Cashu eCash Integration

SatSigner implements the Cashu protocol for privacy-preserving electronic cash (eCash). Connect to Cashu mints to issue, send, receive, and redeem eCash tokens backed by Bitcoin Lightning payments.

What is Cashu?

Cashu is a free and open-source eCash protocol built on Bitcoin and Lightning Network. It provides:

Privacy: Blind signatures prevent mints from tracking spending
Instant settlements: Lightning-backed payments
Offline transactions: Transfer tokens without internet
Interoperability: Standard protocol across implementations

Core Concepts

Mints

Cashu mints are servers that issue eCash tokens:

type EcashMint = {
  url: string              // Mint URL
  name: string             // Mint name
  isConnected: boolean     // Connection status
  keysets: Array<{         // Active keysets
    id: string             // Keyset identifier
    unit: 'sat'            // Unit (satoshis)
    active: boolean        // Active status
  }>
  balance: number          // Your balance at mint
  lastSync: string         // Last sync timestamp
}

Proofs

eCash tokens are cryptographic proofs:

type EcashProof = {
  id: string      // Keyset ID
  amount: number  // Proof amount (sats)
  secret: string  // Blinded secret
  C: string       // Signature (point on curve)
}

Tokens

Proofs are encoded into shareable tokens:

cashuA<base64-encoded-token-data>

Implementation

SatSigner uses the @cashu/cashu-ts library for Cashu operations.

Wallet Management

Wallet Cache: Maintains persistent connections to mints

// apps/mobile/api/ecash.ts:21
const walletCache = new Map<string, Wallet>()

export function getWallet(mintUrl: string): Wallet {
  if (!walletCache.has(mintUrl)) {
    const mint = new Mint(mintUrl)
    const wallet = new Wallet(mint)
    walletCache.set(mintUrl, wallet)
  }
  return walletCache.get(mintUrl)!
}

Connecting to Mints

Adding a Mint

// apps/mobile/api/ecash.ts:95
export async function connectToMint(mintUrl: string): Promise<EcashMint> {
  clearWalletCache(mintUrl)
  const wallet = getWallet(mintUrl)
  await wallet.loadMint()
  const mintInfo = wallet.getMintInfo()
  const keysets = await getKeysetsFromWallet(wallet)
  
  return {
    url: mintUrl,
    name: mintInfo.name || `Mint ${mintUrl}`,
    isConnected: true,
    keysets: keysets.map(ks => ({
      id: ks.id,
      unit: ks.unit,
      active: ks.active
    })),
    balance: 0,
    lastSync: new Date().toISOString()
  }
}

Fetching Keysets

Keysets are cryptographic key sets used by the mint:

// apps/mobile/api/ecash.ts:29
export async function getKeysetsFromWallet(
  wallet: Wallet
): Promise<{ id: string; unit: 'sat'; active: boolean }[]> {
  // Try getKeysets() method first (v3.0.0)
  if (typeof wallet.getKeysets === 'function') {
    const result = await wallet.getKeysets()
    return result.map(ks => ({
      id: ks.id,
      unit: 'sat' as const,
      active: ks.active !== false
    }))
  }
  
  // Fallback: Fetch from mint API
  const response = await fetch(`${mintUrl}/keysets`)
  const keysets = await response.json()
  return keysets.map(ks => ({
    id: ks.id,
    unit: 'sat' as const,
    active: ks.active !== false
  }))
}

Minting eCash (Receiving)

Flow: Lightning → eCash

Step 1: Create Mint Quote Request invoice from mint:

// apps/mobile/api/ecash.ts:115
export async function createMintQuote(
  mintUrl: string,
  amount: number
): Promise<MintQuote> {
  const wallet = getWallet(mintUrl)
  await wallet.loadMint()
  const quote = await wallet.createMintQuote(amount)
  
  return {
    quote: quote.quote,      // Quote ID
    request: quote.request,  // Lightning invoice
    expiry: quote.expiry,    // Expiration time
    paid: false
  }
}

Step 2: Pay Lightning Invoice Pay the returned Lightning invoice through your wallet. Step 3: Check Quote Status

// apps/mobile/api/ecash.ts:130
export async function checkMintQuote(
  mintUrl: string,
  quoteId: string
): Promise<MintQuoteState> {
  const wallet = getWallet(mintUrl)
  await wallet.loadMint()
  const quoteStatus = await wallet.checkMintQuote(quoteId)
  return quoteStatus.state // 'PAID' | 'UNPAID' | 'EXPIRED'
}

Step 4: Mint Proofs Once paid, mint the eCash tokens:

