Generate authenticated url for S3 in ruby

S3QueryStringAuthenticationBuilder

class S3QueryStringAuthenticationBuilder

  def initialize(bucket, access_key, secret_key)
    @bucket = bucket
    @access_key = access_key
    @secret_key = secret_key
  end

  def build_query_string_authentication(file_url, time_to_live=1.minute)
    file_url = "/#{file_url}" unless file_url.start_with?('/')
    expires = (Time.now.utc + 1.minutes).to_i
    signature = build_signature(@secret_key, canonicalized_resource(file_url), expires)
    "#{host}#{file_url}?AWSAccessKeyId=#{@access_key}&Expires=#{expires}&Signature=#{signature}"
  end

  def host
    "https://#{@bucket}.s3.amazonaws.com"
  end

  private

  def canonicalized_resource(file_url)
    "/#{@bucket}#{file_url}"
  end

  def build_signature(secret_key, canonicalized_resource, expires)
    utf8 = Encoding.find('UTF-8')
    url_encode( base64( hmac_sha1(secret_key, string_to_sign(canonicalized_resource, expires).force_encoding(utf8) ) ) )
  end

  def string_to_sign(canonicalized_resource , expires)
    "GET\n\n\n#{expires}\n#{canonicalized_resource}"
  end

  def hmac_sha1(secret_key, string)
    OpenSSL::HMAC.digest( OpenSSL::Digest::SHA1.new, secret_key, string )
  end

  def base64(string)
    Base64.strict_encode64(string)
  end

  def url_encode(string)
    URI.encode(string)
  end

end