Amaury Séchet

On August 1, 2017 at block 478558, Bitcoin’s chain split. The software that produced the fork — Bitcoin ABC (“Adjustable Blocksize Cap”) — was written by Amaury Séchet’s team, with Séchet as lead developer. He had designed the SIGHASH_FORKID replay-protection scheme that prevented the new chain from confusing the old one, and the Emergency Difficulty Adjustment that kept blocks producing on minority hashrate.

Sixteen months later, in November 2018, a second split — this time within BCH itself — pitted Séchet’s Bitcoin ABC against Craig Wright and Calvin Ayre’s nChain.

In November 2020 he led a third split (over a proposed 8% miner-fund tax) into what became eCash (XEC) in 2021.

Séchet (online handle deadalnix) is a French software engineer who spent his pre-Bitcoin career on the Facebook HHVM (HipHop Virtual Machine) team. He began contributing to Bitcoin Core in approximately 2015, aligning with the larger-block faction through the block-size war.

Bitcoin Core contributions (2015–2017)

Séchet’s GitHub history (deadalnix) shows Bitcoin Core contributions from ~2015 onward, primarily in protocol-level code (consensus rules, P2P networking, and validation). He was active in the public discussions that became the block-size war, aligning with the larger-block faction. By 2016 he was visibly arguing in favour of an on-chain block-size increase rather than the SegWit-plus-layer-2 approach that the rest of Bitcoin Core was converging on.

Bitcoin ABC and the Bitcoin Cash fork (2017)

Séchet founded Bitcoin ABC (“Adjustable Blocksize Cap”) in mid-2017 as a clean-room implementation that would activate the larger-block fork without requiring the Bitcoin Core development team’s cooperation. Bitcoin ABC software is what mining pools and nodes ran to produce the August 1, 2017 fork at block 478558. Séchet was the lead developer through this period — designing the SIGHASH_FORKID replay-protection scheme, the Emergency Difficulty Adjustment (EDA) algorithm that allowed the new chain to maintain block production with minority hashrate, and the post-fork governance / development structure.

Hash war with Bitcoin SV (2018)

In November 2018 the BCH community fractured over a proposed protocol upgrade. Séchet led the Bitcoin ABC side; Craig Wright and Calvin Ayre’s nChain led a counter-proposal (Bitcoin SV) with 128 MB blocks and “restored original opcodes.” The two sides committed competing hashpower at the November 15 fork point, producing a hash-war scenario where each chain’s continued survival depended on whether reorganization attacks could be staged. The result was a permanent split: BCH (Bitcoin ABC’s chain) retained the BCH ticker, BSV continued separately under the new SV ticker.

Bitcoin Cash ABC split and eCash (2020–2021)

In November 2020 Séchet led another split — this time within BCH itself — over a proposed 8% miner-fund tax (an “Infrastructure Funding Plan” that would direct a portion of mining rewards to development funding). Most BCH miners and most of the broader BCH community rejected the proposal; Séchet and the Bitcoin ABC team forked off into a separate chain initially called Bitcoin Cash ABC (BCHA), which was later rebranded as eCash (XEC) in mid-2021. The original BCH chain continued under different lead developers; Séchet’s post-2020 work has been on eCash and adjacent projects.

Significance to Bitcoin

Séchet’s record is primarily that of the technical lead of the 2017 BCH hard fork — the only Bitcoin protocol fork that produced a chain with substantive surviving network share. The replay-protection and difficulty-adjustment mechanisms he designed are what allowed BCH to launch viably at all, and they have since been studied and adapted in other contentious-fork events. The 2018 BSV hash war and the 2020 BCHA / eCash split are downstream events in the same lineage. The phase-by-phase chronology of the 2015–2017 dispute that culminated in the BCH fork — including the role of the Bitcoin ABC implementation Séchet led — is collated in the block-size war overview.