status	flip	authors	sponsor	updated
released	45	Greg Santos (greg.santos@dapperlabs.com)	Justin Barry (justin.barry@dapperlabs.com)	2023-05-31

FLIP 45: Flow Client Library (FCL) Specification

Objective

This FLIP proposes a v1.0 specification for FCL Wallet Providers and dApps to trustlessly interact for the purposes of authentication and producing signatures on Flow. It aims to provide a comprehensive description of the communication protocol and the required features and functionality expected to perform these functions.

The goal of this FLIP is to solidify existing implementation patterns and data definitions in a generalized way. Additionally it should open discussion to potential improvements for future versions of the spec in order to evolve the Flow dApp and wallet ecosystem.

In short, this spec describes the data dApps and wallets should send and expect, and how they should communicate in order to interoperate with each other.

Motivation

The problem now is that prospective wallet and SDK developers have only a Draft Specification and several Flow and community maintained implementations to refer to. Formalizing an FCL Specification will make it easier and safer to code libraries in any language, build dApps and wallets, and release on more environments and platforms (mobile, Unity, .NET, etc.).

Developers will benefit from the assurances of a formalized specification and users from improved experiences and greater choice of wallets and platforms.

User Benefit

Formalizing a specification for FCL will improve developer and user experience, support creation of applications and sdks, and help broaden support for Flow by more service providers across more platforms.

Existing SDKs and example implementations of the FCL Specification

Flow Client Library (FCL) JS
Flow JS SDK
Flow Go SDK
FCL Swift
Flow JVM SDK
Flow .NET

Design Proposal

Flow Client Library (FCL) Specification v1.0

Abstract

This specification proposes a standard protocol and schema definition that applications, wallets, libraries, and SDKs can use to interact with the Flow blockchain. It defines a standard, language-agnostic interface for dApps and wallets to communicate with each other. This specification is intended to be a living document, and will be updated as needed to keep up with changes in the Flow protocol and ecosystem. This document describes the data types, data structures, and methods that an FCL compatible wallet must implement in order to be compatible with the Flow Client Library.

An FCL compatible wallet uses and conforms to the Flow Client Library (FCL) Specification.

Background / Overview

Flow Client Library (FCL) enables applications to easily integrate with all FCL-compatible wallets and other services (e.g. profiles, private information, notifications). This offers developers a strong foundation to compose their apps with existing building blocks.

A Wallet Provider handles Authentications and Authorizations. They play a very important role of being the place the users control their information and approve transactions.

One of FCLs core ideals is for the user to be in control of their data, a wallet provider is where many users will do just that.

Flow Client Library (FCL) approaches the idea of blockchain wallets on Flow in a different way than how wallets may be supported on other blockchains. For example, with FCL, a wallet is not necessarily limited to being a browser extension or even a native application on a users device. FCL offers wallet developers the flexibility and freedom to build many different types of applications. Since wallet applications can take on many forms, we needed to create a way for these varying applications to be able to communicate and work together.

FCL acts in many ways as a protocol to facilitate communication and configuration between the different parties involved in a blockchain application. An Application can use FCL to authenticate users, and request authorizations for transactions, as well as mutate and query the Blockchain. An application using FCL offers its Users a way to connect and select any number of Wallet Providers and their Wallet Services. A selected Wallet provides an Application's instance of FCL with configuration information about itself and it's Wallet Services, allowing the User and Application to interact with them.

Specification

This section provides a basic overview of the major components of the FCL Specification, it's core architecture, data model, and representation.

All field names in the specification are case sensitive.

Definitions

This document is written with the perspective that you who are reading this right now are an FCL Wallet Developer. All references to you in this doc are done with this perspective in mind.

Data Types

FCL Primitive data types are based on the types supported by the JSON Schema Specification Wright Draft 00. Note that integer as a type is also supported and is defined as a JSON number without a fraction or exponent part.

Data Structures

In the following description, if a field is not explicitly REQUIRED or described with a MUST or SHALL, it can be considered OPTIONAL.

In this section we define the schema of objects used in the protocol. While they are JavaScript objects, only features supported by JSON should be used. (Meaning that conversion of an object to and from JSON should not result in any loss.)

For the schema definition language we choose TypeScript, so that the schema closely resembles the actual type definitions one would use when making an FCL implementation.

Note that currently there are no official type definitions available for FCL. If you are using TypeScript, you will have to create your own type definitions (possibly based on the schema definitions presented in this document).

FCL Objects

In this section we introduce some common definitions that the individual object definitions will be deriving from and define the FCL objects with each ObjectType.

First, let us define the kinds of FCL objects available:

type ObjectType =
  | "Service"
  | "PollingResponse"
  | "Identity"
  | "ServiceProvider"
  | "AuthnResponse"
  | "Signable"
  | "CompositeSignature"
  | "OpenID"

The fields common to all FCL objects then can be defined as follows:

interface ObjectBase<Version = "1.0.0"> {
  f_vsn: Version
  f_type: ObjectType
}

The f_vsn field is usually 1.0.0 for this specification, but some exceptions will be defined by passing a different Version type parameter to ObjectBase.

