Paper 2026/2328

Automorphism-Compatible NTT and its Application to Homomorphic Encryption

Abstract

For $N$ a power of $2$, the negacyclic number-theoretic transform (NTT) maps a degree $N-1$ polynomial to its evaluations at the $N$ primitive $2N$th roots of unity. In-place butterfly algorithms such as Cooley-Tukey and Gentleman-Sande compute the negacyclic NTT in $O(N\log N)$ time and output the evaluations in a specific, standard order. We give in-place butterfly algorithms for the negacyclic NTT and its inverse that instead output the evaluations in an automorphism-compatible order. These algorithms have the same structure as Cooley-Tukey and Gentleman-Sande, differing only in their precomputed twiddle factors. Our automorphism-compatible butterfly algorithms are applicable to the packed fully homomorphic encryption (FHE) schemes CKKS and BGV/BFV, where the negacyclic NTT is a major component of ciphertext arithmetic. Using our automorphism-compatible butterfly algorithms, Galois automorphisms can be applied to ciphertexts stored in double-CRT format via a simple cyclic shift or adjacent-pair swap of the data. This is contrast to butterfly algorithms currently used that output in the standard order, where applying an automorphism requires performing an arbitrary-looking permutation of the stored data. Our automorphism-compatible butterfly algorithms remove the need for a dedicated automorphism functional unit used in FHE hardware accelerators.