Flow Wallet API

The Flow Wallet API is a REST HTTP service that allows a developer to integrate wallet functionality into a larger Flow application infrastructure. This service can be used by an application that needs to manage Flow user accounts and the assets inside them.

Features

• Create new Flow accounts
• Securely store account private keys
• Send a transaction from an account
• Transfer fungible tokens (e.g. FLOW, FUSD)
• Detect fungible token deposits
• Transfer NFTs (e.g. FLOW, FUSD) (coming soon)
• Detect NFT deposits (coming soon)

View full list of functionality in the API documentation.

Background

Some application developers may wish to manage Flow accounts in a fully-custodial fashion, but without taking on the complexity of building an account management system.

An application may need to support custody of fungible tokens (FLOW, FUSD), non-fungible tokens, or both.

For security and/or legal reasons, some developers need to use a custody service running on-premises as part of their existing infrastructure, rather than a hosted 3rd-party solution.

Example use cases

• FLOW/FUSD Hot Wallet — an application that allows users to convert fiat currency to FLOW or FUSD. A single admin account would be used as a hot wallet for outgoing payments, and additional deposit accounts would be created to accept incoming payments.
• Exchange — a cryptocurrency exchange that is listing FLOW and/or FUSD. Similar to the case above, one or more admin accounts may be used as a hot wallet for outgoing payments, and additional deposit accounts would be created to accept incoming payments.
• Web Wallet — a user-facing wallet application that is compatible with Flow dapps. Each user account would be created and managed by the wallet service.

API Specification

View the Wallet API documentation and OpenAPI (Swagger) specification.

Installation

The Wallet API is provided as a Docker image:

docker pull ghcr.io/flow-hydraulics/flow-wallet-api:latest

Basic example usage

NOTE: This setup is only for demonstrative purposes, please do not run this on production

This setup requires Docker, Docker Compose and the Flow CLI.

Create a configuration file:

cp .env.example .env # and edit

Start the Wallet API, Flow Emulator, Postgres and Redis:

docker-compose up -d

You can now access the API at http://localhost:3000/v1.

Once you're finished, run this to stop the containers:

docker-compose down

Configuration

The application is configured using environment variables. Make sure to prefix variables with "FLOW_WALLET_"

If you have an existing .env file (or you were using -envfile) you can run a command with the variables loaded:

# Replace <command> with the command you want to run
env $(grep -e '^#' .env | xargs) <command>

# For example
env $(grep -e '^#' .env | xargs) go run main.go

Maintenance mode

You can put the service in maintenance mode via the System API by sending the following JSON body as a POST request to /system/settings (example in api-test-scripts/system.http):

{
  "maintenanceMode": true
}

In maintenance mode, all on-chain transactions and event processing are halted. Disabling maintenance mode is done via the same API endpoint ("maintenanceMode": false).

Updates on async requests (webhook)

If you have the possibility to setup a webhook endpoint, you can set FLOW_WALLET_JOB_STATUS_WEBHOOK to receive updates on async requests (requests which return a job). The wallet will send a POST request to this URL containing the job whenever the status of the job is updated.

NOTE: The wallet expects a response with status code 200 and will retry if unsuccessful.

Configuring the server request timeout

When making sync requests it's sometimes required to adjust the server's request timeout. Try increasing FLOW_WALLET_SERVER_REQUEST_TIMEOUT if you're experiencing issues with sync requests, FLOW_WALLET_SERVER_REQUEST_TIMEOUT=180s for example.

NOTE: Using sync requests in production is not recommended, use asynchronous requests & optionally configure a webhook to receive job updates instead.

Enabled fungible tokens

A comma separated list of fungible tokens and their corresponding addresses enabled for this instance. Make sure to name each token exactly as it is in the corresponding cadence code (FlowToken, FUSD etc.). Include at least FlowToken as functionality without it is undetermined.

NOTE: It is necessary to add a 3rd parameter "lowercamelcase" name for each token. For FlowToken this would be "flowToken" and for FUSD "fusd". This is used to construct the vault name, receiver name and balance name in generic transaction templates. Consult the contract code for each token to derive the proper name (search for .*Vault, .*Receiver, .*Balance)

NOTE: Non-fungible tokens can not be enabled using environment variables. Use the API endpoints for that.

Examples:

FLOW_WALLET_ENABLED_TOKENS=FlowToken:0x0ae53cb6e3f42a79:flowToken,FUSD:0xf8d6e0586b0a20c7:fusd

Database

Config variable	Environment variable	Description	Default	Examples
DatabaseType	FLOW_WALLET_DATABASE_TYPE	Type of database driver	sqlite	sqlite, psql, mysql
DatabaseDSN	FLOW_WALLET_DATABASE_DSN	Data source name (DSN) for database connection	wallet.db	See below

Examples of Database DSN

mysql://john:pass@localhost:3306/my_db

postgresql://postgres:postgres@localhost:5432/postgres

user:pass@tcp(127.0.0.1:3306)/dbname?charset=utf8mb4&parseTime=True&loc=Local

host=localhost user=gorm password=gorm dbname=gorm port=9920 sslmode=disable TimeZone=Asia/Shanghai

For more: https://gorm.io/docs/connecting_to_the_database.html

To learn more about database schema versioning and migrations, read MIGRATIONS.md.

