The Time Warp: How Lying About the Time Could Break Bitcoin's Difficulty

The time warp is a known weakness in Bitcoin's difficulty adjustment, described in September 2011 and still open. Each 2,016-block window is measured from its own first block, so a miner with most of the hash rate could stamp the first block of each window far in the past and make the retarget think weeks passed that did not.

The new Rabbit Hole covers:

- how Bitcoin keeps time without a clock (the median-of-eleven floor, the two-hour ceiling, and why about one block in sixty is legitimately stamped earlier than its parent)

- the line in Satoshi's 0.1.0 code that causes both the off-by-one (2,015 gaps) and the seam

- a window-by-window simulation with Bitcoin Core's own retarget arithmetic: with all the hash rate, minimum difficulty in about 39 days from a window boundary, about 37 with the best-timed start (BIP-54 says within 38)

- where it was used: GeistGeld in September 2011 (ArtForz: "it's not a theoretical problem"), and the deliberate demonstration Greg Maxwell planted in testnet3 in July 2012

- what a difficulty rule failing looks like in practice: a 2017 minority fork's emergency rule cut difficulty 43 times in 104 days and ran that chain 10,451 blocks ahead of schedule; and Bitcoin's own worst stretch, four downward retargets in 2021

- every seam on mainnet: ten of 483 negative, worst 18 min 47 s (block 203,616, October 2012), 43 s since 2015. BIP-54's two-hour rule would have rejected none; testnet4's ten-minute rule would have rejected that one 2012 block.

- why the one-rule fix waited from 2011 to BIP-54, and where it stands (complete, not activated)

Both data sets are CSV downloads on the page. Free, no ads, no affiliate links:

https://www.learnbitcoin.com/rabbit-hole/time-warp