Metasurfaces provide a novel platform for information encryption due to their incomparable competence for the manipulation of different optical characteristics. Most current encryption schemes, however, decrypt the hidden information through a direct illumination of the metasurface, and the security of which cannot be fully guaranteed since the number of illumination condition is very limited and vulnerable for exhaustive attack. Herein, a cryptographic system based on the indirect-observation decryption is proposed, the hidden information of which can only be decoded by combining the metasurface-transformed ciphertext and particular decryption devices based on the digit-shift images and spatial light modulator, thus possessing ultra-high confidentiality. In addition, an anti-counterfeiting label is designed to guarantee the information authenticity. This work builds a distinct bridge between metasurface and other light-manipulation elements, nanoprint and hologram, and amplitude- and phase-modulated information, thus opening up a new methodology for constructing cryptographic systems in an indirect-observation way, which may promote the advanced exploration of optical multiplexing and steganography.