Security oriented codes are considered as one of the most efficient countermeasures against fault injection attacks. Their efficiency is usually measured in terms of their error masking probability. This criterion is applicable in cases where it is possible to distinguish between random errors and malicious attacks. In practice, if the induced errors are not fixed for several clock cycles, it is difficult to distinguish between the two. Moreover, a decoder that tries to correct the tampered word can conceal the fact that the device is under attack. This paper defines a new criterion, named t-robustness, for evaluating the efficiency of robust codes that provide both reliability and security. An error correcting code is called t-robust, if it can correct up to t errors and at the same time detect any attack that changes the data. The paper presents a general structure for concatenated codes that have this property.