def gen_signature(data)
      raise ArgumentError unless data

      sig = @priv_key.sign(@digest, data)
      base64sig = Base64.strict_encode64(sig)

      return HTTP_SIGNATURE % [user_path, @fingerprint, @digest_name, base64sig]
    end
def gen_signature(data)
      raise ArgumentError unless data

      sig = @priv_key.sign(@digest, data)
      base64sig = Base64.strict_encode64(sig)

      return HTTP_SIGNATURE % [user_path, @fingerprint, @digest_name, base64sig]
    end
Given a chunk of data, creates an HTTP signature which the Manta service
 understands and uses for authentication.