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Provable Authn

In order to improve UX/DX and encourage seamless integration with App backends and services, fcl.authenticate has been upgraded.

Additional data is sent in the body of FCL:VIEW:READY:RESPONSE. This data includes what the wallet needs to build a message for signing with the user’s private key/s. The signature can be returned as part of an optional account-proof service with the FCL:VIEW:RESPONSE.

When provided by the wallet, this signature and additional account-proof data is available to the App via fcl.currentUser services. The service data can be used to recreate the message, and verify the signature on the Flow Blockchain.

For example, it can be sent to the App’s backend and after validating the signature and the other account-proof data, it can safely associate the included account address to a user and log them in.


TL;DR Wallet Provider

  1. Wallet receives Authn FCL:VIEW:READY:RESPONSE request and parses out the appIdentifier, and nonce.
  2. The wallet authenticates the user however they choose to do, and determines the user's account address
  3. Wallet prepares and signs the message:
    • Encodes the appIdentifier, nonce, and address along with the "FCL-ACCOUNT-PROOF-V0.0" domain separation tag, using the encoding scheme described below.
    • Signs the message with the signatureAlgorithm and hashAlgorithm specified on user's key. It is highly recommended that the wallet display the message data and receive user approval before signing.
  4. Wallet sends back this new service and data along with the other service configuration when completing Authn.

Account Proof Message Encoding

The account proof message is encoded as follows:

MESSAGE = 
  USER_DOMAIN_TAG ||
  RLP_ENCODE([
    APP_IDENTIFIER, 
    ADDRESS, 
    NONCE
  ])

with the following values:

  • ACCOUNT_PROOF_DOMAIN_TAG is the constant "FCL-ACCOUNT-PROOF-V0.0", encoded as UTF-8 byte array and right-padded with zero bytes to a length of 32 bytes.
  • APP_IDENTIFIER is an arbitrary length string.
  • ADDRESS is a byte array containing the address bytes, left-padded with zero bytes to a length of 8 bytes.
  • NONCE is an byte array with a minimum length of 32 bytes.

RLP_ENCODE is a function that performs RLP encoding and returns the encoded value as bytes.

JavaScript Signing Example

// Using WalletUtils
import {WalletUtils} from "@onflow/fcl"

const message = WalletUtils.encodeAccountProof(
  appIdentifier, // A human readable string to identify your application during signing
  address,       // Flow address of the user authenticating
  nonce,         // minimum 32-btye nonce
)

sign(privateKey, message)

// Without using FCL WalletUtils
const ACCOUNT_PROOF_DOMAIN_TAG = rightPaddedHexBuffer(
  Buffer.from("FCL-ACCOUNT-PROOF-V0.0").toString("hex"),
  32
)
const message =  rlp([appIdentifier, address, nonce])
const prependUserDomainTag = (message) => ACCOUNT_PROOF_DOMAIN_TAG + message

sign(privateKey, prependUserDomainTag(message))
// Authentication Proof Service
{
  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],
  }
}