Decoding Homomorphically Encrypted Flac Audio without Decryption

Homomorphic Encryption (HE) allows processing cipher-text data, but it is a challenge to enable complex methods such as multimedia decompression in the HE domain. In this paper, we propose a novel scheme to enable FLAC (Free Lossless Audio Codec) decompression in the HE domain. FLAC applies linear prediction to predict the current sample and Golomb coding to encode residuals. FLAC decoding relies heavily on dynamic controls that HE does not support due to unknown values of control variables after encryption. Our scheme regularizes dynamic controls in FLAC decoding with static controls by calculating an encrypted matching bit for each possible value of a control variable and producing candidate results as if it were a match. The summation of each possible value’s candidate results multiplied by its matching bit is equivalent to selecting the results of the matched control value. Our FLAC decoding scheme enables Single-Instruction Multiple-Data (SIMD): multiple (e.g., 256) plaintexts are packed and encrypted into a single ciphertext, and decoding one encrypted frame corresponds to decoding multiple plaintext frames. Our scheme is applicable to other audio compression standards based on similar technologies. Experimental results are also reported.