content: /*
   * 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.
       */
  
      public 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;
          }
      }
  
  }
  