Yesod.Auth.BcryptDB

A Yesod authentication plugin designed to look users up in a Persistent database where the salted hash of their password is stored. This is based on Yesod.Auth.HashDB plugin, but it uses BCrypt to hash and salt the passwords.

To use this in a Yesod application, the foundation data type must be an instance of YesodPersist, and the username and hashed passwords should be added to the database. The following steps give an outline of what is required.

You need a database table to store user records: in a scaffolded site it might look like:

User
    name Text             -- user name used to uniquely identify users
    password Text Maybe   -- password hash for BCryptDB
    UniqueUser name

Create an instance of BCryptDBUser for this data type:

import Yesod.Auth.BcryptDB (BcryptDBUser(..))

instance BcryptDBUser User where
    userPasswordSaltedHash = userPassword
    setPasswordSaltedHash h u = u { userPassword = Just h }

In the YesodAuth instance declaration for your app, include authBcryptDB like so:

import Yesod.Auth.BcryptDB (authBcryptDB)

instance YesodAuth App where
    authPlugins _ = [authBcryptDB (Just . UniqueUser), ....]

The argument to authBcryptDB is a function which takes a Text and produces a Maybe containing a Unique value to look up in the User table. The example (Just . UniqueUser) shown here works for the model outlined above.

For a real application, the developer should provide some sort of of administrative interface for setting passwords; it needs to call setPassword and save the result in the database. However, if you need to initialize the database by hand, you can generate the correct password hash as follows:

$ ghci -XOverloadedStrings
> import Crypto.BCrypt
> hashPasswordUsingPolicy slowerBcryptHashingPolicy "mypassword"

Custom Login Form

Instead of using the built-in HTML form, a custom one can be supplied by using authBcryptDBWithForm instead of authBcryptDB.

The custom form needs to be given as a function returning a Widget, since it has to build in the supplied "action" URL, and it must provide two text fields called "username" and "password". For example, the following modification of the outline code given above would replace the default form with a very minimal one which has no labels and a simple layout.

instance YesodAuth App where
    authPlugins _ = [authBcryptDBWithForm myform (Just . UniqueUser), ....]

myform :: Route App -> Widget
myform action = $(whamletFile "templates/loginform.hamlet")

where templates/loginform.hamlet contains

<form method="post" action="@{action}">
    <input name="username">
    <input type="password" name="password">
    <input type="submit" value="Login">

If a CSRF token needs to be embedded in a custom form, code must be included in the widget to add it - see defaultForm in the source code.

JSON Interface

This plugin provides sufficient tools to build a complete JSON-based authentication flow. We assume that a design goal is to avoid URLs being built into the client, so all of the URLs needed are passed in JSON data.

To start the process, Yesod's defaultErrorHandler produces a JSON response if the HTTP Accept header gives "application/json" precedence over HTML. For a NotAuthenticated error, the status is 401 and the response contains the URL to use for authentication: this is the route which will be handled by the loginHandler method of the YesodAuth instance, which normally returns a login form.

Leaving the loginHandler aside for a moment, the final step - supported by this plugin since version 1.6 - is to POST the credentials for authentication in a JSON object. This object must include the properties "username" and "password". In the HTML case this would be the form submission, but here we want to use JSON instead.

In a JSON interface, the purpose of the loginHandler is to tell the client the URL for submitting the credentials. This requires a custom loginHandler, since the default one generates HTML only. It can find the correct URL by using the submitRouteBcryptDB function defined in this module.

Writing the loginHandler is made a little messy by the fact that its type allows only HTML content. A work-around is to send JSON as a short-circuit response, but we still make the choice using selectRep so as to get its matching of content types. Here is an example which is geared around using BcryptDB on its own, supporting both JSON and HTML clients:

instance YesodAuth App where
   loginHandler = do
        submission <- submitRouteBcryptDB
        render <- lift getUrlRender
        typedContent@(TypedContent ct _) <- selectRep $ do
            provideRepType typeHtml $ return emptyContent
                           -- Dummy: the real Html version is at the end
            provideJson $ object [("loginUrl", toJSON $ render submission)]
        when (ct == typeJson) $
            sendResponse typedContent   -- Short-circuit JSON response
        defaultLoginHandler             -- Html response