Fiat Module

Note: all numeric values, amounts, rates, and identifiers in the examples are illustrative and provided for demonstration purposes only. Real values depend on market conditions, account configuration, and the parameters of a specific transaction.

Overview

Fiat Module is a subsystem of the CryptoCash platform that allows a merchant to accept fiat payments from end users and receive cryptocurrency to their wallet. Integration is performed through signed API requests. Payments are processed via a payment provider (Mercuryo). The end user completes the payment in the CryptoCash widget, where they select a payment method. After the payment is processed, the merchant receives a webhook with the final status. Supported features:

Payment by card, Apple Pay, Google Pay, SEPA, and other payment methods
Signed requests (ED25519 or LEGACY/SHA256) for merchant authentication
Webhooks to the merchant on every transaction status change
Automatic calculation of the merchant’s earnings based on partner_fee

High-level flow

Merchant sends a signed API request → receives a payment URL
User opens the URL → sees the CryptoCash widget
User selects a payment method
Payment provider processes the payment
Merchant receives a webhook on every status change
After successful payment, the merchant’s earnings are calculated automatically

Two payment flows in the widget:

Payment method	Flow
Apple Pay (native)	The user pays directly in the widget — without being redirected to an external page
Card / other methods	The user is redirected to the payment provider’s page to complete the payment

For a detailed description of the native Apple Pay flow, see the Apple Native Pay section.

Merchant setup and Fiat API Key

What the merchant configures

After the administrator has prepared the account, the merchant generates a Fiat API Key in the dashboard. When creating the key, the merchant sets parameters that apply to all payments associated with this key:

Parameter	Description
`keyType`	Signature algorithm: `ED25519` (recommended) or `LEGACY` (`SHA256`)
`keyName`	Custom key name
`partnerFee`	Share of the provider fee that goes to the merchant; configured by the administrator on the provider account
`paymentMethods`	Payment methods available in the widget, for example `["card", "apple_pay"]`
`webhookUrl`	URL for receiving payment status webhooks
`logoUrl`	Merchant logo displayed in the widget
`ipRestrictionEnabled`	Whether to restrict API access by IP address
`allowedIpAddresses`	List of allowed IP addresses or CIDR ranges
`expiresAt`	Specific key expiration date
`neverExpires`	If `true`, the key does not expire (`expiresAt` = `null`); by default, the key validity period is 1 year

The response returns a publicKey / privateKey pair. The private key is stored on the platform in encrypted form and is shown to the merchant only once — during creation. Save it immediately.

If two-factor authentication is enabled on the merchant account, 2FA confirmation is required to generate the key.

Fiat API Key fields

Field	Type	Description
`publicKey`	`string`	Key identifier, passed in every request
`privateKey`	`string`	Secret for signing requests; stored encrypted on the platform
`keyType`	`ED25519`	`LEGACY`	Signature algorithm
`isActive`	`boolean`	Whether the key is active
`expiresAt`	`datetime`	`null`	Expiration time (`null` — no expiration)
`paymentMethods`	`string[]`	Allowed payment methods
`webhookUrl`	`string`	`null`	URL for webhook delivery
`logoUrl`	`string`	`null`	Logo URL
`ipRestrictionEnabled`	`boolean`	Whether IP restriction is enabled
`allowedIpAddresses`	`string[]`	Allowed IP addresses

ED25519 vs LEGACY

	`ED25519`	`LEGACY`
Algorithm	Asymmetric cryptography (Ed25519)	SHA256-based signature
Key relation	`publicKey` is mathematically derived from `privateKey`	`publicKey` is an identifier; `privateKey` is a shared secret used for signing
Verification	Signature is verified using `publicKey` without knowing `privateKey`	The platform uses the stored `privateKey` to verify the signature
Recommended	Yes	Only for compatibility with legacy integrations

What is checked on each request

When receiving a signed request, the platform checks:

The key is active
The key has not expired
The IP of the API caller (merchant server) is included in the allowed list if IP restriction is enabled
The signature is valid for the selected algorithm

Important: IP restriction applies to the address of the server making the API request, not to the end user’s IP in the payment payload ip field.

Signed requests

All merchant API requests must be signed. The signature algorithm is defined by the keyType value selected when creating the key.

Request structure

{
  "data": "<base64-encoded JSON payload>",
  "signature": "<base64-encoded signature>"
}

publicKey is passed either in the x-public-key header or inside the data object. If both are present, the header takes priority.

