# File lib/aws/s3/encryption_utils.rb, line 76
      def check_encryption_materials mode, key
        rsa = OpenSSL::PKey::RSA
        case key
        when rsa
          unless key.private? or mode == :encrypt
            msg = "invalid key, #{rsa} requires a private key"
            raise ArgumentError, msg
          end
        when String # no problem
        else
          msg = "invalid key, must be an #{rsa} or a cipher key string"
          raise ArgumentError, msg
        end
      end