Vitalik Buterin eyes 'big FOCIL' and encrypted mempools to prevent centralization in 'block building pipeline'

Over the past several days, Vitalik Buterin has been going deep on his plans for Ethereum’s development — from laying down a "quantum resistance roadmap" to explaining upcoming execution layer changes.

On Monday, Buterin added another tome to his series, this time discussing Ethereum's "block building pipeline," in particular focusing on ways Ethereum developers can mitigate centralization concerns that may result from an upcoming upgrade.

In the forthcoming Glamsterdam upgrade, expected in the first half of 2026, Ethereum developers plan to roll out “enshrined Proposer-Builder Separation” (ePBS), a protocol-level mechanism that allows proposers to outsource block building to a permissionless market.

"This [ePBS] ensures that block builder centralization does not creep into staking centralization, but it leaves the question: what do we do about block builder centralization? And what are the _other_ problems in the block building pipeline that need to be addressed, and how?" Buterin wrote.

In other words, in developing a system to try to prevent staking concentration, ePBS may itself lead to centralized block building.

There’s a tendency for block building to centralize around a few sophisticated actors who can extract maximum value by optimizing the transaction orders. Buterin notes this process can “creep” into staking because stakers are incentivized to delegate to or join large pools affiliated with dominant builders to maximize their earnings.

ePBS addresses this by separating the roles of block proposers, including stakers and validators, from block builders by outsourcing block construction to a permissionless, open market of specialized builders.

Additionally, Ethereum devs are introducing Forward Obligatory Commitment to Inclusion Lists, better known as FOCIL, a mechanism to ensure Ethereum remains censorship-resistant, to Glamsterdam. At the start, FOCIL will involve 16 randomly selected attesters to mandate that all transactions — even controversial ones — make it into a block.

FOCIL aims to ensure transactions can't be fully censored because transactions “*must* be included somewhere in the block (the block gets rejected otherwise),” Buterin notes. “This means that even if 100% of block building is taken over by one hostile actor, they cannot prevent transactions from being included, because the FOCILers will push them in.”

'Big FOCIL' and encrypted mempools

On Monday, Buterin discussed expanding this idea to something called "Big FOCIL," which, as the name implies, would make "FOCILs bigger, so they can include all of the transactions in the block."

Buterin argues that FOCILs could expand to handle all transactions in a block, thereby diminishing the block building role to just “MEV-relevant” transactions and state computation, effectively commoditizing block building.

Furthermore, developers could encrypt the mempool — where transactions wait to be included — to address the problem of "toxic MEV," like sandwiching and frontrunning attacks, Buterin noted. "If a transaction is encrypted until it's included, no one gets the opportunity to ‘wrap’ it in a hostile way," he added.

Relatedly, Buterin argues that the so-called “Transaction Ingress Layer,” essentially “what happens in between a user sending out a transaction, and that transaction making it into a block,” should include some form of anonymization to protect from exploitation. He notes that “there has recently been increasing work on network-layer anonymization,” including tools like Tor and Ethereum-focused mixnets like Flashnet.

“This is an open design space, I expect the kohaku initiative @ncsgy will be interested in integrating pluggable support for such protocols, like it is for onchain privacy protocols,” Buterin said, highlighting the Ethereum Foundation’s Kohaku privacy initiative.

All of this theorizing comes amid significant ongoing upgrades for Ethereum and ambitious proposals, including an idea to swap out Ethereum’s native EVM virtual machine for RISC-V. For his part, Buterin wants Ethereum to become both “leaner” and “harder,” and has become an increasingly vocal champion for an ambitious developer cycle.

Earlier this year, Buterin noted he would be selling ETH to finance open source development as the Ethereum Foundation entered a "period of mild austerity."