Module refresh 

Proactive share refresh for P-256 threshold shares (Herzberg et al. 1995).

Each party generates a random polynomial g_i(x) of degree T-1 with g_i(0) = 0, distributes evaluations g_i(j) to party j, and each party adds the received refresh contributions to their existing share.

The key invariant: sum_i g_i(0) = 0, so the aggregate secret is unchanged and the public key doesn’t change. Previously compromised shares become useless after refresh because the attacker doesn’t know the refresh contributions.

Usage

use confium_tc_cmp20::inprocess;
use confium_tc_cmp20::refresh;

// 1. Initial keygen.
let kg = inprocess::keygen(2, 3).unwrap();

// 2. Each party generates refresh contributions.
let contributions = refresh::generate_refresh_contributions(2, 3);

// 3. Each party applies the refresh to their share.
let refreshed_shares: Vec<Vec<u8>> = kg.shares.iter().enumerate()
    .map(|(i, share)| refresh::apply_to_share(share, i as u32 + 1, &contributions))
    .collect();

// 4. Sign with refreshed shares — same joint public key.
let sig = inprocess::sign(&refreshed_shares[..2], 2, b"refreshed").unwrap();

Structs

RefreshContribution

One party’s refresh contribution: (source_party_index, target_party_index, refresh_scalar_bytes).

Functions

apply_to_share

Apply refresh contributions to a CMP20 share blob. The share’s internal scalar x_i is replaced with x_i + sum(g_j(i)) for all contributions directed at party_index.

generate_refresh_contributions

Generate refresh contributions for all (N-1) × N party pairs.

verify_zero_sum

Verify that a set of refresh contributions preserves the aggregate secret: sum_i g_i(0) must be zero. If this check fails, the refresh round is malformed and the shares are compromised.