verifier.cpp

/*
 * Copyright (C) 2008 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.
 */

#include "verifier.h"

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <algorithm>
#include <functional>
#include <memory>
#include <vector>

#include <android-base/logging.h>
#include <openssl/bn.h>
#include <openssl/ecdsa.h>
#include <openssl/obj_mac.h>

#include "asn1_decoder.h"
#include "print_sha1.h"

static constexpr size_t MiB = 1024 * 1024;

/*
 * Simple version of PKCS#7 SignedData extraction. This extracts the
 * signature OCTET STRING to be used for signature verification.
 *
 * For full details, see http://www.ietf.org/rfc/rfc3852.txt
 *
 * The PKCS#7 structure looks like:
 *
 *   SEQUENCE (ContentInfo)
 *     OID (ContentType)
 *     [0] (content)
 *       SEQUENCE (SignedData)
 *         INTEGER (version CMSVersion)
 *         SET (DigestAlgorithmIdentifiers)
 *         SEQUENCE (EncapsulatedContentInfo)
 *         [0] (CertificateSet OPTIONAL)
 *         [1] (RevocationInfoChoices OPTIONAL)
 *         SET (SignerInfos)
 *           SEQUENCE (SignerInfo)
 *             INTEGER (CMSVersion)
 *             SEQUENCE (SignerIdentifier)
 *             SEQUENCE (DigestAlgorithmIdentifier)
 *             SEQUENCE (SignatureAlgorithmIdentifier)
 *             OCTET STRING (SignatureValue)
 */
static bool read_pkcs7(const uint8_t* pkcs7_der, size_t pkcs7_der_len,
                       std::vector<uint8_t>* sig_der) {
  CHECK(sig_der != nullptr);
  sig_der->clear();

  asn1_context ctx(pkcs7_der, pkcs7_der_len);

  std::unique_ptr<asn1_context> pkcs7_seq(ctx.asn1_sequence_get());
  if (pkcs7_seq == nullptr || !pkcs7_seq->asn1_sequence_next()) {
    return false;
  }

  std::unique_ptr<asn1_context> signed_data_app(pkcs7_seq->asn1_constructed_get());
  if (signed_data_app == nullptr) {
    return false;
  }

  std::unique_ptr<asn1_context> signed_data_seq(signed_data_app->asn1_sequence_get());
  if (signed_data_seq == nullptr ||
      !signed_data_seq->asn1_sequence_next() ||
      !signed_data_seq->asn1_sequence_next() ||
      !signed_data_seq->asn1_sequence_next() ||
      !signed_data_seq->asn1_constructed_skip_all()) {
    return false;
  }

  std::unique_ptr<asn1_context> sig_set(signed_data_seq->asn1_set_get());
  if (sig_set == nullptr) {
    return false;
  }

  std::unique_ptr<asn1_context> sig_seq(sig_set->asn1_sequence_get());
  if (sig_seq == nullptr ||
      !sig_seq->asn1_sequence_next() ||
      !sig_seq->asn1_sequence_next() ||
      !sig_seq->asn1_sequence_next() ||
      !sig_seq->asn1_sequence_next()) {
    return false;
  }

  const uint8_t* sig_der_ptr;
  size_t sig_der_length;
  if (!sig_seq->asn1_octet_string_get(&sig_der_ptr, &sig_der_length)) {
    return false;
  }

  sig_der->resize(sig_der_length);
  std::copy(sig_der_ptr, sig_der_ptr + sig_der_length, sig_der->begin());
  return true;
}

/*
 * Looks for an RSA signature embedded in the .ZIP file comment given the path to the zip. Verifies
 * that it matches one of the given public keys. A callback function can be optionally provided for
 * posting the progress.
 *
 * Returns VERIFY_SUCCESS or VERIFY_FAILURE (if any error is encountered or no key matches the
 * signature).
 */
int verify_file(const unsigned char* addr, size_t length, const std::vector<Certificate>& keys,
                const std::function<void(float)>& set_progress) {
  if (set_progress) {
    set_progress(0.0);
  }

  // An archive with a whole-file signature will end in six bytes:
  //
  //   (2-byte signature start) $ff $ff (2-byte comment size)
  //
  // (As far as the ZIP format is concerned, these are part of the archive comment.) We start by
  // reading this footer, this tells us how far back from the end we have to start reading to find
  // the whole comment.

