delvingbitcoin
murch
Zawy’s Alternating Timestamp Attack
Posted on: August 9, 2024 16:00 UTC
Zawy's Alternating Timestamp Attack, as explained by @zawy in a recent discussion, presents a methodical approach to exploit the Bitcoin Testnet 4 through a sophisticated manipulation of block timestamps.
This attack diverges from the known Timewarp Attack by not depending on the non-overlapping durations of difficulty periods. Instead, it requires the attacker to control the majority of block timestamps and perform a calculated alternation between future and past timestamps on specific blocks to manipulate difficulty adjustments. The essence of the attack lies in its ability to significantly increase the production of blocks beyond what is achievable under normal mining conditions by honest miners.
The attack unfolds in stages, initially requiring the attacker to mine with timestamps set as low as possible, adhering to the rules that only necessitate incrementing the timestamp by a minimum interval, which is notably less than the actual time taken to find a block. Through strategic manipulation, including setting a future timestamp on the last block of a difficulty period, the attacker can drastically reduce the difficulty for subsequent periods. This manipulation enables the mining of blocks at an accelerated pace due to the lowered difficulty, further compounding the effect by repeating this process to continue reducing the difficulty exponentially.
A variant of Zawy's Attack was also proposed, which involves more granular control over the timestamp adjustments across difficulty periods, potentially allowing the attacker to decrease the difficulty even more aggressively. This variation suggests increasing the timestamp in two steps rather than alternating between the minimum and a significant future timestamp, leading to an exponential reduction in difficulty and, consequently, an increased rate of block generation.
The potential repercussions of such attacks are profound, encompassing the invalidation of weeks of transaction activity on the public network, reorganization of thousands of blocks, and the usurpation of block rewards and transaction fees. To mitigate these and similar attack vectors, the proposal of a softfork introducing a new requirement for timestamps has been discussed. This requirement would ensure that the last block in any given difficulty period has a higher timestamp than the first block of that same period, indirectly enforcing a consistent increase in timestamps across difficulty periods and thus safeguarding against manipulative practices that could destabilize the network's intended functioning.