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Expose internals to support randomization (simplified) (#152) 

* changes required for randomization

* simplified version

* simplify compute_signature_share return

* add frost-rerandomized crate

* move rerandomized tests to frost-rerandomized from reddsa; remove unused deps

* Update frost-core/src/frost.rs

Co-authored-by: Deirdre Connolly <durumcrustulum@gmail.com>
This commit is contained in:
Conrado Gouvea 2023-01-06 01:26:13 -03:00 committed by GitHub
parent 0fffd6517d
commit ffe5c57a17
No known key found for this signature in database
GPG Key ID: 4AEE18F83AFDEB23
15 changed files with 537 additions and 65 deletions
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1
Cargo.toml
View File

@ -8,5 +8,6 @@ members = [
"frost-p256",
"frost-ristretto255",
"frost-secp256k1",
"frost-rerandomized",
"gendoc"
]

2
frost-core/Cargo.toml
View File

@ -23,6 +23,7 @@ digest = "0.10"
hex = { version = "0.4.3", features = ["serde"] }
rand_core = "0.6"
thiserror = "1.0"
visibility = "0.0.1"
zeroize = { version = "1.5.4", default-features = false, features = ["derive"] }
# Test dependencies used with the test-impl feature
@ -44,3 +45,4 @@ nightly = []
default = []
# Exposes ciphersuite-generic tests for other crates to use
test-impl = ["proptest", "proptest-derive", "serde_json"]
internals = []