#define FOOTER_SIZE 6

  if (length < FOOTER_SIZE) {
    LOG(ERROR) << "not big enough to contain footer";
    return VERIFY_FAILURE;
  }

  const unsigned char* footer = addr + length - FOOTER_SIZE;

  if (footer[2] != 0xff || footer[3] != 0xff) {
    LOG(ERROR) << "footer is wrong";
    return VERIFY_FAILURE;
  }

  size_t comment_size = footer[4] + (footer[5] << 8);
  size_t signature_start = footer[0] + (footer[1] << 8);
  LOG(INFO) << "comment is " << comment_size << " bytes; signature is " << signature_start
            << " bytes from end";

  if (signature_start > comment_size) {
    LOG(ERROR) << "signature start: " << signature_start << " is larger than comment size: "
               << comment_size;
    return VERIFY_FAILURE;
  }

  if (signature_start <= FOOTER_SIZE) {
    LOG(ERROR) << "Signature start is in the footer";
    return VERIFY_FAILURE;
  }

#define EOCD_HEADER_SIZE 22

  // The end-of-central-directory record is 22 bytes plus any comment length.
  size_t eocd_size = comment_size + EOCD_HEADER_SIZE;

  if (length < eocd_size) {
    LOG(ERROR) << "not big enough to contain EOCD";
    return VERIFY_FAILURE;
  }

  // Determine how much of the file is covered by the signature. This is everything except the
  // signature data and length, which includes all of the EOCD except for the comment length field
  // (2 bytes) and the comment data.
  size_t signed_len = length - eocd_size + EOCD_HEADER_SIZE - 2;

  const unsigned char* eocd = addr + length - eocd_size;

  // If this is really is the EOCD record, it will begin with the magic number $50 $4b $05 $06.
  if (eocd[0] != 0x50 || eocd[1] != 0x4b || eocd[2] != 0x05 || eocd[3] != 0x06) {
    LOG(ERROR) << "signature length doesn't match EOCD marker";
    return VERIFY_FAILURE;
  }

  for (size_t i = 4; i < eocd_size-3; ++i) {
    if (eocd[i] == 0x50 && eocd[i+1] == 0x4b && eocd[i+2] == 0x05 && eocd[i+3] == 0x06) {
      // If the sequence $50 $4b $05 $06 appears anywhere after the real one, libziparchive will
      // find the later (wrong) one, which could be exploitable. Fail the verification if this
      // sequence occurs anywhere after the real one.
      LOG(ERROR) << "EOCD marker occurs after start of EOCD";
      return VERIFY_FAILURE;
    }
  }

  bool need_sha1 = false;
  bool need_sha256 = false;
  for (const auto& key : keys) {
    switch (key.hash_len) {
      case SHA_DIGEST_LENGTH: need_sha1 = true; break;
      case SHA256_DIGEST_LENGTH: need_sha256 = true; break;
    }
  }

  SHA_CTX sha1_ctx;
  SHA256_CTX sha256_ctx;
  SHA1_Init(&sha1_ctx);
  SHA256_Init(&sha256_ctx);

  double frac = -1.0;
  size_t so_far = 0;
  while (so_far < signed_len) {
    // On a Nexus 5X, experiment showed 16MiB beat 1MiB by 6% faster for a
    // 1196MiB full OTA and 60% for an 89MiB incremental OTA.
    // http://b/28135231.
    size_t size = std::min(signed_len - so_far, 16 * MiB);

    if (need_sha1) SHA1_Update(&sha1_ctx, addr + so_far, size);
    if (need_sha256) SHA256_Update(&sha256_ctx, addr + so_far, size);
    so_far += size;

    if (set_progress) {
      double f = so_far / (double)signed_len;
      if (f > frac + 0.02 || size == so_far) {
        set_progress(f);
        frac = f;
      }
    }
  }

  uint8_t sha1[SHA_DIGEST_LENGTH];
  SHA1_Final(sha1, &sha1_ctx);
  uint8_t sha256[SHA256_DIGEST_LENGTH];
  SHA256_Final(sha256, &sha256_ctx);

  const uint8_t* signature = eocd + eocd_size - signature_start;
  size_t signature_size = signature_start - FOOTER_SIZE;

