/*
 * Copyright (C) 2018 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.util.apk;

import static android.util.apk.ApkSigningBlockUtils.CONTENT_DIGEST_VERITY_CHUNKED_SHA256;
import static android.util.apk.ApkSigningBlockUtils.SIGNATURE_DSA_WITH_SHA256;
import static android.util.apk.ApkSigningBlockUtils.SIGNATURE_ECDSA_WITH_SHA256;
import static android.util.apk.ApkSigningBlockUtils.SIGNATURE_ECDSA_WITH_SHA512;
import static android.util.apk.ApkSigningBlockUtils.SIGNATURE_RSA_PKCS1_V1_5_WITH_SHA256;
import static android.util.apk.ApkSigningBlockUtils.SIGNATURE_RSA_PKCS1_V1_5_WITH_SHA512;
import static android.util.apk.ApkSigningBlockUtils.SIGNATURE_RSA_PSS_WITH_SHA256;
import static android.util.apk.ApkSigningBlockUtils.SIGNATURE_RSA_PSS_WITH_SHA512;
import static android.util.apk.ApkSigningBlockUtils.SIGNATURE_VERITY_DSA_WITH_SHA256;
import static android.util.apk.ApkSigningBlockUtils.SIGNATURE_VERITY_ECDSA_WITH_SHA256;
import static android.util.apk.ApkSigningBlockUtils.SIGNATURE_VERITY_RSA_PKCS1_V1_5_WITH_SHA256;
import static android.util.apk.ApkSigningBlockUtils.compareSignatureAlgorithm;
import static android.util.apk.ApkSigningBlockUtils.getContentDigestAlgorithmJcaDigestAlgorithm;
import static android.util.apk.ApkSigningBlockUtils.getLengthPrefixedSlice;
import static android.util.apk.ApkSigningBlockUtils.getSignatureAlgorithmContentDigestAlgorithm;
import static android.util.apk.ApkSigningBlockUtils.getSignatureAlgorithmJcaKeyAlgorithm;
import static android.util.apk.ApkSigningBlockUtils.getSignatureAlgorithmJcaSignatureAlgorithm;
import static android.util.apk.ApkSigningBlockUtils.readLengthPrefixedByteArray;

import android.os.Build;
import android.util.ArrayMap;
import android.util.Pair;

import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.BufferUnderflowException;
import java.nio.ByteBuffer;
import java.security.DigestException;
import java.security.InvalidAlgorithmParameterException;
import java.security.InvalidKeyException;
import java.security.KeyFactory;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.security.PublicKey;
import java.security.Signature;
import java.security.SignatureException;
import java.security.cert.CertificateEncodingException;
import java.security.cert.CertificateException;
import java.security.cert.CertificateFactory;
import java.security.cert.X509Certificate;
import java.security.spec.AlgorithmParameterSpec;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.X509EncodedKeySpec;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashSet;
import java.util.List;
import java.util.Map;

APK Signature Scheme v3 verifier.
@hide
for internal use only./**
 * APK Signature Scheme v3 verifier.
 *
 * @hide for internal use only.
 */
public class ApkSignatureSchemeV3Verifier {

    
ID of this signature scheme as used in X-Android-APK-Signed header used in JAR signing.
/**
     * ID of this signature scheme as used in X-Android-APK-Signed header used in JAR signing.
     */
    public static final int SF_ATTRIBUTE_ANDROID_APK_SIGNED_ID = 3;

    private static final int APK_SIGNATURE_SCHEME_V3_BLOCK_ID = 0xf05368c0;

    
Returns true if the provided APK contains an APK Signature Scheme V3 signature. 
NOTE: This method does not verify the signature.
/**
     * Returns {@code true} if the provided APK contains an APK Signature Scheme V3 signature.
     *
     * <p><b>NOTE: This method does not verify the signature.</b>
     */
    public static boolean hasSignature(String apkFile) throws IOException {
        try (RandomAccessFile apk = new RandomAccessFile(apkFile, "r")) {
            findSignature(apk);
            return true;
        } catch (SignatureNotFoundException e) {
            return false;
        }
    }