126
frost-core/src/frost.rs
View File

@ -12,11 +12,11 @@
use std::{
collections::{BTreeMap, HashMap},
convert::TryFrom,
fmt::{self, Debug},
ops::Index,
};
#[cfg(any(test, feature = "test-impl"))]
use hex::FromHex;
mod identifier;
@ -75,6 +75,12 @@ impl<C> BindingFactorList<C>
where
C: Ciphersuite,
{
/// Create a new [`BindingFactorList`] from a vector of binding factors.
#[cfg(feature = "internals")]
pub fn new(binding_factors: BTreeMap<Identifier<C>, BindingFactor<C>>) -> Self {
Self(binding_factors)
}
/// Return iterator through all factors.
pub fn iter(&self) -> impl Iterator<Item = (&Identifier<C>, &BindingFactor<C>)> {
self.0.iter()
@ -97,26 +103,28 @@ where
}
}
impl<C> From<&SigningPackage<C>> for BindingFactorList<C>
/// [`compute_binding_factors`] in the spec
///
/// [`compute_binding_factors`]: https://www.ietf.org/archive/id/draft-irtf-cfrg-frost-10.html#section-4.4
#[cfg_attr(feature = "internals", visibility::make(pub))]
pub(crate) fn compute_binding_factor_list<C>(
signing_package: &SigningPackage<C>,
additional_prefix: &[u8],
) -> BindingFactorList<C>
where
C: Ciphersuite,
{
// [`compute_binding_factors`] in the spec
//
// [`compute_binding_factors`]: https://www.ietf.org/archive/id/draft-irtf-cfrg-frost-11.html#section-4.4
fn from(signing_package: &SigningPackage<C>) -> BindingFactorList<C> {
let preimages = signing_package.binding_factor_preimages();
let preimages = signing_package.binding_factor_preimages(additional_prefix);
BindingFactorList(
preimages
.iter()
.map(|(identifier, preimage)| {
let binding_factor = C::H1(preimage);
(*identifier, BindingFactor(binding_factor))
})
.collect(),
)
}
BindingFactorList(
preimages
.iter()
.map(|(identifier, preimage)| {
let binding_factor = C::H1(preimage);
(*identifier, BindingFactor(binding_factor))
})
.collect(),
)
}
#[cfg(any(test, feature = "test-impl"))]
@ -138,6 +146,7 @@ where
// TODO: pub struct Lagrange<C: Ciphersuite>(Scalar);
/// Generates the lagrange coefficient for the i'th participant.
#[cfg_attr(feature = "internals", visibility::make(pub))]
fn derive_lagrange_coeff<C: Ciphersuite>(
signer_id: &Identifier<C>,
signing_package: &SigningPackage<C>,
@ -221,15 +230,20 @@ where
&self.message
}
/// Compute the preimages to H3 to compute the per-signer binding factors
/// Compute the preimages to H1 to compute the per-signer binding factors
// We separate this out into its own method so it can be tested
pub fn binding_factor_preimages(&self) -> Vec<(Identifier<C>, Vec<u8>)> {
#[cfg_attr(feature = "internals", visibility::make(pub))]
pub fn binding_factor_preimages(
&self,
additional_prefix: &[u8],
) -> Vec<(Identifier<C>, Vec<u8>)> {
let mut binding_factor_input_prefix = vec![];
binding_factor_input_prefix.extend_from_slice(C::H4(self.message.as_slice()).as_ref());
binding_factor_input_prefix.extend_from_slice(
C::H5(&round1::encode_group_commitments(self.signing_commitments())[..]).as_ref(),
);
binding_factor_input_prefix.extend_from_slice(additional_prefix);
self.signing_commitments()
.iter()
@ -246,9 +260,20 @@ where
/// The product of all signers' individual commitments, published as part of the
/// final signature.
#[derive(PartialEq, Eq)]
#[derive(Clone, PartialEq, Eq)]
pub struct GroupCommitment<C: Ciphersuite>(pub(super) Element<C>);
impl<C> GroupCommitment<C>
where
C: Ciphersuite,
{
/// Return the underlying element.
#[cfg(feature = "internals")]
pub fn to_element(self) -> <C::Group as Group>::Element {
self.0
}
}
// impl<C> Debug for GroupCommitment<C> where C: Ciphersuite {
// fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
