/*
* Copyright (C) 2020 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.stamp;
import static com.android.apksig.internal.apk.ApkSigningBlockUtils.encodeAsLengthPrefixedElement;
import static com.android.apksig.internal.apk.ApkSigningBlockUtils.encodeAsSequenceOfLengthPrefixedElements;
import static com.android.apksig.internal.apk.ApkSigningBlockUtils.encodeAsSequenceOfLengthPrefixedPairsOfIntAndLengthPrefixedBytes;
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.util.Pair;
import java.security.InvalidKeyException;
import java.security.NoSuchAlgorithmException;
import java.security.SignatureException;
import java.security.cert.CertificateEncodingException;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
import java.util.Map;
/**
* SourceStamp signer.
*
* <p>SourceStamp improves traceability of apps with respect to unauthorized distribution.
*
* <p>The stamp is part of the APK that is protected by the signing block.
*
* <p>The APK contents hash is signed using the stamp key, and is saved as part of the signing
* block.
*
* <p>V1 of the source stamp allows signing the digest of at most one signature scheme only.
*/
public abstract class V1SourceStampSigner {
public static final int V1_SOURCE_STAMP_BLOCK_ID =
SourceStampConstants.V1_SOURCE_STAMP_BLOCK_ID;
/** Hidden constructor to prevent instantiation. */
private V1SourceStampSigner() {}
public static Pair<byte[], Integer> generateSourceStampBlock(
SignerConfig sourceStampSignerConfig, Map<ContentDigestAlgorithm, byte[]> digestInfo)
throws SignatureException, NoSuchAlgorithmException, InvalidKeyException {
if (sourceStampSignerConfig.certificates.isEmpty()) {
throw new SignatureException("No certificates configured for signer");
}
List<Pair<Integer, byte[]>> digests = new ArrayList<>();
for (Map.Entry<ContentDigestAlgorithm, byte[]> digest : digestInfo.entrySet()) {
digests.add(Pair.of(digest.getKey().getId(), digest.getValue()));
}
Collections.sort(digests, Comparator.comparing(Pair::getFirst));
SourceStampBlock sourceStampBlock = new SourceStampBlock();
try {
sourceStampBlock.stampCertificate =
sourceStampSignerConfig.certificates.get(0).getEncoded();
} catch (CertificateEncodingException e) {
throw new SignatureException(
"Retrieving the encoded form of the stamp certificate failed", e);
}
byte[] digestBytes =
encodeAsSequenceOfLengthPrefixedPairsOfIntAndLengthPrefixedBytes(digests);
sourceStampBlock.signedDigests =
ApkSigningBlockUtils.generateSignaturesOverData(
sourceStampSignerConfig, digestBytes);
// FORMAT:
// * length-prefixed bytes: X.509 certificate (ASN.1 DER encoded)
// * length-prefixed sequence of length-prefixed signatures:
// * uint32: signature algorithm ID
// * length-prefixed bytes: signature of signed data
byte[] sourceStampSignerBlock =
encodeAsSequenceOfLengthPrefixedElements(
new byte[][] {
sourceStampBlock.stampCertificate,
encodeAsSequenceOfLengthPrefixedPairsOfIntAndLengthPrefixedBytes(
sourceStampBlock.signedDigests),
});
// FORMAT:
// * length-prefixed stamp block.
return Pair.of(encodeAsLengthPrefixedElement(sourceStampSignerBlock),
SourceStampConstants.V1_SOURCE_STAMP_BLOCK_ID);
}
private static final class SourceStampBlock {
public byte[] stampCertificate;
public List<Pair<Integer, byte[]>> signedDigests;
}
}