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 initialise 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 of this module for an example.

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

Documentation

class BCryptDBUser user where

The type representing user information stored in the database should be an instance of this class. It just provides the getter and setter used by the functions in this module.

Minimal complete definition

setPasswordSaltedHash, userPasswordSaltedHash

Methods

setPasswordSaltedHash :: Text -> user -> user

Setter used by setPassword and upgradePasswordHash. Produces a version of the user data with the hash set to the new value.

userPasswordSaltedHash :: user -> Maybe Text

Getter used by validatePass and upgradePasswordHash to retrieve the password hash from user data

setPassword

Arguments

:: BCryptDBUser user

=> Text

Password

-> user

-> IO user

Set password for user. This function should be used for setting passwords. It generates random salt and calculates proper hashes.

setPasswordWithHashingPolicy

Arguments

:: BCryptDBUser user

=> HashingPolicy

-> Text

Password

-> user

-> IO user

Set password for user. This function should be used for setting passwords with an specified hashing policy. It generates random salt and calculates proper hashes.

Interface to database and Yesod.Auth

authBCryptDB :: BCryptDBPersist master user => (Text -> Maybe (Unique user)) -> AuthPlugin master

Prompt for username and password, validate that against a database which holds the username and a salted hash of the password

authBCryptDBWithForm :: BCryptDBPersist master user => (Route master -> WidgetT master IO ()) -> (Text -> Maybe (Unique user)) -> AuthPlugin master

Like authBCryptDB, but with an extra parameter to supply a custom HTML form.

The custom form should be specified as a function which takes a route to use as the form action, and returns a Widget containing the form. The form must use the supplied route as its action URL, and, when submitted, it must send two text fields called "username" and "password".

Please see the example in the documentation at the head of this module.

submitRouteBcryptDB :: YesodAuth site => HandlerT Auth (HandlerT site IO) (Route site)

The route, in the parent site, to which the username and password should be sent in order to log in. This function is particularly useful in constructing a loginHandler function which provides a JSON response. See the "JSON Interface" section above for more details.

validateCreds

Arguments

:: BCryptDBPersist master user

=> Unique user

User unique identifier

-> Text

Password given

-> HandlerT master IO Bool

Given a user ID and password in plain text, validate them against the database values.