Chaumian eCash
Chaumian eCash is a privacy-preserving digital cash system built on top of Bitcoin. It uses blind signatures to allow a mint to authenticate tokens without knowing who holds them or how they are spent. Cashu is the most widely adopted open-source implementation, settling against the Lightning Network.
Core Technology: Blind Signatures
Invented by David Chaum in 1982, blind signatures allow a signer (the mint) to validate a token without seeing its contents:
- User blinds a token (mathematically obscures it)
- Mint signs the blinded token
- User unblinds the signature
- The signature is valid, but the mint cannot link it to the original request
This means internal transfers within a single mint are perfectly private — the mint sees deposits and redemptions, but cannot connect them.
How Cashu Works
- Deposit — User sends bitcoin via Lightning to a Cashu mint
- Issue — Mint returns signed ecash tokens
- Transfer — User sends tokens to another person via QR, link, or Nostr DM
- Redeem — Recipient redeems tokens back into Lightning sats
Privacy via Mint Swaps
| Scenario | Privacy Level |
|---|---|
| Internal transfer (same mint) | Perfect — mint sees nothing |
| Inter-mint swap | Strong — receiving mint doesn't know origin |
| Deposit + redeem via different wallets | Strong — link erased at Lightning layer |
Offline Use
Cashu tokens can be transferred without internet access:
- Copy the Base64 token to clipboard
- Send via mesh networks (Bitchat BLE, Meshtastic LoRa, Briar) while offline
- Recipient redeems when back online
This makes Cashu useful for disaster scenarios, censorship events, or areas with unreliable connectivity.
Trust Model
- The mint custodies the underlying bitcoin — trusted, but for small amounts
- Verify mint reserves via proof-of-reserves where available
- Use multiple mints to reduce single-point-of-failure
- Community-run mints lower trust compared to unknown operators
Comparison to Other Privacy Tools
| Tool | Mechanism | Online Required | Custody |
|---|---|---|---|
| Cashu | Blind signatures | No (token transfer) | Mint custodial |
| Lightning | HTLCs | Yes | Self-custodial |
| CoinJoin | Transaction mixing | Yes | Self-custodial |
| PayJoin | Interactive transaction | Yes | Self-custodial |
Wallets & Mints
- Nutstash — web wallet (wallet.nutstash.app)
- Cashu.me — browser wallet with Nostr integration
- eNuts — Android wallet
- Community mints — anyone can run a mint with a Lightning node
Related
- Cashu — Cashu protocol entity
- Nutstash — Web wallet for Cashu
- Privacy Stacks — Broader privacy practices
- Lightning Network — Settlement layer for Cashu
- Peer-to-Peer Exchange — Acquiring bitcoin to fund Cashu