// apps/mobile/api/ecash.ts:140
export async function mintProofs(
  mintUrl: string,
  amount: number,
  quoteId: string
): Promise<EcashMintResult> {
  const wallet = getWallet(mintUrl)
  await wallet.loadMint()
  const proofs = await wallet.mintProofs(amount, quoteId)
  
  return {
    proofs,           // Array of EcashProof
    totalAmount: amount
  }
}

Sending eCash

Creating Tokens

// apps/mobile/api/ecash.ts:222
export async function sendEcash(
  mintUrl: string,
  amount: number,
  proofs: EcashProof[],
  memo?: string
): Promise<EcashSendResult> {
  // Validate proofs before sending
  const { validProofs, spentProofs } = await validateProofs(mintUrl, proofs)
  
  if (spentProofs.length > 0) {
    throw new Error('Token already spent')
  }
  
  const totalProofAmount = validProofs.reduce(
    (sum, proof) => sum + proof.amount,
    0
  )
  
  if (totalProofAmount < amount) {
    throw new Error('Insufficient balance')
  }
  
  const wallet = getWallet(mintUrl)
  await wallet.loadMint()
  
  // Split proofs into keep and send
  const { keep, send } = await wallet.send(amount, validProofs, {
    includeFees: true
  })
  
  // Encode proofs into token
  const token = getEncodedTokenV4({
    mint: mintUrl,
    proofs: send,
    unit: 'sat',
    memo
  })
  
  return {
    token,  // Encoded token string
    keep,   // Proofs you keep
    send    // Proofs being sent
  }
}

Token Format (v4):

cashuAeyJ0b2tlbiI6W3sibWludCI6Imh0dHBzOi8vbWludC5leGFtcGxlLmNvbSIsInByb29mcyI6W3siYW1vdW50IjoxLCJpZCI6IjAwYWQwMjVhMjY0NTQwNSIsInNlY3JldCI6ImZvbyIsIkMiOiJiYXIifV19XSwibWVtbyI6IlRlc3QgdG9rZW4ifQ==

Receiving eCash

Redeeming Tokens

// apps/mobile/api/ecash.ts:278
export async function receiveEcash(
  mintUrl: string,
  token: string
): Promise<EcashReceiveResult> {
  const wallet = getWallet(mintUrl)
  await wallet.loadMint()
  
  // Decode and validate token
  const decodedToken = getDecodedToken(token)
  if (decodedToken.mint !== mintUrl) {
    throw new Error('Token mint URL does not match current mint')
  }
  
  // Receive proofs
  const proofs = await wallet.receive(token)
  const totalAmount = proofs.reduce(
    (sum, proof) => sum + proof.amount,
    0
  )
  
  return {
    proofs,                  // Received proofs
    totalAmount,             // Total amount
    memo: decodedToken.memo  // Optional memo
  }
}

Melting eCash (Spending)

Flow: eCash → Lightning

Step 1: Create Melt Quote

// apps/mobile/api/ecash.ts:154
export async function createMeltQuote(
  mintUrl: string,
  invoice: string  // Lightning invoice to pay
): Promise<MeltQuote> {
  const wallet = getWallet(mintUrl)
  const quote = await wallet.createMeltQuote(invoice)
  
  return {
    quote: quote.quote,           // Quote ID
    amount: quote.amount,         // Amount to pay
    fee_reserve: quote.fee_reserve, // Reserved for fees
    paid: false,
    expiry: quote.expiry
  }
}

Step 2: Melt Proofs Pay the Lightning invoice with your proofs:

// apps/mobile/api/ecash.ts:169
export async function meltProofs(
  mintUrl: string,
  quote: MeltQuote,
  proofs: EcashProof[],
  description?: string,
  originalInvoice?: string
): Promise<EcashMeltResult> {
  const wallet = getWallet(mintUrl)
  await wallet.loadMint()
  
  // Recreate quote with original invoice
  const invoiceToUse = originalInvoice || quote.quote
  const meltQuote = await wallet.createMeltQuote(invoiceToUse)
  
  // Melt proofs to pay invoice
  const result = await wallet.meltProofs(meltQuote, proofs)
  
  return {
    paid: true,
    preimage: result.preimage || result.payment_preimage,
    change: result.change  // Unused proofs returned as change
  }
}

Proof Validation

Check if proofs are still valid (unspent):

// apps/mobile/api/ecash.ts:199
export async function validateProofs(
  mintUrl: string,
  proofs: EcashProof[]
): Promise<{ validProofs: EcashProof[]; spentProofs: EcashProof[] }> {
  const wallet = getWallet(mintUrl)
  await wallet.loadMint()
  
  const proofStates = await wallet.checkProofsStates(proofs)
  
  const validProofs: EcashProof[] = []
  const spentProofs: EcashProof[] = []
  
  proofStates.forEach((state, index) => {
    if (state.state === 'UNSPENT' || state.state === 'PENDING') {
      validProofs.push(proofs[index])
    } else if (state.state === 'SPENT') {
      spentProofs.push(proofs[index])
    }
  })
  
  return { validProofs, spentProofs }
}

Proof States:

