Linear Bandwidth Naccache-Stern Encryption

The Naccache-Stern (ns) knapsack cryptosystem is an original yet little-known public-key encryption scheme. In this scheme, the ciphertext is obtained by multiplying public-keys indexed by the message bits modulo a prime p. The cleartext is recovered by factoring the ciphertext raised to a secret power modulo p. ns encryption requires a multiplication per two plaintext bits on the average. Decryption is roughly as costly as an rsa decryption. However, ns features a bandwidth sublinear in log p, namely log p/ log log p. As an example, for a 2048-bit prime p, ns encryption features a 233-bit bandwidth for a 59-kilobyte public key size. This paper presents new ns variants achieving bandwidths linear in log p. As linear bandwidth claims a public-key of size log p/ log log p, we recommend to combine our scheme with other bandwidth optimization techniques presented here. For a 2048-bit prime p, we obtain figures such as 169-bit plaintext for a 10-kilobyte public key, 255-bit plaintext for a 20-kilobyte public key or a 781-bit plaintext for a 512-kilobyte public key. Encryption and decryption remain unaffected by our optimizations: As an example, the 781-bit variant requires 152 multiplications per encryption.