Cashu eCash

The module supports Cashu eCash tokens as an alternative payment method to Lightning invoices.

Standard Mode vs P2PK Mode
Standard Mode P2PK Mode
How it works Calls wallet.receive() → contacts mint to swap tokens Verifies token locked to proxy's public key locally
Speed Slower (blocks on mint API call per request) Fast (milliseconds — no mint call!)
Best for Low-traffic or simple setups High-traffic production deployments
Extra requirement None CASHU_WHITELISTED_MINTS is required

Standard Mode Setup

 
Environment=CASHU_ECASH_SUPPORT=true
Environment=CASHU_DB_PATH=/var/lib/nginx/cashu_tokens.db
# BIP39 wallet mnemonic (NUT-13 backup phrase); unset = auto-generated on first run
Environment=CASHU_WALLET_MNEMONIC="word1 word2 ... word12"

# Optional: Whitelist specific mints (comma-separated)
Environment=CASHU_WHITELISTED_MINTS=https://mint1.example.com,https://mint2.example.com

# Optional: Auto-redeem to Lightning
Environment=CASHU_REDEEM_ON_LIGHTNING=true
Environment=CASHU_REDEMPTION_INTERVAL_SECS=3600

⚠️ Security: CASHU_WALLET_MNEMONIC is the BIP39 phrase that derives the wallet seed (NUT-13) — the only backup of your wallet. Anyone with it can steal your tokens, and losing it loses the funds. It's a 12/24-word phrase (not a hex secret); leave it unset to auto-generate and persist one on first run, and never commit it to Git. On startup the module records a seed fingerprint next to the DB and refuses to start if a later mnemonic doesn't match — delete wallet.fingerprint to switch wallets intentionally.

P2PK Mode Setup (High Performance)

 
Environment=CASHU_P2PK_MODE=true
Environment=CASHU_P2PK_PRIVATE_KEY=<your-private-key-hex>
# CASHU_WHITELISTED_MINTS is REQUIRED in P2PK mode
Environment=CASHU_WHITELISTED_MINTS=https://mint1.example.com

⚠️ Security: CASHU_P2PK_PRIVATE_KEY is equally critical. Anyone with this key can spend tokens locked to your public key. Generate with openssl rand -hex 32.

How P2PK mode works per request:

  1. Proxy derives a public key from CASHU_P2PK_PRIVATE_KEY and sends it to clients via the X-Cashu header (NUT-24)
  2. Client creates P2PK-locked tokens to that public key
  3. Proxy verifies tokens are locked to its public key (NUT-11) — local check, no network call
  4. Proxy unlocks proofs with private key (local cryptographic operation)
  5. Unlocked proofs are stored directly in CDK database via wallet.receive_proofs()
  6. Background redemption task finds proofs via wallet.get_unspent_proofs() and redeems to Lightning via wallet.melt()

Redemption Fee Configuration

 
# Minimum balance to attempt melting (default: 10 sats)
Environment=CASHU_MELT_MIN_BALANCE_SATS=10

# Percentage to reserve for fees (default: 1%)
Environment=CASHU_MELT_FEE_RESERVE_PERCENT=1

# Minimum fee reserve when percentage is small (default: 4 sats)
Environment=CASHU_MELT_MIN_FEE_RESERVE_SATS=4

# Maximum proofs per melt operation (default: 0 = unlimited)
# Use this if your mint has a per-melt proof limit (e.g. mint.coinos.io = 1000)
Environment=CASHU_MAX_PROOFS_PER_MELT=1000

Fee calculation: fee_reserve = max(total_amount × percent/100, min_fee_sats)

Example 1 — Large balance (500 sats) with 1% fee reserve:

  • Percentage fee: 500 × 1% = 5 sats
  • Minimum fee: 4 sats
  • Used reserve: max(5, 4) = 5 sats
  • Redeemable: 500 - 5 = 495 sats

Example 2 — Small balance (50 sats) with 1% fee reserve:

  • Percentage fee: 50 × 1% = 0.5 sats
  • Minimum fee: 4 sats
  • Used reserve: max(0.5, 4) = 4 sats ← Minimum kicks in!
  • Redeemable: 50 - 4 = 46 sats

Example 3 — Proof count limiting when exceeding mint limit (CASHU_MAX_PROOFS_PER_MELT=1000):

  • Scenario: 1282 proofs worth 13,588 sats total
  • Check: 1282 proofs > 1000 limit → Limiting triggered
  • Action: Select first 1000 proofs worth ~10,600 sats
  • Invoice: Generate invoice for 10,600 sats
  • Remaining: 282 proofs (~2,988 sats) stay for next cycle
  • Next cycle: 282 proofs < 1000 limit → all remaining proofs melted

Actual melt quote fees are verified against the reserve; warnings appear if the reserve was insufficient.

Note on CASHU_WHITELISTED_MINTS: If not configured, all mints are accepted in standard mode. In P2PK mode, whitelisted mints are REQUIRED for security and the payment request (NUT-24).

SQLite Database Setup

 
# One-time setup — persists across restarts
sudo mkdir -p /var/lib/nginx
sudo chown nginx:nginx /var/lib/nginx
sudo chmod 755 /var/lib/nginx

The cdk-sqlite crate automatically creates the database file and tables. Database location: /var/lib/nginx/cashu_tokens.db