Litecoin Cash is a Bitcoin Core clone which uses a hybrid Proof-of-Work/Proof-of-Stake consensus algorithm in an attempt to aleviate 51% attacks on its network. LCC's PoW algorithm is SHA256 but its network hashrate is many orders of magnitude smaller than Bitcoin's, making it highly vulnerable to 51% attacks, as was demonstrated last year. The LCC whitepaper describes a system they call "Hive Mining", which is effectively a PoS lottery in which users can purchase "bees" (lottery tickets) that have the potential to be eligible to propose a PoS block for each new PoW block. In the paper, the authors claim this scheme provides "protection" from 51% attacks by interlacing PoW and PoS blocks, and giving PoS blocks more relative weight than PoW blocks in the chain-work calculation for selecting the most-work block.
The deep reorgs are summarised below. Each event gives the time the reorg occured from our LCC full node's perspective, the number of blocks removed from the main chain in the reorg, and the number of attacker blocks that replaced them. Also provided are the high-level details of any double-spends caused by the reorg. All times are in GMT.
- Thursday, 4 July 2019 08:57:24 - 48 blocks removed, 41 blocks added
- No double-spends detected
- Friday, 5 July 2019 08:42:58 - 102 blocks removed, 64 blocks added
- 28,000 LCC double-spent (~$317)
- Coins originally sent to
MKKbmesxvDY1rYbJ8GojY7azorLZyNf5Cc
- Returned to
CaBtdSSrXBn8k4MjH58QNqGtNfaYAyKXqZ
in transaction 145a8e54e8e6c866a3976d9d3233dc3390147acab29d73e7fa757fdb06534056
- Friday, 5 July 2019 15:26:55 - 63 blocks removed, 50 blocks added
- 66,000 LCC double-spent (~$747)
- Coins originally sent to
MKKbmesxvDY1rYbJ8GojY7azorLZyNf5Cc
- Returned to
CaBtdSSrXBn8k4MjH58QNqGtNfaYAyKXqZ
in transaction 48459dd4aef12806176ab49bc79751751403645f1b7d205ec5bb5708c8c29fa9
- Saturday, 6 July 2019 01:55:06 - 73 blocks removed, 57 blocks added
- 80,000 LCC double-spent (~$903)
- Coins originally sent to
MKKbmesxvDY1rYbJ8GojY7azorLZyNf5Cc
- Returned to
CaBtdSSrXBn8k4MjH58QNqGtNfaYAyKXqZ
in transaction b26d6b77ea25090a2e7b2d9037577340bdbe2570ab1446b6020edaf5010406c8
- Saturday, 6 July 2019 08:19:53 - 91 blocks removed, 68 blocks added
- 160,000 LCC double-spent (~$1827)
- Coins originally sent to
MKKbmesxvDY1rYbJ8GojY7azorLZyNf5Cc
- Returned to
CaBtdSSrXBn8k4MjH58QNqGtNfaYAyKXqZ
in transaction 46197e2068f8b7511add3e7791fdb06ce44bb5ab0be7320531ada6ef79097cfa
- Sunday, 7 July 2019 03:14:56 - 37 blocks removed, 19 blocks added
- 150,000 LCC double-spent (~$1717)
- Coins originally sent to
MKKbmesxvDY1rYbJ8GojY7azorLZyNf5Cc
- Returned to
CaBtdSSrXBn8k4MjH58QNqGtNfaYAyKXqZ
in transaction 23aedc186c22852e641fd498b68703aa6580198afff581e7e57551d0dfe2e4de
In each case the attacker did not mine any PoS blocks, producing their higher-work chain purely through PoW blocks. The increase in difficulty this caused while producing their alternative chain is the reason for the attacker's chain being shorter in terms of blocks than the original chain it replaced, which was interlaced with PoS blocks. Each reorg appears to have been performed in sequence, with the set of attacker blocks from the prior reorg containing the transaction to be double-spent in the subsequent reorg. It is unclear who the victim address (MKKbmesxvDY1rYbJ8GojY7azorLZyNf5Cc
) belongs to, but the pattern of attacker behaviour of repeated deposits and double-spends suggests it is an attempt to drain an exchange wallet of coins.
We observe that the LCC code does not appear to include a higher relative weight for PoS blocks as described in the paper, which perhaps made the job of the attacker easier. However, there is no requirement for a certain number of PoS blocks to be generated per PoW block beyond the difficulty adjustment algorithm, and the attacker was able to simply rent more hashrate to compensate for the increased PoW difficulty, which would remain possible at any reasonable relative weighting.
James,
Thank you for this detailed coverage, which we'll reference in our whitepaper on Hive 1.1, which aims to correct the issues which led to these reorgs.