Calle - E-cash | Dev Masterclasses 2026

Biography

Calle (GitHub: callebtc) is the creator of Cashu, an open-source Chaumian e-cash protocol built on Bitcoin and the Lightning Network. He holds a PhD in physics, bringing rigorous scientific methodology to cryptographic protocol design.

Calle launched Cashu in September 2022, drawing on David Chaum's seminal work on blind signatures from the 1980s but reimagining it for the Bitcoin era. The key innovation is that Cashu mints can issue and redeem tokens backed by Bitcoin (via Lightning), providing perfect transaction privacy for the sender.

The project has grown rapidly, with Calle authoring 29 NUT (Notation, Usage, and Terminology) specifications that define the Cashu protocol. These specifications are community-driven and have attracted implementations in multiple programming languages.

Calle is also an active participant in the Nostr ecosystem, building bridges between e-cash and decentralized social protocols. He has received an OpenSats Long-Term Support (LTS) grant to continue his work on Cashu, and has delivered over 15 conference talks explaining e-cash concepts to developers worldwide.

Technical Contributions

Calle's work spans protocol design, reference implementations, and ecosystem tools:

Cashu Protocol: Designed the complete Cashu e-cash protocol, including the blind signature scheme, token format, mint operations (minting, melting, splitting), and Lightning integration.
NUT Specifications: Authored 29 NUT specs that formally define every aspect of the protocol, from basic token operations to advanced features like multi-path payments and token state management.
Nutshell: Built the reference implementation of a Cashu mint and wallet in Python, serving as the canonical implementation that other developers build against.
Cashu.me: Created a Progressive Web App (PWA) wallet that lets users interact with Cashu mints directly from their browser, no installation required.
Bitchat: Developed a mesh networking experiment for Bitcoin-backed communication, exploring the intersection of e-cash and messaging.
Nostr Integration: Built bridges between Cashu and the Nostr protocol, enabling e-cash tips and payments in decentralized social media.

Masterclass: E-cash

This masterclass explores Chaumian e-cash as a privacy layer for Bitcoin. While Bitcoin transactions are pseudonymous, they are not private — the blockchain is a public ledger. E-cash protocols like Cashu solve this by using cryptographic blind signatures to create truly private digital cash.

Chaumian E-cash on Bitcoin

Privacy Layer • Mar 17, 2026

Topics likely covered include:

David Chaum's blind signature scheme and its modern implementation
The Cashu protocol: minting, melting, and token operations
NUT specifications: understanding the protocol documentation
Building a Cashu mint from scratch
Lightning integration: backing e-cash tokens with real sats
Privacy tradeoffs: trust model, mint custody, and user sovereignty

Resources & Preparation

Cashu NUT Specifications

The 29 NUT specs that define the Cashu protocol. Essential reading.

Nutshell

Reference implementation of a Cashu mint and wallet in Python.

Cashu.space

Official Cashu project page with protocol overview and ecosystem links.

Cashu.me Wallet

Progressive Web App wallet. Try Cashu e-cash directly in your browser.

Blind Signatures

Background on the cryptographic primitive that powers Chaumian e-cash.

Chaumian E-cash Tutorial

Educational implementation of Chaum's blind signature protocol.