  LOG(INFO) << "signature (offset: " << std::hex << (length - signature_start) << ", length: "
            << signature_size << "): " << print_hex(signature, signature_size);

  std::vector<uint8_t> sig_der;
  if (!read_pkcs7(signature, signature_size, &sig_der)) {
    LOG(ERROR) << "Could not find signature DER block";
    return VERIFY_FAILURE;
  }

  // Check to make sure at least one of the keys matches the signature. Since any key can match,
  // we need to try each before determining a verification failure has happened.
  size_t i = 0;
  for (const auto& key : keys) {
    const uint8_t* hash;
    int hash_nid;
    switch (key.hash_len) {
      case SHA_DIGEST_LENGTH:
        hash = sha1;
        hash_nid = NID_sha1;
        break;
      case SHA256_DIGEST_LENGTH:
        hash = sha256;
        hash_nid = NID_sha256;
        break;
      default:
        continue;
    }

    // The 6 bytes is the "(signature_start) $ff $ff (comment_size)" that the signing tool appends
    // after the signature itself.
    if (key.key_type == Certificate::KEY_TYPE_RSA) {
      if (!RSA_verify(hash_nid, hash, key.hash_len, sig_der.data(), sig_der.size(),
                      key.rsa.get())) {
        LOG(INFO) << "failed to verify against RSA key " << i;
        continue;
      }

      LOG(INFO) << "whole-file signature verified against RSA key " << i;
      return VERIFY_SUCCESS;
    } else if (key.key_type == Certificate::KEY_TYPE_EC && key.hash_len == SHA256_DIGEST_LENGTH) {
      if (!ECDSA_verify(0, hash, key.hash_len, sig_der.data(), sig_der.size(), key.ec.get())) {
        LOG(INFO) << "failed to verify against EC key " << i;
        continue;
      }

      LOG(INFO) << "whole-file signature verified against EC key " << i;
      return VERIFY_SUCCESS;
    } else {
      LOG(INFO) << "Unknown key type " << key.key_type;
    }
    i++;
  }

  if (need_sha1) {
    LOG(INFO) << "SHA-1 digest: " << print_hex(sha1, SHA_DIGEST_LENGTH);
  }
  if (need_sha256) {
    LOG(INFO) << "SHA-256 digest: " << print_hex(sha256, SHA256_DIGEST_LENGTH);
  }
  LOG(ERROR) << "failed to verify whole-file signature";
  return VERIFY_FAILURE;
}

std::unique_ptr<RSA, RSADeleter> parse_rsa_key(FILE* file, uint32_t exponent) {
    // Read key length in words and n0inv. n0inv is a precomputed montgomery
    // parameter derived from the modulus and can be used to speed up
    // verification. n0inv is 32 bits wide here, assuming the verification logic
    // uses 32 bit arithmetic. However, BoringSSL may use a word size of 64 bits
    // internally, in which case we don't have a valid n0inv. Thus, we just
    // ignore the montgomery parameters and have BoringSSL recompute them
    // internally. If/When the speedup from using the montgomery parameters
    // becomes relevant, we can add more sophisticated code here to obtain a
    // 64-bit n0inv and initialize the montgomery parameters in the key object.
    uint32_t key_len_words = 0;
    uint32_t n0inv = 0;
    if (fscanf(file, " %i , 0x%x", &key_len_words, &n0inv) != 2) {
        return nullptr;
    }

    if (key_len_words > 8192 / 32) {
        LOG(ERROR) << "key length (" << key_len_words << ") too large";
        return nullptr;
    }