All FCL objects carry an f_type field so that their types can be identified at runtime.

We also define the union of them to mean any FCL object:

type FclObject =
  | Service
  | PollingResponse
  | Identity
  | ServiceProvider
  | AuthnResponse
  | Signable
  | CompositeSignature
  | OpenID

`Service`

type ServiceType =
  | "authn"
  | "authz"
  | "user-signature"
  | "pre-authz"
  | "open-id"
  | "back-channel-rpc"
  | "authn-refresh"
  | "account-proof"

type ServiceMethod =
  | "HTTP/POST"
  | "IFRAME/RPC"
  | "POP/RPC"
  | "TAB/RPC"
  | "EXT/RPC"
  | "DATA"

interface Service extends ObjectBase {
  f_type: "Service"
  type: ServiceType
  method: ServiceMethod
  uid: string
  endpoint: string
  id: string
  identity: Identity
  provider?: ServiceProvider
  data?: FclObject
}

The meaning of the fields is as follows.

type: The type of this service.
method: The service method this service uses. DATA means that the purpose of this service is just to provide the information in this Service object, and no active communication services are provided.
uid: A unique identifier for the service. A common scheme for deriving this is to use 'wallet-name#${type}', where ${type} refers to the type of this service.
endpoint: Defines where to communicate with the service.
- When method is EXT/RPC, this can be an arbitrary unique string, and the extension will need to use it to identify its own services. A common scheme for deriving the endpoint is to use 'ext:${address}', where ${address} refers to the wallet's address. (See ServiceProvider for more information.)
id: The wallet's internal identifier for the user. If no other identifier is used, simply the user's flow account address can be used here.
identity: Information about the identity of the user.
provider: Information about the wallet.
data: Additional information used with a service of type open-id.

Service Object Example

{
  "f_type": "Service"
  "type": "ServiceType"
  "method": "ServiceMethod"
  "uid": "string"
  "endpoint": "string"
  "id": "string"
  "identity": "Identity"
  "provider": "ServiceProvider"
  "data": "FclObject"
}

`PollingResponse`

interface PollingResponse extends ObjectBase {
  f_type: "PollingResponse"
  status: "APPROVED" | "DECLINED" | "PENDING" | "REDIRECT"
  reason: string | null
  data?: FclObject
  updates?: FclObject
  local?: FclObject
}

Each response back to FCL must be "wrapped" in a PollingResponse. The status field determines the meaning of the response:

An APPROVED status means that the request has been approved. The data field should be present.
A DECLINED status means that the request has been declined. The reason field should contain a human readable reason for the refusal.
A PENDING status means that the request is being processed. More PENDING responses may follow, but eventually a non-pending status should be returned. The updates and local fields may be present.
The REDIRECT status is reserved, and should not be used by wallet services.

In summary, zero or more PENDING responses should be followed by a non-pending response. It is entirely acceptable for your service to immediately return an APPROVED Polling Response, skipping a PENDING state.

`Identity`

This object is used to define the identity of the user.

interface Identity extends ObjectBase {
  f_type: "Identity"
  address: string
  keyId?: number
}

Note: The 'keyId' is required when returning a proposer identity.

The meaning of the fields is as follows.

address: The flow account address of the user.
keyId: The id of the key associated with this account that will be used for signing.

`ServiceProvider`

This object is used to communicate information about a wallet.

interface ServiceProvider extends ObjectBase {
  f_type: "ServiceProvider"
  address: string
  name?: string
  description?: string
  icon?: string
  website?: string
  supportUrl?: string
  supportEmail?: string
}

The meaning of the fields is as follows.

address: A flow account address owned by the wallet. It is unspecified what this will be used for.
name: The name of the wallet.
description: A short description for the wallet.
icon: An image URL for the wallet's icon.
website: The wallet's website.
supportUrl: A URL the user can use to get support with the wallet.
supportEmail: An e-mail address the user can use to get support with the wallet.

`AuthnResponse`

This object is used to inform FCL about the services a wallet provides.

interface AuthnResponse extends ObjectBase {
  f_type: "AuthnResponse"
  addr: string
  services: Service[]
}

The meaning of the fields is as follows.

addr: The flow account address of the user.
services: The list of services provided by the wallet.

`Signable`

interface Signable extends ObjectBase<"1.0.1"> {
  f_type: "Signable"
  addr: string
  keyId: number
  voucher: {
    cadence: string
    refBlock: string
    computeLimit: number
    arguments: {
      type: string
      value: unknown
    }[]
    proposalKey: {
      address: string
      keyId: number
      sequenceNum: number
    }
    payer: string
    authorizers: string[]
  }
}

The WalletUtils.encodeMessageFromSignable function can be used to calculate the message that needs to be signed.

