  Pre-trained Large Language Models (LLMs) are an integral part of modern AI
that have led to breakthrough performances in complex AI tasks. Major AI
companies with expensive infrastructures are able to develop and train these
large models with billions and millions of parameters from scratch. Third
parties, researchers, and practitioners are increasingly adopting these
pre-trained models and fine-tuning them on their private data to accomplish
their downstream AI tasks. However, it has been shown that an adversary can
extract/reconstruct the exact training samples from these LLMs, which can lead
to revealing personally identifiable information. The issue has raised deep
concerns about the privacy of LLMs. Differential privacy (DP) provides a
rigorous framework that allows adding noise in the process of training or
fine-tuning LLMs such that extracting the training data becomes infeasible
(i.e., with a cryptographically small success probability). While the
theoretical privacy guarantees offered in most extant studies assume learning
models from scratch through many training iterations in an asymptotic setting,
this assumption does not hold in fine-tuning scenarios in which the number of
training iterations is significantly smaller. To address the gap, we present
\ewtune, a DP framework for fine-tuning LLMs based on Edgeworth accountant with
finite-sample privacy guarantees. Our results across four well-established
natural language understanding (NLU) tasks show that while \ewtune~adds privacy
guarantees to LLM fine-tuning process, it directly contributes to decreasing
the induced noise to up to 5.6\% and improves the state-of-the-art LLMs
performance by up to 1.1\% across all NLU tasks. We have open-sourced our
implementations for wide adoption and public testing purposes.
