login

DuraShare: BIP39-Native Threshold Backup with Full Manual Fallback and Per-Share Audit
.ical
2026-11-21 –, Momentum - Hack Zone Stage

This session’s header image

TL;DR: Picture this: on an air-gapped device, you create a 3-of-5 backup of your BIP39 seed. Years later, you recover it using just pen, paper and a calculator. This is one of the many cool things DuraShare can do. In this hands-on workshop, you’ll see a live air-gapped ceremony in action, perform an individual share audit, and manually reconstruct a toy model seed (3 words plus checksum) step-by-step to experience true software-independent recovery firsthand.

Abstract

Long-horizon cryptocurrency self-custody faces a dual failure model: exposing or losing a master BIP39 mnemonic, or surviving decades only to find recovery software, operating systems, or specialized hardware obsolete or unavailable. Existing approaches may use custom share encodings, alternative wallet-derivation models, or high manual friction.

This workshop presents DuraShare, a BIP39-native k-of-n threshold backup with full manual fallback and per-share audit capability. For everyday convenience we adopt a software-assisted model, while ensuring that all the core math remains feasible for recovery, requiring nothing beyond paper, a calculator, and specific instructions.

DuraShare combines three existing, well-established cryptographic techniques: (1) Shamir Secret Sharing over GF(2053), the smallest prime field containing the 2048-word BIP39 list; (2) 2D error detection, applying row and column checksums to each share to protect the data against transcription errors and physical damage; and (3) an optional affine inner-product MAC over the same finite field that, when the corresponding key material is separately held and uncompromised, provides bounded detection of active share substitution before secret reconstruction.

To demonstrate recovery independence, the workshop includes two live demos: the whole process using a single-file HTML tool (designed for air-gapped environments like Tails OS), followed by a hands-on, step-by-step paper-and-calculator recovery of a single row (3 words and a checksum) using printed templates.

DuraShare uses Shamir secret sharing to split a standard BIP39 recovery phrase into k-of-n durable, human-readable shares in an offline, software-assisted experience, while keeping all the math executable manually on paper. It also allows individual geographically distributed shares to be verified before recovery, without gathering a threshold or revealing the secret.

DuraShare modifies existing, well-established cryptographic techniques for human-friendly threshold backup. This implementation is thoroughly tested, published in good faith as is, and has not been independently audited.

What is this workshop about?

Can a Bitcoin backup survive a lost share, damaged paper, and the original tool disappearing? Self-custody often hangs on one BIP39 recovery phrase. Multisig answers who may spend; it does not fully answer how to back up the seed material so recovery remains possible without the original setup.

DuraShare is an open protocol for BIP39-native threshold backup: it splits an existing recovery phrase into k-of-n durable shares, with:

  • human-readable durable share artifacts (paper / metal / long-lived media);
  • a separate Share Audit Ceremony so one geographically distributed share can be checked without gathering a threshold or revealing the mnemonic;
  • a software-assisted path (offline / air-gapped HTML reference tool) and a specified full manual fallback (tables, modular arithmetic, precomputed Lagrange coefficients) so recovery does not permanently depend on one app or vendor.

This workshop is show-and-do: concepts are introduced while running ceremonies, not in a long lecture first. We use dummy material only. Implementations are published as-is and are not independently audited — we will not use real funds or real recovery phrases.

Pedagogical design: a short narrated software-assisted ceremony shows the real product shape (12-word, 3-of-5). The centerpiece is a hands-on manual recovery: attendees recover a complete row (3 words plus checksum) from three 3-of-5 shares using a calculator and printed templates.

Goals: invite protocol review, attract contributors, and gather concrete usability feedback by doing the core recovery exercise rather than sitting through a long preamble.

What would an attendee learn from this workshop?

  • What a software-assisted Sharing → Recovery ceremony looks like in practice (12-word, 3-of-5)
  • How DuraShare’s row-based GF(2053) works: one row of 3 BIP39 indices → share cells + row checksum; why a full mnemonic is mostly that loop repeated plus column/GIC/transcription.
  • Hands-on: recover a DuraShare row (3 words plus checksum) from 3 shares.
  • How Share Audit differs from Recovery, and why that matters for geographically distributed custody.
  • How the software-assisted path relates to manual fallback (recommended everyday path vs sovereignty backstop).
  • Where to read the whitepaper/specs, verify release artifacts, and engage as a reviewer or contributor.
  • Clear safety posture: unaudited software; dummy material only; do not use with real funds.

Is there anything attendees should read up on before they attend this talk?

Helpful but not required:

Is there anything attendees should set up before the workshop?

  • For the manual lab (row recovery): a simple calculator (mobile phone works for the event, with a strong recommendation not to use for real funds). Any necessary printed worksheets will be provided.
  • Do not bring real seed phrases. Software demo and manual lab use dummy material only.

Room/attendee cap: depends on room size, works best in a room with tables.

Relevant Links

Prior workshop delivery

  • Casa21, São Paulo (2026-09-10): 4 participants completed a toy 3-of-5 Sharing and Recovery exercise by hand.
  • TABConf 8, Atlanta (2026-10-13): around 20 participants manually shared and recovered a complete 3-of-5 DuraShare row (3 words plus checksum).

Level of difficulty

Accessible to self-custodians

Familiarity with BIP39 and self-custody helps, but is not required. The manual recovery exercise is guided step by step; calculator arithmetic and the relevant intuition are introduced during the workshop. This is best for attendees curious about how Bitcoin backups can survive real-world loss and long-term recovery.

About the Speaker

Renato Schiavinato Lopez — Founder of GRIFORTIS and protocol author of DuraShare. Professional background as an Agile Coach with extensive experience designing and facilitating hands-on workshops and ceremonies.

Solo presenter. The session will be delivered in English.

Social Links

Reference Video

https://youtu.be/waiqQoT6WGo

Recorded against a previous version of the protocol. It does not present all current features, but it illustrates the intent and the show-and-do style.

Project:

DureShare

The speaker's profile picture
Renato Schiavinato Lopez

Renato Schiavinato Lopez builds security processes for the inconvenient moments: when software is gone, the instructions are old, the people involved are stressed, and the person who designed the system is not there.

He is the founder of GRIFORTIS and the creator of DuraShare, an open protocol for splitting and recovering Bitcoin BIP39 backups with threshold cryptography, offline tools, paper artifacts, and a manual fallback. His question is not merely whether a backup is cryptographically sound, but whether ordinary people can operate it years later without turning recovery into a leap of faith.

He also created SeReCoLiPrInGo—a framework for treating self-custody as seven competing concerns: Security, Resilience, Coercion, Liquidity, Privacy, Inheritance, and Governance. It is an argument against the idea that a hardware wallet, a seed phrase, or multisig alone can solve custody.

Before moving into Bitcoin self-custody, Renato was an Agile Coach. He designed and facilitated hundreds of workshops, trainings, and working sessions in Portuguese and English—getting groups to make decisions, surface assumptions, build shared understanding, and leave with a process they could actually use. That background now shapes his security work: DuraShare treats custody as a human coordination problem as much as a cryptographic one.

In 2026, he brought DuraShare to TABConf, UFMG, Casa21, and Brazilian BitDevs—not as a product pitch, but as something participants could calculate, audit, and recover themselves.