`CompositeSignature`

interface CompositeSignature extends ObjectBase {
  f_type: "CompositeSignature"
  addr: string
  keyId: number
  signature: string
}

`PreSignable`

interface PreSignable extends ObjectBase<"1.0.1"> {
  f_type: "PreSignable"
  roles: {
    proposer: boolean
    authorizer: boolean
    payer: boolean
    param: boolean
  }
  voucher: {
    cadence: string
    refBlock: string
    computeLimit: number
    arguments: {
      type: string
      value: unknown
    }[]
    proposalKey: {
      address: string
      keyId: number
      sequenceNum: number
    }
    payer: string
    authorizers: string[]
  }
}

Miscellaneous Objects

`ErrorResponse`

interface ErrorResponse {
  id: number
  error: {
    code: number
    message: string
  }
}

`Message`

type MessageType =
  | "FCL:VIEW:READY"
  | "FCL:VIEW:READY:RESPONSE"
  | "FCL:VIEW:RESPONSE"
  | "FCL:VIEW:CLOSE"

type Message = {
  type: MessageType
}

A message that indicates the status of the protocol invocation.

This type is sometimes used as part of an intersection type. For example, the type Message & PollingResponse means a PollingResponse extended with the type field from Message.

`ExtensionServiceInitiationMessage`

type ExtensionServiceInitiationMessage = {
  service: Service
}

This object is used to invoke a service when the EXT/RPC service method is used.

Supported Services and Methods

Service Methods

FCL Services are your way as a Wallet Provider of configuring FCL with information about what your wallet can do. FCL uses what it calls Service Methods to perform your supported FCL services. Service Methods are the ways FCL can talk to your wallet. Your wallet gets to decide which of these service methods each of your supported services use to communicate with you.

Sometimes services just configure FCL and that's it. An example of this can be seen with the Authentication Service and the OpenID Service. With those two services you are simply telling FCL "here is a bunch of info about the current user". (You will see that those two services both have a method: "DATA" field in them. Currently these are the only two cases that can be a data service.)

Other services can be a little more complex. For example, they might require a back and forth communication between FCL and the Service in question.

Ultimately we want to do this back and forth via a secure back-channel (https requests to servers), but in some situations that isn't a viable option, so there is also a front-channel option.

Where possible, you should aim to provide a back-channel support for services, and only fall back to a front-channel if absolutely necessary.

Back-channel communications use method: "HTTP/POST", while front-channel communications use method: "IFRAME/RPC", method: "POP/RPC", method: "TAB/RPC and method: "EXT/RPC".

Service Method	Front	Back
HTTP/POST	⛔	✅
IFRAME/RPC	✅	⛔
POP/RPC	✅	⛔
TAB/RPC	✅	⛔
EXT/RPC	✅	⛔

It's important to note that regardless of the method of communication, the data that is sent back and forth between the parties involved is the same.

IFRAME/RPC (Front Channel)

IFRAME/RPC is the easiest to explain, so we will start with it:

An iframe is rendered (comes from the endpoint in the service).
The rendered iframe adds a listener and sends the "FCL:VIEW:READY" message. This can be simplified WalletUtils.ready(callback)
FCL will send the data to be dealt with:
- Where body is the stuff you care about, params and data are additional information you can provide in the service object.
The wallet sends back an "APPROVED" or "DECLINED" post message. (It will be a f_type: "PollingResponse", which we will get to in a bit). This can be simplified using WalletUtils.approve and WalletUtils.decline
- If it's approved, the polling response's data field will need to be what FCL is expecting.
- If it's declined, the polling response's reason field should say why it was declined.

export const WalletUtils.approve = data => {
  sendMsgToFCL("FCL:VIEW:RESPONSE", {
    f_type: "PollingResponse",
    f_vsn: "1.0.0",
    status: "APPROVED",
    reason: null,
    data: data,
  })
}

export const WalletUtils.decline = reason => {
  sendMsgToFCL("FCL:VIEW:RESPONSE", {
    f_type: "PollingResponse",
    f_vsn: "1.0.0",
    status: "DECLINED",
    reason: reason,
    data: null,
  })
}

POP/RPC | TAB/RPC (Front Channel)

POP/RPC and TAB/RPC work in an almost entirely similar way to IFRAME/RPC, except instead of rendering the method in an iframe, we render it in a popup or new tab. The same communication protocol between the rendered view and FCL applies.

HTTP/POST (Back Channel)

HTTP/POST initially sends a post request to the endpoint specified in the service, which should immediately return a f_type: "PollingResponse".

Like IFRAME/RPC, POP/RPC or TAB/RPC, our goal is to eventually get an APPROVED or DECLINED polling response, and technically this endpoint could return one of those immediately.

But more than likely that isn't the case and it will be in a PENDING state (PENDING is not available to IFRAME/RPC, POP/RPC or TAB/RPC). When the polling response is PENDING it requires an updates field that includes a service, BackChannelRpc, that FCL can use to request an updated PollingResponse from. FCL will use that BackChannelRpc to request a new PollingResponse which itself can be APPROVED, DECLINED or PENDING. If it is APPROVED FCL will return, otherwise if it is DECLINED FCL will error. However, if it is PENDING, it will use the BackChannelRpc supplied in the new PollingResponse updates field. It will repeat this cycle until it is either APPROVED or DECLINED.

