FIRO Coin and Its Lelantus Spark Privacy Protocol

Last year, in August 2021, Firo – a privacy cryptocurrency protocol, formerly known as Zcoin, unveiled the details of its new privacy protocol.

Called Lelantus Spark, it’s the protocol’s next major update that intends to greatly improve the privacy, as well as the overall flexibility of its transactions. With this in mind, let’s dive a bit deeper into it.

What is Lelantus Spark?

Spark represents the logical extension of the team’s work on Lelantus v1/v2, and it also retains many important features which include but are not limited to:

• High anonymity sets
• Straightforward construction
• Efficient support for batch verification
• No trusted setup
• It also relies on well-established cryptographic assumptions

Furthermore, the protocol introduces new privacy-preserving capabilities, including:

• Efficient multi-sig operations
• Incoming and full view keys,
• Spark addresses
• Modular design

Firo’s team continues to explore new avenues to expand the protocol’s functionality, such as enhanced addressing, as well as the ability to show proof of payment to various merchants.

Speaking on the matter, Reuben Yap, Project Steward of Firo, said:

“There are only a handful of cryptocurrency privacy protocols in meaningful use today, each with different trade-offs. […] Firo has always been at the forefront of privacy tech development and we believe Lelantus Spark represents a holistic balance of high anonymity, simplicity, and flexibility while remaining true to the spirit of trustlessness in cryptocurrencies.”

Lelantus Spark’s key ideas have also been used in Monero’s upcoming privacy framework Seraphis in an effort to scale its ring sizes.

It’s worth noting that Monero has had its fair share of challenges through the years. Firo’s implementation of Spark focuses on much higher anonymity sets and utilizes sliding windows between large sets, avoiding many of the issues of decoy selection present in ring-based privacy. Firo’s core team has published a detailed article comparing Spark with other privacy protocols.

Breaking Down the Features

Spark Addresses

The Lelantus Spark protocol brings forward a non-interactive addressing system that’s designed to increase the privacy of the recipient of the transaction. The funds kept in these Spark addresses are also kept hidden, disallowing anyone from knowing the user’s exact holdings.

Previously, publicly-shared addresses could be searched on a block explorer directly, and anyone could check when said addresses received a payment. Even with the amounts being hidden, the payment information itself was leaked. With Spark addresses, users are allowed to share their addresses without them being searchable on the blockchain.

Multi-Signature

Multi-sig operations will enable multiple mutually non-trusting parties to generate, receive, and also authorize, in a cooperative manner, transactions associated with an address that supports the multi-sig.

The Spark protocol is designed to support efficient signing and multi-sig operations as it uses the modified Chaum-Pedersen discrete logarithm proof.

Modular Design

The modular design introduced by Spark follows well-known cryptographic building blocks such as the Pedersen commitments, zero-knowledge one-of-many proofs, range proofs, as well as discrete logarithm equality proofs that are designed to allow straightforward security analysis without needing any trusted setup processes.

View Keys

This feature comes with a few practical applications for organizations, charities, and individuals, that include:

• Benefactors are able to view balances and transaction values
• Offloaded scan services are allowed on online and mobile wallets
• The balances can be determined for both auditing and accounting purposes

In summary, the Lelantus Spark protocol is different than some of the most popular ones used at the moment. For instance, the Ring-CT-based protocol that’s used in Monero (XMR) limits the sender anonymity because of the space and time scaling of the underlying signature scheme. Triptych, on the other hand, has a very cumbersome and complex multi-sig process, while zkSNARKs requires complex math and a trusted setup.