Authentication

Blockstack Auth provides single sign on and authentication without third parties or remote servers.

Authentication flow

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

A decentralized application and the Blockstack App communicate during the authentication flow by passing back and forth two tokens. The requesting application sends the Blockstack App an authRequest token. Once a user approves a sign-in, the Blockstack App 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 authenticate a decentralized application, it calls the doOpenAuth() method which sends an authRequest to the Blockstack App. Blockstack passes the token in via a URL query string in the authRequest parameter:

https://app.blockstack.org/#/sign-up?authRequest=j902120cn829n1jnvoa...

When the Blockstack App receives the request, it generates an (authResponse) token to the application using an ephemeral transit key . The ephemeral transit key is just used for the particular instance of the application, in this case, to sign the authRequest. The application stores the ephemeral transit key during the request generation. The public portion of the transit key is passed in the authRequest token. The Blockstack App uses the public portion of the key to encrypt an app-private key which is returned via the authResponse.

During sign in, the Blockstack App generates the app-private key from the user's identity-address private key and the application's appDomain. The app private key serves three functions:

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

Finally, the app private key is deterministic, meaning that for a given user ID and domain name, the same private key is generated each time.

Scopes

Scopes define the permissions requested by an app for granting during authentication.

Apps may request any of the following scopes:

ScopeDefinition

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.

The permissions scope should be specified through the AppConfig object. If no scopes array is provided to the redirectToSignIn or makeAuthRequest functions, the default is to request ['store_write'].

Manifest file

Decentralized 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" } ] }{ "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 App retrieves the manifest file from the app during the authentication process and displays the information in it such as the app name and to the user during sign in. 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: *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 three public-private key pairs used in the authentication process:

  • 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 App to the decentralized 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 App 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.

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 username 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 App.

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 username and domain_name, the same private key is generated each time.

The app private key is securely shared with the app on each authentication, encrypted by the Blockstack App with the transit public key.

JSON Web Token signatures

Both the authRequest and the authResponse tokens are JSON Web Tokens, and they are passed via URL query strings.

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. Blockstack provide both JavaScript and Ruby JWT libraries with support for this signing algorithm.

Example: authRequest 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 the Blockstack App redirects user on auth approval - must be hosted on app origin version, // version tuple do_not_include_profile, // a boolean flag asking Blockstack App 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 };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 the Blockstack App redirects user on auth approval - must be hosted on app origin version, // version tuple do_not_include_profile, // a boolean flag asking Blockstack App 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};

Example: authResponse 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 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 };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 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};

Decode authRequest

To decode the token and see what information it holds:

  1. Copy the authRequest string from the URL.

  2. Navigate to jwt.io.

  3. Paste the full token there.

    The output should look similar to below:

    { "jti": "f65f02db-9f42-4523-bfa9-8034d8edf459", "iat": 1555641911, "exp": 1555645511, "iss": "did:btc-addr:1ANL7TNdT7TTcjVnrvauP7Mq3tjcb8TsUX", "public_keys": ["02f08d5541bf611ded745cc15db08f4447bfa55a55a2dd555648a1de9759aea5f9"], "domain_name": "http://localhost:8080", "manifest_uri": "http://localhost:8080/manifest.json", "redirect_uri": "http://localhost:8080", "version": "1.3.1", "do_not_include_profile": true, "supports_hub_url": true, "scopes": ["store_write", "publish_data"] }{ "jti": "f65f02db-9f42-4523-bfa9-8034d8edf459", "iat": 1555641911, "exp": 1555645511, "iss": "did:btc-addr:1ANL7TNdT7TTcjVnrvauP7Mq3tjcb8TsUX", "public_keys": ["02f08d5541bf611ded745cc15db08f4447bfa55a55a2dd555648a1de9759aea5f9"], "domain_name": "http://localhost:8080", "manifest_uri": "http://localhost:8080/manifest.json", "redirect_uri": "http://localhost:8080", "version": "1.3.1", "do_not_include_profile": true, "supports_hub_url": true, "scopes": ["store_write", "publish_data"]}

    The iss property is a decentralized identifier or did. This identifies the user and the user name to the application. The specific did is a btc-addr.