ED25519 (recommended)

payload    → JSON.stringify(payload)
json       → Buffer.from(json, 'utf8').toString('base64')  → data
sign       → Ed25519.sign(Buffer.from(data), privateKeyBytes) → signatureBytes
signature  → signatureBytes.toString('base64')

LEGACY (SHA256)

payload    → JSON.stringify(payload)
json       → Buffer.from(json, 'utf8').toString('base64')  → data
hash       → SHA256(privateKey + data) → hashHex
signature  → Buffer.from(hashHex, 'utf8').toString('base64')

Full example

Payload for form/retrieve:

{
  "publicKey": "a1b2c3d4...",
  "fiatCurrency": "SGD",
  "fiatAmount": "100",
  "currency": "USDT",
  "network": "TRC20",
  "address": "TXxxx...",
  "ip": "203.0.113.5"
}

Final body of the signed request:

{
  "data": "eyJwdWJsaWNLZXkiOiJhMWIyYzNkNC4uLiIsImZpYXRDdXJyZW5jeSI6IlNHRCIsImZpYXRBbW91bnQiOiIxMDAiLCJjdXJyZW5jeSI6IlVTRFQiLCJuZXR3b3JrIjoiVFJDMjAiLCJhZGRyZXNzIjoiVFh4eHguLi4iLCJpcCI6IjIwMy4wLjExMy41In0=",
  "signature": "base64-encoded-signature"
}

Payment creation — `form/retrieve`

Endpoint

POST https://rates.crypto-cash.world/merchant/api/v1/pay-fiat/form/retrieve

Signed request. The signature and IP are checked before the request is processed.

Request

The data field of the signed request contains the following JSON:

Field	Type	Required	Description
`ticker`	string	✓*	Crypto asset and network ticker, for example `"USDTTRC20"`
`currency`	string	✓*	Cryptocurrency code, for example `"USDT"`
`network`	string	✓*	Blockchain network, for example `"TRC20"`
`fiatCurrency`	string	✓	Fiat currency code, for example `"SGD"`
`fiatAmount`	string	✓	Fiat amount as a string, for example `"100"`
`address`	string	✓	Wallet address for cryptocurrency crediting
`ip`	string	✓	End user’s IP address
`email`	string	—	Customer email
`redirectUrl`	string	—	URL to return the user to after payment
`lang`	string	—	Widget language, default `"en"`
`externalId`	string	—	External order ID on the merchant side; must be unique within the key

* To select a crypto asset, pass either ticker or the currency + network pair.

Response

{
  "code": 200,
  "data": {
    "item": {
      "id": "550e8400-e29b-41d4-a716-446655440000",
      "externalId": "order-123",
      "url": "https://buy.cryptocash.world/merchant/payment/550e8400-e29b-41d4-a716-446655440000"
    }
  }
}

Field	Description
`id`	Internal transaction UUID
`externalId`	External ID from the request or `null`
`url`	CryptoCash widget URL — redirect the user to this URL

What happens after the request

Signature and IP are checked.
externalId uniqueness is checked within the merchant key.
The fiat amount is checked against the minimum allowed amount for the selected currency.
The network value is normalized case-insensitively.
A transaction is created with the New status.
The widget URL is generated.
A payment::created webhook is sent to the merchant.
id and url are returned.

	CryptoCash widget	Payment provider page
URL	`https://buy.cryptocash.world/merchant/payment/{id}`	Generated inside the widget
Branding	Merchant logo	Provider styling
Purpose	Payment method selection, rate preview, native Apple Pay	Direct payment processing

The merchant always receives the CryptoCash widget URL. The provider page URL is generated inside the widget after the payment method is selected and is not returned to the merchant.

Merchant Public API

Retrieve payment

POST https://rates.crypto-cash.world/merchant/api/v1/pay-fiat/payments/retrieve

Signed request. The data field contains one of the following:

{ "internalId": "550e8400-..." }

{ "externalId": "order-123" }

Response:

{
  "code": 200,
  "data": {
    "item": {
      "id": "550e8400-e29b-41d4-a716-446655440000",
      "externalId": "order-123",
      "status": "Paid",
      "fiatCurrency": "SGD",
      "fiatAmount": "100",
      "cryptoCurrency": "USDT",
      "network": "TRC20",
      "cryptoAmount": "73.21",
      "address": "TXxxx...",
      "hash": "blockchain-hash",
      "widgetUrl": "https://buy.cryptocash.world/merchant/payment/550e8400-...",
      "createdAt": "2026-04-16T10:00:00.000Z",
      "updatedAt": "2026-04-16T10:07:00.000Z"
    }
  }
}

