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Title:
Seamless Switching Between PBS and WoPBS for Scalable TFHE
Authors: Rostin Shokri, Nektarios Georgios Tsoutsos
Abstract:Fully Homomorphic Encryption (FHE) enables arbitrary and unlimited computations directly on encrypted data. Notably, the TFHE scheme allows users to encrypt bits or small numbers (4-6 bits) and compute any univariate function using programmable bootstrapping (PBS), while simultaneously refreshing the ciphertext noise. Since both linear and non-linear functions can be evaluated using PBS, it is possible to compute arbitrary functions and circuits of unlimited depth without any accuracy loss. Nevertheless, a major limitation of TFHE, compared to other FHE schemes, is that it operates on a single ciphertext at a time, and the underlying message size remains small. For larger messages with longer bit sizes, the execution overhead of PBS grows exponentially with the number of message bits. A recent approach, called Without-padding PBS (WoPBS), enables computation of much larger lookup tables (10-28 bits), with the execution cost scaling linearly with the number of message bits. The significant encoding mismatch between the PBS and WoPBS, however, complicates the use of both approaches within the same circuit execution.
In this work, we introduce novel switching algorithms that enable ciphertexts to be converted back and forth between the PBS and WoPBS contexts without impacting the input noise. Moreover, we introduce a new method to bootstrap ciphertexts within the WoPBS context, allowing for unlimited XOR operations at negligible cost. To enhance runtime, we further introduce optimized parameters for both contexts. We validate our techniques through the homomorphic evaluation of AES encryption and decryption, demonstrating transciphering applications that outperform related works.
ePrint: https://eprint.iacr.org/2025/777
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