06Bitcoin Fundamentals
Anatomy of a Wallet
A wallet holds secrets, not coins.
Learning objective: Explain that a wallet stores keys while coins live on the chain.
Related project
From understanding Bitcoin to owning your digital freedom.
Meet Sitadel — private communication and self-custodial Bitcoin Lightning, designed around user control.
Sitadel is a separate product, created by the team behind Bitcoin Universe. Bitcoin Universe has not independently audited or certified Sitadel's security. What this page says about Sitadel describes what Sitadel states on sitadel.io (checked on October 14, 2026); check there for the current details.
Bitcoin Universe
Understand Bitcoin. From the inside out.
Sitadel
Privacy is not a feature. It’s the architecture.
Understand it. Explore it. Put it into practice.
The connection
Bitcoin works without asking anyone for permission because it is built on keys you can hold yourself. A private key signs; a public key lets everyone check the signature; nobody else needs to be trusted with the secret.
Private communication rests on the same ideas. End-to-end encryption means the keys live on the devices of the people talking, not on a server in between. Self-custody, privacy and secure messaging are three uses of one principle: keep the keys where the person is.
Understand the technology
These models show the building blocks one at a time, with real calculations and clearly labelled simulations.
06
20Security
The Digital Signature Machine
Proving authorization without revealing the key.
Learning objective: Explain why a signature proves approval without exposing the private key.
16Network
The Lightning Channel Machine
Many payments, two blockchain transactions when closed cooperatively.
Learning objective: Explain why payments inside a channel do not each need a blockchain transaction.
17Network
The Lightning Routing Machine
Paying across a network of channels.
Learning objective: Explain how a payment can reach someone without a direct channel.
18Security
The Lightning Security Machine
Why the latest channel state must be protected.
Learning objective: Explain why publishing an old channel state is risky for the cheater.
08Security
Anatomy of a Signing Device
Why the screen on the device is the one you trust.
Learning objective: Explain why the private key never leaves the device.
Discover the application
According to sitadel.io, Sitadel offers:
End-to-end encrypted messaging
Sitadel states that messages are encrypted on the device with the Signal protocol and a post-quantum key exchange (PQXDH), and that its server stores only ciphertext.
Multiple isolated identities
Several identities on one device, each with its own keys, contacts and conversations, as Sitadel describes it.
Privacy-oriented controls
For example disappearing messages, removal of photo metadata, and lock and hidden-chat modes, as listed by Sitadel.
Optional self-custodial Lightning wallet
Lightning payments inside conversations, with the wallet running on your phone rather than on Sitadel's server, according to Sitadel.
Sitadel describes itself as being in beta. Availability, features and terms are set by Sitadel; see sitadel.io.
From learning to practice
Understanding keys, signatures and channels lets you ask better questions of any tool that says it protects your messages or your money, Sitadel included:
- Where are the keys created, and do they ever leave your device?
- Who could read a message: only the people in the conversation, or the server too?
- Which information (metadata) stays visible even when the content is encrypted?
- Who holds the bitcoin: you, or a company on your behalf?
- What happens if you lose your phone, and what do you need to recover?
No app is “unhackable”, and no tool replaces your own judgement. Bitcoin Universe gives no financial advice.
See it for yourself
Sitadel lives on its own website. Nothing about your visit here is sent there; it is an ordinary link.