/*
* Copyright (C) 2016 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 com.android.apksig.internal.apk.v2;
import static com.android.apksig.Constants.MAX_APK_SIGNERS;
import static com.android.apksig.internal.apk.ApkSigningBlockUtils.encodeAsSequenceOfLengthPrefixedElements;
import static com.android.apksig.internal.apk.ApkSigningBlockUtils.encodeAsSequenceOfLengthPrefixedPairsOfIntAndLengthPrefixedBytes;
import static com.android.apksig.internal.apk.ApkSigningBlockUtils.encodeCertificates;
import static com.android.apksig.internal.apk.ApkSigningBlockUtils.encodePublicKey;
import com.android.apksig.internal.apk.ApkSigningBlockUtils;
import com.android.apksig.internal.apk.ApkSigningBlockUtils.SignerConfig;
import com.android.apksig.internal.apk.ContentDigestAlgorithm;
import com.android.apksig.internal.apk.SignatureAlgorithm;
import com.android.apksig.internal.util.Pair;
import com.android.apksig.util.DataSource;
import com.android.apksig.util.RunnablesExecutor;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.ByteOrder;
import java.security.InvalidKeyException;
import java.security.NoSuchAlgorithmException;
import java.security.PublicKey;
import java.security.SignatureException;
import java.security.cert.CertificateEncodingException;
import java.security.interfaces.ECKey;
import java.security.interfaces.RSAKey;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Map;
/**
* APK Signature Scheme v2 signer.
*
* <p>APK Signature Scheme v2 is a whole-file signature scheme which aims to protect every single
* bit of the APK, as opposed to the JAR Signature Scheme which protects only the names and
* uncompressed contents of ZIP entries.
*
* @see <a href="https://source.android.com/security/apksigning/v2.html">APK Signature Scheme v2</a>
*/
public abstract class V2SchemeSigner {
/*
* The two main goals of APK Signature Scheme v2 are:
* 1. Detect any unauthorized modifications to the APK. This is achieved by making the signature
* cover every byte of the APK being signed.
* 2. Enable much faster signature and integrity verification. This is achieved by requiring
* only a minimal amount of APK parsing before the signature is verified, thus completely
* bypassing ZIP entry decompression and by making integrity verification parallelizable by
* employing a hash tree.
*
* The generated signature block is wrapped into an APK Signing Block and inserted into the
* original APK immediately before the start of ZIP Central Directory. This is to ensure that
* JAR and ZIP parsers continue to work on the signed APK. The APK Signing Block is designed for
* extensibility. For example, a future signature scheme could insert its signatures there as
* well. The contract of the APK Signing Block is that all contents outside of the block must be
* protected by signatures inside the block.
*/
public static final int APK_SIGNATURE_SCHEME_V2_BLOCK_ID =
V2SchemeConstants.APK_SIGNATURE_SCHEME_V2_BLOCK_ID;
/** Hidden constructor to prevent instantiation. */
private V2SchemeSigner() {}
/**
* Gets the APK Signature Scheme v2 signature algorithms to be used for signing an APK using the
* provided key.
*
* @param minSdkVersion minimum API Level of the platform on which the APK may be installed (see
* AndroidManifest.xml minSdkVersion attribute).
* @throws InvalidKeyException if the provided key is not suitable for signing APKs using APK
* Signature Scheme v2
*/
public static List<SignatureAlgorithm> getSuggestedSignatureAlgorithms(PublicKey signingKey,
int minSdkVersion, boolean verityEnabled, boolean deterministicDsaSigning)
throws InvalidKeyException {
String keyAlgorithm = signingKey.getAlgorithm();
if ("RSA".equalsIgnoreCase(keyAlgorithm)) {
// Use RSASSA-PKCS1-v1_5 signature scheme instead of RSASSA-PSS to guarantee
// deterministic signatures which make life easier for OTA updates (fewer files
// changed when deterministic signature schemes are used).
// Pick a digest which is no weaker than the key.
int modulusLengthBits = ((RSAKey) signingKey).getModulus().bitLength();
if (modulusLengthBits <= 3072) {
// 3072-bit RSA is roughly 128-bit strong, meaning SHA-256 is a good fit.
List<SignatureAlgorithm> algorithms = new ArrayList<>();
algorithms.add(SignatureAlgorithm.RSA_PKCS1_V1_5_WITH_SHA256);
if (verityEnabled) {
algorithms.add(SignatureAlgorithm.VERITY_RSA_PKCS1_V1_5_WITH_SHA256);
}
return algorithms;
} else {
// Keys longer than 3072 bit need to be paired with a stronger digest to avoid the
// digest being the weak link. SHA-512 is the next strongest supported digest.
return Collections.singletonList(SignatureAlgorithm.RSA_PKCS1_V1_5_WITH_SHA512);
}
} else if ("DSA".equalsIgnoreCase(keyAlgorithm)) {
// DSA is supported only with SHA-256.
