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Cashu

Cashu is a free and open-source Chaumian ecash system built for Bitcoin. It lets users hold and transfer ecash tokens offline, settling against the Lightning Network when needed. Nobody needs to know who you are, how much you hold, or with whom you transact.

Core Technology: Chaumian Blind Signatures​

Invented by David Chaum in 1982, blind signatures allow a mint to sign a token without knowing its contents. This means:

  • The mint can verify a token is authentic
  • The mint cannot link issuance to redemption
  • Internal transfers within a single mint are perfectly private

How It Works​

  1. Mint — an online Lightning node custodies sats and issues signed ecash tokens
  2. Mint — a user receives a set of tokens from the mint (funded via Lightning)
  3. Transfer — the user sends tokens to another user via QR, link, or Nostr DM
  4. Redeem — the recipient redeems tokens back into Lightning sats (or keeps them as ecash)

Privacy via Mint Swaps​

ScenarioPrivacy Level
Internal transfer (same mint)Perfect — mint sees nothing
Inter-mint swapStrong — receiving mint doesn't know origin
Deposit + redeem via different walletsStrong — link erased at Lightning layer

Offline Use Cases​

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

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

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

Resources​