forked from 626_privacy/tensorflow_privacy
297 lines
8.9 KiB
Python
297 lines
8.9 KiB
Python
# Copyright 2018 The TensorFlow Authors. All Rights Reserved.
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#
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# Licensed under the Apache License, Version 2.0 (the "License");
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# you may not use this file except in compliance with the License.
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# You may obtain a copy of the License at
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#
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# http://www.apache.org/licenses/LICENSE-2.0
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#
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# Unless required by applicable law or agreed to in writing, software
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# distributed under the License is distributed on an "AS IS" BASIS,
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# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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# See the License for the specific language governing permissions and
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# limitations under the License.
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# ==============================================================================
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"""RDP analysis of the Sampled Gaussian Mechanism.
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Functionality for computing Renyi differential privacy (RDP) of an additive
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Sampled Gaussian Mechanism (SGM). Its public interface consists of two methods:
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compute_rdp(q, noise_multiplier, T, orders) computes RDP for SGM iterated
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T times.
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get_privacy_spent(orders, rdp, target_eps, target_delta) computes delta
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(or eps) given RDP at multiple orders and
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a target value for eps (or delta).
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Example use:
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Suppose that we have run an SGM applied to a function with l2-sensitivity 1.
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Its parameters are given as a list of tuples (q1, sigma1, T1), ...,
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(qk, sigma_k, Tk), and we wish to compute eps for a given delta.
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The example code would be:
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max_order = 32
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orders = range(2, max_order + 1)
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rdp = np.zeros_like(orders, dtype=float)
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for q, sigma, T in parameters:
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rdp += rdp_accountant.compute_rdp(q, sigma, T, orders)
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eps, _, opt_order = rdp_accountant.get_privacy_spent(rdp, target_delta=delta)
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"""
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from __future__ import absolute_import
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from __future__ import division
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from __future__ import print_function
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import math
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import sys
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import numpy as np
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from scipy import special
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import six
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########################
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# LOG-SPACE ARITHMETIC #
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########################
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def _log_add(logx, logy):
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"""Add two numbers in the log space."""
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a, b = min(logx, logy), max(logx, logy)
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if a == -np.inf: # adding 0
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return b
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# Use exp(a) + exp(b) = (exp(a - b) + 1) * exp(b)
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return math.log1p(math.exp(a - b)) + b # log1p(x) = log(x + 1)
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def _log_sub(logx, logy):
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"""Subtract two numbers in the log space. Answer must be non-negative."""
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if logx < logy:
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raise ValueError("The result of subtraction must be non-negative.")
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if logy == -np.inf: # subtracting 0
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return logx
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if logx == logy:
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return -np.inf # 0 is represented as -np.inf in the log space.
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try:
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# Use exp(x) - exp(y) = (exp(x - y) - 1) * exp(y).
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return math.log(math.expm1(logx - logy)) + logy # expm1(x) = exp(x) - 1
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except OverflowError:
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return logx
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def _log_print(logx):
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"""Pretty print."""
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if logx < math.log(sys.float_info.max):
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return "{}".format(math.exp(logx))
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else:
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return "exp({})".format(logx)
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def _compute_log_a_int(q, sigma, alpha):
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"""Compute log(A_alpha) for integer alpha. 0 < q < 1."""
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assert isinstance(alpha, six.integer_types)
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# Initialize with 0 in the log space.
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log_a = -np.inf
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for i in range(alpha + 1):
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log_coef_i = (
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math.log(special.binom(alpha, i)) + i * math.log(q) +
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(alpha - i) * math.log(1 - q))
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s = log_coef_i + (i * i - i) / (2 * (sigma**2))
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log_a = _log_add(log_a, s)
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return float(log_a)
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def _compute_log_a_frac(q, sigma, alpha):
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"""Compute log(A_alpha) for fractional alpha. 0 < q < 1."""
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# The two parts of A_alpha, integrals over (-inf,z0] and [z0, +inf), are
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# initialized to 0 in the log space:
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log_a0, log_a1 = -np.inf, -np.inf
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i = 0
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z0 = sigma**2 * math.log(1 / q - 1) + .5
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while True: # do ... until loop
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coef = special.binom(alpha, i)
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log_coef = math.log(abs(coef))
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j = alpha - i
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log_t0 = log_coef + i * math.log(q) + j * math.log(1 - q)
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log_t1 = log_coef + j * math.log(q) + i * math.log(1 - q)
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log_e0 = math.log(.5) + _log_erfc((i - z0) / (math.sqrt(2) * sigma))
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log_e1 = math.log(.5) + _log_erfc((z0 - j) / (math.sqrt(2) * sigma))
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log_s0 = log_t0 + (i * i - i) / (2 * (sigma**2)) + log_e0
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log_s1 = log_t1 + (j * j - j) / (2 * (sigma**2)) + log_e1
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if coef > 0:
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log_a0 = _log_add(log_a0, log_s0)
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log_a1 = _log_add(log_a1, log_s1)
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else:
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log_a0 = _log_sub(log_a0, log_s0)
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log_a1 = _log_sub(log_a1, log_s1)
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i += 1
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if max(log_s0, log_s1) < -30:
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break
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return _log_add(log_a0, log_a1)
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def _compute_log_a(q, sigma, alpha):
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"""Compute log(A_alpha) for any positive finite alpha."""
