Privacy Random Variable (PRV) Accountant

A fast algorithm to optimally compose privacy guarantees of differentially private (DP) algorithms to arbitrary accuracy. Our method is based on the notion of privacy loss random variables to quantify the privacy loss of DP algorithms. For more details see [1].

Installation

pip install prv-accountant

Mechanisms

Currently the following mechanisms are supported:

Subsampled Gaussian Mechanism

from prv_accountant import PoissonSubsampledGaussianMechanism
prv = PoissonSubsampledGaussianMechanism(noise_multiplier, sampling_probability)

which computes the privacy curve

 \delta \left ( \mathcal{N}(0, \sigma^2) \| (1-p) \mathcal{N}(0, \sigma^2) + p \mathcal{N}(1, \sigma^2) \right ), 

where p is the sampling probability and \sigma is the noise multiplier. The second argument represents a mixture distribution.

Gaussian Mechanism

from prv_accountant import GaussianMechanism
prv = GaussianMechanism(noise_multiplier)

which computes the privacy curve

 \delta \left ( \mathcal{N}(0, \sigma^2) \| \mathcal{N}(1, \sigma^2) \right ), 

where \sigma is the noise multiplier.

Laplace Mechanism

from prv_accountant import LaplaceMechanism
prv = LaplaceMechanism(mu)

which computes the privacy curve

 \delta \left ( \textsf{Lap}(0, 1) \| \textsf{Lap}(\mu, 1) \right ). 

Pure-DP and Approximate-DP

It is also possible to compose DP guarantees directly

• pure \varepsilon-DP guarantees using prv_accountant.PureDPMechanism(epsilon)
• approximate (\varepsilon, \delta)-DP guarantees using prv_accountant.ApproximateDPMechanism(epsilon, delta)

Custom Mechanisms

It is also possible to add custom mechanisms for the composition computation. An example can be found in this notebook. All we need is to implement the CDF of the privacy loss distribution.

Example

Heterogeneous Composition

It is possible to compose different mechanisms. The following example will compute the composition of three different mechanism M^{(a)}, M^{(b)} and M^{(c)} composed with themselves m, n and o times, respectively.

An application for such a composition is DP-SGD training with increasing batch size and therefore increasing sampling probability. After m+n+o training steps, the resulting privacy mechanism M for the whole training process is given by M = M_1^{(a)} \circ \dots \circ M_m^{(a)} \circ M_1^{(b)} \circ \dots \circ M_n^{(b)} \circ M_1^{(c)} \circ \dots \circ M_o^{(c)}.

Using the prv_accountant we need to create a privacy random variable for each mechanism

from prv_accountant.privacy_random_variables import PoissonSubsampledGaussianMechanism, GaussianMechanism, LaplaceMechanism

prv_a = PoissonSubsampledGaussianMechanism(noise_multiplier=0.8, sampling_probability=5e-3)
prv_b = GaussianMechanism(noise_multiplier=8.0)
prv_c = LaplaceMechanism(mu=0.1)

m = 100
n = 200
o = 100

Next, we need to create an accountant instance. The accountant will take care of most of the numerical intricacies such as finding the support of the PRV and discretisation. In order to find a suitable domain, the accountant needs to know about the largest number of compositions of each PRV with itself that will be computed. Larger values of max_self_compositions lead to larger domains which can cause slower performance. In the case of DP-SGD, a reasonable choice of max_self_compositions would be the total number of training steps. Additionally, the desired error bounds for \varepsilon and \delta are required.

from prv_accountant import PRVAccountant

accountant = PRVAccountant(
    prvs=[prv_a, prv_b, prv_c],
    max_self_compositions=[1_000, 1_000, 1_000],
    eps_error=0.1,
    delta_error=1e-10
)

Finally, we're ready to compute the composition. The final bounds and estimates for \varepsilon for the mechanism M are

eps_low, eps_est, eps_up = accountant.compute_epsilon(delta=1e-6, num_self_compositions=[m, n, o])

DP-SGD

For homogeneous DP-SGD (i.e. constant noise multiplier and constant sampling probability) things are even simpler. We provide a simple command line utility for getting epsilon estimates.

compute-dp-epsilon --sampling-probability 5e-3 --noise-multiplier 0.8 --delta 1e-6 --num-compositions 1000

Or, use it in python code

from prv_accountant.dpsgd import DPSGDAccountant

accountant = DPSGDAccountant(
    noise_multiplier=0.8,
    sampling_probability=5e-3,
    delta=1e-6,
    eps_error=0.1,
    delta_error=1e-10,
    max_compositions=1000
)

eps_low, eps_estimate, eps_upper = accountant.compute_epsilon(num_compositions=1000)

For more examples, have a look in the notebooks directory.

References

[1] Sivakanth Gopi, Yin Tat Lee, Lukas Wutschitz. (2021). Numerical composition of differential privacy. Advances in Neural Information Processing Systems

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