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Understand Blockstack authentication

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Blockstack Auth provides single sign on and authentication without third parties or remote servers. On this page, you'll get an overview of authentication from an developer and user perspective.

User experience flow

Blockstack Auth is a bearer token-based authentication system. From an application user's perspective, Blockstack authentication is similar to legacy third-party authentication techniques that they're familiar with. Applications present users with a Sign in with Blockstack button.

Assume a user, Alice, clicks the Sign in with Blockstack button on an app. She is redirected to her copy of the Blockstack Browser. If the user has signed into the DApp previously. The actual Blockstack sign-in dialog depends on whether the user already has an existing session in the Blockstack Browser.

Alice can choose to authenticate as one of her Blockstack IDs by selecting the ID and clicking the Approve button. The Blockstack Browser shows Alice an approval dialog with information about your app including:

The origin your app was served from
Your app's name
Your app's logo
The types of permissions and data your app is requesting

Signing in with an identity is the means by which the user grants the DApp access. Access depends on the scope requested by the DApp. The default store_write scope allows the DApp to read the user profile and read/write user data for the DApp. Data is encrypted at a unique URL on a Gaia storage hub.

When she clicks approve, Alice is redirected back to the DApp where she is logged in.

Application-authentication workflow

For an application developer, the application flow is different from the typical client-server flow used by centralized sign in services (e.g., OAuth). Rather, with Blockstack, the authentication flow happens entirely client-side.

A decentralized application (DApp) and the Blockstack Browser communicate during the authentication flow by passing back and forth two tokens. The requesting application sends the Blockstack Browser an authRequest token. Once a user approves a sign-in, the Blockstack Browser responds to the application with an authResponse token. These tokens are JSON Web Tokens, and they are passed via URL query strings.

When a user chooses to Sign in with Blockstack on a DApp, calls the redirectToSignIn() method which sends the user to the Blockstack Browser. When Blockstack Browser is provided an ID, it generates an The browser responds with an authentication token and an app private key.

The app private key is application-specific. It is generated from the user's identity address private key using the appDomain as input. The key is ephemeral, it is generated for each execution of a key establishment process. This key is just used for the particular instance of the application, in this case to sign a sign-in request.

This app private key is also deterministic, meaning that for a given Blockstack ID and domain name, the same private key is generated each time. The app private key is securely shared with the app on each authentication and encrypted by the Blockstack Browser. The key serves three functions, it:

is used to create the credentials that give an app access to the Gaia hub storage bucket for that specific app
is used in the end-to-end encryption of files stored for the app on the user's Gaia hub
serves as a cryptographic secret that apps can use to perform other cryptographic functions

A Stacks node also generates a public key token which is sent to the browser as an authRequest from the browser to the core node. The signed authentication request is sent to Blockstack through a JSON Web Token (JWT). Blocktack passes the token in via a URL query string in the authRequest parameter:

https://browser.blockstack.org/auth?authRequest=j902120cn829n1jnvoa...

When the Blockstack node receives the authRequest, it generates a session token and returns an authentication response (authResponse) to the application. Similar to the authRequest, the authResponse token includes a private key intended only for the application. This allows the application to encrypt data on user's personal Blockstack storage.

{% include note.html content="The Blockstack JWT implementation is different from other implementations because of the underlying cryptography we employ. There are libraries in Javascript and Ruby available on the Blockstack Github to allow you to work with these tokens." %}

Manifest file

Blockstack apps have a manifest file. This file is based on the W3C web app manifest specification. The following is an example manifest file.

{
  "name": "Todo App",
  "start_url": "http://blockstack-todos.appartisan.com",
  "description": "A simple todo app build on blockstack",
  "icons": [{
    "src": "http://blockstack-todos.appartisan.com/logo.png",
    "sizes": "400x400",
    "type": "image/png"
  }]
}

The Blockstack Browser retrieves the manifest file from the app during the authentication process and displays some of the information in it such as the app name and icon to the user. The location of the app manifest file is specific in the authentication request token and must be on the same origin as the app requesting authentication.

The manifest file must have Cross-origin resource sharing (CORS) headers that allow the manifest file to be fetched from any arbitrary source. This usually means returning a header like this:

Access-Control-Allow-Origin: *

How you implement cors depends in part on which platform/service you use to serve your application. For example, Netlify and Firebase have two different ways of configuring CORS. Consult your vendor documentation for more information.

Key pairs

Blockstack Auth makes extensive use of public key cryptography. Blockstack uses ECDSA with the secp256k1 curve. The following sections describe the various public-private key pairs used in the authentication process including:

how they're generated,
where they're used
to whom the private key is disclosed.

Transit private key

The transit private is an ephemeral key that is used to encrypt secrets that need to be passed from the Blockstack Browser to the app during the authentication process. It is randomly generated by the app at the beginning of the authentication response.

The public key that corresponds to the transit private key is stored in a single element array in the public_keys key of the authentication request token. The Blockstack Browser encrypts secret data such as the app private key using this public key and sends it back to the app when the user signs in to the app. The transit private key signs the app authentication request.

Blockstack ID Identity address private key

The identity address private key is derived from the user's keychain phrase and is the private key of the Blockstack ID that the user chooses to use to sign in to the app. It is a secret owned by the user and never leaves the user's instance of the Blockstack Browser. This private key signs the authentication response token for an app to indicate that the user approves sign in to that app.

App private key

The app private key is an app-specific private key that is generated from the user's identity address private key using the domain_name as input. It is deterministic in that for a given Blockstack ID and domain_name, the same private key will be generated each time. The app private key is securely shared with the app on each authentication, encrypted by the Blockstack Browser with the transit public key.

