Add differential privacy library and algorithms (0d1bc332) · Commits · e / os / android_frameworks_base

Android.bp

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		@@ -671,6 +671,7 @@ java_library {
		"libphonenumber-platform",
		"nist-sip",
		"tagsoup",
		"rappor",
		],
		dxflags: ["--core-library"],
		}

core/java/android/privacy/DifferentialPrivacyConfig.java

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		/*
		* Copyright 2017 The Android Open Source Project
		*
		* Licensed under the Apache License, Version 2.0 (the "License");
		* you may not use this file except in compliance with the License.
		* You may obtain a copy of the License at
		*
		* http://www.apache.org/licenses/LICENSE-2.0
		*
		* Unless required by applicable law or agreed to in writing, software
		* distributed under the License is distributed on an "AS IS" BASIS,
		* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
		* See the License for the specific language governing permissions and
		* limitations under the License.
		*/

		package android.privacy;

		/**
		* An interface for differential privacy configuration.
		* {@link DifferentialPrivacyEncoder} will apply this configuration to do differential privacy
		* encoding.
		*
		* @hide
		*/
		public interface DifferentialPrivacyConfig {

		/**
		* Returns the name of the algorithm used in differential privacy config.
		*
		* @return The name of the algorithm
		*/
		String getAlgorithm();
		}

core/java/android/privacy/DifferentialPrivacyEncoder.java

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		/*
		* Copyright 2017 The Android Open Source Project
		*
		* Licensed under the Apache License, Version 2.0 (the "License");
		* you may not use this file except in compliance with the License.
		* You may obtain a copy of the License at
		*
		* http://www.apache.org/licenses/LICENSE-2.0
		*
		* Unless required by applicable law or agreed to in writing, software
		* distributed under the License is distributed on an "AS IS" BASIS,
		* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
		* See the License for the specific language governing permissions and
		* limitations under the License.
		*/

		package android.privacy;

		/**
		* An interface for differential privacy encoder.
		* Applications can use it to convert privacy sensitive data to privacy protected report.
		* There is no decoder implemented in Android as it is not possible decode a single report by
		* design.
		*
		* <p>Each type of log should have its own encoder, otherwise it may leak
		* some information about Permanent Randomized Response(PRR, is used to create a “noisy”
		* answer which is memoized by the client and permanently reused in place of the real answer).
		*
		* <p>Some encoders may not support all encoding methods, and it will throw {@link
		* UnsupportedOperationException} if you call unsupported encoding method.
		*
		* <p><b>WARNING:</b> Privacy protection works only when encoder uses a suitable DP configuration,
		* and the configuration and algorithm that is suitable is highly dependent on the use case.
		* If the configuration is not suitable for the use case, it may hurt privacy or utility or both.
		*
		* @hide
		*/
		public interface DifferentialPrivacyEncoder {

		/**
		* Apply differential privacy to encode a string.
		*
		* @param original An arbitrary string
		* @return Differential privacy encoded bytes derived from the string
		*/
		byte[] encodeString(String original);

		/**
		* Apply differential privacy to encode a boolean.
		*
		* @param original An arbitrary boolean.
		* @return Differential privacy encoded bytes derived from the boolean
		*/
		byte[] encodeBoolean(boolean original);

		/**
		* Apply differential privacy to encode sequence of bytes.
		*
		* @param original An arbitrary byte array.
		* @return Differential privacy encoded bytes derived from the bytes
		*/
		byte[] encodeBits(byte[] original);

		/**
		* Returns the configuration that this encoder is using.
		*/
		DifferentialPrivacyConfig getConfig();

		/**
		* Return True if the output from encoder is NOT securely randomized, otherwise encoder should
		* be secure to randomize input.
		*
		* <b> A non-secure encoder is intended only for testing only and must not be used to process
		* real data.
		* </b>
		*/
		boolean isInsecureEncoderForTest();
		}

core/java/android/privacy/internal/longitudinalreporting/LongitudinalReportingConfig.java

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		/*
		* Copyright 2017 The Android Open Source Project
		*
		* Licensed under the Apache License, Version 2.0 (the "License");
		* you may not use this file except in compliance with the License.
		* You may obtain a copy of the License at
		*
		* http://www.apache.org/licenses/LICENSE-2.0
		*
		* Unless required by applicable law or agreed to in writing, software
		* distributed under the License is distributed on an "AS IS" BASIS,
		* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
		* See the License for the specific language governing permissions and
		* limitations under the License.
		*/

		package android.privacy.internal.longitudinalreporting;

		import android.privacy.DifferentialPrivacyConfig;
		import android.privacy.internal.rappor.RapporConfig;
		import android.text.TextUtils;

		import com.android.internal.util.Preconditions;