    
Verifies APK Signature Scheme v3 signatures of the provided APK and returns the certificates
associated with each signer.
Throws:
SignatureNotFoundException – if the APK is not signed using APK Signature Scheme v3.
SecurityException – if the APK Signature Scheme v3 signature of this APK does not
verify.
IOException – if an I/O error occurs while reading the APK file./**
     * Verifies APK Signature Scheme v3 signatures of the provided APK and returns the certificates
     * associated with each signer.
     *
     * @throws SignatureNotFoundException if the APK is not signed using APK Signature Scheme v3.
     * @throws SecurityException if the APK Signature Scheme v3 signature of this APK does not
     * verify.
     * @throws IOException if an I/O error occurs while reading the APK file.
     */
    public static VerifiedSigner verify(String apkFile)
            throws SignatureNotFoundException, SecurityException, IOException {
        return verify(apkFile, true);
    }

    
Returns the certificates associated with each signer for the given APK without verification.
This method is dangerous and should not be used, unless the caller is absolutely certain the
APK is trusted.  Specifically, verification is only done for the APK Signature Scheme v3
Block while gathering signer information.  The APK contents are not verified.
Throws:
SignatureNotFoundException – if the APK is not signed using APK Signature Scheme v3.
IOException – if an I/O error occurs while reading the APK file./**
     * Returns the certificates associated with each signer for the given APK without verification.
     * This method is dangerous and should not be used, unless the caller is absolutely certain the
     * APK is trusted.  Specifically, verification is only done for the APK Signature Scheme v3
     * Block while gathering signer information.  The APK contents are not verified.
     *
     * @throws SignatureNotFoundException if the APK is not signed using APK Signature Scheme v3.
     * @throws IOException if an I/O error occurs while reading the APK file.
     */
    public static VerifiedSigner plsCertsNoVerifyOnlyCerts(String apkFile)
            throws SignatureNotFoundException, SecurityException, IOException {
        return verify(apkFile, false);
    }

    private static VerifiedSigner verify(String apkFile, boolean verifyIntegrity)
            throws SignatureNotFoundException, SecurityException, IOException {
        try (RandomAccessFile apk = new RandomAccessFile(apkFile, "r")) {
            return verify(apk, verifyIntegrity);
        }
    }

    
Verifies APK Signature Scheme v3 signatures of the provided APK and returns the certificates
associated with each signer.
Throws:
SignatureNotFoundException – if the APK is not signed using APK Signature Scheme v3.
SecurityException – if an APK Signature Scheme v3 signature of this APK does not
        verify.
IOException – if an I/O error occurs while reading the APK file./**
     * Verifies APK Signature Scheme v3 signatures of the provided APK and returns the certificates
     * associated with each signer.
     *
     * @throws SignatureNotFoundException if the APK is not signed using APK Signature Scheme v3.
     * @throws SecurityException if an APK Signature Scheme v3 signature of this APK does not
     *         verify.
     * @throws IOException if an I/O error occurs while reading the APK file.
     */
    private static VerifiedSigner verify(RandomAccessFile apk, boolean verifyIntegrity)
            throws SignatureNotFoundException, SecurityException, IOException {
        SignatureInfo signatureInfo = findSignature(apk);
        return verify(apk, signatureInfo, verifyIntegrity);
    }

    
Returns the APK Signature Scheme v3 block contained in the provided APK file and the
additional information relevant for verifying the block against the file.
Throws:
SignatureNotFoundException – if the APK is not signed using APK Signature Scheme v3.
IOException – if an I/O error occurs while reading the APK file./**
     * Returns the APK Signature Scheme v3 block contained in the provided APK file and the
     * additional information relevant for verifying the block against the file.
     *
     * @throws SignatureNotFoundException if the APK is not signed using APK Signature Scheme v3.
     * @throws IOException if an I/O error occurs while reading the APK file.
     */
    private static SignatureInfo findSignature(RandomAccessFile apk)
            throws IOException, SignatureNotFoundException {
        return ApkSigningBlockUtils.findSignature(apk, APK_SIGNATURE_SCHEME_V3_BLOCK_ID);
    }