List<SignatureAlgorithm> algorithms = new ArrayList<>();
algorithms.add(
deterministicDsaSigning ?
SignatureAlgorithm.DETDSA_WITH_SHA256 :
SignatureAlgorithm.DSA_WITH_SHA256);
if (verityEnabled) {
algorithms.add(SignatureAlgorithm.VERITY_DSA_WITH_SHA256);
}
return algorithms;
} else if ("EC".equalsIgnoreCase(keyAlgorithm)) {
// Pick a digest which is no weaker than the key.
int keySizeBits = ((ECKey) signingKey).getParams().getOrder().bitLength();
if (keySizeBits <= 256) {
// 256-bit Elliptic Curve is roughly 128-bit strong, meaning SHA-256 is a good fit.
List<SignatureAlgorithm> algorithms = new ArrayList<>();
algorithms.add(SignatureAlgorithm.ECDSA_WITH_SHA256);
if (verityEnabled) {
algorithms.add(SignatureAlgorithm.VERITY_ECDSA_WITH_SHA256);
}
return algorithms;
} else {
// Keys longer than 256 bit need to be paired with a stronger digest to avoid the
// digest being the weak link. SHA-512 is the next strongest supported digest.
return Collections.singletonList(SignatureAlgorithm.ECDSA_WITH_SHA512);
}
} else {
throw new InvalidKeyException("Unsupported key algorithm: " + keyAlgorithm);
}
}
public static ApkSigningBlockUtils.SigningSchemeBlockAndDigests
generateApkSignatureSchemeV2Block(RunnablesExecutor executor,
DataSource beforeCentralDir,
DataSource centralDir,
DataSource eocd,
List<SignerConfig> signerConfigs,
boolean v3SigningEnabled)
throws IOException, InvalidKeyException, NoSuchAlgorithmException,
SignatureException {
return generateApkSignatureSchemeV2Block(executor, beforeCentralDir, centralDir, eocd,
signerConfigs, v3SigningEnabled, null);
}
public static ApkSigningBlockUtils.SigningSchemeBlockAndDigests
generateApkSignatureSchemeV2Block(
RunnablesExecutor executor,
DataSource beforeCentralDir,
DataSource centralDir,
DataSource eocd,
List<SignerConfig> signerConfigs,
boolean v3SigningEnabled,
List<byte[]> preservedV2SignerBlocks)
throws IOException, InvalidKeyException, NoSuchAlgorithmException,
SignatureException {
Pair<List<SignerConfig>, Map<ContentDigestAlgorithm, byte[]>> digestInfo =
ApkSigningBlockUtils.computeContentDigests(
executor, beforeCentralDir, centralDir, eocd, signerConfigs);
return new ApkSigningBlockUtils.SigningSchemeBlockAndDigests(
generateApkSignatureSchemeV2Block(
digestInfo.getFirst(), digestInfo.getSecond(), v3SigningEnabled,
preservedV2SignerBlocks),
digestInfo.getSecond());
}
private static Pair<byte[], Integer> generateApkSignatureSchemeV2Block(
List<SignerConfig> signerConfigs,
Map<ContentDigestAlgorithm, byte[]> contentDigests,
boolean v3SigningEnabled,
List<byte[]> preservedV2SignerBlocks)
throws NoSuchAlgorithmException, InvalidKeyException, SignatureException {
// FORMAT:
// * length-prefixed sequence of length-prefixed signer blocks.