There is an additional optional feature that HTTP/POST enables in the first PollingResponse that is returned. This optional feature is the ability for FCL to render an iframe, popup or new tab, and it can be triggered by supplying a service type: "VIEW/IFRAME", type: "VIEW/POP" or type: "VIEW/TAB" and the endpoint that the wallet wishes to render in the local field of the PollingResponse. This is a great way for a wallet provider to switch to a webpage if displaying a UI is necessary for the service it is performing.

EXT/RPC (Front Channel)

EXT/RPC is used to enable and communicate between FCL and an installed web browser extension. (Though this specification is geared towards Chromium based browsers, it should be implementable in any browser with similar extension APIs available. From now on we will be using the word Chrome to refer to Chromium based browsers.)

An implementation of EXT/RPC needs to somehow enable communication between the application and the extension context. Implementing this is a bit more complex and usually relies on 3 key scripts to allow message passing between an installed extension and FCL. The separation of contexts enforced by Chrome and the availability of different Chrome APIs within those contexts require these scripts to be set up in a particular sequence so that the communication channels needed by FCL's EXT/RPC service method will work.

The following is an overview of these scripts and the functionality they need to support FCL:

background.js: Used to launch the extension popup with chrome.windows.create if selected by the user from Discovery or set directly via fcl.config.discovery.wallet
content.js: Used to proxy messages between the application to the extension via chrome.runtime.sendMessage.
script.js: Injected by content.js into the application's HTML page. It appends the extension authn service to the window.fcl_extensions array on page load. This allows FCL to confirm installation and send extension details to Discovery or launch your wallet as the default wallet.

An example and guide showing how to build an FCL compatible wallet extension on Flow can be found here.

Once the extension is enabled (for example when the user selects it through the discovery service), the following communication protocol applies. (The term send should specifically refer to using window.postMessage in the application context, as this is the only interface between the application and the extension. Note that since window.postMessage broadcasts messages to all message event handlers, care should be taken by each party to filter only the messages targeted at them.)

An ExtensionServiceInitiationMessage object is sent by FCL. It is the extension's responsibility to inspect the endpoint field of the service, and only activate itself (e.g. by opening a popup) if it is the provider of this service.
The extension should respond by sending a Message with type FCL:VIEW:READY. (Usually this message will originate from the extension popup, and be relayed to the application context.)
FCL will send a Message with type FCL:VIEW:READY:RESPONSE. Additional fields specific to the service (such as body, params or data) are usually present. See the section on the specific service for a description of these fields.
The wallet sends back a Message & PollingResponse with type FCL:VIEW:RESPONSE with either an APPROVED or DECLINED status.
- If it's approved, the polling response's data field will need to be what FCL is expecting.
- If it's declined, the polling response's reason field should say why it was declined.

The extension can send a Message with type FCL:VIEW:CLOSE at any point during this protocol to indicate an interruption. This will halt FCL's current routine. On the other hand, once a PollingResponse with either an APPROVED or DECLINED status was sent, the protocol is considered finished, and the extension should not send any further messages as part of this exchange.

Conversely, when FCL sends a new ExtensionServiceInitiationMessage, the previous routine is interrupted. (This is the case even when the new service invocation is targeted at a different extension.)

Note that as a consequence of the above restrictions, only single service invocation can be in progress at a time.

Here is a code example for how an extension popup might send its response:

chrome.tabs.sendMessage(tabs[0].id, {
  f_type: "PollingResponse",
  f_vsn: "1.0.0",
  status: "APPROVED",
  reason: null,
  data: {
    f_type: "AuthnResponse",
    f_vsn: "1.0.0",
    addr: address,
    services: services,
  },
})

Service Method Plugins

TODO

Service Types

Here we explore ways in which you can integrate with FCL by providing implementations of various FCL services.

The following services will be covered:

Service	type
Authentication Service	`authn`
Pre-Authz Service	`pre-authz`
Authorization Service	`authz`
User Signature Service	`user-signature`
Authentication Refresh Service	`authn-refresh`
Authentication Proof Service	`account-proof`

Authentication Service `authn`

In the following examples, we'll walk you through the process of building an authentication service.

In FCL, wallets are configured by passing in a wallet provider's authentication URL or extension endpoint as the discovery.wallet config variable.

You will need to make and expose a webpage or API hosted at an authentication endpoint that FCL will use.

// IN APPLICATION
// configuring fcl to point at a wallet looks like this
import { config } from "@onflow/fcl"

config({
  "discovery.wallet": "url-or-endpoint-fcl-will-use-for-authentication", // FCL Discovery endpoint, wallet provider's authentication URL or extension endpoint
  "discovery.wallet.method": "IFRAME/RPC", // Optional. Available methods are "IFRAME/RPC", "POP/RPC", "TAB/RPC", "EXT/RPC" or "HTTP/POST", defaults to "IFRAME/RPC".
})

If the method specified is IFRAME/RPC, POP/RPC or TAB/RPC, then the URL specified as discovery.wallet will be rendered as a webpage. If the configured method is EXT/RPC, discovery.wallet should be set to the extension's authn endpoint. Otherwise, if the method specified is HTTP/POST, then the authentication process will happen over HTTP requests. (While authentication can be accomplished using any of those service methods, this example will use the IFRAME/RPC service method.)

