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We present a scale and/or rotation invariant ID tag to achieve real time automatic vehicle identification for inventory or security purposes. We focus our attention to achieve invariance with respect to scale variations and in-plane rotations of the ID tag. Both distortions must be taken into account in the design of a vehicle identification system operating from above the area of interest (e.g., aerial detection and authentication). In our proposal, distortion invariance is achieved by both multiplexing the ID tag a priori information, and developing an appropriate topology for encoding the information on the ID tag. To increase security, the designed ID tag consists of an optical code containing an encrypted signature which identifies the vehicle. The applied encryption procedure follows the double phase encoding technique, which provides robustness to different degradation sources such as noise, occlusion, scratches, etc. that may affect the ID tag. Once the ID tag is captured by the processor, decryption of the hidden signature is carried out, and its correlation with a previously stored reference signal allows either the identification or the rejection of the authorized vehicle. Numerical results are provided to show the feasibility of the system. The proposed system may have broad applications in transportation, homeland security, and remote vehicle inventory control.