// f.debug_tuple("GroupCommitment")
@ -257,43 +282,41 @@ pub struct GroupCommitment<C: Ciphersuite>(pub(super) Element<C>);
// }
// }
impl<C> TryFrom<&SigningPackage<C>> for GroupCommitment<C>
/// Generates the group commitment which is published as part of the joint
/// Schnorr signature.
///
/// Implements [`compute_group_commitment`] from the spec.
///
/// [`compute_group_commitment`]: https://www.ietf.org/archive/id/draft-irtf-cfrg-frost-10.html#section-4.5
#[cfg_attr(feature = "internals", visibility::make(pub))]
fn compute_group_commitment<C>(
signing_package: &SigningPackage<C>,
binding_factor_list: &BindingFactorList<C>,
) -> Result<GroupCommitment<C>, Error<C>>
where
C: Ciphersuite,
{
type Error = Error<C>;
let identity = <C::Group as Group>::identity();
/// Generates the group commitment which is published as part of the joint
/// Schnorr signature.
///
/// Implements [`compute_group_commitment`] from the spec.
///
/// [`compute_group_commitment`]: https://www.ietf.org/archive/id/draft-irtf-cfrg-frost-11.html#section-4.5
fn try_from(signing_package: &SigningPackage<C>) -> Result<GroupCommitment<C>, Error<C>> {
let binding_factor_list: BindingFactorList<C> = signing_package.into();
let mut group_commitment = <C::Group as Group>::identity();
let identity = <C::Group>::identity();
let mut group_commitment = <C::Group>::identity();
// Ala the sorting of B, just always sort by identifier in ascending order
//
// https://github.com/cfrg/draft-irtf-cfrg-frost/blob/master/draft-irtf-cfrg-frost.md#encoding-operations-dep-encoding
for commitment in signing_package.signing_commitments() {
// The following check prevents a party from accidentally revealing their share.
// Note that the '&&' operator would be sufficient.
if identity == commitment.binding.0 || identity == commitment.hiding.0 {
return Err(Error::IdentityCommitment);
}
let binding_factor = binding_factor_list[commitment.identifier].clone();
group_commitment = group_commitment
+ (commitment.hiding.0 + (commitment.binding.0 * binding_factor.0));
// Ala the sorting of B, just always sort by identifier in ascending order
//
// https://github.com/cfrg/draft-irtf-cfrg-frost/blob/master/draft-irtf-cfrg-frost.md#encoding-operations-dep-encoding
for commitment in signing_package.signing_commitments() {
// The following check prevents a party from accidentally revealing their share.
// Note that the '&&' operator would be sufficient.
if identity == commitment.binding.0 || identity == commitment.hiding.0 {
return Err(Error::IdentityCommitment);
}
Ok(GroupCommitment(group_commitment))
let binding_factor = binding_factor_list[commitment.identifier].clone();
group_commitment =
group_commitment + (commitment.hiding.0 + (commitment.binding.0 * binding_factor.0));
}
Ok(GroupCommitment(group_commitment))
}
////////////////////////////////////////////////////////////////////////////////
@ -325,10 +348,11 @@ where
{
// Encodes the signing commitment list produced in round one as part of generating [`BindingFactor`], the
// binding factor.
let binding_factor_list: BindingFactorList<C> = signing_package.into();
let binding_factor_list: BindingFactorList<C> =
compute_binding_factor_list(signing_package, &[]);
// Compute the group commitment from signing commitments produced in round one.
let group_commitment = GroupCommitment::<C>::try_from(signing_package)?;
let group_commitment = compute_group_commitment(signing_package, &binding_factor_list)?;
// Compute the per-message challenge.
let challenge = crate::challenge::<C>(