Once the Authentication webpage is rendered, the extension popup is enabled, or the API is ready, you then need to tell FCL that it is ready. You will do this by sending a message to FCL, and FCL will send back a message with some additional information that you can use about the application requesting authentication on behalf of the user.

The following example is using the IFRAME/RPC method. Your authentication webpage will likely resemble the following code:

// IN WALLET AUTHENTICATION FRAME
import {WalletUtils} from "@onflow/fcl"

function callback(data) {
  if (typeof data != "object") return
  if (data.type !== "FCL:VIEW:READY:RESPONSE") return

  ... // Do authentication things ...

  // Send back AuthnResponse
  WalletUtils.sendMsgToFCL("FCL:VIEW:RESPONSE", {
    f_type: "PollingResponse",
    f_vsn: "1.0.0",
    status: "APPROVED",
    data: {
      f_type: "AuthnResponse",
      f_vsn: "1.0.0"
      ...
    }
  })

  // Alternatively be sent using WalletUtils.approve (or WalletUtils.decline)
  // which will wrap AuthnResponse in a PollingResponse
  WalletUtils.approve({
    f_type: "AuthnResponse",
    f_vsn: "1.0.0"
    ...
  })
}
// add event listener first
WalletUtils.onMsgFromFCL("FCL:VIEW:READY:RESPONSE", callback)

// tell fcl the wallet is ready
WalletUtils.sendMsgToFCL("FCL:VIEW:READY")

// alternatively adds "FCL:VIEW:READY:RESPONSE" listener and sends "FCL:VIEW:READY"
WalletUtils.ready(callback)

During authentication, the application has a chance to request to you what they would like you to send back to them. These requests are included in the FCL:VIEW:READY:RESPONSE message sent to the wallet from FCL.

An example of such a request is the OpenID service. The application can request for example that you to send them the email address of the current user. The application requesting this information does not mean you need to send it. It's entirely optional for you to do so. However, some applications may depend on you sending the requested information back, and should you decline to do so it may cause the application to not work.

In the config they can also tell you a variety of things about them, such as the name of their application or a url for an icon of their application. You can use these pieces of information to customize your wallet's user experience should you desire to do so.

Your wallet having a visual distinction from the application, but still a seamless and connected experience is our goal here.

Whether your authentication process happens using a webpage with the IFRAME/RPC, POP/RPC or TAB/RPC methods, via an enabled extension using the EXT/RPC method, or using a backchannel to an API with the HTTP/POST method, the handshake is the same. The same messages are sent in all methods, however the transport mechanism changes. For IFRAME/RPC, POP/RPC, TAB/RPC or EXT/RPC methods, the transport is window.postMessage(), with the HTTP/POST method, the transport is HTTP post messages.

As always, you must never trust anything you receive from an application. Always do your due-diligence and be alert as you are the user's first line of defense against potentially malicious applications.

Authenticate your User

It's important that you are confident that the user is who the user claims to be.

Have them provide enough proof to you that you are okay with passing their details back to FCL. Using Blocto as an example, an authentication code is sent to the email a user enters at login. This code can be used as validation and is everything Blocto needs to be confident in the user's identity.

Once you know who your User is

Once you're confident in the user's identity, we can complete the authentication process.

The authentication process is complete once FCL receives back a response that configures FCL with FCL Services for the current user. This response is extremely important to FCL. At its core it tells FCL who the user is, and then via the included services it tells FCL how the user authenticated, how to request transaction signatures, how to get a personal message signed and the user's email and other details if requested. In the future it may also include many more things!

You can kind of think of FCL as a plugin system. But since those plugins exist elsewhere outside of FCL, FCL needs to be configured with information on how to communicate with them.

What you are sending back to FCL is everything that it needs to communicate with the plugins that you are supplying. Your wallet is like a plugin to FCL, and these details tell FCL how to use you as a plugin.

Here is an example of an authentication response:

// IN WALLET AUTHENTICATION FRAME
import {WalletUtils} from "@onflow/fcl"

WalletUtils.approve({
  f_type: "AuthnResponse",
  f_vsn: "1.0.0",
  addr: "0xUSER",                      // The user's flow address