if (signerConfigs.size() > MAX_APK_SIGNERS) {
throw new IllegalArgumentException(
"APK Signature Scheme v2 only supports a maximum of " + MAX_APK_SIGNERS + ", "
+ signerConfigs.size() + " provided");
}
List<byte[]> signerBlocks = new ArrayList<>(signerConfigs.size());
if (preservedV2SignerBlocks != null && preservedV2SignerBlocks.size() > 0) {
signerBlocks.addAll(preservedV2SignerBlocks);
}
int signerNumber = 0;
for (SignerConfig signerConfig : signerConfigs) {
signerNumber++;
byte[] signerBlock;
try {
signerBlock = generateSignerBlock(signerConfig, contentDigests, v3SigningEnabled);
} catch (InvalidKeyException e) {
throw new InvalidKeyException("Signer #" + signerNumber + " failed", e);
} catch (SignatureException e) {
throw new SignatureException("Signer #" + signerNumber + " failed", e);
}
signerBlocks.add(signerBlock);
}
return Pair.of(
encodeAsSequenceOfLengthPrefixedElements(
new byte[][] {
encodeAsSequenceOfLengthPrefixedElements(signerBlocks),
}),
V2SchemeConstants.APK_SIGNATURE_SCHEME_V2_BLOCK_ID);
}
private static byte[] generateSignerBlock(
SignerConfig signerConfig,
Map<ContentDigestAlgorithm, byte[]> contentDigests,
boolean v3SigningEnabled)
throws NoSuchAlgorithmException, InvalidKeyException, SignatureException {
if (signerConfig.certificates.isEmpty()) {
throw new SignatureException("No certificates configured for signer");
}
PublicKey publicKey = signerConfig.certificates.get(0).getPublicKey();
byte[] encodedPublicKey = encodePublicKey(publicKey);
V2SignatureSchemeBlock.SignedData signedData = new V2SignatureSchemeBlock.SignedData();
try {
signedData.certificates = encodeCertificates(signerConfig.certificates);
} catch (CertificateEncodingException e) {
throw new SignatureException("Failed to encode certificates", e);
}
List<Pair<Integer, byte[]>> digests =
new ArrayList<>(signerConfig.signatureAlgorithms.size());
for (SignatureAlgorithm signatureAlgorithm : signerConfig.signatureAlgorithms) {
ContentDigestAlgorithm contentDigestAlgorithm =
signatureAlgorithm.getContentDigestAlgorithm();
byte[] contentDigest = contentDigests.get(contentDigestAlgorithm);
if (contentDigest == null) {
throw new RuntimeException(
contentDigestAlgorithm
+ " content digest for "
+ signatureAlgorithm
+ " not computed");
}
digests.add(Pair.of(signatureAlgorithm.getId(), contentDigest));
}
signedData.digests = digests;
signedData.additionalAttributes = generateAdditionalAttributes(v3SigningEnabled);
V2SignatureSchemeBlock.Signer signer = new V2SignatureSchemeBlock.Signer();
// FORMAT:
// * length-prefixed sequence of length-prefixed digests:
// * uint32: signature algorithm ID
// * length-prefixed bytes: digest of contents
// * length-prefixed sequence of certificates:
// * length-prefixed bytes: X.509 certificate (ASN.1 DER encoded).
// * length-prefixed sequence of length-prefixed additional attributes:
// * uint32: ID
// * (length - 4) bytes: value
signer.signedData =
encodeAsSequenceOfLengthPrefixedElements(
new byte[][] {
encodeAsSequenceOfLengthPrefixedPairsOfIntAndLengthPrefixedBytes(
signedData.digests),
encodeAsSequenceOfLengthPrefixedElements(signedData.certificates),
signedData.additionalAttributes,
new byte[0],
});
signer.publicKey = encodedPublicKey;
signer.signatures = new ArrayList<>();
signer.signatures =
ApkSigningBlockUtils.generateSignaturesOverData(signerConfig, signer.signedData);
// FORMAT:
// * length-prefixed signed data
// * length-prefixed sequence of length-prefixed signatures:
// * uint32: signature algorithm ID
// * length-prefixed bytes: signature of signed data
// * length-prefixed bytes: public key (X.509 SubjectPublicKeyInfo, ASN.1 DER encoded)
return encodeAsSequenceOfLengthPrefixedElements(
new byte[][] {
signer.signedData,
encodeAsSequenceOfLengthPrefixedPairsOfIntAndLengthPrefixedBytes(
signer.signatures),
signer.publicKey,
});
}
private static byte[] generateAdditionalAttributes(boolean v3SigningEnabled) {
if (v3SigningEnabled) {
// FORMAT (little endian):
// * length-prefixed bytes: attribute pair
// * uint32: ID - STRIPPING_PROTECTION_ATTR_ID in this case
// * uint32: value - 3 (v3 signature scheme id) in this case
int payloadSize = 4 + 4 + 4;
ByteBuffer result = ByteBuffer.allocate(payloadSize);
result.order(ByteOrder.LITTLE_ENDIAN);
result.putInt(payloadSize - 4);
result.putInt(V2SchemeConstants.STRIPPING_PROTECTION_ATTR_ID);
result.putInt(ApkSigningBlockUtils.VERSION_APK_SIGNATURE_SCHEME_V3);
return result.array();
} else {
return new byte[0];
}
}
private static final class V2SignatureSchemeBlock {
private static final class Signer {
public byte[] signedData;
public List<Pair<Integer, byte[]>> signatures;
public byte[] publicKey;
}
private static final class SignedData {
public List<Pair<Integer, byte[]>> digests;
public List<byte[]> certificates;
public byte[] additionalAttributes;
}
}
}
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