    
Verifies the contents of the provided APK file against the provided APK Signature Scheme v3
Block.
Params:
signatureInfo – APK Signature Scheme v3 Block and information relevant for verifying it
       against the APK file./**
     * Verifies the contents of the provided APK file against the provided APK Signature Scheme v3
     * Block.
     *
     * @param signatureInfo APK Signature Scheme v3 Block and information relevant for verifying it
     *        against the APK file.
     */
    private static VerifiedSigner verify(
            RandomAccessFile apk,
            SignatureInfo signatureInfo,
            boolean doVerifyIntegrity) throws SecurityException, IOException {
        int signerCount = 0;
        Map<Integer, byte[]> contentDigests = new ArrayMap<>();
        VerifiedSigner result = null;
        CertificateFactory certFactory;
        try {
            certFactory = CertificateFactory.getInstance("X.509");
        } catch (CertificateException e) {
            throw new RuntimeException("Failed to obtain X.509 CertificateFactory", e);
        }
        ByteBuffer signers;
        try {
            signers = getLengthPrefixedSlice(signatureInfo.signatureBlock);
        } catch (IOException e) {
            throw new SecurityException("Failed to read list of signers", e);
        }
        while (signers.hasRemaining()) {
            try {
                ByteBuffer signer = getLengthPrefixedSlice(signers);
                result = verifySigner(signer, contentDigests, certFactory);
                signerCount++;
            } catch (PlatformNotSupportedException e) {
                // this signer is for a different platform, ignore it.
                continue;
            } catch (IOException | BufferUnderflowException | SecurityException e) {
                throw new SecurityException(
                        "Failed to parse/verify signer #" + signerCount + " block",
                        e);
            }
        }

        if (signerCount < 1 || result == null) {
            throw new SecurityException("No signers found");
        }

        if (signerCount != 1) {
            throw new SecurityException("APK Signature Scheme V3 only supports one signer: "
                    + "multiple signers found.");
        }

        if (contentDigests.isEmpty()) {
            throw new SecurityException("No content digests found");
        }

        if (doVerifyIntegrity) {
            ApkSigningBlockUtils.verifyIntegrity(contentDigests, apk, signatureInfo);
        }

        if (contentDigests.containsKey(CONTENT_DIGEST_VERITY_CHUNKED_SHA256)) {
            byte[] verityDigest = contentDigests.get(CONTENT_DIGEST_VERITY_CHUNKED_SHA256);
            result.verityRootHash = ApkSigningBlockUtils.parseVerityDigestAndVerifySourceLength(
                    verityDigest, apk.length(), signatureInfo);
        }

        return result;
    }

    private static VerifiedSigner verifySigner(
            ByteBuffer signerBlock,
            Map<Integer, byte[]> contentDigests,
            CertificateFactory certFactory)
            throws SecurityException, IOException, PlatformNotSupportedException {
        ByteBuffer signedData = getLengthPrefixedSlice(signerBlock);
        int minSdkVersion = signerBlock.getInt();
        int maxSdkVersion = signerBlock.getInt();

        if (Build.VERSION.SDK_INT < minSdkVersion || Build.VERSION.SDK_INT > maxSdkVersion) {
            // this signature isn't meant to be used with this platform, skip it.
            throw new PlatformNotSupportedException(
                    "Signer not supported by this platform "
                    + "version. This platform: " + Build.VERSION.SDK_INT
                    + ", signer minSdkVersion: " + minSdkVersion
                    + ", maxSdkVersion: " + maxSdkVersion);
        }

