Core engineering and protocols for Encrypted Threshold Comparison.
Homomorphic cryptosystems natively support only addition and multiplication. Evaluating whether a match score s exceeds an authentication threshold T requires a step function, which is non-linear and non-polynomial. FHEID utilizes high-degree Chebyshev polynomial approximations to compute continuous step approximations homomorphically.
The output of the homomorphic comparison is an encrypted single bit representing success or failure. The verifying terminal decrypts this single bit using its private key: if 1, access is granted; if 0, access is denied. Neither the cloud server nor potential eavesdroppers learn the actual biometric match score, preventing biometric hill-climbing attacks.