Scopes

Scopes define the information and permissions an app requests from the user during authentication. Requested scopes may be any of the following:

Scope	Definition
`store_write`	Read and write data to the user's Gaia hub in an app-specific storage bucket.
`publish_data`	Publish data so that other users of the app can discover and interact with the user.
`email`	Requests the user's email if available.

If no scopes array is provided to the redirectToSignIn or makeAuthRequest functions, the default is to request ['store_write'].

Authentication tokens

The app and the Blockstack Browser communicate during the authentication flow by passing back and forth two tokens, the authRequest and the authResponse token. The requesting application sends the Blockstack Browser an authRequest token. Once a user approves a sign in, the Blockstack Browser responds to the application with an authResponse token.

These tokens are JSON Web Tokens, and they are passed via URL query strings.

JSON Web Token signatures

Blockstack's authentication tokens are based on the RFC 7519 OAuth JSON Web Token (JWT) with additional support for the secp256k1 curve used by Bitcoin and many other cryptocurrencies.

This signature algorithm is indicated by specifying ES256K in the token's alg key, specifying that the JWT signature uses ECDSA with the secp256k1 curve. We provide both JavaScript and Ruby JWT libraries with support for this signing algorithm.

Authentication request payload schema

const requestPayload = {
    jti, // UUID
    iat, // JWT creation time in seconds
    exp, // JWT expiration time in seconds
    iss, // legacy decentralized identifier generated from transit key
    public_keys, // single entry array with public key of transit key
    domain_name, // app origin
    manifest_uri, // url to manifest file - must be hosted on app origin
    redirect_uri, // url to which browser redirects user on auth approval - must be hosted on app origin
    version, // version tuple
    do_not_include_profile, // a boolean flag asking browser to send profile url instead of profile object
    supports_hub_url, // a boolean flag indicating gaia hub support
    scopes // an array of string values indicating scopes requested by the app
  }

Authentication response payload schema

    const responsePayload = {
    jti, // UUID
    iat, // JWT creation time in seconds
    exp, // JWT expiration time in seconds
    iss, // legacy decentralized identifier (string prefix + identity address) - this uniquely identifies the user
    private_key, // encrypted private key payload
    public_keys, // single entry array with public key
    profile, // profile object or null if passed by profile_url
    username, // blockstack id username (if any)
    core_token, // encrypted core token payload
    email, // email if email scope is requested & email available
    profile_url, // url to signed profile token
    hubUrl, // url pointing to user's gaia hub
    version // version tuple
  }

`blockstack:` custom protocol handler

The blockstack: custom protocol handler is how Blockstack apps send their authentication requests to the Blockstack Browser. When the Blockstack Browser is installed on a user's computer, it registers itself as the handler for the blockstack: customer protocol.

When an application calls redirectToSignIn or redirectToSignInWithAuthRequest, blockstack.js checks if a blockstack: protocol handler is installed and, if so, redirects the user to blockstack:<authRequestToken>. This passes the authentication request token from the app to the Blockstack Browser, which will in turn validate the request and display an authentication dialog.

Adding Blockstack Authentication to your app

The way you can add Blockstack Authentication to you app depends on whether your app is a modern decentralized Blockstack App where code runs client-side without trusted servers or a legacy client-server app where a server is trusted.

Authentication in Client-side apps

This method is appropriate for decentralized client-side apps where the user's zone of trust - the parts of the app that the user is trusting - begins and ends with the code running on their own computer. In apps like these, any code the app interacts with that's not on their own computer such as external servers does not need to know who she is.

Blockstack.js provides API methods that help you to implement Blockstack Authentication in your client-side app.

Standard flow

The preferred way to implement authentication in these apps is to use the standard flow. This flow hides much of the process behind a few easy function calls and makes it very fast to get up and running.

In this process you'll use these four functions:

When your app wants to start the sign in process, typically when the user clicks a "Sign in with Blockstack" button, your app will call the redirectToSignIn method of blockstack.js.

This creates an ephemeral transit key, stores it in the web browser's localStorage, uses it to create an authentication request token and finally redirects the user to the Blockstack Browser to approve the sign in request.

Handling an authentication response

When a user approves a sign in request, the Blockstack Browser will return the signed authentication response token to the redirectURI specified in redirectToSignIn.

To check for the presence of this token, your app should call isSignInPending. If this returns true, the app should then call handlePendingSignIn. This decodes the token, returns the signed-in-user's data, and simultaneously storing it to localStorage so that it can be retrieved later with loadUserData.

import * as blockstack from 'blockstack'

if (blockstack.UserSession.isSignInPending()) {
    blockstack.UserSession.handlePendingSignIn()
    .then(userData => {
        const profile = userData.profile
    })
}

Manual flow

Alternatively, you can manually generate your own transit private key and/or authentication request token. This gives you more control over the experience.

For example, you could use the following code to generate an authentication request on https://alice.example.com or https://bob.example.com for an app running on origin https://example.com.


const transitPrivateKey = generateAndStoreTransitKey()
const redirectURI = 'https://example.com/authLandingPage'
const manifestURI = 'https://example.com/manifest.json'
const scopes = ['scope_write', 'publish_data']
const appDomain = 'https://example.com'

const authRequest = makeAuthRequest(transitPrivateKey, redirectURI, manifestURI, scopes, appDomain)

redirectToSignInWithAuthRequest(authRequest)

Authentication in client-server apps

Note: Client-server authentication requires using a library written in the language of your server app. There are private methods in blockstack.js that can be accomplish this on node.js server apps, but they are not currently part of our public, supported API.

Using Blockstack Authentication in client-server apps is very similar to client-side apps. You generate the authentication request using the same code in the client as described above.

The main difference is that you need to verify the authentication response token on the server after the user approves sign in to your app.

For an example of how verification can be done server side, take a look at the blockstack-ruby library.

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