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if float(alpha).is_integer():
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return _compute_log_a_int(q, sigma, int(alpha))
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else:
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return _compute_log_a_frac(q, sigma, alpha)
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def _log_erfc(x):
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"""Compute log(erfc(x)) with high accuracy for large x."""
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try:
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return math.log(2) + special.log_ndtr(-x * 2**.5)
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except NameError:
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# If log_ndtr is not available, approximate as follows:
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r = special.erfc(x)
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if r == 0.0:
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# Using the Laurent series at infinity for the tail of the erfc function:
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# erfc(x) ~ exp(-x^2-.5/x^2+.625/x^4)/(x*pi^.5)
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# To verify in Mathematica:
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# Series[Log[Erfc[x]] + Log[x] + Log[Pi]/2 + x^2, {x, Infinity, 6}]
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return (-math.log(math.pi) / 2 - math.log(x) - x**2 - .5 * x**-2 +
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.625 * x**-4 - 37. / 24. * x**-6 + 353. / 64. * x**-8)
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else:
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return math.log(r)
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def _compute_delta(orders, rdp, eps):
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"""Compute delta given a list of RDP values and target epsilon.
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Args:
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orders: An array (or a scalar) of orders.
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rdp: A list (or a scalar) of RDP guarantees.
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eps: The target epsilon.
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Returns:
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Pair of (delta, optimal_order).
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Raises:
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ValueError: If input is malformed.
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"""
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orders_vec = np.atleast_1d(orders)
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rdp_vec = np.atleast_1d(rdp)
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if len(orders_vec) != len(rdp_vec):
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raise ValueError("Input lists must have the same length.")
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deltas = np.exp((rdp_vec - eps) * (orders_vec - 1))
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idx_opt = np.argmin(deltas)
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return min(deltas[idx_opt], 1.), orders_vec[idx_opt]
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def _compute_eps(orders, rdp, delta):
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"""Compute epsilon given a list of RDP values and target delta.
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Args:
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orders: An array (or a scalar) of orders.
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rdp: A list (or a scalar) of RDP guarantees.
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delta: The target delta.
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Returns:
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Pair of (eps, optimal_order).
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Raises:
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ValueError: If input is malformed.
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"""
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orders_vec = np.atleast_1d(orders)
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rdp_vec = np.atleast_1d(rdp)
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if len(orders_vec) != len(rdp_vec):
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raise ValueError("Input lists must have the same length.")
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eps = rdp_vec - math.log(delta) / (orders_vec - 1)
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idx_opt = np.nanargmin(eps) # Ignore NaNs
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return eps[idx_opt], orders_vec[idx_opt]
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def _compute_rdp(q, sigma, alpha):
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"""Compute RDP of the Sampled Gaussian mechanism at order alpha.
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Args:
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q: The sampling rate.
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sigma: The std of the additive Gaussian noise.
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alpha: The order at which RDP is computed.
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Returns:
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RDP at alpha, can be np.inf.
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"""
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if q == 0:
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return 0
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if q == 1.:
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return alpha / (2 * sigma**2)
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if np.isinf(alpha):
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return np.inf
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return _compute_log_a(q, sigma, alpha) / (alpha - 1)
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def compute_rdp(q, noise_multiplier, steps, orders):
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"""Compute RDP of the Sampled Gaussian Mechanism.
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Args:
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q: The sampling rate.
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noise_multiplier: The ratio of the standard deviation of the Gaussian noise
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to the l2-sensitivity of the function to which it is added.
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steps: The number of steps.
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orders: An array (or a scalar) of RDP orders.
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Returns:
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The RDPs at all orders, can be np.inf.
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"""
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if np.isscalar(orders):
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rdp = _compute_rdp(q, noise_multiplier, orders)
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else:
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rdp = np.array([_compute_rdp(q, noise_multiplier, order)
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for order in orders])
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return rdp * steps
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def get_privacy_spent(orders, rdp, target_eps=None, target_delta=None):
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"""Compute delta (or eps) for given eps (or delta) from RDP values.
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Args:
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orders: An array (or a scalar) of RDP orders.
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rdp: An array of RDP values. Must be of the same length as the orders list.
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target_eps: If not None, the epsilon for which we compute the corresponding
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delta.
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target_delta: If not None, the delta for which we compute the corresponding
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epsilon. Exactly one of target_eps and target_delta must be None.
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Returns:
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eps, delta, opt_order.
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Raises:
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ValueError: If target_eps and target_delta are messed up.
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"""
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if target_eps is None and target_delta is None:
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raise ValueError(
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"Exactly one out of eps and delta must be None. (Both are).")
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if target_eps is not None and target_delta is not None:
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raise ValueError(
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"Exactly one out of eps and delta must be None. (None is).")
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if target_eps is not None:
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delta, opt_order = _compute_delta(orders, rdp, target_eps)
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return target_eps, delta, opt_order
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else:
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eps, opt_order = _compute_eps(orders, rdp, target_delta)
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return eps, target_delta, opt_order
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