components/proximity_auth/cryptauth/fake_secure_message_delegate.cc

   1 // Copyright 2015 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #include "components/proximity_auth/cryptauth/fake_secure_message_delegate.h"
   6
   7 #include <algorithm>
   8
   9 #include "base/callback.h"
  10 #include "base/md5.h"
  11 #include "base/strings/string_util.h"
  12
  13 namespace proximity_auth {
  14
  15 namespace {
  16
  17 const char kKeyPrefix[] = "fake_key_";
  18 const char kPrivateKeyPrefix[] = "private_";
  19
  20 // Encrypts the |plaintext| with the |key| using the |encryption_scheme| and
  21 // returns the ciphertext.
  22 std::string Encrypt(const std::string& plaintext,
  23                     const std::string& key,
  24                     const securemessage::EncScheme& encryption_scheme) {
  25   if (encryption_scheme == securemessage::NONE)
  26     return plaintext;
  27
  28   // Encrypt with a Vigenere cipher.
  29   std::string ciphertext;
  30   ciphertext.resize(plaintext.size());
  31   for (size_t i = 0; i < plaintext.size(); ++i) {
  32     unsigned char plaintext_char = plaintext[i];
  33     unsigned char key_char = key[i % key.size()];
  34     ciphertext[i] = plaintext_char + key_char;
  35   }
  36   return ciphertext;
  37 }
  38
  39 // Decrypts the |ciphertext| with the |key| using the |encryption_scheme| and
  40 // returns the plaintext.
  41 std::string Decrypt(const std::string& ciphertext,
  42                     const std::string& key,
  43                     const securemessage::EncScheme& encryption_scheme) {
  44   if (encryption_scheme == securemessage::NONE)
  45     return ciphertext;
  46
  47   // Decrypt with a Vigenere cipher.
  48   std::string plaintext;
  49   plaintext.resize(ciphertext.size());
  50   for (size_t i = 0; i < ciphertext.size(); ++i) {
  51     unsigned char ciphertext_char = ciphertext[i];
  52     unsigned char key_char = key[i % key.size()];
  53     plaintext[i] = ciphertext_char - key_char;
  54   }
  55   return plaintext;
  56 }
  57
  58 // Signs the |payload| and |associated_data| with the |key| using the
  59 // |signature_scheme| and returns the signature.
  60 std::string Sign(const std::string& payload,
  61                  const std::string& associated_data,
  62                  const std::string& key) {
  63   return base::MD5String(payload + "|" + associated_data + "|" + key);
  64 }
  65
  66 // Verifies that the |signature| for the the |payload| and |associated_data| is
  67 // valid for the given the |key| and |signature_scheme|.
  68 bool Verify(const std::string& signature,
  69             const std::string& payload,
  70             const std::string& associated_data,
  71             const std::string& key,
  72             const securemessage::SigScheme& signature_scheme) {
  73   std::string signing_key;
  74
  75   if (signature_scheme == securemessage::HMAC_SHA256) {
  76     signing_key = key;
  77   } else {
  78     std::string prefix = kPrivateKeyPrefix;
  79     bool is_private_key = base::StartsWithASCII(key, prefix, true);
  80     signing_key = is_private_key ? key.substr(prefix.size()) : prefix + key;
  81   }
  82
  83   std::string expected_signature = Sign(payload, associated_data, signing_key);
  84   return signature == expected_signature;
  85 }
  86
  87 }  // namespace
  88
  89 FakeSecureMessageDelegate::FakeSecureMessageDelegate()
  90     : next_public_key_(std::string(kKeyPrefix) + "0") {
  91 }
  92
  93 FakeSecureMessageDelegate::~FakeSecureMessageDelegate() {
  94 }
  95
  96 void FakeSecureMessageDelegate::GenerateKeyPair(
  97     const GenerateKeyPairCallback& callback) {
  98   std::string public_key = next_public_key_;
  99
 100   // The private key is simply the public key prepended with "private_".