        ByteBuffer signatures = getLengthPrefixedSlice(signerBlock);
        byte[] publicKeyBytes = readLengthPrefixedByteArray(signerBlock);

        int signatureCount = 0;
        int bestSigAlgorithm = -1;
        byte[] bestSigAlgorithmSignatureBytes = null;
        List<Integer> signaturesSigAlgorithms = new ArrayList<>();
        while (signatures.hasRemaining()) {
            signatureCount++;
            try {
                ByteBuffer signature = getLengthPrefixedSlice(signatures);
                if (signature.remaining() < 8) {
                    throw new SecurityException("Signature record too short");
                }
                int sigAlgorithm = signature.getInt();
                signaturesSigAlgorithms.add(sigAlgorithm);
                if (!isSupportedSignatureAlgorithm(sigAlgorithm)) {
                    continue;
                }
                if ((bestSigAlgorithm == -1)
                        || (compareSignatureAlgorithm(sigAlgorithm, bestSigAlgorithm) > 0)) {
                    bestSigAlgorithm = sigAlgorithm;
                    bestSigAlgorithmSignatureBytes = readLengthPrefixedByteArray(signature);
                }
            } catch (IOException | BufferUnderflowException e) {
                throw new SecurityException(
                        "Failed to parse signature record #" + signatureCount,
                        e);
            }
        }
        if (bestSigAlgorithm == -1) {
            if (signatureCount == 0) {
                throw new SecurityException("No signatures found");
            } else {
                throw new SecurityException("No supported signatures found");
            }
        }

        String keyAlgorithm = getSignatureAlgorithmJcaKeyAlgorithm(bestSigAlgorithm);
        Pair<String, ? extends AlgorithmParameterSpec> signatureAlgorithmParams =
                getSignatureAlgorithmJcaSignatureAlgorithm(bestSigAlgorithm);
        String jcaSignatureAlgorithm = signatureAlgorithmParams.first;
        AlgorithmParameterSpec jcaSignatureAlgorithmParams = signatureAlgorithmParams.second;
        boolean sigVerified;
        try {
            PublicKey publicKey =
                    KeyFactory.getInstance(keyAlgorithm)
                            .generatePublic(new X509EncodedKeySpec(publicKeyBytes));
            Signature sig = Signature.getInstance(jcaSignatureAlgorithm);
            sig.initVerify(publicKey);
            if (jcaSignatureAlgorithmParams != null) {
                sig.setParameter(jcaSignatureAlgorithmParams);
            }
            sig.update(signedData);
            sigVerified = sig.verify(bestSigAlgorithmSignatureBytes);
        } catch (NoSuchAlgorithmException | InvalidKeySpecException | InvalidKeyException
                | InvalidAlgorithmParameterException | SignatureException e) {
            throw new SecurityException(
                    "Failed to verify " + jcaSignatureAlgorithm + " signature", e);
        }
        if (!sigVerified) {
            throw new SecurityException(jcaSignatureAlgorithm + " signature did not verify");
        }

        // Signature over signedData has verified.

        byte[] contentDigest = null;
        signedData.clear();
        ByteBuffer digests = getLengthPrefixedSlice(signedData);
        List<Integer> digestsSigAlgorithms = new ArrayList<>();
        int digestCount = 0;
        while (digests.hasRemaining()) {
            digestCount++;
            try {
                ByteBuffer digest = getLengthPrefixedSlice(digests);
                if (digest.remaining() < 8) {
                    throw new IOException("Record too short");
                }
                int sigAlgorithm = digest.getInt();
                digestsSigAlgorithms.add(sigAlgorithm);
                if (sigAlgorithm == bestSigAlgorithm) {
                    contentDigest = readLengthPrefixedByteArray(digest);
                }
            } catch (IOException | BufferUnderflowException e) {
                throw new IOException("Failed to parse digest record #" + digestCount, e);
            }
        }

