A true random number generator with on-line testability

True random number generators (TRNGs) are widely used throughout cryptography. They are used in the challenge response authentication procedures, key generation and for hardening measures against power analysis attacks. An important feature of each TRNG is true randomness. Such randomness can be obtained from random physical effects like noise. In order to make a TRNG usable for different semiconductor technologies (including FPGAs) only digital standard library cells can be used. One possible type of TRNG that can be realized with only standard library cells is based on ring os-cillators. Such a TRNG was implemented in a test chip and investigated in combination with a simple post processing. Different on-line testability measures of the TRNG are proposed. The solution is compared with other ones.