Google KMS setup

Note: In order to use Google KMS for remote key management you'll need a Google Cloud Platform account.

Pre-requisites:

1. Create a new Project if you don't have one already. You'll need the Project ID later.
2. Enable Cloud Key Management Service (KMS) API for the project, Security -> Cryptographic Keys.
3. Create a new Key Ring for your wallet (or use an existing Key Ring), Security -> Cryptographic Keys -> Create Key Ring, you'll need the Location ID (or Location) and Key Ring ID (or Name) later.

Using a Service Account to access the KMS API (see official docs for more);

1. Create a new Service Account, IAM & Admin -> Service Accounts -> Create Service Account
2. Use the roles Cloud KMS Admin & Cloud KMS Signer/Verifier or grant the required permissions through a custom role (NOTE: deletion not supported yet):
   • cloudkms.cryptoKeyVersions.useToSign
   • cloudkms.cryptoKeyVersions.viewPublicKey
   • cloudkms.cryptoKeys.create
3. After creating the Service Account, select Manage Keys from the Actions menu in the Service Account listing.
4. Create a new key, Add Key -> Create New key, and select JSON as the key type.
5. Save the JSON file.

Configure the Google KMS client library by setting the environment variable GOOGLE_APPLICATION_CREDENTIALS:

export GOOGLE_APPLICATION_CREDENTIALS="/home/example/path/to/service-account-file.json"

Configure Google KMS as the key storage for flow-wallet-api and set the necessary environment variables:

Config variable	Environment variable	Description	Default	Examples
DefaultKeyType	FLOW_WALLET_DEFAULT_KEY_TYPE	Default key type	local	local, google_kms
ProjectID	FLOW_WALLET_GOOGLE_KMS_PROJECT_ID	GCP Project ID	-	flow-wallet-example
LocationID	FLOW_WALLET_GOOGLE_KMS_LOCATION_ID	GCP Location ID	-	europe-north1, us-west1
KeyRingID	FLOW_WALLET_GOOGLE_KMS_KEYRING_ID	GCP Key Ring ID	-	example-wallet-keyring

Google KMS for admin account

If you want to use a key stored in Google KMS for the admin account, just pass the resource identifier as the private key (FLOW_WALLET_ADMIN_PRIVATE_KEY) and set FLOW_WALLET_ADMIN_KEY_TYPE to google_kms.

Creating an account on testnet via the faucet:

1. When generating the key, choose "Asymmetric sign" as the purpose and "Elliptic Curve P-256 - SHA256 Digest" as the key type and algorithm (other combinations may work but these have been confirmed to work)
2. Download the public key from Google KMS in PEM format (should have a .pub ending)
3. Run it through flow keys decode pem --from-file <filename>
4. Copy the "Public Key" part
5. Go to https://testnet-faucet-v2.onflow.org
6. Paste the copied public key in the form
7. IMPORTANT: Choose SHA2_256 as the hash algorithm (SHA3_256 won't work with the key setup above)
8. Copy the new address and use it as the FLOW_WALLET_ADMIN_ADDRESS
9. Set FLOW_WALLET_ADMIN_PRIVATE_KEY to the resource id of the key
10. Set FLOW_WALLET_ADMIN_KEY_TYPE to google_kms

Example environment:

FLOW_WALLET_ADMIN_ADDRESS=0x1234567890123456
FLOW_WALLET_ADMIN_PRIVATE_KEY=projects/<project_id>/locations/<location_id>/keyRings/<keyring_id>/cryptoKeys/<key_name>/cryptoKeyVersions/<version_number> # Make sure this ends with the version number
FLOW_WALLET_ADMIN_KEY_TYPE=google_kms

NOTE: This will mess up the docker-compose setup (emulator won't start) as it uses FLOW_WALLET_ADMIN_PRIVATE_KEY as FLOW_SERVICEPRIVATEKEY. It will cause an encoding error on the emulator.

Google KMS key for database encryption

Before configuring a Google KMS key for database encryption please refer to the official guide for setting up a symmetric encryption key;

https://cloud.google.com/kms/docs/encrypt-decrypt#before_you_begin

If you want to use an Google KMS symmetric encryption key for encrypting the stored account keys, please refer to the following configuration settings;

Config variable	Environment variable	Description	Default	Examples value for Google KMS
EncryptionKeyType	FLOW_WALLET_ENCRYPTION_KEY_TYPE	Encryption key type	local	google_kms
EncryptionKey	FLOW_WALLET_ENCRYPTION_KEY	KMS encryption key resource name	-	projects/my-project/locations/us-west1/keyRings/my-keyring/cryptoKeys/my-encryption-key

AWS KMS setup

Note: In order to use AWS KMS for remote key management you'll need an AWS account. Note: Custom key stores are not supported.