        if (!signaturesSigAlgorithms.equals(digestsSigAlgorithms)) {
            throw new SecurityException(
                    "Signature algorithms don't match between digests and signatures records");
        }
        int digestAlgorithm = getSignatureAlgorithmContentDigestAlgorithm(bestSigAlgorithm);
        byte[] previousSignerDigest = contentDigests.put(digestAlgorithm, contentDigest);
        if ((previousSignerDigest != null)
                && (!MessageDigest.isEqual(previousSignerDigest, contentDigest))) {
            throw new SecurityException(
                    getContentDigestAlgorithmJcaDigestAlgorithm(digestAlgorithm)
                    + " contents digest does not match the digest specified by a preceding signer");
        }

        ByteBuffer certificates = getLengthPrefixedSlice(signedData);
        List<X509Certificate> certs = new ArrayList<>();
        int certificateCount = 0;
        while (certificates.hasRemaining()) {
            certificateCount++;
            byte[] encodedCert = readLengthPrefixedByteArray(certificates);
            X509Certificate certificate;
            try {
                certificate = (X509Certificate)
                        certFactory.generateCertificate(new ByteArrayInputStream(encodedCert));
            } catch (CertificateException e) {
                throw new SecurityException("Failed to decode certificate #" + certificateCount, e);
            }
            certificate = new VerbatimX509Certificate(
                    certificate, encodedCert);
            certs.add(certificate);
        }

        if (certs.isEmpty()) {
            throw new SecurityException("No certificates listed");
        }
        X509Certificate mainCertificate = certs.get(0);
        byte[] certificatePublicKeyBytes = mainCertificate.getPublicKey().getEncoded();
        if (!Arrays.equals(publicKeyBytes, certificatePublicKeyBytes)) {
            throw new SecurityException(
                    "Public key mismatch between certificate and signature record");
        }

        int signedMinSDK = signedData.getInt();
        if (signedMinSDK != minSdkVersion) {
            throw new SecurityException(
                    "minSdkVersion mismatch between signed and unsigned in v3 signer block.");
        }

        int signedMaxSDK = signedData.getInt();
        if (signedMaxSDK != maxSdkVersion) {
            throw new SecurityException(
                    "maxSdkVersion mismatch between signed and unsigned in v3 signer block.");
        }

        ByteBuffer additionalAttrs = getLengthPrefixedSlice(signedData);
        return verifyAdditionalAttributes(additionalAttrs, certs, certFactory);
    }

    private static final int PROOF_OF_ROTATION_ATTR_ID = 0x3ba06f8c;

    private static VerifiedSigner verifyAdditionalAttributes(ByteBuffer attrs,
            List<X509Certificate> certs, CertificateFactory certFactory) throws IOException {
        X509Certificate[] certChain = certs.toArray(new X509Certificate[certs.size()]);
        VerifiedProofOfRotation por = null;

        while (attrs.hasRemaining()) {
            ByteBuffer attr = getLengthPrefixedSlice(attrs);
            if (attr.remaining() < 4) {
                throw new IOException("Remaining buffer too short to contain additional attribute "
                        + "ID. Remaining: " + attr.remaining());
            }
            int id = attr.getInt();
            switch(id) {
                case PROOF_OF_ROTATION_ATTR_ID:
                    if (por != null) {
                        throw new SecurityException("Encountered multiple Proof-of-rotation records"
                                + " when verifying APK Signature Scheme v3 signature");
                    }
                    por = verifyProofOfRotationStruct(attr, certFactory);
                    // make sure that the last certificate in the Proof-of-rotation record matches
                    // the one used to sign this APK.
                    try {
                        if (por.certs.size() > 0
                                && !Arrays.equals(por.certs.get(por.certs.size() - 1).getEncoded(),
                                        certChain[0].getEncoded())) {
                            throw new SecurityException("Terminal certificate in Proof-of-rotation"
                                    + " record does not match APK signing certificate");
                        }
                    } catch (CertificateEncodingException e) {
                        throw new SecurityException("Failed to encode certificate when comparing"
                                + " Proof-of-rotation record and signing certificate", e);
                    }

