  Face recognition has recently become ubiquitous in many scenes for
authentication or security purposes. Meanwhile, there are increasing concerns
about the privacy of face images, which are sensitive biometric data that
should be carefully protected. Software-based cryptosystems are widely adopted
nowadays to encrypt face images, but the security level is limited by
insufficient digital secret key length or computing power. Hardware-based
optical cryptosystems can generate enormously longer secret keys and enable
encryption at light speed, but most reported optical methods, such as double
random phase encryption, are less compatible with other systems due to system
complexity. In this study, a plain yet high-efficient speckle-based optical
cryptosystem is proposed and implemented. A scattering ground glass is
exploited to generate physical secret keys of gigabit length and encrypt face
images via seemingly random optical speckles at light speed. Face images can
then be decrypted from the random speckles by a well-trained decryption neural
network, such that face recognition can be realized with up to 98% accuracy.
The proposed cryptosystem has wide applicability, and it may open a new avenue
for high-security complex information encryption and decryption by utilizing
optical speckles.
