Add dealer key generation crate

crypto/dkg/dealer/src/lib.rs

@@ -0,0 +1,68 @@
+#![cfg_attr(docsrs, feature(doc_auto_cfg))]
+#![doc = include_str!("../README.md")]
+#![no_std]
+
+use core::ops::Deref;
+use std_shims::{vec::Vec, collections::HashMap};
+
+use zeroize::{Zeroize, Zeroizing};
+use rand_core::{RngCore, CryptoRng};
+
+use ciphersuite::{
+  group::ff::{Field, PrimeField},
+  Ciphersuite,
+};
+pub use dkg::*;
+
+/// Create a key via a dealer key generation protocol.
+pub fn key_gen<R: RngCore + CryptoRng, C: Ciphersuite>(
+  rng: &mut R,
+  threshold: u16,
+  participants: u16,
+) -> Result<HashMap<Participant, ThresholdKeys<C>>, DkgError> {
+  let mut coefficients = Vec::with_capacity(usize::from(participants));
+  // `.max(1)` so we always generate the 0th coefficient which we'll share
+  for _ in 0 .. threshold.max(1) {
+    coefficients.push(Zeroizing::new(C::F::random(&mut *rng)));
+  }
+
+  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
+  }
+
+  let group_key = C::generator() * coefficients[0].deref();
+  let mut secret_shares = HashMap::with_capacity(participants as usize);
+  let mut verification_shares = HashMap::with_capacity(participants as usize);
+  for i in 1 ..= participants {
+    let i = Participant::new(i).expect("non-zero u16 wasn't a valid Participant index");
+    let secret_share = polynomial(&coefficients, i);
+    secret_shares.insert(i, secret_share.clone());
+    verification_shares.insert(i, C::generator() * *secret_share);
+  }
+
+  let mut res = HashMap::with_capacity(participants as usize);
+  for (i, secret_share) in secret_shares {
+    let keys = ThresholdKeys::new(
+      ThresholdParams::new(threshold, participants, i)?,
+      Interpolation::Lagrange,
+      secret_share,
+      verification_shares.clone(),
+    )?;
+    debug_assert_eq!(keys.group_key(), group_key);
+    res.insert(i, keys);
+  }
+  Ok(res)
+}