Here’s What BIP 91 Activation Means (and What It Does Not)

It looks as if Bitcoin is getting Segregated Witness.

Bitcoin Improvement Proposal 91 (BIP 91) just locked in. Up to 90 percent of all hash power signaled support for this soft fork, which implies miners intend, in turn, to trigger Segregated Witness (SegWit) activation. By extension, this should make BIP 148 obsolete and August 1 a non-event.

But SegWit is not certain. In fact, on a technical level, SegWit is not any closer to activation at all.

  BIP 91

Segregated Witness, defined by BIP 141, locks in if at least 95 percent of miners (by hash power) signal support for the upgrade within a two-week difficulty period. To do so, miners need to embed a piece of data called “bit 1” in the blocks they mine.

Importantly, this is technically the only way for SegWit to activate right now. And this threshold has not yet been met.

But there are alternative strategies to try and reach this threshold “indirectly” — like BIP 91.

BIP 91 is a Bitcoin Improvement Proposal proposed by Bitmain Warranty engineer James Hilliard. It is compatible with the New York Agreement and backed by a number of Bitcoin companies and mining pools. It is also compatible with BIP 148, another strategy to meet the BIP 141 threshold indirectly.

Miners have been signaling support for BIP 91 over the past couple of days through another piece of data, “bit 4.” Once 269 blocks within a 336-block window include bit 4, this BIP 91 soft fork gets locked in. This threshold was just met.

As long as a majority of hash power enforces BIP 91, this majority should eventually control the longest valid chain according to all Bitcoin nodes. And as this chain consists of SegWit-signaling blocks only, it would in turn activate SegWit on all SegWit-ready nodes.

In that case, BIP 141 should lock in by mid-August, and SegWit should be live on the Bitcoin network after a two-week “grace period” by the end of that month.

If all goes well …

What Could Go Wrong?

Although well over 80 percent of hash power has signaled bit 4 for BIP 91 activation, this doesn’t actually guarantee anything. Most importantly, it doesn’t in itself mean that these miners will signal bit 1 for SegWit.

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