3
frost-core/src/frost/identifier.rs
View File

@ -20,7 +20,8 @@ where
C: Ciphersuite,
{
/// Serialize the identifier using the ciphersuite encoding.
pub fn serialize(&self) -> <<C::Group as Group>::Field as Field>::Serialization {
#[cfg_attr(feature = "internals", visibility::make(pub))]
pub(crate) fn serialize(&self) -> <<C::Group as Group>::Field as Field>::Serialization {
<<C::Group as Group>::Field>::serialize(&self.0)
}

5
frost-core/src/frost/keys.rs
View File

@ -9,7 +9,9 @@ use std::{
iter,
};
#[cfg(any(test, feature = "test-impl"))]
use hex::FromHex;
use rand_core::{CryptoRng, RngCore};
use zeroize::{DefaultIsZeroes, Zeroize};
@ -542,7 +544,8 @@ pub(crate) fn generate_secret_shares<C: Ciphersuite>(
let (coefficients, commitment) =
generate_secret_polynomial(secret, max_signers, min_signers, coefficients)?;
for id in (1..=max_signers as u16).map_while(|i| Identifier::<C>::try_from(i).ok()) {
for idx in 1..=max_signers as u16 {
let id = Identifier::<C>::try_from(idx)?;
let value = evaluate_polynomial(id, &coefficients);
secret_shares.push(SecretShare {

3
frost-core/src/frost/round1.rs
View File

@ -2,7 +2,9 @@
use std::fmt::{self, Debug};
#[cfg(any(test, feature = "test-impl"))]
use hex::FromHex;
use rand_core::{CryptoRng, RngCore};
use zeroize::Zeroize;
@ -225,6 +227,7 @@ where
/// Computes the [signature commitment share] from these round one signing commitments.
///
/// [signature commitment share]: https://www.ietf.org/archive/id/draft-irtf-cfrg-frost-11.html#name-signature-share-verificatio
#[cfg_attr(feature = "internals", visibility::make(pub))]
pub(super) fn to_group_commitment_share(
self,
binding_factor: &frost::BindingFactor<C>,