    // Read the modulus.
    std::unique_ptr<uint32_t[]> modulus(new uint32_t[key_len_words]);
    if (fscanf(file, " , { %u", &modulus[0]) != 1) {
        return nullptr;
    }
    for (uint32_t i = 1; i < key_len_words; ++i) {
        if (fscanf(file, " , %u", &modulus[i]) != 1) {
            return nullptr;
        }
    }

    // Cconvert from little-endian array of little-endian words to big-endian
    // byte array suitable as input for BN_bin2bn.
    std::reverse((uint8_t*)modulus.get(),
                 (uint8_t*)(modulus.get() + key_len_words));

    // The next sequence of values is the montgomery parameter R^2. Since we
    // generally don't have a valid |n0inv|, we ignore this (see comment above).
    uint32_t rr_value;
    if (fscanf(file, " } , { %u", &rr_value) != 1) {
        return nullptr;
    }
    for (uint32_t i = 1; i < key_len_words; ++i) {
        if (fscanf(file, " , %u", &rr_value) != 1) {
            return nullptr;
        }
    }
    if (fscanf(file, " } } ") != 0) {
        return nullptr;
    }

    // Initialize the key.
    std::unique_ptr<RSA, RSADeleter> key(RSA_new());
    if (!key) {
      return nullptr;
    }

    key->n = BN_bin2bn((uint8_t*)modulus.get(),
                       key_len_words * sizeof(uint32_t), NULL);
    if (!key->n) {
      return nullptr;
    }

    key->e = BN_new();
    if (!key->e || !BN_set_word(key->e, exponent)) {
      return nullptr;
    }

    return key;
}

struct BNDeleter {
  void operator()(BIGNUM* bn) const {
    BN_free(bn);
  }
};

std::unique_ptr<EC_KEY, ECKEYDeleter> parse_ec_key(FILE* file) {
    uint32_t key_len_bytes = 0;
    if (fscanf(file, " %i", &key_len_bytes) != 1) {
        return nullptr;
    }

    std::unique_ptr<EC_GROUP, void (*)(EC_GROUP*)> group(
        EC_GROUP_new_by_curve_name(NID_X9_62_prime256v1), EC_GROUP_free);
    if (!group) {
        return nullptr;
    }

    // Verify that |key_len| matches the group order.
    if (key_len_bytes != BN_num_bytes(EC_GROUP_get0_order(group.get()))) {
        return nullptr;
    }

    // Read the public key coordinates. Note that the byte order in the file is
    // little-endian, so we convert to big-endian here.
    std::unique_ptr<uint8_t[]> bytes(new uint8_t[key_len_bytes]);
    std::unique_ptr<BIGNUM, BNDeleter> point[2];
    for (int i = 0; i < 2; ++i) {
        unsigned int byte = 0;
        if (fscanf(file, " , { %u", &byte) != 1) {
            return nullptr;
        }
        bytes[key_len_bytes - 1] = byte;

        for (size_t i = 1; i < key_len_bytes; ++i) {
            if (fscanf(file, " , %u", &byte) != 1) {
                return nullptr;
            }
            bytes[key_len_bytes - i - 1] = byte;
        }

        point[i].reset(BN_bin2bn(bytes.get(), key_len_bytes, nullptr));
        if (!point[i]) {
            return nullptr;
        }

        if (fscanf(file, " }") != 0) {
            return nullptr;
        }
    }

    if (fscanf(file, " } ") != 0) {
        return nullptr;
    }

    // Create and initialize the key.
    std::unique_ptr<EC_KEY, ECKEYDeleter> key(EC_KEY_new());
    if (!key || !EC_KEY_set_group(key.get(), group.get()) ||
        !EC_KEY_set_public_key_affine_coordinates(key.get(), point[0].get(),
                                                  point[1].get())) {
        return nullptr;
    }