  services: [                          // All the stuff that configures FCL

      // Authentication Service - REQUIRED
      {
          f_type: "Service",                                         // It's a service!
          f_vsn: "1.0.0",                                            // Follows the v1.0.0 spec for the service
          type: "authn",                                             // the type of service it is
          method: "DATA",                                            // It's data!
          uid: "amazing-wallet#authn",                               // A unique identifier for the service
          endpoint: "your-url-that-fcl-will-use-for-authentication", // should be the same as was passed into the config
          id: "0xUSER",                                              // the wallet's internal id for the user, use flow address if you don't have one
          // The User's Info
          identity: {
              f_type: "Identity",  // It's an Identity!
              f_vsn: "1.0.0",      // Follows the v1.0.0 spec for an identity
              address: "0xUSER",   // The user's address
              keyId: 0,            // OPTIONAL - The User's KeyId they will use
          },
          // The Wallet's Info
          provider: {
              f_type: "ServiceProvider",      // It's a Service Provider
              f_vsn: "1.0.0",                 // Follows the v1.0.0 spec for service providers
              address: "0xWallet",            // A flow address owned by the wallet
              name: "Amazing Wallet",         // OPTIONAL - The name of your wallet. ie: "Dapper Wallet" or "Blocto Wallet"
              description: "The best wallet", // OPTIONAL - A short description for your wallet
              icon: "https://___",            // OPTIONAL - Image url for your wallet's icon
              website: "https://___",         // OPTIONAL - Your wallet's website
              supportUrl: "https://___",      // OPTIONAL - An url the user can use to get support from you
              supportEmail: "help@aw.com",    // OPTIONAL - An email the user can use to get support from you
          },
      },

      // Authorization Service
      {
          f_type: "Service",
          f_vsn: "1.0.0",
          type: "authz",
          uid: "amazing-wallet#authz",
          ...
          // We will cover this at length in the authorization section of this guide
      },

      // User Signature Service
      {
          f_type: "Service",
          f_vsn: "1.0.0",
          type: "user-signature",
          uid: "amazing-wallet#user-signature",
          ...
          // We will cover this at length in the user signature section of this guide
      },

      // OpenID Service
      {
          f_type: "Service",
          f_vsn: "1.0.0",
          type: "open-id",
          uid: "amazing-wallet#open-id",
          method: "DATA",
          data: { // only include data that was request, ideally only if the user approves the sharing of data, everything is optional
              f_type: "OpenID",
              f_vsn: "1.0.0",
              profile: {
                  name: "Jeff",
                  family_name: "D", // icky underscored names because of OpenID Connect spec
                  given_name: "Jeffrey",
                  middle_name: "FakeMiddleName",
                  nickname: "JeffJeff",
                  preferred_username: "Jeff",
                  profile: "https://www.jeff.jeff/",
                  picture: "https://avatars.onflow.org/avatar/jeff",
                  website: "https://www.jeff.jeff/",
                  gender: "male",
                  birthday: "1900-01-01", // can use 0000 for year if year is not known
                  zoneinfo: "America/Vancouver",
                  locale: "en",
                  updated_at: "1625588304427"
              },
              email: {
                  email: "jeff@jeff.jeff",
                  email_verified: false,
              }
          },
      }
  ]
})

Stopping an Authentication Process

From any frame, you can send a FCL:VIEW:CLOSE post message to FCL, which will halt FCL's current routine and close the frame.

import { WalletUtils } from "@onflow/fcl"

WalletUtils.sendMsgToFCL("FCL:VIEW:CLOSE")

Authorization Service `authz`

Authorization services are depicted with with a type: "authz", and a method of either HTTP/POST, IFRAME/RPC, POP/RPC, TAB/RPC or EXT/RPC. They are expected to eventually return a f_type: "CompositeSignature".

An authorization service is expected to know the Account and the Key that will be used to sign the transaction at the time the service is sent to FCL (during authentication).

{
  f_type: "Service",
  f_vsn: "1.0.0",
  type: "authz",               // say it's an authorization service
  uid: "amazing-wallet#authz", // standard service uid
  method: "HTTP/POST",         // can also be `IFRAME/RPC` or `POP/RPC`
  endpoint: "https://____",    // where to talk to the service
  identity: {
    f_type: "Identity",
    f_vsn: "1.0.0",
    address: "0xUser",         // the address that the signature will be for
    keyId: 0,                  // the key for the address that the signature will be for
  },
  data: {},
  params: {},
}

FCL will use the method provided to request an array of composite signature from authorization service (Wrapped in a PollingResponse). The authorization service will be sent a Signable. The service is expected to construct an encoded message to sign from Signable.voucher. It then needs to hash the encoded message, and prepend a required transaction domain tag. Finally it signs the payload with the user/s keys, producing a signature. This signature, as a HEX string, is sent back to FCL as part of the CompositeSignature which includes the user address and keyID in the data property of a PollingResponse.

signature =
  signable.voucher
    |> encode
    |> hash
    |> tag
    |> sign
    |> convert_to_hex

The eventual response back from the authorization service should resolve to something like this:

{
  f_type: "PollingResponse",
  f_vsn: "1.0.0",
  status: "APPROVED",
  data: {
    f_type: "CompositeSignature",
    f_vsn: "1.0.0",
    addr: "0xUSER",
    keyId: 0,
    signature: "signature as hex value"
  }
}

A CompositeSignature can alternatively be constructed using WalletUtils

import {WalletUtils} from "@onflow/fcl"

WalletUtils.CompositeSignature(addr: String, keyId: Number, signature: Hex)

User Signature Service `user-signature`

User Signature services are depicted with a type: "user-signature" and a method of either HTTP/POST, IFRAME/RPC, POP/RPC, TAB/RPC or EXT/RPC. They are expected to eventually return an array of f_type: "CompositeSignature".