39
frost-core/src/frost/round2.rs
View File

@ -113,6 +113,25 @@ where
}
}
/// Compute the signature share for a signing operation.
#[cfg_attr(feature = "internals", visibility::make(pub))]
fn compute_signature_share<C: Ciphersuite>(
signer_nonces: &round1::SigningNonces<C>,
binding_factor: BindingFactor<C>,
lambda_i: <<<C as Ciphersuite>::Group as Group>::Field as Field>::Scalar,
key_package: &keys::KeyPackage<C>,
challenge: Challenge<C>,
) -> SignatureShare<C> {
let z_share: <<C::Group as Group>::Field as Field>::Scalar = signer_nonces.hiding.0
+ (signer_nonces.binding.0 * binding_factor.0)
+ (lambda_i * key_package.secret_share.0 * challenge.0);
SignatureShare::<C> {
identifier: *key_package.identifier(),
signature: SignatureResponse::<C> { z_share },
}
}
// // Zeroizes `SignatureShare` to be the `Default` value on drop (when it goes out
// // of scope). Luckily the derived `Default` includes the `Default` impl of
// // Scalar, which is four 0u64's under the hood, and u16, which is
@ -138,12 +157,13 @@ pub fn sign<C: Ciphersuite>(
) -> Result<SignatureShare<C>, Error<C>> {
// Encodes the signing commitment list produced in round one as part of generating [`BindingFactor`], the
// binding factor.
let binding_factor_list: frost::BindingFactorList<C> = signing_package.into();
let binding_factor_list: BindingFactorList<C> =
compute_binding_factor_list(signing_package, &[]);
let binding_factor: frost::BindingFactor<C> =
binding_factor_list[key_package.identifier].clone();
// Compute the group commitment from signing commitments produced in round one.
let group_commitment = GroupCommitment::<C>::try_from(signing_package)?;
let group_commitment = compute_group_commitment(signing_package, &binding_factor_list)?;
// Compute Lagrange coefficient.
let lambda_i = frost::derive_lagrange_coeff(key_package.identifier(), signing_package)?;
@ -156,14 +176,13 @@ pub fn sign<C: Ciphersuite>(
);
// Compute the Schnorr signature share.
let z_share = signer_nonces.hiding.0
+ (signer_nonces.binding.0 * binding_factor.0)
+ (lambda_i * key_package.secret_share.0 * challenge.0);
let signature_share = SignatureShare::<C> {
identifier: *key_package.identifier(),
signature: SignatureResponse::<C> { z_share },
};
let signature_share = compute_signature_share(
signer_nonces,
binding_factor,
lambda_i,
key_package,
challenge,
);
Ok(signature_share)
}

12
frost-core/src/lib.rs
View File

@ -239,6 +239,17 @@ pub trait Ciphersuite: Copy + Clone + PartialEq + Debug {
#[derive(Clone)]
pub struct Challenge<C: Ciphersuite>(pub(crate) <<C::Group as Group>::Field as Field>::Scalar);
impl<C> Challenge<C>
where
C: Ciphersuite,
{
/// Return the underlying scalar.
#[cfg(feature = "internals")]
pub fn to_scalar(self) -> <<<C as Ciphersuite>::Group as Group>::Field as Field>::Scalar {
self.0
}
}
impl<C> Debug for Challenge<C>
where
C: Ciphersuite,
@ -261,6 +272,7 @@ where
///
/// [FROST]: https://www.ietf.org/archive/id/draft-irtf-cfrg-frost-11.html#name-signature-challenge-computa
/// [RFC]: https://www.ietf.org/archive/id/draft-irtf-cfrg-frost-11.html#section-3.2
#[cfg_attr(feature = "internals", visibility::make(pub))]
fn challenge<C>(R: &Element<C>, verifying_key: &Element<C>, msg: &[u8]) -> Challenge<C>
where
C: Ciphersuite,

9
frost-core/src/signature.rs
View File

@ -20,6 +20,15 @@ where
C::Group: Group,
<C::Group as Group>::Field: Field,
{
/// Create a new Signature.
#[cfg(feature = "internals")]
pub fn new(
R: <C::Group as Group>::Element,
z: <<C::Group as Group>::Field as Field>::Scalar,
) -> Self {
Self { R, z }
}
/// Converts bytes as [`Ciphersuite::SignatureSerialization`] into a `Signature<C>`.
pub fn from_bytes(bytes: C::SignatureSerialization) -> Result<Self, Error<C>> {
// To compute the expected length of the encoded point, encode the generator