UNSPENT: Proof is valid and spendable
PENDING: Proof is being processed
SPENT: Proof has already been redeemed

Token Validation

Validate encoded tokens:

// apps/mobile/api/ecash.ts:308
export async function validateEcashToken(
  token: string,
  mintUrl: string
): Promise<{ isValid: boolean; isSpent?: boolean; details?: string }> {
  try {
    const decodedToken = getDecodedToken(token)
    const wallet = getWallet(mintUrl)
    const proofs = decodedToken.proofs || []
    
    if (proofs.length === 0) {
      return { isValid: false, details: 'No proofs found in token' }
    }
    
    const proofStates = await wallet.checkProofsStates(proofs)
    
    const spentProofs = proofStates.filter(s => s.state === 'SPENT')
    const unspentProofs = proofStates.filter(s => s.state === 'UNSPENT')
    const pendingProofs = proofStates.filter(s => s.state === 'PENDING')
    
    if (spentProofs.length === proofs.length) {
      return {
        isValid: true,
        isSpent: true,
        details: 'All proofs have been spent'
      }
    } else if (unspentProofs.length === proofs.length) {
      return {
        isValid: true,
        isSpent: false,
        details: 'All proofs are unspent'
      }
    } else {
      return {
        isValid: true,
        isSpent: false,
        details: `Mixed: ${spentProofs.length} spent, ${unspentProofs.length} unspent`
      }
    }
  } catch {
    return {
      isValid: false,
      details: 'Failed to check proof states'
    }
  }
}

Balance Management

Calculate balance from proofs:

// apps/mobile/api/ecash.ts:301
export async function getMintBalance(
  mintUrl: string,
  proofs: EcashProof[]
): Promise<number> {
  return proofs.reduce((sum, proof) => sum + proof.amount, 0)
}

Privacy Features

Cashu uses blind signatures to ensure privacy:

Client: Creates blinded secret
Mint: Signs blinded secret (cannot see original)
Client: Unblinds signature to get valid proof
Mint: Cannot link signature to original request

No Tracking

Mints cannot track how tokens are spent
Tokens can be split and combined freely
No transaction graph like Bitcoin
Offline transfers possible

Multi-Mint Strategy

For enhanced privacy:

Use multiple mints simultaneously
Swap between mints periodically
Split large amounts across mints
Diversify custodial risk

Security Considerations

Custodial Risk

eCash is custodial - the mint holds the Bitcoin:

Only use trusted mints
Don’t store large amounts
Monitor mint reputation
Diversify across multiple mints

Proof Management

Never share proofs - they are bearer assets
Backup proofs securely
Track spent vs unspent proofs
Validate tokens before accepting

Token Security

Tokens are like cash - anyone with the token can spend it
Send tokens through secure channels
Verify mint URL before redeeming
Be cautious with public tokens

Error Handling

Common errors and solutions: “Token already spent”

Proofs in token have been redeemed
Double-spend attempt detected
Cannot recover - proofs are spent

“Insufficient balance”

Not enough valid proofs for amount
Check proof states
Mint more tokens or receive from others

“Token mint URL does not match”

Token is from a different mint
Cannot redeem at current mint
Connect to correct mint first

Implementation Reference

Core API: apps/mobile/api/ecash.ts:1 Type Definitions: apps/mobile/types/models/Ecash.ts:1 UI Component: apps/mobile/components/SSEcashTransactionCard.tsx:1

Cashu Protocol Versions

SatSigner supports:

Token v4: Current standard with memo support
Keyset Fetching: Dynamic keyset discovery
Proof States: UNSPENT, PENDING, SPENT tracking

Overview

Getting Started

Core Features

Advanced Features

Security

Resources

​Cashu eCash Integration

​What is Cashu?

​Core Concepts

​Mints

​Proofs

​Tokens

​Implementation

​Wallet Management

​Connecting to Mints

​Adding a Mint

​Fetching Keysets

​Minting eCash (Receiving)

​Flow: Lightning → eCash

​Sending eCash

​Creating Tokens

​Receiving eCash

​Redeeming Tokens

​Melting eCash (Spending)

​Flow: eCash → Lightning

​Proof Validation

​Token Validation

​Balance Management

​Privacy Features

​Blind Signatures

​No Tracking

​Multi-Mint Strategy

​Security Considerations

​Custodial Risk

​Proof Management

​Token Security

​Error Handling

​Implementation Reference

​Cashu Protocol Versions

​Resources

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