The User Signature service is a stock/standard service.

{
  f_type: "Service",
  f_vsn: "1.0.0",
  type: "user-signature",               // say it's an user-signature service
  uid: "amazing-wallet#user-signature", // standard service uid
  method: "HTTP/POST",                  // can also be `IFRAME/RPC`
  endpoint: "https://___",              // where to talk to the service
  data: {},
  params: {},
}

FCL will use the method provided to request an array of composite signatures from the user signature service (Wrapped in a PollingResponse). The user signature service will be sent a Signable. The service is expected to tag the Signable.message and then sign it with enough keys to produce a full weight. The signatures need to be sent back to FCL as HEX strings in an array of CompositeSignatures.

// Pseudocode:
// For every required signature
import { WalletUtils } from "@onflow/fcl"

const encoded = WalletUtils.encodeMessageFromSignable(signable, signerAddress)
const taggedMessage = tagMessage(encoded) // Tag the message to sign
const signature = signMessage(taggedMessage) // Sign the message
const hexSignature = signatureToHex(signature) // Convert the signature to hex, if required.

return hexSignature

The eventual response back from the user signature service should resolve to something like this:

{
  f_type: "PollingResponse",
  f_vsn: "1.0.0",
  status: "APPROVED",
  data: [
    {
      f_type: "CompositeSignature",
      f_vsn: "1.0.0",
      addr: "0xUSER",
      keyId: 0,
      signature: "signature as hex value"
    },
    {
      f_type: "CompositeSignature",
      f_vsn: "1.0.0",
      addr: "0xUSER",
      keyId: 1,
      signature: "signature as hex value"
    }
  ]
}

PreAuthz Service `pre-authz`

This is a strange one, but extremely powerful. This service should be used when a wallet is responsible for an account that's signing as multiple roles of a transaction, and wants the ability to change the accounts on a per role basis.

Pre Authz Services are depicted with a type: "pre-authz" and a method of either HTTP/POST, IFRAME/RPC, POP/RPC, TAB/RPC or EXT/RPC. They are expected to eventually return a f_type: "PreAuthzResponse".

The Pre Authz Service is a stock/standard service.

{
  f_type: "Service",
  f_vsn: "1.0.0",
  type: "pre-authz",               // say it's a pre-authz service
  uid: "amazing-wallet#pre-authz", // standard service uid
  method: "HTTP/POST",             // can also be IFRAME/RPC, POP/RPC, TAB/RPC
  endpoint: "https://___",         // where to talk to the service
  data: {},
  params: {},
}

FCL will use the method provided to request a PreAuthzReponse (Wrapped in a PollingResponse). The Authorizations service will be sent a PreSignable. The pre-authz service is expected to look at the PreSignable and determine the breakdown of accounts to be used. The pre-authz service is expected to return Authz services for each role it is responsible for. A pre-authz service can only supply roles it is responsible for. If a pre-authz service is responsible for multiple roles, but it wants the same account to be responsible for all the roles, it will need to supply an Authz service per role.

The eventual response back from the pre-authz service should resolve to something like this:

{
  f_type: "PollingResponse",
  f_vsn: "1.0.0",
  status: "APPROVED",
  data: {
    f_type: "PreAuthzResponse",
    f_vsn: "1.0.0",
    proposer: {              // A single Authz Service
      f_type: "Service",
      f_vsn: "1.0.0",
      type: "authz",
      ...
    },
    payer: [                // An array of Authz Services
      {
        f_type: "Service",
        f_vsn: "1.0.0",
        type: "authz",
        ...
      }
    ],
    authorization: [       // An array of Authz Services (it's singular because it only represents a singular authorization)
      {
        f_type: "Service",
        f_vsn: "1.0.0",
        type: "authz",
        ...
      }
    ],
  }
}

Authentication Refresh Service `authn-refresh`

Since synchronization of a user's session is important to provide a seamless user experience when using an app and transacting with the Flow Blockchain, a way to confirm, extend, and refresh a user session can be provided by the wallet.

Authentication Refresh Services should include a type: "authn-refresh", endpoint, and supported method (HTTP/POST, IFRAME/RPC, POP/RPC, or EXT/RPC).

FCL will use the endpoint and service method provided to request updated authentication data. The authn-refresh service should refresh the user's session if necessary and return updated authentication configuration and user session data.

The service is expected to return a PollingResponse with a new AuthnResponse as data. If user input is required, a PENDING PollingResponse can be returned with a local view for approval/re-submission of user details.

The Authentication Refresh Service is a stock/standard service.

  {
    "f_type": "Service",
    "f_vsn": "1.0.0",
    "type": "authn-refresh",
    "uid": "uniqueDedupeKey",
    "endpoint": "https://rawr",
    "method": "HTTP/POST",  // "HTTP/POST", // HTTP/POST | IFRAME/RPC | HTTP/RPC
    "id": "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx", // wallet's internal id for the user
    "data": {}, // included in body of request
    "params": {}, // included as query params on endpoint url
  }

The provided data and params should include all the wallet needs to identify and re-authenticate the user if necessary.

