Introducing Post-quantum Security to Signatures on Substrate

Introducing Post-quantum Security to Signatures on Substrate

Authored by Elizabeth

ChainSafe is proud to announce the completion of a collaboration with xx network, one of the world's first quantum-resistant and privacy-focused blockchain ecosystems.

A few months back, xx network enlisted our help to **** implement Winternitz One Time Signatures (W-OTS+) for Substrate-based chains and automated Placards generation. The primary goal of this engagement was to introduce post-quantum-security into signatures on Substrate.

Technical highlightsFor context, W-OTS+ is a quantum-resistant digital signature scheme that uses relatively small key and signature sizes. And the xx network is a layer one blockchain co-founded by cryptography pioneer David Chaum. XX network is unique in its efforts to guard against cryptography-breaking quantum computing.

This collaboration had two focal points. The first is an implementation of an independent W-OTS+ crypto library that can be used in any context. The second is a Substrate Pallet and Off Chain-Worker Implementation to introduce the W-OTS+ signature scheme and Placards into a Substrate runtime environment.

Both implementations are novel to the Rust-Substrate ecosystem. There's an existing Golang implementation, but to foster interoperability with Substrate-based chains, the W-OTS+ scheme has been written in Rust.

This package of technologies, therefore, can be regarded as an attractive option for future implementers from various language backgrounds.

Broader implications

W-OTS+ is suitable for post-quantum environments as it's a hash-based signature scheme. Unlike the current signature schemes generally used in blockchains (e.g., ECDSA, Schnorr, BLS, etc.), which are based on the hardness of the discrete logarithm problem, hash-based schemes are based on the hardness of finding the preimage of a hash.

Unlike the former, there is no efficient quantum algorithm for the latter, making hash-based signature schemes quantum-resistant.

Hash-based cryptography is not the only approach being proposed for a post-quantum world. Isogeny and lattice-based approaches both hope to address the one-time-use limitation of hash-based signature schemes.

However, following the unexpected critical vulnerabilities recently discovered in a NIST finalist SIDH, hash-based cryptography remains a safe approach.

Given that W-OTS+ are one-time signatures, each time a signature is revealed, part of the private key can potentially be recovered. Thus, re-using a private key for multiple signatures eventually leaks the entire private key.

To work around this issue, for the xx network's use case, a set of keys derived from the same seed and merkleized is published as a validator's public key. Then, to validate a signature, a merkle proof of inclusion within the tree with the published root is used. The merkle tree of keys is referred to as a "Placard" in the implementation, which can be seen as a simplified version of the XMSS signature scheme.


As the need for post-quantum cryptography draws closer, the xx network is ensuring its privacy-preserving tech stack is quantum-secure.

This stack includes a private messenger app backed by a mixnet; a novel wallet generation mechanism called Sleeve, which allows embedding a W-OTS+ public key as a backup for any other cryptocurrency wallet - and now, the first step towards integrating quantum secure cryptography into the xx network's Substrate-based blockchain.

The newly developed W-OTS+ crypto library already empowered the xx network team to implement Sleeve wallet generation in Rust and cross-compile it to WASM for usage in web browsers. Any user can now generate a quantum-ready wallet for any cryptocurrency directly in the xx network web-based wallet and staking app.

About xx networkA community dedicated to the preservation of private and secure communication for all. The xx movement is a diverse group of people from around the globe that care about protecting personal data from bad actors.

Website | Twitter | Discord | GitHub | GitLab (mixnet) | Youtube

This project was completed by ChainSafe Solutions, a team dedicated to solving some of the hardest problems in the blockchain space.

Want to know more? Check out our Solutions Hub or get in touch!

About ChainSafe

ChainSafe is a leading blockchain research and development firm specializing in infrastructure solutions for web3. Alongside its contributions to major ecosystems such as Ethereum, Polkadot, Filecoin, Mina, and more, ChainSafe creates solutions for developers and teams across the web3 space utilizing our expertise in gaming, bridging, NFTs, and decentralized storage. As part of its mission to build innovative products for users and improved tooling for developers, ChainSafe embodies an open source and community-oriented ethos to advance the future of the internet.

Website | Youtube | Medium | Twitter | Linkedin | GitHub | Discord