#amber

TILNovember 26, 2019by Igor Alexandrov

How to add HTTP Basic auth to Amber application

As you already may know, one of our projects have a Crystal application in production. It is created with Amber framework and works just perfect.

The only thing that I don't personally like in Amber is not clear and sometimes outdated documentation, after about 11 years with Rails I still think that Rails Guides are the number one developer documentation in the world.

I had a task to add HTTP Basic auth to a couple of URLs in Amber application, and after studying documentation found that Amber doesn't provide necessary Pipe out of the box. Ok, next place to search for the answer was Gitter and after about an hour Dru Jensen helped me with a code example.

Amber uses internally HTTP::Handler for Pipes as well as Kemal does for Middlewares, so we can easily use code from Basic Auth for Kemal.


# src/pipes/http_basic_auth_pipe.cr



require "crypto/subtle"



class HTTPBasicAuthPipe

  include HTTP::Handler

  BASIC = "Basic"

  AUTH = "Authorization"

  AUTH_MESSAGE = "Could not verify your access level for that URL.\nYou have to login with proper credentials"

  HEADER_LOGIN_REQUIRED = "Basic realm=\"Login Required\""



  property credentials : Credentials?



  def initialize(@credentials : Credentials)

  end



  def initialize(username : String, password : String)

    initialize({ username => password })

  end



  def initialize(hash : Hash(String, String))

    initialize(Credentials.new(hash))

  end



  def initialize

    if ENV["HTTP_BASIC_USERNAME"]? && ENV["HTTP_BASIC_PASSWORD"]?

      initialize(ENV["HTTP_BASIC_USERNAME"], ENV["HTTP_BASIC_PASSWORD"])

    end

  end



  def call(context)

    if credentials

      if context.request.headers[AUTH]?

        if value = context.request.headers[AUTH]

          if value.size > 0 && value.starts_with?(BASIC)

            return call_next(context) if authorized?(value)

          end

        end

      end

      headers = HTTP::Headers.new

      context.response.status_code = 401

      context.response.headers["WWW-Authenticate"] = HEADER_LOGIN_REQUIRED

      context.response.print AUTH_MESSAGE

    else

      call_next(context)

    end

  end



  private def authorized?(value)

    username, password = Base64.decode_string(value[BASIC.size + 1..-1]).split(":")

    credentials.not_nil!.authorize?(username, password)

  end



  class Credentials

    def initialize(@entries : Hash(String, String) = Hash(String, String).new)

    end



    def authorize?(username : String, given_password : String) : String?

      test_password = find_password(username, given_password)

      if Crypto::Subtle.constant_time_compare(test_password, given_password)

        username

      else

        nil

      end

    end



    private def find_password(username, given_password)

      # return a password that cannot possibly be correct if the username is wrong

      pw = "not #{given_password}"



      # iterate through each possibility to not leak info about valid usernames

      @entries.each do |(user, password)|

        if Crypto::Subtle.constant_time_compare(user, username)

          pw = password

        end

      end



      pw

    end

  end

end

And in routes.cr:


  # ...

  pipeline :api do

    # ...

    plug HTTPBasicAuthPipe.new

  end

  # ...