path: root/src/main/java/com/android/apksig/internal/apk/v2/V2SchemeSigner.java

/*
 * 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.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.
     */

    private static final int APK_SIGNATURE_SCHEME_V2_BLOCK_ID = 0x7109871a;

    /** 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 apkSigningBlockPaddingSupported)
            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 (apkSigningBlockPaddingSupported) {
                    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(SignatureAlgorithm.DSA_WITH_SHA256);
            if (apkSigningBlockPaddingSupported) {
                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 (apkSigningBlockPaddingSupported) {
                    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 Pair<byte[], Integer> generateApkSignatureSchemeV2Block(
            RunnablesExecutor executor,
            DataSource beforeCentralDir,
            DataSource centralDir,
            DataSource eocd,
            List<SignerConfig> signerConfigs,
            boolean v3SigningEnabled)
                    throws IOException, InvalidKeyException, NoSuchAlgorithmException,
                            SignatureException {
        Pair<List<SignerConfig>,
                Map<ContentDigestAlgorithm, byte[]>> digestInfo =
                ApkSigningBlockUtils.computeContentDigests(
                        executor, beforeCentralDir, centralDir, eocd, signerConfigs);
        return generateApkSignatureSchemeV2Block(
                digestInfo.getFirst(), digestInfo.getSecond(),v3SigningEnabled);
    }

    private static Pair<byte[], Integer> generateApkSignatureSchemeV2Block(
            List<SignerConfig> signerConfigs,
            Map<ContentDigestAlgorithm, byte[]> contentDigests,
            boolean v3SigningEnabled)
                    throws NoSuchAlgorithmException, InvalidKeyException, SignatureException {
        // FORMAT:
        // * length-prefixed sequence of length-prefixed signer blocks.

        List<byte[]> signerBlocks = new ArrayList<>(signerConfigs.size());
        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),
                }), 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,
                });
    }

    // Attribute to check whether a newer APK Signature Scheme signature was stripped
    protected static final int STRIPPING_PROTECTION_ATTR_ID = 0xbeeff00d;

    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(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;
        }
    }

}