Kyber AHE: An Easy-to-Implement Additive Homomorphic Encryption Scheme Based on Kyber and Its Application in Biometric Template Protection

Homomorphic encryption solutions tend to be costly in terms of memory and computational resources, making them difficult to implement. In this paper, we present Kyber AHE, a lightweight additive homomorphic encryption scheme for computing the addition modulo 2 of two binary strings in the encrypted domain. It is based on the CRYSTALS-Kyber public key encryption (PKE) scheme, which is the basis of the NIST module-lattice-based key-encapsulation mechanism standard. Apart from being quantum-safe, Kyber PKE has other interesting features such as the use of compressed ciphertexts, reduced sizes of keys, low execution times, and the ability to easily increase the security level. The operations performed in the encrypted domain by Kyber AHE are the decompression of ciphertexts, the component-wise modulo q addition of polynomials, and the compression of the results. A great advantage of Kyber AHE is that it can be easily implemented along with CRYSTALS-Kyber without the need for additional libraries. Among the applications of homomorphic encryption, biometric template protection schemes are a promising solution to provide data privacy by comparing biometric features in the encrypted domain. Therefore, we present the application of Kyber AHE for the protection of biometric templates. Experimental results have been obtained using Kyber AHE in an iris biometric template protection scheme with 256-byte features using Kyber512, Kyber768, and Kyber1024 instances. The sizes of the encrypted iris features are 6.0, 8.5, and 12.5 kB for NIST security levels I, III, and V, respectively. Using a commercial laptop, the encryption ranges from 0.755 to 1.73 ms, the evaluation from 0.096 to 0.161 ms, and the decryption from 0.259 to 0.415 ms, depending on the security level.