		/**
		* A class to store {@link LongitudinalReportingEncoder} configuration.
		*
		* <ul>
		* <li> f is probability to flip input value, used in IRR.
		* <li> p is probability to override input value, used in PRR1.
		* <li> q is probability to set input value as 1 when result of PRR(p) is true, used in PRR2.
		* </ul>
		*
		* @hide
		*/
		public class LongitudinalReportingConfig implements DifferentialPrivacyConfig {

		private static final String ALGORITHM_NAME = "LongitudinalReporting";

		// Probability to flip input value.
		private final double mProbabilityF;

		// Probability to override original value.
		private final double mProbabilityP;
		// Probability to override value with 1.
		private final double mProbabilityQ;

		// IRR config to randomize original value
		private final RapporConfig mIRRConfig;

		private final String mEncoderId;

		/**
		* Constructor to create {@link LongitudinalReportingConfig} used for {@link
		* LongitudinalReportingEncoder}
		*
		* @param encoderId Unique encoder id.
		* @param probabilityF Probability F used in Longitudinal Reporting algorithm.
		* @param probabilityP Probability P used in Longitudinal Reporting algorithm. This will be
		* quantized to the nearest 1/256.
		* @param probabilityQ Probability Q used in Longitudinal Reporting algorithm. This will be
		* quantized to the nearest 1/256.
		*/
		public LongitudinalReportingConfig(String encoderId, double probabilityF,
		double probabilityP, double probabilityQ) {
		Preconditions.checkArgument(probabilityF >= 0 && probabilityF <= 1,
		"probabilityF must be in range [0.0, 1.0]");
		this.mProbabilityF = probabilityF;
		Preconditions.checkArgument(probabilityP >= 0 && probabilityP <= 1,
		"probabilityP must be in range [0.0, 1.0]");
		this.mProbabilityP = probabilityP;
		Preconditions.checkArgument(probabilityQ >= 0 && probabilityQ <= 1,
		"probabilityQ must be in range [0.0, 1.0]");
		this.mProbabilityQ = probabilityQ;
		Preconditions.checkArgument(!TextUtils.isEmpty(encoderId), "encoderId cannot be empty");
		mEncoderId = encoderId;
		mIRRConfig = new RapporConfig(encoderId, 1, 0.0, probabilityF, 1 - probabilityF, 1, 1);
		}

		@Override
		public String getAlgorithm() {
		return ALGORITHM_NAME;
		}

		RapporConfig getIRRConfig() {
		return mIRRConfig;
		}

		double getProbabilityP() {
		return mProbabilityP;
		}

		double getProbabilityQ() {
		return mProbabilityQ;
		}

		String getEncoderId() {
		return mEncoderId;
		}

		@Override
		public String toString() {
		return String.format("EncoderId: %s, ProbabilityF: %.3f, ProbabilityP: %.3f"
		+ ", ProbabilityQ: %.3f",
		mEncoderId, mProbabilityF, mProbabilityP, mProbabilityQ);
		}
		}

core/java/android/privacy/internal/longitudinalreporting/LongitudinalReportingEncoder.java

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		/*
		* Copyright 2017 The Android Open Source Project
		*
		* Licensed under the Apache License, Version 2.0 (the "License");
		* you may not use this file except in compliance with the License.
		* You may obtain a copy of the License at
		*
		* http://www.apache.org/licenses/LICENSE-2.0
		*
		* Unless required by applicable law or agreed to in writing, software
		* distributed under the License is distributed on an "AS IS" BASIS,
		* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
		* See the License for the specific language governing permissions and
		* limitations under the License.
		*/

		package android.privacy.internal.longitudinalreporting;

		import android.privacy.DifferentialPrivacyEncoder;
		import android.privacy.internal.rappor.RapporConfig;
		import android.privacy.internal.rappor.RapporEncoder;

		import com.android.internal.annotations.VisibleForTesting;

		/**
		* Differential privacy encoder by using Longitudinal Reporting algorithm.
		*
		* <b>
		* Notes: It supports encodeBoolean() only for now.
		* </b>
		*
		* <p>
		* Definition:
		* PRR = Permanent Randomized Response
		* IRR = Instantaneous Randomized response
		*
		* Algorithm:
		* Step 1: Create long-term secrets x(ignoreOriginalInput)=Ber(P), y=Ber(Q), where Ber denotes
		* Bernoulli distribution on {0, 1}, and we use it as a long-term secret, we implement Ber(x) by
		* using PRR(2x, 0) when x < 1/2, PRR(2(1-x), 1) when x >= 1/2.
		* Step 2: If x is 0, report IRR(F, original), otherwise report IRR(F, y)
		* </p>
		*
		* Reference: go/bit-reporting-with-longitudinal-privacy
		* TODO: Add a public blog / site to explain how it works.
		*
		* @hide
		*/
		public class LongitudinalReportingEncoder implements DifferentialPrivacyEncoder {

