2026-07-02 –, Amphitheater 122
Shamir's Secret Sharing (SSS) has been trusted for decades by organizations like ICANN (DNSSEC root key ceremonies), Trezor (SLIP-39), and Coinbase ; yet it remains largely inaccessible to individual practitioners and small teams who need cold storage for cryptocurrency wallets, SSH keys, infra recovery keys, or root CA credentials.
This talk introduces Fractum, an open-source (MIT) CLI tool that combines AES-256-GCM authenticated encryption with Shamir's Secret Sharing over GF(2^8) to split sensitive files into K-of-N threshold shares. Designed as an air-gapped, portable & offline-first tool with zero network dependencies, This tool brings information-theoretic security to anyone with a terminal.
I will walk through the cryptographic design decisions: why GCM over CBC, how polynomial interpolation in GF(256) actually works at the byte level, how we handle entropy collection from multiple sources, and the trade-offs of implementing memory protection (SecureString with mlock and multi-pass overwrite) in a garbage-collected language like Python. A pre-recorded demo will show a full encrypt-split-distribute-reconstruct cycle running inside a network-isolated Docker container.
Attendees will take away: a clear mental model of how threshold cryptography works in practice, an understanding of the security properties (and honest limitations) of implementing SSS in Python, and a free tool they can use immediately for their own cold storage needs.
The gap no one talks about (3 min):
There is a missing category between "encrypt it and hope you don't lose the key" and "$50K HSM setup." Most practitioners fall back on copying encrypted files to multiple locations, which means a single key compromise exposes everything. I will frame the cold storage problem: cryptocurrency wallets, root CA keys, disaster recovery credentials, digital inheritance ; all scenarios where you need security measured in years, not sessions.
How Shamir's Secret Sharing actually works (5 min):
No hand-waving. I will walk through polynomial construction over GF(2^8), Lagrange interpolation for reconstruction, and why the information-theoretic security guarantee is fundamentally different from computational security. If you have K-1 shares, every possible secret is equally likely ; this is not a bruteforce problem, it is a mathematical impossibility. Real-world precedents: ICANN DNSSEC ceremonies, Trezor SLIP-39, Ledger Recover, military grade algos.
Building it in Python: the honest version (4 min):
- Memory protection with SecureString: ctypes.memset(), mlock(), multi-pass overwrite
- Honest limitations: Python string immutability, garbage collection timing, no side-channel resistance
- Air-gapped design: '--network=none' Docker guarantee, no telemetry, self-contained share archives
- Supply chain considerations: minimal dependencies, SHA-256 integrity checking
Demo: encrypt, split, reconstruct (4 min):
Pre-recorded terminal session inside a '--network=none' Docker container. Encrypt a file, split into 3-of-5 shares, attempt reconstruction with 2 shares (fails, by design), reconstruct with 3 shares (succeeds). Inspect the share metadata and integrity verification.
What is missing and what comes next (4 min):
Open discussion of limitations: no formal verification of the SSS implementation, no side-channel analysis, Python GC constraints. Roadmap items: DPSS (Dynamic-committee Proactive Secret Sharing), HSM integration. Open questions for the community: share verification without reconstruction, HSM integration, formal verification approaches for Python crypto.
Resources:
- GitHub: https://github.com/katvio/fractum
- Documentation: https://fractum.katvio.com
- Security Architecture: https://fractum.katvio.com/security-architecture/
Cédric is a seasoned DevSecOps and infrastructure engineer with 10yrs of experience spanning defense, blockchain, and cloud-native environments. He began his career working on safety-critical software in the defense industry, then moved into the blockchain space as a DevSecOps & SRE for the Tezos Foundation. He now runs his own digital agency, delivering security, and key management services to key accounts across banking and SaaS. His expertise sits at the intersection of cybersec, supply chain security, and applied cryptography, with a particular interest in privacy-preserving technologies. Company: Katvio.com