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We present the implementation of an experimental optical encryption configuration in the Fresnel domain using the thermal lens (TL) effect to modulate the phase of the illumination beam. We corroborate the basic performance of our cryptosystem by performing the encryption-decryption procedure of a single object. Then, we test the tolerance of the proposed scheme to decryption with keys registered for different modulations of the illumination beam induced by the phase generated due to TL effect. These different phase modulations are obtained by introducing axial displacements of a sample, where the TL effect is induced by an excitation beam (EB). Considering the phase modulation changes in the illumination due to sample displacements, we propose an undercover procedure to further protect the target information using decoy data. The key associated with the target information is obtained when the probe beam (PB) and EB are focused on the same point of the sample, while the decoy object key is registered when the beams are not focused on the same region of the sample. Finally, we analyzed the resistance of the undercover ciphertext to some of the attacks reported in the literature. All experimental results presented in this work demonstrate the basic capabilities, viability, and versatility of our proposals. [ABSTRACT FROM AUTHOR]