5
frost-core/src/tests/vectors.rs
View File

@ -280,11 +280,12 @@ pub fn check_sign_with_test_vectors<C: Ciphersuite>(json_vectors: &Value) {
let signing_package = frost::SigningPackage::new(signer_commitments_vec, message_bytes);
for (identifier, input) in signing_package.binding_factor_preimages().iter() {
for (identifier, input) in signing_package.binding_factor_preimages(&[]).iter() {
assert_eq!(*input, binding_factor_inputs[identifier]);
}
let binding_factor_list: frost::BindingFactorList<C> = (&signing_package).into();
let binding_factor_list: frost::BindingFactorList<C> =
compute_binding_factor_list(&signing_package, &[]);
for (identifier, binding_factor) in binding_factor_list.iter() {
assert_eq!(*binding_factor, binding_factors[identifier]);

13
frost-core/src/verifying_key.rs
View File

@ -1,5 +1,6 @@
use std::fmt::{self, Debug};
#[cfg(any(test, feature = "test-impl"))]
use hex::FromHex;
use crate::{Challenge, Ciphersuite, Element, Error, Group, Signature};
@ -23,6 +24,18 @@ where
// VerifyingKey { element }
// }
/// Create a new VerifyingKey from the given element.
#[cfg(feature = "internals")]
pub fn new(element: <C::Group as Group>::Element) -> Self {
Self { element }
}
/// Return the underlying element.
#[cfg(feature = "internals")]
pub fn to_element(self) -> <C::Group as Group>::Element {
self.element
}
/// Deserialize from bytes
pub fn from_bytes(
bytes: <C::Group as Group>::Serialization,

31
frost-rerandomized/Cargo.toml Normal file
View File

@ -0,0 +1,31 @@
[package]
name = "frost-rerandomized"
edition = "2021"
# When releasing to crates.io:
# - Update html_root_url
# - Update CHANGELOG.md
# - Create git tag.
version = "0.1.0"
authors = ["Deirdre Connolly <durumcrustulum@gmail.com>", "Chelsea Komlo <me@chelseakomlo.com>",
"Conrado Gouvea <conradoplg@gmail.com>"]
readme = "README.md"
license = "MIT OR Apache-2.0"
repository = "https://github.com/ZcashFoundation/frost"
categories = ["cryptography"]
keywords = ["cryptography", "crypto", "threshold", "signature", "schnorr", "randomized"]
description = "Types and traits to support implementing a re-randomized variant of Flexible Round-Optimized Schnorr Threshold signature schemes (FROST)."
[package.metadata.docs.rs]
features = ["nightly"]
[dependencies]
frost-core = { path = "../frost-core", features = ["internals"] }
rand_core = "0.6"
[dev-dependencies]
[features]
nightly = []
default = []
# Exposes ciphersuite-generic tests for other crates to use
test-impl = ["frost-core/test-impl"]

23
frost-rerandomized/README.md Normal file
View File

@ -0,0 +1,23 @@
# FROST (Flexible Round-Optimised Schnorr Threshold signatures) Rerandomized
Base traits and types in Rust that implement ['Two-Round Threshold Schnorr Signatures with
FROST'](https://datatracker.ietf.org/doc/draft-irtf-cfrg-frost/) generically for
`frost-core::Ciphersuite` implementations, with support for Zcash-compatible
RedDSA rerandomized signatures.
## Status ⚠
The FROST specification is not yet finalized, and this codebase has not yet been audited or
released. The APIs and types in `frost-rerandomized` are subject to change.
## Usage
`frost-rerandomized` is similar to `frost-core`, but provides different
`sign()` and `aggregate()` functions adding support for rerandomized signatures.
End-users should not use `frost-rerandomized` if they want to sign and verify signatures, they
should use the crate specific to their ciphersuite/curve parameters that uses `frost-rerandomized` as a
dependency, such as [`reddsa`](https://github.com/ZcashFoundation/reddsa/).
## Example
See ciphersuite-specific modules, e.g. the ones in [`reddsa`](https://github.com/ZcashFoundation/reddsa/).

