_generic.rs

@@ -529,7 +529,7 @@ where
 }
 
 /// Check that calling dkg::part3() with distinct sets of participants fail.
-fn check_part3_different_participants<C: Ciphersuite>(
+pub fn check_part3_different_participants<C: Ciphersuite>(
     max_signers: u16,
     round2_secret_packages: BTreeMap<Identifier<C>, frost::keys::dkg::round2::SecretPackage<C>>,
     received_round1_packages: BTreeMap<