Problem

DoubleRatchet.skipMessages enforced a per-call cap (maxSkip, default MAX_SKIP = 100) on the size of a single burst of skipped messages within one receive chain:

if (until - s.recvMessageNumber > this.maxSkip) {
    throw new Error(`Too many skipped messages (${until - s.recvMessageNumber} > ${this.maxSkip})`);
}
while (s.recvMessageNumber < until) {
    const [mk, next] = kdfChain(s.chainKeyRecv);
    s.skippedKeys.set(skippedKeyId(s.dhRemotePublic!, s.recvMessageNumber), mk);
    s.chainKeyRecv = next;
    s.recvMessageNumber++;
}

But the skippedKeys map is not reset on a DH ratchet step (it is intentionally kept so out-of-order messages from old chains can still be decrypted). A long-lived session whose peer repeatedly forces fresh DH rotations while leaving small out-of-order tails behind in each chain accumulates skipped message keys indefinitely — memory grows in O(total skipped across the lifetime of the ratchet) rather than O(per-chain burst).

The Python core implementation already pins this: agent-governance-python/agent-mesh/src/agentmesh/encryption/ratchet.py defines _MAX_SKIPPED_KEYS_TOTAL: int = 2000 and FIFO-evicts the oldest entries. The TypeScript implementation had no equivalent.

Fix

Add a module-level MAX_SKIPPED_KEYS_TOTAL = 2000 and FIFO-evict the oldest entries from state.skippedKeys whenever it exceeds the cap. Map iteration order is insertion order in ES2015+, so s.skippedKeys.keys().next().value returns the oldest surviving insertion — the same pattern Python's next(iter(d)) uses. The cap is intentionally kept identical to Python (2000) so the two SDKs have matching memory bounds for the same wire protocol.

Test

Added a regression test in tests/encryption.test.ts that:

1. Raises maxSkip to 2051 via DoubleRatchet.fromState, so the per-chain cap does not gate the burst.
2. Has Alice encrypt 2050 messages that Bob will skip, then a final one we deliver.
3. Asserts the final decrypts correctly AND that bRatchet.getState().skippedKeys.size === 2000 after delivery (the oldest 50 were evicted FIFO).

PASS  tests/encryption.test.ts
  ...
  Tests:       26 passed, 26 total

Recipe: cd agent-governance-typescript && npx jest tests/encryption.test.ts --no-coverage.

Surfaced during independent audit conducted by @finnoybu (Ken Tannenbaum, AEGIS Initiative); [LOW, TypeScript].