209
frost-rerandomized/src/lib.rs Normal file
View File

@ -0,0 +1,209 @@
//! Randomized FROST support.
//!
#![allow(non_snake_case)]
#[cfg(any(test, feature = "test-impl"))]
pub mod tests;
pub use frost_core;
use frost_core::{
frost::{self, keys::PublicKeyPackage},
Ciphersuite, Error, Field, Group, VerifyingKey,
};
use rand_core::{CryptoRng, RngCore};
/// Performed once by each participant selected for the signing operation.
///
/// Implements [`sign`] from the spec.
///
/// Receives the message to be signed and a set of signing commitments and a set
/// of randomizing commitments to be used in that signing operation, including
/// that for this participant.
///
/// Assumes the participant has already determined which nonce corresponds with
/// the commitment that was assigned by the coordinator in the SigningPackage.
///
/// [`sign`]: https://www.ietf.org/archive/id/draft-irtf-cfrg-frost-10.html#name-round-two-signature-share-g
pub fn sign<C: Ciphersuite>(
signing_package: &frost::SigningPackage<C>,
signer_nonces: &frost::round1::SigningNonces<C>,
key_package: &frost::keys::KeyPackage<C>,
randomizer_point: &<C::Group as Group>::Element,
) -> Result<frost::round2::SignatureShare<C>, Error<C>> {
let public_key = key_package.group_public.to_element() + *randomizer_point;
// Encodes the signing commitment list produced in round one as part of generating [`Rho`], the
// binding factor.
let binding_factor_list = frost::compute_binding_factor_list(
signing_package,
<C::Group as Group>::serialize(randomizer_point).as_ref(),
);
let rho: frost::BindingFactor<C> = binding_factor_list[key_package.identifier].clone();
// Compute the group commitment from signing commitments produced in round one.
let group_commitment = frost::compute_group_commitment(signing_package, &binding_factor_list)?;
// Compute Lagrange coefficient.
let lambda_i = frost::derive_lagrange_coeff(key_package.identifier(), signing_package)?;
// Compute the per-message challenge.
let challenge = frost_core::challenge::<C>(
&group_commitment.to_element(),
&public_key,
signing_package.message().as_slice(),
);
// Compute the Schnorr signature share.
let signature_share = frost::round2::compute_signature_share(
signer_nonces,
rho,
lambda_i,
key_package,
challenge,
);
Ok(signature_share)
}
/// Verifies each participant's signature share, and if all are valid,
/// aggregates the shares into a signature to publish.
///
/// Resulting signature is compatible with verification of a plain SpendAuth
/// signature.
///
/// This operation is performed by a coordinator that can communicate with all
/// the signing participants before publishing the final signature. The
/// coordinator can be one of the participants or a semi-trusted third party
/// (who is trusted to not perform denial of service attacks, but does not learn
/// any secret information). Note that because the coordinator is trusted to
/// report misbehaving parties in order to avoid publishing an invalid
/// signature, if the coordinator themselves is a signer and misbehaves, they
/// can avoid that step. However, at worst, this results in a denial of
/// service attack due to publishing an invalid signature.
pub fn aggregate<C>(
signing_package: &frost::SigningPackage<C>,
signature_shares: &[frost::round2::SignatureShare<C>],
pubkeys: &frost::keys::PublicKeyPackage<C>,
randomized_params: &RandomizedParams<C>,
) -> Result<frost_core::Signature<C>, Error<C>>
where
C: Ciphersuite,
{
let public_key = pubkeys.group_public.to_element() + *randomized_params.randomizer_point();
// Encodes the signing commitment list produced in round one as part of generating [`Rho`], the
// binding factor.
let binding_factor_list = frost::compute_binding_factor_list(
signing_package,
<C::Group as Group>::serialize(randomized_params.randomizer_point()).as_ref(),
);
// Compute the group commitment from signing commitments produced in round one.
let group_commitment = frost::compute_group_commitment(signing_package, &binding_factor_list)?;
// Compute the per-message challenge.
let challenge = frost_core::challenge::<C>(
&group_commitment.clone().to_element(),
&public_key,
signing_package.message().as_slice(),
);
// Verify the signature shares.
for signature_share in signature_shares {
// Look up the public key for this signer, where `signer_pubkey` = _G.ScalarBaseMult(s[i])_,
// and where s[i] is a secret share of the constant term of _f_, the secret polynomial.
let signer_pubkey = pubkeys
.signer_pubkeys
.get(&signature_share.identifier)
.unwrap();
// Compute Lagrange coefficient.
let lambda_i = frost::derive_lagrange_coeff(&signature_share.identifier, signing_package)?;
let rho = binding_factor_list[signature_share.identifier].clone();
// Compute the commitment share.
let R_share = signing_package
.signing_commitment(&signature_share.identifier)
.to_group_commitment_share(&rho);
// Compute relation values to verify this signature share.
signature_share.verify(&R_share, signer_pubkey, lambda_i, &challenge)?;
}
// The aggregation of the signature shares by summing them up, resulting in
// a plain Schnorr signature.
//
// Implements [`aggregate`] from the spec.
//
// [`aggregate`]: https://www.ietf.org/archive/id/draft-irtf-cfrg-frost-10.html#section-5.3
let mut z = <<C::Group as Group>::Field as Field>::zero();
for signature_share in signature_shares {
z = z + signature_share.signature.z_share;
}
z = z + challenge.to_scalar() * randomized_params.randomizer;
Ok(frost_core::Signature::new(group_commitment.to_element(), z))
}
/// Randomized params for a signing instance of randomized FROST.
pub struct RandomizedParams<C: Ciphersuite> {
/// The randomizer, also called `alpha`
randomizer: frost_core::Scalar<C>,
/// The generator multiplied by the randomizer.
randomizer_point: <C::Group as Group>::Element,
/// The randomized group public key. The group public key added to the randomizer point.
randomized_group_public_key: frost_core::VerifyingKey<C>,
}
impl<C> RandomizedParams<C>
where
C: Ciphersuite,
{
/// Create a new RandomizedParams for the given [`PublicKeyPackage`]
pub fn new<R: RngCore + CryptoRng>(
public_key_package: &PublicKeyPackage<C>,
mut rng: R,
) -> Self {
let randomizer = <<C::Group as Group>::Field as Field>::random(&mut rng);
let randomizer_point = <C::Group as Group>::generator() * randomizer;
let group_public_point = public_key_package.group_public.to_element();
let randomized_group_public_point = group_public_point + randomizer_point;
let randomized_group_public_key = VerifyingKey::new(randomized_group_public_point);
Self {
randomizer,
randomizer_point,
randomized_group_public_key,
}
}
/// Return the randomizer.
///
/// It can be useful to the coordinator, e.g. to generate the ZK proof
/// in Zcash. It MUST NOT be sent to other parties.
pub fn randomizer(&self) -> &frost_core::Scalar<C> {
&self.randomizer
}
/// Return the randomizer point.
///
/// It must be sent by the coordinator to each participant when signing.
pub fn randomizer_point(&self) -> &<C::Group as Group>::Element {
&self.randomizer_point
}
/// Return the randomized group public key.
///
/// It can be used to verify the final signature.
pub fn randomized_group_public_key(&self) -> &frost_core::VerifyingKey<C> {
&self.randomized_group_public_key
}
}

