The EVM CoreISSUE #3 of 48

sync committees · light-client support

SoliaVSThe Reentrancy Reaper

Solia saysA 512-validator sync committee rotates every ~27 hours and signs block headers — letting light clients follow the chain without downloading every attestation.

Light clients want to follow Ethereum without storing state or replaying every attestation. The sync committee is the answer: a randomly-selected set of 512 validators per ~27-hour period that sign every block header. A light client verifies those 512 BLS signatures per block and knows the head it can trust.

Each sync committee serves 256 epochs (~27 hours), then rotates. The per-period odds of any individual validator being on sync duty are tiny (512 / ~1M ≈ 0.05%), so it is a rare but well-rewarded duty. The demo computes the period and selection odds.

Power-ups you unlock

Sync committee size: 512 validators
Rotation: every 256 epochs (~27 hours)
Signs block headers → light clients verify with 512 sigs only
Per-validator odds per period: 512 / active set (~0.05%)
Sync duties are rare but pay extra to compensate

The Reentrancy Reaper attacks — common mistakes

Confusing the sync committee with the per-slot attestation committee
Assuming light clients can ignore the sync committee (they depend on it)
Forgetting sync committee membership is rotated randomly
Treating sync sigs as finality (they verify head, not FFG)

Boss battleCompute the sync-committee period in hours and a validator’s expected number of sync periods per year for a 1M-validator active set.

Example code

<!doctype html><html><head><meta charset="utf-8"></head>
<body style="background:#06040d;color:#e6e0ff;font-family:monospace;padding:20px"><pre id="o"></pre>
<script>
const SYNC_COMMITTEE_SIZE = 512;
const EPOCHS_PER_SYNC_PERIOD = 256;
const slotsPerEpoch = 32, slotSec = 12;
const periodSec = EPOCHS_PER_SYNC_PERIOD * slotsPerEpoch * slotSec;
const periodHours = periodSec / 3600;
const periodsPerYear = (365 * 24) / periodHours;
const N = 1000000;
const probPerPeriod = SYNC_COMMITTEE_SIZE / N;
const expectedPeriodsPerYearPerValidator = periodsPerYear * probPerPeriod;
document.getElementById('o').textContent = [
  'sync committee period: ' + EPOCHS_PER_SYNC_PERIOD + ' epochs ≈ ' + periodHours.toFixed(1) + ' hours',
  'sync committee size:   ' + SYNC_COMMITTEE_SIZE + ' validators',
  'periods per year:      ' + Math.round(periodsPerYear),
  '',
  'odds a given validator is on sync duty:',
  '  per period:   ' + (probPerPeriod * 100).toFixed(4) + '% (' + SYNC_COMMITTEE_SIZE + ' of ' + N.toLocaleString() + ')',
  '  per year:     ~' + expectedPeriodsPerYearPerValidator.toFixed(3) + ' sync periods expected',
  '',
  'sync committees sign block headers → light clients follow with just 512 sigs'
].join('\n');
</script></body></html>

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