  Sharding enhances blockchain scalability by dividing the network into shards,
each managing specific unspent transaction outputs or accounts. As an
introduced new transaction type, cross-shard transactions pose a critical
challenge to the security and efficiency of sharding blockchains. Currently,
there is a lack of a generic sharding consensus pattern that achieves both
security and low overhead. In this paper, we present Kronos, a secure sharding
blockchain consensus achieving optimized overhead. In particular, we propose a
new secure sharding consensus pattern, based on a buffer managed jointly by
shard members. Valid transactions are transferred to the payee via the buffer,
while invalid ones are rejected through happy or unhappy paths. Kronos is
proved to achieve security with atomicity under malicious clients with optimal
intra-shard overhead $kB$ ($k$ for involved shard number and $B$ for a
Byzantine fault tolerance (BFT) cost). Besides, we propose secure cross-shard
certification methods based on batch certification and reliable cross-shard
transfer. The former combines hybrid trees or vector commitments, while the
latter integrates erasure coding. Handling $b$ transactions, Kronos is proved
to achieve reliability with low cross-shard overhead $O(n b \lambda)$ ($n$ for
shard size and $\lambda$ for the security parameter). Notably, Kronos imposes
no restrictions on BFT and does not rely on time assumptions, offering optional
constructions in various modules. We implement Kronos using two prominent BFT
protocols: asynchronous Speeding Dumbo and partial synchronous Hotstuff.
Extensive experiments demonstrate Kronos scales the consensus nodes to
thousands, achieving a substantial throughput of 320 ktx/sec with 2.0 sec
latency. Compared with the past solutions, Kronos outperforms, achieving up to
a 12* improvement in throughput and a 50% reduction in latency.