                    break;
                default:
                    // not the droid we're looking for, move along, move along.
                    break;
            }
        }
        return new VerifiedSigner(certChain, por);
    }

    private static VerifiedProofOfRotation verifyProofOfRotationStruct(
            ByteBuffer porBuf,
            CertificateFactory certFactory)
            throws SecurityException, IOException {
        int levelCount = 0;
        int lastSigAlgorithm = -1;
        X509Certificate lastCert = null;
        List<X509Certificate> certs = new ArrayList<>();
        List<Integer> flagsList = new ArrayList<>();

        // Proof-of-rotation struct:
        // A uint32 version code followed by basically a singly linked list of nodes, called levels
        // here, each of which have the following structure:
        // * length-prefix for the entire level
        //     - length-prefixed signed data (if previous level exists)
        //         * length-prefixed X509 Certificate
        //         * uint32 signature algorithm ID describing how this signed data was signed
        //     - uint32 flags describing how to treat the cert contained in this level
        //     - uint32 signature algorithm ID to use to verify the signature of the next level. The
        //         algorithm here must match the one in the signed data section of the next level.
        //     - length-prefixed signature over the signed data in this level.  The signature here
        //         is verified using the certificate from the previous level.
        // The linking is provided by the certificate of each level signing the one of the next.

        try {

            // get the version code, but don't do anything with it: creator knew about all our flags
            porBuf.getInt();
            HashSet<X509Certificate> certHistorySet = new HashSet<>();
            while (porBuf.hasRemaining()) {
                levelCount++;
                ByteBuffer level = getLengthPrefixedSlice(porBuf);
                ByteBuffer signedData = getLengthPrefixedSlice(level);
                int flags = level.getInt();
                int sigAlgorithm = level.getInt();
                byte[] signature = readLengthPrefixedByteArray(level);

                if (lastCert != null) {
                    // Use previous level cert to verify current level
                    Pair<String, ? extends AlgorithmParameterSpec> sigAlgParams =
                            getSignatureAlgorithmJcaSignatureAlgorithm(lastSigAlgorithm);
                    PublicKey publicKey = lastCert.getPublicKey();
                    Signature sig = Signature.getInstance(sigAlgParams.first);
                    sig.initVerify(publicKey);
                    if (sigAlgParams.second != null) {
                        sig.setParameter(sigAlgParams.second);
                    }
                    sig.update(signedData);
                    if (!sig.verify(signature)) {
                        throw new SecurityException("Unable to verify signature of certificate #"
                                + levelCount + " using " + sigAlgParams.first + " when verifying"
                                + " Proof-of-rotation record");
                    }
                }

                signedData.rewind();
                byte[] encodedCert = readLengthPrefixedByteArray(signedData);
                int signedSigAlgorithm = signedData.getInt();
                if (lastCert != null && lastSigAlgorithm != signedSigAlgorithm) {
                    throw new SecurityException("Signing algorithm ID mismatch for certificate #"
                            + levelCount + " when verifying Proof-of-rotation record");
                }
                lastCert = (X509Certificate)
                        certFactory.generateCertificate(new ByteArrayInputStream(encodedCert));
                lastCert = new VerbatimX509Certificate(lastCert, encodedCert);

                lastSigAlgorithm = sigAlgorithm;
                if (certHistorySet.contains(lastCert)) {
                    throw new SecurityException("Encountered duplicate entries in "
                            + "Proof-of-rotation record at certificate #" + levelCount + ".  All "
                            + "signing certificates should be unique");
                }
                certHistorySet.add(lastCert);
                certs.add(lastCert);
                flagsList.add(flags);
            }
        } catch (IOException | BufferUnderflowException e) {
            throw new IOException("Failed to parse Proof-of-rotation record", e);
        } catch (NoSuchAlgorithmException | InvalidKeyException
                | InvalidAlgorithmParameterException | SignatureException e) {
            throw new SecurityException(
                    "Failed to verify signature over signed data for certificate #"
                            + levelCount + " when verifying Proof-of-rotation record", e);
        } catch (CertificateException e) {
            throw new SecurityException("Failed to decode certificate #" + levelCount
                    + " when verifying Proof-of-rotation record", e);
        }
        return new VerifiedProofOfRotation(certs, flagsList);
    }

