Parallelized PBS
This document describes the implementation and benefits of parallelized Programmable Bootstrapping (PBS) in TFHE-rs, including code examples for using multi-bit PBS parameters and ensuring deterministic execution.
Parallelized Programmable Bootstrapping
Programmable Bootstrapping is inherently a sequential operation. However, some recent results showed that introducing parallelism is feasible at the expense of larger keys, thereby enhancing the performance of PBS. This new PBS is called a multi-bit PBS.
TFHE-rs can already perform parallel execution of integer homomorphic operations. Activating this feature can lead to performance improvements, particularly in the case of high core-count CPUs when enough cores are available, or when dealing with operations that require small input message precision.
The following example shows how to use parallelized bootstrapping by choosing multi-bit PBS parameters:
Deterministic parallelized Programmable Bootstrapping
By nature, the parallelized PBS might not be deterministic: while the resulting ciphertext will always decrypt to the correct plaintext, the order of the operations could vary, resulting in different output ciphertext. To ensure a consistent ciphertext output regardless of execution order, add the with_deterministic_execution()
suffix to the parameters.
Here's an example:
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