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//! Ciphersuite-generic test functions for re-randomized FROST.
use std::collections::HashMap;
use crate::{frost_core::frost, frost_core::Ciphersuite, RandomizedParams};
use frost_core::{Signature, VerifyingKey};
use rand_core::{CryptoRng, RngCore};
/// Test re-randomized FROST signing with trusted dealer with a Ciphersuite.
/// Returns the signed message, generated signature, and the randomized public key
/// so that the caller can verify the signature with their own implementation.
pub fn check_randomized_sign_with_dealer<C: Ciphersuite, R: RngCore + CryptoRng>(
mut rng: R,
) -> (Vec<u8>, Signature<C>, VerifyingKey<C>) {
////////////////////////////////////////////////////////////////////////////
// Key generation
////////////////////////////////////////////////////////////////////////////
let max_signers = 5;
let min_signers = 3;
let (shares, pubkeys) =
frost::keys::keygen_with_dealer(max_signers, min_signers, &mut rng).unwrap();
// Verifies the secret shares from the dealer
let key_packages: HashMap<frost::Identifier<C>, frost::keys::KeyPackage<C>> = shares
.into_iter()
.map(|share| {
(
share.identifier,
frost::keys::KeyPackage::try_from(share).unwrap(),
)
})
.collect();
let mut nonces: HashMap<frost::Identifier<C>, frost::round1::SigningNonces<C>> = HashMap::new();
let mut commitments: HashMap<frost::Identifier<C>, frost::round1::SigningCommitments<C>> =
HashMap::new();
let randomizer_params = RandomizedParams::new(&pubkeys, &mut rng);
////////////////////////////////////////////////////////////////////////////
// Round 1: generating nonces and signing commitments for each participant
////////////////////////////////////////////////////////////////////////////
for participant_index in 1..(min_signers as u16 + 1) {
let participant_identifier = participant_index.try_into().expect("should be nonzero");
// Generate one (1) nonce and one SigningCommitments instance for each
// participant, up to _min_signers_.
let (nonce, commitment) = frost::round1::commit(
participant_identifier,
key_packages
.get(&participant_identifier)
.unwrap()
.secret_share(),
&mut rng,
);
nonces.insert(participant_identifier, nonce);
commitments.insert(participant_identifier, commitment);
}
// This is what the signature aggregator / coordinator needs to do:
// - decide what message to sign
// - take one (unused) commitment per signing participant
let mut signature_shares: Vec<frost::round2::SignatureShare<_>> = Vec::new();
let message = "message to sign".as_bytes();
let comms = commitments.clone().into_values().collect();
let signing_package = frost::SigningPackage::new(comms, message.to_vec());
////////////////////////////////////////////////////////////////////////////
// Round 2: each participant generates their signature share
////////////////////////////////////////////////////////////////////////////
for participant_identifier in nonces.keys() {
let key_package = key_packages.get(participant_identifier).unwrap();
let nonces_to_use = &nonces.get(participant_identifier).unwrap();
// Each participant generates their signature share.
let signature_share = crate::sign(
&signing_package,
nonces_to_use,
key_package,
randomizer_params.randomizer_point(),
)
.unwrap();
signature_shares.push(signature_share);
}
////////////////////////////////////////////////////////////////////////////
// Aggregation: collects the signing shares from all participants,
// generates the final signature.
////////////////////////////////////////////////////////////////////////////
// Aggregate (also verifies the signature shares)
let group_signature_res = crate::aggregate(
&signing_package,
&signature_shares[..],
&pubkeys,
&randomizer_params,
);
assert!(group_signature_res.is_ok());
let group_signature = group_signature_res.unwrap();
// Check that the threshold signature can be verified by the randomized group public
// key (the verification key).
assert!(randomizer_params
.randomized_group_public_key()
.verify(message, &group_signature)
.is_ok());
// Note that key_package.group_public can't be used to verify the signature
// since those are non-randomized.
(
message.to_owned(),
group_signature,
*randomizer_params.randomized_group_public_key(),
)
}