    static byte[] getVerityRootHash(String apkPath)
            throws IOException, SignatureNotFoundException, SecurityException {
        try (RandomAccessFile apk = new RandomAccessFile(apkPath, "r")) {
            SignatureInfo signatureInfo = findSignature(apk);
            VerifiedSigner vSigner = verify(apk, false);
            return vSigner.verityRootHash;
        }
    }

    static byte[] generateApkVerity(String apkPath, ByteBufferFactory bufferFactory)
            throws IOException, SignatureNotFoundException, SecurityException, DigestException,
                   NoSuchAlgorithmException {
        try (RandomAccessFile apk = new RandomAccessFile(apkPath, "r")) {
            SignatureInfo signatureInfo = findSignature(apk);
            return ApkSigningBlockUtils.generateApkVerity(apkPath, bufferFactory, signatureInfo);
        }
    }

    static byte[] generateFsverityRootHash(String apkPath)
            throws NoSuchAlgorithmException, DigestException, IOException,
                   SignatureNotFoundException {
        try (RandomAccessFile apk = new RandomAccessFile(apkPath, "r")) {
            SignatureInfo signatureInfo = findSignature(apk);
            VerifiedSigner vSigner = verify(apk, false);
            if (vSigner.verityRootHash == null) {
                return null;
            }
            return ApkVerityBuilder.generateFsverityRootHash(
                    apk, ByteBuffer.wrap(vSigner.verityRootHash), signatureInfo);
        }
    }

    private static boolean isSupportedSignatureAlgorithm(int sigAlgorithm) {
        switch (sigAlgorithm) {
            case SIGNATURE_RSA_PSS_WITH_SHA256:
            case SIGNATURE_RSA_PSS_WITH_SHA512:
            case SIGNATURE_RSA_PKCS1_V1_5_WITH_SHA256:
            case SIGNATURE_RSA_PKCS1_V1_5_WITH_SHA512:
            case SIGNATURE_ECDSA_WITH_SHA256:
            case SIGNATURE_ECDSA_WITH_SHA512:
            case SIGNATURE_DSA_WITH_SHA256:
            case SIGNATURE_VERITY_RSA_PKCS1_V1_5_WITH_SHA256:
            case SIGNATURE_VERITY_ECDSA_WITH_SHA256:
            case SIGNATURE_VERITY_DSA_WITH_SHA256:
                return true;
            default:
                return false;
        }
    }

    
Verified processed proof of rotation.
@hide
for internal use only./**
     * Verified processed proof of rotation.
     *
     * @hide for internal use only.
     */
    public static class VerifiedProofOfRotation {
        public final List<X509Certificate> certs;
        public final List<Integer> flagsList;

        public VerifiedProofOfRotation(List<X509Certificate> certs, List<Integer> flagsList) {
            this.certs = certs;
            this.flagsList = flagsList;
        }
    }

    
Verified APK Signature Scheme v3 signer, including the proof of rotation structure.
@hide
for internal use only./**
     * Verified APK Signature Scheme v3 signer, including the proof of rotation structure.
     *
     * @hide for internal use only.
     */
    public static class VerifiedSigner {
        public final X509Certificate[] certs;
        public final VerifiedProofOfRotation por;

        public byte[] verityRootHash;

        public VerifiedSigner(X509Certificate[] certs, VerifiedProofOfRotation por) {
            this.certs = certs;
            this.por = por;
        }

    }

    private static class PlatformNotSupportedException extends Exception {

        PlatformNotSupportedException(String s) {
            super(s);
        }
    }
}