    return key;
}

// Reads a file containing one or more public keys as produced by
// DumpPublicKey:  this is an RSAPublicKey struct as it would appear
// as a C source literal, eg:
//
//  "{64,0xc926ad21,{1795090719,...,-695002876},{-857949815,...,1175080310}}"
//
// For key versions newer than the original 2048-bit e=3 keys
// supported by Android, the string is preceded by a version
// identifier, eg:
//
//  "v2 {64,0xc926ad21,{1795090719,...,-695002876},{-857949815,...,1175080310}}"
//
// (Note that the braces and commas in this example are actual
// characters the parser expects to find in the file; the ellipses
// indicate more numbers omitted from this example.)
//
// The file may contain multiple keys in this format, separated by
// commas.  The last key must not be followed by a comma.
//
// A Certificate is a pair of an RSAPublicKey and a particular hash
// (we support SHA-1 and SHA-256; we store the hash length to signify
// which is being used).  The hash used is implied by the version number.
//
//       1: 2048-bit RSA key with e=3 and SHA-1 hash
//       2: 2048-bit RSA key with e=65537 and SHA-1 hash
//       3: 2048-bit RSA key with e=3 and SHA-256 hash
//       4: 2048-bit RSA key with e=65537 and SHA-256 hash
//       5: 256-bit EC key using the NIST P-256 curve parameters and SHA-256 hash
//
// Returns true on success, and appends the found keys (at least one) to certs.
// Otherwise returns false if the file failed to parse, or if it contains zero
// keys. The contents in certs would be unspecified on failure.
bool load_keys(const char* filename, std::vector<Certificate>& certs) {
  std::unique_ptr<FILE, decltype(&fclose)> f(fopen(filename, "re"), fclose);
  if (!f) {
    PLOG(ERROR) << "error opening " << filename;
    return false;
  }

  while (true) {
    certs.emplace_back(0, Certificate::KEY_TYPE_RSA, nullptr, nullptr);
    Certificate& cert = certs.back();
    uint32_t exponent = 0;

    char start_char;
    if (fscanf(f.get(), " %c", &start_char) != 1) return false;
    if (start_char == '{') {
      // a version 1 key has no version specifier.
      cert.key_type = Certificate::KEY_TYPE_RSA;
      exponent = 3;
      cert.hash_len = SHA_DIGEST_LENGTH;
    } else if (start_char == 'v') {
      int version;
      if (fscanf(f.get(), "%d {", &version) != 1) return false;
      switch (version) {
        case 2:
          cert.key_type = Certificate::KEY_TYPE_RSA;
          exponent = 65537;
          cert.hash_len = SHA_DIGEST_LENGTH;
          break;
        case 3:
          cert.key_type = Certificate::KEY_TYPE_RSA;
          exponent = 3;
          cert.hash_len = SHA256_DIGEST_LENGTH;
          break;
        case 4:
          cert.key_type = Certificate::KEY_TYPE_RSA;
          exponent = 65537;
          cert.hash_len = SHA256_DIGEST_LENGTH;
          break;
        case 5:
          cert.key_type = Certificate::KEY_TYPE_EC;
          cert.hash_len = SHA256_DIGEST_LENGTH;
          break;
        default:
          return false;
      }
    }

    if (cert.key_type == Certificate::KEY_TYPE_RSA) {
      cert.rsa = parse_rsa_key(f.get(), exponent);
      if (!cert.rsa) {
        return false;
      }

      LOG(INFO) << "read key e=" << exponent << " hash=" << cert.hash_len;
    } else if (cert.key_type == Certificate::KEY_TYPE_EC) {
      cert.ec = parse_ec_key(f.get());
      if (!cert.ec) {
        return false;
      }
    } else {
      LOG(ERROR) << "Unknown key type " << cert.key_type;
      return false;
    }

    // if the line ends in a comma, this file has more keys.
    int ch = fgetc(f.get());
    if (ch == ',') {
      // more keys to come.
      continue;
    } else if (ch == EOF) {
      break;
    } else {
      LOG(ERROR) << "unexpected character between keys";
      return false;
    }
  }
  return true;
}