This month, we’re celebrating five years of Sebastian Falbesoner (GitHub, X) working on Bitcoin Core and libsecp256k1 at Brink. Sebastian’s progress followed an arc similar to other contributors by starting with often unglamorous test work, steadily widening his scope, and eventually stepping up to steward impactful privacy proposals. We are glad to have supported that journey, and optimistic about where it’s heading.
Part-Time Testing
Sebastian joined Brink in July 2021 as part of our second round of developer grants. At the time he was contributing in his spare time around a full-time software engineering job, and the grant gave him a dedicated slice of each week to focus purely on Bitcoin Core.
From the beginning, his contributions were grounded in the parts of the project that tend to attract fewer headlines but provide the project’s foundation: code review, testing, and the functional test framework. His early work added missing checks for consensus reject reasons, improved RPC error coverage, cleaned up outdated terminology (such as converting tests from the pre-segwit concept of “block size” to “block weight”), and expanded the use of MiniWallet so that more tests could run without the Bitcoin Core wallet compiled. He also helped Hennadii’s efforts by chipping away at the codebase’s overuse of recursive mutexes, either replacing them with non-recursive ones or removing them entirely where possible.
BIP324
In 2022 and 2023, Sebastian became increasingly involved in the review of BIP324, the version 2 P2P transport protocol that added encryption to Bitcoin’s peer-to-peer layer. It was the first project where he really leaned into the cryptographic side of Bitcoin, reviewing everything from the ElligatorSwift key exchange primitives to the network-layer integration. Reviewers noticed that he had a habit of re-implementing pieces of the protocol independently to cross-check the Bitcoin Core implementation, which is particularly valuable for cryptographic code.
On top of his review work, Sebastian built bip324-proxy, a proof-of-concept that makes it easier for light clients to use v2 encrypted transport without implementing the protocol themselves.
Libsecp256k1
Sebastian describes libsecp256k1 as “one of the most fascinating projects in the Bitcoin ecosystem due to its low-level, math-like and highly optimized character, as well as its importance for both nodes and wallets”.
Since his initial libsecp256k1 work with BIP324, he has contributed by adding exhaustive test coverage for the ElligatorSwift module, reviewing the MuSig2 module that shipped in v0.6.0, and reviving an old pull request to ensure sensitive memory (like private key material) is properly cleared without being optimized out by the compiler. While working on that fix, he also identified and fixed a place in Bitcoin Core where private key data could be leaked during xprv parsing.
In 2024 he co-authored BIP374 (DLEQ proofs for secp256k1), writing the reference implementation and test vectors and helping refine the specification. By the end of the year, Jonas Nick described him as “a key contributor to the project” whose reviews “carry significant weight.”
Going Full-Time
In April 2025, after years of contributing part-time, Sebastian left his day job to work on Bitcoin full-time. The shift was visible in his output.
Silent Payments
The most significant project of Sebastian’s full-time year has been silent payments. BIP352 defines a reusable-address scheme that lets a recipient publish a single address while each payment lands at a distinct, unlinkable on-chain output. This is a meaningful privacy improvement for anyone who needs to accept payments publicly or without interactivity.
“Thinking back, my early work in Bitcoin Core focused to a large degree on improving test coverage and extending the functional test framework. Over the years, I became more and more drawn into work on privacy-related protocols that make use of elliptic-curve cryptography, such as peer-to-peer encryption (BIP-324), MuSig2 (BIP-327), FROST (BIP-445), and Silent Payments (BIP-352).”
When the original author of the libsecp256k1 silent payments pull request stepped back in late 2025, Sebastian took over just as a “worst-case scanning attack” was discovered that could stall a scanning node for several minutes on pathological transactions. Sebastian spent the following months working with other reviewers to benchmark alternative scanning approaches and open pull requests for two candidate designs.
Jonas Nick (libsecp256k1 maintainer) described the impact:
“theStack’s impact on libsecp256k1 has been significant… He has recently taken over the Silent Payments pull request. This is a high-priority project for both libsecp256k1 and Bitcoin Core. His maintenance of that PR has been effective in driving it forward… He works effectively with very little direction. When he encounters design questions, he proactively writes code or benchmarks to help resolve them.”
MuSig2 in the Wallet
Sebastian was one of the primary reviewers of the pull requests that brought MuSig2 multisignature spending via schnorr signatures into the Core wallet. His familiarity with the underlying libsecp256k1 musig2 module allowed him to verify that the signing workflow was implemented correctly and that the wallet avoided nonce reuse which could leak secret keys and lead to loss of funds. His review was thorough enough that it surfaced improvements which were folded back into BIP373 itself.
SwiftSync
After hearing Ruben Somsen present the SwiftSync proposal, Sebastian built the first proof-of-concept implementation, which demonstrated roughly a 5x speedup of initial block download. This work could take the time for a new node operator to sync from days to hours.
Secp256k1lab
Sebastian also co-created secp256k1lab, a Python library extracted from work on the ChillDKG reference implementation. Secp256k1lab is intended as a shared playground for prototyping cryptographic BIPs and it’s already been adopted as a dependency by the FROST signing and DKG BIP repositories. It’s a small but useful piece of infrastructure for the people working on Bitcoin’s cryptographic proposals.
For good measure, he also ran a historical benchmarking study comparing ECDSA signature verification performance in libsecp256k1 against OpenSSL over the last decade since the switch, discussed on the Bitcoin Optech podcast.
Looking Ahead
Sebastian’s top priority is getting silent payments merged into libsecp256k1 and then activating the DLEQ module that sits on top of it. Beyond that, he plans to dig into BIP54 (the Consensus Cleanup), with particular focus on reviewing the timewarp attack fix and the legacy signature operations limit in Bitcoin Inquisition. He’ll also continue his work on FROST threshold signatures and help with the ongoing effort to remove libevent as a Bitcoin Core dependency.
In his own words, reflecting on the past five years:
“I am looking forward to continuing to support current and upcoming privacy proposals, with a strong focus on code review in general, and occasional unexpected side journeys, such as implementing the first SwiftSync PoC. Thanks to Brink for supporting me throughout this exciting journey.”
Thank you, Sebastian, for five years of careful, quiet, and increasingly consequential work.
About Brink
Brink is a Bitcoin research and development centre, founded in 2020 to support independent open source protocol developers and mentor new contributors. Find out more about our grants and fellowship programs, and how to support us.
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