Payment list

POST https://rates.crypto-cash.world/merchant/api/v1/pay-fiat/payments/list

Signed request. The data field:

Field	Type	Default	Description
`page`	number	1	Page number
`pageSize`	number	100	Number of items per page
`dateAfter`	datetime	—	Filter: created after this date
`dateBefore`	datetime	—	Filter: created before this date

Response:

{
  "code": 200,
  "data": {
    "items": [ /* array of payments in the same structure as retrieve */ ],
    "pagination": {
      "page": 1,
      "pageSize": 100,
      "total": 42
    }
  }
}

Currency lists

POST https://rates.crypto-cash.world/merchant/api/v1/fiat-currencies/list
POST https://rates.crypto-cash.world/merchant/api/v1/pay-fiat/crypto-currencies/list

Both endpoints use signed requests. Minimum payload in data:

{ "publicKey": "..." }

crypto-currencies/list returns the cryptocurrencies available to the merchant together with networks and tickers. Values from tickers can be passed to form/retrieve instead of the currency + network pair.

Payment response fields

Field	Type	Nullable	Description
`id`	string		Transaction UUID
`externalId`	string	✓	Merchant external ID
`status`	string		Current status
`fiatCurrency`	string		Fiat currency code
`fiatAmount`	string		Fiat amount
`cryptoCurrency`	string		Cryptocurrency code
`network`	string		Blockchain network
`cryptoAmount`	string	✓	Received crypto amount
`address`	string		Wallet address
`hash`	string	✓	Blockchain transaction hash
`widgetUrl`	string	✓	Widget URL
`createdAt`	datetime		Creation time
`updatedAt`	datetime		Last update time

Transaction statuses

Status	Description
`New`	Transaction created, awaiting user action
`Waiting`	Payment accepted and being processed
`Paid`	Cryptocurrency sent to the wallet address
`Fail`	Payment failed
`Canceled`	Payment canceled

Statuses are set by the platform based on payment provider events. The merchant cannot change the status via API.

Error handling

All API errors are returned in one of two formats: Format A — coded error:

{ "code": 2020, "message": "..." }

Format B — error list used for decoding errors:

{ "errors": [2010] }

Error codes

Code	Description
`1000`	Bad request
`1102`	Invalid request: unsupported network, amount below minimum, duplicate `externalId`
`1110`	Request validation error
`2010`	`data` decoding error
`2011`	`signature` decoding error
`2020`	Invalid signature
`2021`	API key blocked
`2030`	IP address forbidden
`3001`	Order not found
`3002`	Transaction not found

Apple Pay

The CryptoCash widget supports the native Apple Pay flow: the iOS payment sheet opens directly in the widget without redirecting to an external page. For a detailed description of native mode availability conditions, fallback behavior, and error handling, see the Apple Native Pay section.

Fiat Module

Fiat Module

Overview

High-level flow

Merchant setup and Fiat API Key

What the merchant configures

Fiat API Key fields

ED25519 vs LEGACY

What is checked on each request

Signed requests

Request structure

ED25519 (recommended)

LEGACY (SHA256)

Full example

Payment creation — `form/retrieve`

Endpoint

Request

Response

What happens after the request

CryptoCash widget and provider page

Merchant Public API

Retrieve payment

Payment list

Currency lists

Payment response fields

Transaction statuses

Error handling

Error codes

Apple Pay

Diagrams

General payment flow

​Fiat Module

​Overview

​High-level flow

​Merchant setup and Fiat API Key

​What the merchant configures

​Fiat API Key fields

​ED25519 vs LEGACY

​What is checked on each request

​Signed requests

​Request structure

​ED25519 (recommended)

​LEGACY (SHA256)

​Full example

​Payment creation — form/retrieve

​Endpoint

​Request

​Response

​What happens after the request

​CryptoCash widget and provider page

​Merchant Public API

​Retrieve payment

​Payment list

​Currency lists

​Payment response fields

​Transaction statuses

​Error handling

​Error codes

​Apple Pay

​Diagrams

​General payment flow

Fiat Module

Overview

High-level flow

Merchant setup and Fiat API Key

What the merchant configures

Fiat API Key fields

ED25519 vs LEGACY

What is checked on each request

Signed requests

Request structure

ED25519 (recommended)

LEGACY (SHA256)

Full example

Payment creation — `form/retrieve`

Endpoint

Request

Response

What happens after the request

CryptoCash widget and provider page

Merchant Public API

Retrieve payment

Payment list

Currency lists

Payment response fields

Transaction statuses

Error handling

Error codes

Apple Pay

Diagrams

General payment flow