		// Suffix that will be added to Rappor's encoder id. There's a (relatively) small risk some
		// other Rappor encoder may re-use the same encoder id.
		private static final String PRR1_ENCODER_ID = "prr1_encoder_id";
		private static final String PRR2_ENCODER_ID = "prr2_encoder_id";

		private final LongitudinalReportingConfig mConfig;

		// IRR encoder to encode input value.
		private final RapporEncoder mIRREncoder;

		// A value that used to replace original value as input, so there's always a chance we are
		// doing IRR on a fake value not actual original value.
		// Null if original value does not need to be replaced.
		private final Boolean mFakeValue;

		// True if encoder is securely randomized.
		private final boolean mIsSecure;

		/**
		* Create {@link LongitudinalReportingEncoder} with
		* {@link LongitudinalReportingConfig} provided.
		*
		* @param config Longitudinal Reporting parameters to encode input
		* @param userSecret User generated secret that used to generate PRR
		* @return {@link LongitudinalReportingEncoder} instance
		*/
		public static LongitudinalReportingEncoder createEncoder(LongitudinalReportingConfig config,
		byte[] userSecret) {
		return new LongitudinalReportingEncoder(config, true, userSecret);
		}

		/**
		* Create <strong>insecure</strong> {@link LongitudinalReportingEncoder} with
		* {@link LongitudinalReportingConfig} provided.
		* Should not use it to process sensitive data.
		*
		* @param config Rappor parameters to encode input.
		* @return {@link LongitudinalReportingEncoder} instance.
		*/
		@VisibleForTesting
		public static LongitudinalReportingEncoder createInsecureEncoderForTest(
		LongitudinalReportingConfig config) {
		return new LongitudinalReportingEncoder(config, false, null);
		}

		private LongitudinalReportingEncoder(LongitudinalReportingConfig config,
		boolean secureEncoder, byte[] userSecret) {
		mConfig = config;
		mIsSecure = secureEncoder;
		final boolean ignoreOriginalInput = getLongTermRandomizedResult(config.getProbabilityP(),
		secureEncoder, userSecret, config.getEncoderId() + PRR1_ENCODER_ID);

		if (ignoreOriginalInput) {
		mFakeValue = getLongTermRandomizedResult(config.getProbabilityQ(),
		secureEncoder, userSecret, config.getEncoderId() + PRR2_ENCODER_ID);
		} else {
		// Not using fake value, so IRR will be processed on real input value.
		mFakeValue = null;
		}

		final RapporConfig irrConfig = config.getIRRConfig();
		mIRREncoder = secureEncoder
		? RapporEncoder.createEncoder(irrConfig, userSecret)
		: RapporEncoder.createInsecureEncoderForTest(irrConfig);
		}

		@Override
		public byte[] encodeString(String original) {
		throw new UnsupportedOperationException();
		}

		@Override
		public byte[] encodeBoolean(boolean original) {
		if (mFakeValue != null) {
		// Use the fake value generated in PRR.
		original = mFakeValue.booleanValue();
		}
		return mIRREncoder.encodeBoolean(original);
		}

		@Override
		public byte[] encodeBits(byte[] bits) {
		throw new UnsupportedOperationException();
		}

		@Override
		public LongitudinalReportingConfig getConfig() {
		return mConfig;
		}

		@Override
		public boolean isInsecureEncoderForTest() {
		return !mIsSecure;
		}

		/**
		* Get PRR result that with probability p is 1, probability 1-p is 0.
		*/
		@VisibleForTesting
		public static boolean getLongTermRandomizedResult(double p, boolean secureEncoder,
		byte[] userSecret, String encoderId) {
		// Use Rappor to get PRR result. Rappor's P and Q are set to 0 and 1 so IRR will not be
		// effective.
		// As Rappor has rapporF/2 chance returns 0, rapporF/2 chance returns 1, and 1-rapporF
		// chance returns original input.
		// If p < 0.5, setting rapporF=2p and input=0 will make Rappor has p chance to return 1
		// P(output=1 \| input=0) = rapporF/2 = 2p/2 = p.
		// If p >= 0.5, setting rapporF=2(1-p) and input=1 will make Rappor has p chance
		// to return 1.
		// P(output=1 \| input=1) = rapporF/2 + (1 - rapporF) = 2(1-p)/2 + (1 - 2(1-p)) = p.
		final double effectiveF = p < 0.5f ? p * 2 : (1 - p) * 2;
		final boolean prrInput = p < 0.5f ? false : true;
		final RapporConfig prrConfig = new RapporConfig(encoderId, 1, effectiveF,
		0, 1, 1, 1);
		final RapporEncoder encoder = secureEncoder
		? RapporEncoder.createEncoder(prrConfig, userSecret)
		: RapporEncoder.createInsecureEncoderForTest(prrConfig);
		return encoder.encodeBoolean(prrInput)[0] > 0;
		}
		}