These two classes are the two strategies I went about to verify Minecraft accounts for Civtrade http://civtrade.herokuapp.com.

1-minecraft_account_verifier.rb

class MinecraftAccountVerifier
  require 'net/http'
  require 'uri'

  AUTH_URI = URI.parse('https://login.minecraft.net/').freeze
  CLIENT_VERSION = 13

  attr_reader :error

  # Public: verify an account as a true Minecraft account this user has access to.
  #
  # login    - the username or email used to log into the Minecraft client
  # username - both the username for this service and their in-game identity
  # password - password used to log into the Minecraft client
  def initialize(login, username, password, http_requester=Net::HTTP)
    @login = login
    @username = username
    @password = password
    @http_requester = http_requester
  end

  def authentic?
    response = login.body

    if response =~ username_regex
      true
    else
      @error = login.body.chomp
      false
    end
  end

  private
  def username_regex
    Regexp.new(@username, 'i')
  end

  def request_parameters
    {
      'user' => @login,
      'password' => @password,
      'version' => CLIENT_VERSION
    }
  end

  def login
    request = Net::HTTP::Post.new(AUTH_URI.request_uri)
    request.set_form_data(request_parameters)

    http = Net::HTTP.new(AUTH_URI.host, AUTH_URI.port)
    http.use_ssl = true
    http.verify_mode = OpenSSL::SSL::VERIFY_PEER

    http.request(request)
  end
end

2-minecraft_account_verifier.rb

class MinecraftAccountVerifier
  require 'net/http'
  require 'uri'

  SKINS_S3_BUCKET = 's3.amazonaws.com'.freeze

  attr_reader :error

  # Public: verify an account as a true Minecraft account this user has access to.
  #
  # username - both the username for this service and their in-game identity
  def initialize(username)
    @username = username
  end

  def authentic?
    if user_skin
      return true if skin_difference == 0.0

      @error = "Skin does not match verification skin. Please wait a minute or try uploading the skin again. (#{skin_difference}% different)"
      false
    else
      @error = 'Your skin was not found. Please note that your username is case sensitive'
      false
    end
  end

  private
  def skin_difference
    diffs = []
    user_skin.height.times do |y|
      user_skin.row(y).each_with_index do |pixel, x|
        diffs << [x, y] unless pixel == verification_skin[x, y]
      end
    end
    diffs.length.to_f / verification_skin.pixels.length * 100
  end

  def user_skin
    @user_skin ||=
      Net::HTTP.start(SKINS_S3_BUCKET) do |http|
        response = http.get("/MinecraftSkins/#{@username}.png")
        if response.code == '200'
          datastream = ChunkyPNG::Datastream.from_blob(response.body)
          ChunkyPNG::Image.from_datastream(datastream)
        end
      end
  end

  def verification_skin
    @verification_skin ||=
      ChunkyPNG::Image.from_file(Rails.root.join('public/verification_skin.png'))
  end
end