1 //
2 // Copyright (C) 2014 The Android Open Source Project
3 //
4 // Licensed under the Apache License, Version 2.0 (the "License");
5 // you may not use this file except in compliance with the License.
6 // You may obtain a copy of the License at
7 //
8 // http://www.apache.org/licenses/LICENSE-2.0
9 //
10 // Unless required by applicable law or agreed to in writing, software
11 // distributed under the License is distributed on an "AS IS" BASIS,
12 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 // See the License for the specific language governing permissions and
14 // limitations under the License.
15 //
16
17 #include "update_engine/payload_consumer/payload_verifier.h"
18
19 #include <utility>
20 #include <vector>
21
22 #include <base/logging.h>
23 #include <openssl/pem.h>
24
25 #include "update_engine/common/constants.h"
26 #include "update_engine/common/hash_calculator.h"
27 #include "update_engine/common/utils.h"
28 #include "update_engine/update_metadata.pb.h"
29
30 using std::string;
31
32 namespace chromeos_update_engine {
33
34 namespace {
35
36 // The ASN.1 DigestInfo prefix for encoding SHA256 digest. The complete 51-byte
37 // DigestInfo consists of 19-byte SHA256_DIGEST_INFO_PREFIX and 32-byte SHA256
38 // digest.
39 //
40 // SEQUENCE(2+49) {
41 // SEQUENCE(2+13) {
42 // OBJECT(2+9) id-sha256
43 // NULL(2+0)
44 // }
45 // OCTET STRING(2+32) <actual signature bytes...>
46 // }
47 const uint8_t kSHA256DigestInfoPrefix[] = {
48 0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, 0x48, 0x01,
49 0x65, 0x03, 0x04, 0x02, 0x01, 0x05, 0x00, 0x04, 0x20,
50 };
51
52 } // namespace
53
VerifySignature(const string & signature_proto,const string & pem_public_key,const brillo::Blob & sha256_hash_data)54 bool PayloadVerifier::VerifySignature(const string& signature_proto,
55 const string& pem_public_key,
56 const brillo::Blob& sha256_hash_data) {
57 Signatures signatures;
58 LOG(INFO) << "signature blob size = " << signature_proto.size();
59 TEST_AND_RETURN_FALSE(signatures.ParseFromString(signature_proto));
60
61 if (!signatures.signatures_size()) {
62 LOG(ERROR) << "No signatures stored in the blob.";
63 return false;
64 }
65
66 std::vector<brillo::Blob> tested_hashes;
67 // Tries every signature in the signature blob.
68 for (int i = 0; i < signatures.signatures_size(); i++) {
69 const Signatures::Signature& signature = signatures.signatures(i);
70 brillo::Blob sig_data(signature.data().begin(), signature.data().end());
71 brillo::Blob sig_hash_data;
72 if (!GetRawHashFromSignature(sig_data, pem_public_key, &sig_hash_data))
73 continue;
74
75 brillo::Blob padded_hash_data = sha256_hash_data;
76 if (PadRSASHA256Hash(&padded_hash_data, sig_hash_data.size()) &&
77 padded_hash_data == sig_hash_data) {
78 LOG(INFO) << "Verified correct signature " << i + 1 << " out of "
79 << signatures.signatures_size() << " signatures.";
80 return true;
81 }
82 tested_hashes.push_back(sig_hash_data);
83 }
84 LOG(ERROR) << "None of the " << signatures.signatures_size()
85 << " signatures is correct. Expected hash before padding:";
86 utils::HexDumpVector(sha256_hash_data);
87 LOG(ERROR) << "But found decrypted hashes:";
88 for (const auto& sig_hash_data : tested_hashes) {
89 utils::HexDumpVector(sig_hash_data);
90 }
91 return false;
92 }
93
GetRawHashFromSignature(const brillo::Blob & sig_data,const string & pem_public_key,brillo::Blob * out_hash_data)94 bool PayloadVerifier::GetRawHashFromSignature(const brillo::Blob& sig_data,
95 const string& pem_public_key,
96 brillo::Blob* out_hash_data) {
97 // The code below executes the equivalent of:
98 //
99 // openssl rsautl -verify -pubin -inkey <(echo |pem_public_key|)
100 // -in |sig_data| -out |out_hash_data|
101
102 BIO* bp = BIO_new_mem_buf(pem_public_key.data(), pem_public_key.size());
103 char dummy_password[] = {' ', 0}; // Ensure no password is read from stdin.
104 RSA* rsa = PEM_read_bio_RSA_PUBKEY(bp, nullptr, nullptr, dummy_password);
105 BIO_free(bp);
106
107 TEST_AND_RETURN_FALSE(rsa != nullptr);
108 unsigned int keysize = RSA_size(rsa);
109 if (sig_data.size() > 2 * keysize) {
110 LOG(ERROR) << "Signature size is too big for public key size.";
111 RSA_free(rsa);
112 return false;
113 }
114
115 // Decrypts the signature.
116 brillo::Blob hash_data(keysize);
117 int decrypt_size = RSA_public_decrypt(
118 sig_data.size(), sig_data.data(), hash_data.data(), rsa, RSA_NO_PADDING);
119 RSA_free(rsa);
120 TEST_AND_RETURN_FALSE(decrypt_size > 0 &&
121 decrypt_size <= static_cast<int>(hash_data.size()));
122 hash_data.resize(decrypt_size);
123 out_hash_data->swap(hash_data);
124 return true;
125 }
126
PadRSASHA256Hash(brillo::Blob * hash,size_t rsa_size)127 bool PayloadVerifier::PadRSASHA256Hash(brillo::Blob* hash, size_t rsa_size) {
128 TEST_AND_RETURN_FALSE(hash->size() == kSHA256Size);
129 TEST_AND_RETURN_FALSE(rsa_size == 256 || rsa_size == 512);
130
131 // The following is a standard PKCS1-v1_5 padding for SHA256 signatures, as
132 // defined in RFC3447 section 9.2. It is prepended to the actual signature
133 // (32 bytes) to form a sequence of 256|512 bytes (2048|4096 bits) that is
134 // amenable to RSA signing. The padded hash will look as follows:
135 //
136 // 0x00 0x01 0xff ... 0xff 0x00 ASN1HEADER SHA256HASH
137 // |-----------205|461----------||----19----||----32----|
138 size_t padding_string_size =
139 rsa_size - hash->size() - sizeof(kSHA256DigestInfoPrefix) - 3;
140 brillo::Blob padded_result = brillo::CombineBlobs({
141 {0x00, 0x01},
142 brillo::Blob(padding_string_size, 0xff),
143 {0x00},
144 brillo::Blob(kSHA256DigestInfoPrefix,
145 kSHA256DigestInfoPrefix + sizeof(kSHA256DigestInfoPrefix)),
146 *hash,
147 });
148
149 *hash = std::move(padded_result);
150 TEST_AND_RETURN_FALSE(hash->size() == rsa_size);
151 return true;
152 }
153
154 } // namespace chromeos_update_engine
155