create_data_bag.rb

require 'rubygems'
require 'chef/encrypted_data_bag_item'

secret = Chef::EncryptedDataBagItem.load_secret('data_bag_key')
data = {"id" => "mysql", "root" => "some secret password"}
encrypted_data = Chef::EncryptedDataBagItem.encrypt_data_bag_item(data, secret)

FileUtils.mkpath('data_bags/passwords')
File.open('data_bags/passwords/mysql.json', 'w') do |f|
  f.print encrypted_data.to_json
end