The eventual response back from the authn-refresh service should resolve to an AuthnResponse and look something like this:

{
  f_type: "PollingResponse",
  f_vsn: "1.0.0",
  status: "APPROVED",
  data: {
    f_type: "AuthnResponse",
    f_vsn: "1.0.0",
    addr: "0xUSER",
    services: [
      // Authentication Service - REQUIRED
      {
        f_type: "Service",
        f_vsn: "1.0.0",
        type: "authn",
        ...
      },
      // Authorization Service
      {
        f_type: "Service",
        f_vsn: "1.0.0",
        type: "authz",
        ...
      },
      // Authentication Refresh Service
      {
        f_type: "Service",
        f_vsn: "1.0.0",
        type: "authn-refresh",
        ...
      }
      // Additional Services
    ],
  }
}

Authentication Proof Service `account-proof`

An application may wish to prove that their current user controls the account authenticated with. To do so, the application can supply a app identifier (string), and a nonce (hex string), which can be used by the wallet to return an optional account-proof service as part of user authentication. The application can use this service to verify that the current user is in control of the account they authenticated with.

{
  f_type: "Service",                       // Its a service!
  f_vsn: "1.0.0",                          // Follows the v1.0.0 spec for the service
  type: "account-proof",                   // the type of service it is
  method: "DATA",                          // Its data!
  uid: "awesome-wallet#account-proof",     // A unique identifier for the service
  data: {
    f_type: "account-proof",
    f_vsn: "1.0.0"
    // The user's address (8 bytes, i.e 16 hex characters)
    address: "0xf8d6e0586b0a20c7",
    // Nonce signed by the current account-proof (minimum 32 bytes in total, i.e 64 hex characters)
    nonce: "75f8587e5bd5f9dcc9909d0dae1f0ac5814458b2ae129620502cb936fde7120a",
    signatures: [CompositeSignature],
  }
}

OpenIDService

TODO

FCL allows wallet to act as identity provider
Sign message as identity token
provide verifiable user claims

Service `data` and `params`

data and params are information that the wallet can provide in the service config that FCL will pass back to the service.

params will be added onto the endpoint as query params.
data will be included in the body of the HTTP/POST request or in the FCL:VIEW:READY:RESPONSE for a IFRAME/RPC, POP/RPC, TAB/RPC or EXT/RPC.

Integrating with FCL Discovery

TODO

When building an dApp on Flow using @onflow/fcl, Discovery eliminates the need for dApp developers to write code to integrate their user's preferred wallet into their application. Instead, @onflow/fcl and this repo uses a secure discovery protocol that wallets can implement to connect to @onflow/fcl. The end result is dApp using @onflow/fcl automatically integrate all compatible wallets without their developers needing to write any custom code!

To read more about consuming/using this repo via FCL, visit the Discovery docs.

Wallet Compliance Guide

Dependencies

This proposal adds no new dependencies to Flow and is backwards compatible with existing implementations.

Files

20221108-fcl-specification.md

Latest commit

History

20221108-fcl-specification.md

File metadata and controls

FLIP 45: Flow Client Library (FCL) Specification

Objective

Motivation

User Benefit

Design Proposal

Flow Client Library (FCL) Specification v1.0

Table of Contents

Abstract

Background / Overview

Specification

Definitions

Data Types

Data Structures

FCL Objects

Service

Service Object Example

PollingResponse

Identity

ServiceProvider

AuthnResponse

Signable

CompositeSignature

PreSignable

Miscellaneous Objects

ErrorResponse

Message

ExtensionServiceInitiationMessage

open-id

frame

local-view

Supported Services and Methods

Service Methods

IFRAME/RPC (Front Channel)

POP/RPC | TAB/RPC (Front Channel)

HTTP/POST (Back Channel)

EXT/RPC (Front Channel)

Service Method Plugins

Service Types

Authentication Service authn

Authenticate your User

Once you know who your User is

Stopping an Authentication Process

Authorization Service authz

User Signature Service user-signature

PreAuthz Service pre-authz

Authentication Refresh Service authn-refresh

Authentication Proof Service account-proof

OpenIDService

Service data and params

Integrating with FCL Discovery

Dependencies

Engineering Impact

Best Practices

Tutorials and Examples

Compatibility

User Impact

Related Issues

Prior Art

Questions and Discussion Topics

`Service`

`PollingResponse`

`Identity`

`ServiceProvider`

`AuthnResponse`

`Signable`

`CompositeSignature`

`PreSignable`

`ErrorResponse`

`Message`

`ExtensionServiceInitiationMessage`

`open-id`

`frame`

`local-view`

Authentication Service `authn`

Authorization Service `authz`

User Signature Service `user-signature`

PreAuthz Service `pre-authz`

Authentication Refresh Service `authn-refresh`

Authentication Proof Service `account-proof`

Service `data` and `params`