Pre-requisites:

1. AWS credentials for an account that has access to KMS

Environment variable	Description	Default	Examples
AWS_PROFILE	AWS profile name, when using a shared credentials file	-	my-aws-profile-name
AWS_REGION	AWS KMS Region	-	eu-central-1, us-west-1
AWS_ACCESS_KEY_ID	AWS access key ID	-	AKIAXXX123FOOBAR1234
AWS_SECRET_ACCESS_KEY	AWS secret access key	-	FooBaRBaZ12345...

Configure AWS KMS as the key storage for flow-wallet-api and set the necessary environment variables, with the default key type as aws_kms:

Config variable	Environment variable	Description	Default	Value for AWS KMS
DefaultKeyType	FLOW_WALLET_DEFAULT_KEY_TYPE	Default key type	local	aws_kms

AWS KMS for admin account

If you want to use a key stored in AWS KMS for the admin account, please refer to the following configuration settings;

Config variable	Environment variable	Description	Default	Example value for AWS KMS
AdminKeyType	FLOW_WALLET_ADMIN_KEY_TYPE	Admin key type	local	aws_kms
AdminPrivateKey	FLOW_WALLET_ADMIN_PRIVATE_KEY	Admin private key ARN	-	arn:aws:kms:eu-central-1:012345678910:key/00000000-aaaa-bbbb-cccc-12345678910

When testing make sure to add the key to the admin account. You can convert the AWS public key (e.g. aws.pem) you downloaded/copied from AWS with flow-cli;

flow keys decode pem --from-file=aws.pem --sig-algo "ECDSA_secp256k1"

AWS KMS for encrypting stored keys

If you want to use an AWS KMS symmetric encryption key for encrypting the stored account keys, please refer to the following configuration settings;

Config variable	Environment variable	Description	Default	Examples value for AWS KMS
EncryptionKeyType	FLOW_WALLET_ENCRYPTION_KEY_TYPE	Encryption key type	local	aws_kms
EncryptionKey	FLOW_WALLET_ENCRYPTION_KEY	KMS encryption key ARN	-	arn:aws:kms:eu-central-1:012345678910:key/00000000-aaaa-bbbb-cccc-12345678910

Idempotency middleware

Idempotency middleware ensures that POST requests are idempotent. When the middleware is enabled an Idempotency-Key HTTP header is required for POST requests. The header value should be a unique identifier for the request (UUID or similar is recommended). Trying to send a request with a duplicate idempotency key will result in a 409 Conflict HTTP response.

To configure the middleware set the following configuration settings;

Config variable	Environment variable	Description	Default	Examples
DisableIdempotencyMiddleware	FLOW_WALLET_DISABLE_IDEMPOTENCY_MIDDLEWARE	Disable the idempotency middleware entirely	false	true, false
IdempotencyMiddlewareDatabaseType	FLOW_WALLET_IDEMPOTENCY_MIDDLEWARE_DATABASE_TYPE	Database type for idempotency key middleware	local	local, shared, redis
IdempotencyMiddlewareRedisURL	FLOW_WALLET_IDEMPOTENCY_MIDDLEWARE_REDIS_URL	Redis URL for idempotency key middleware storage	-	redis://walletapi:wallet-api-redis@localhost:6379/

NOTE:

• The local option for IdempotencyMiddlewareDatabaseType does not support multiple instances.
• The provided docker-compose.yml provides a basic Redis instance for local development purposes, with basic configuration files in the redis-config directory.
• There is currently no automatic cleanup of old idempotency keys when using the shared (sql) database. Redis is recommended for production use.

Log level

The default log level of the service is info. You can change the log level by setting the environment variable FLOW_WALLET_LOG_LEVEL.

Valid log leves are (case-insensitive):

panic
fatal
error
warn, warning
info
debug
trace

Multiple keys for custodial accounts

To enable multiple keys for custodial accounts you'll need to set FLOW_WALLET_DEFAULT_ACCOUNT_KEY_COUNT to the number of keys each account should have. When a new account is created the auto-generated account key is cloned so that the total number of keys matches the configured value.

NOTE: Changing FLOW_WALLET_DEFAULT_ACCOUNT_KEY_COUNT does not affect existing accounts.

All possible configuration variables

Refer to configs/configs.go for details and documentation.

Credit

The Flow Wallet API is developed and maintained by Equilibrium, with support from the Flow core contributors.

Testing

You can run a fully dockerized test suite if you have Docker and Docker Compose installed.

# Run the test suite
make run-test-suite

# If you don't want to leave background services running (database, redis, flow-emulator)
make stop-test-suite