 101   std::string private_key(kPrivateKeyPrefix + public_key);
 102
 103   next_public_key_ = std::string(kKeyPrefix) + base::MD5String(public_key);
 104
 105   callback.Run(public_key, private_key);
 106 }
 107
 108 void FakeSecureMessageDelegate::DeriveKey(const std::string& private_key,
 109                                           const std::string& public_key,
 110                                           const DeriveKeyCallback& callback) {
 111   // To ensure that the same symmetric key is derived for DeriveKey(private1,
 112   // public2) and DeriveKey(private2, public1), we remove the prefix from the
 113   // private key so it is equal to its corresponding public key.
 114   std::string prefix(kPrivateKeyPrefix);
 115   std::string normalized_private_key =
 116       base::StartsWithASCII(private_key, prefix, true)
 117           ? private_key.substr(prefix.size())
 118           : private_key;
 119
 120   std::vector<std::string> keys;
 121   keys.push_back(normalized_private_key);
 122   keys.push_back(public_key);
 123   std::sort(keys.begin(), keys.end());
 124   callback.Run(base::MD5String(keys[0] + "|" + keys[1]));
 125 }
 126
 127 void FakeSecureMessageDelegate::CreateSecureMessage(
 128     const std::string& payload,
 129     const std::string& key,
 130     const CreateOptions& create_options,
 131     const CreateSecureMessageCallback& callback) {
 132   securemessage::Header header;
 133   header.set_signature_scheme(create_options.signature_scheme);
 134   header.set_encryption_scheme(create_options.encryption_scheme);
 135   if (!create_options.public_metadata.empty())
 136     header.set_public_metadata(create_options.public_metadata);
 137   if (!create_options.verification_key_id.empty())
 138     header.set_verification_key_id(create_options.verification_key_id);
 139   if (!create_options.decryption_key_id.empty())
 140     header.set_decryption_key_id(create_options.decryption_key_id);
 141
 142   securemessage::HeaderAndBody header_and_body;
 143   header_and_body.mutable_header()->CopyFrom(header);
 144   header_and_body.set_body(
 145       Encrypt(payload, key, create_options.encryption_scheme));
 146   std::string serialized_header_and_body;
 147   header_and_body.SerializeToString(&serialized_header_and_body);
 148
 149   securemessage::SecureMessage secure_message;
 150   secure_message.set_header_and_body(serialized_header_and_body);
 151   secure_message.set_signature(
 152       Sign(payload, create_options.associated_data, key));
 153
 154   std::string serialized_secure_message;
 155   secure_message.SerializeToString(&serialized_secure_message);
 156   callback.Run(serialized_secure_message);
 157 }
 158
 159 void FakeSecureMessageDelegate::UnwrapSecureMessage(
 160     const std::string& serialized_message,
 161     const std::string& key,
 162     const UnwrapOptions& unwrap_options,
 163     const UnwrapSecureMessageCallback& callback) {
 164   securemessage::SecureMessage secure_message;
 165   if (!secure_message.ParseFromString(serialized_message)) {
 166     LOG(ERROR) << "Failed to parse SecureMessage.";
 167     callback.Run(false, std::string(), securemessage::Header());
 168     return;
 169   }
 170
 171   securemessage::HeaderAndBody header_and_body;
 172   if (!header_and_body.ParseFromString(secure_message.header_and_body())) {
 173     LOG(ERROR) << "Failed to parse secure message HeaderAndBody.";
 174     callback.Run(false, std::string(), securemessage::Header());
 175     return;
 176   }
 177
 178   const securemessage::Header& header = header_and_body.header();
 179   std::string payload =
 180       Decrypt(header_and_body.body(), key, unwrap_options.encryption_scheme);
 181
 182   bool verified = Verify(secure_message.signature(), payload,
 183                          unwrap_options.associated_data, key,
 184                          unwrap_options.signature_scheme);
 185   if (verified) {
 186     callback.Run(true, payload, header);
 187   } else {
 188     callback.Run(false, std::string(), securemessage::Header());
 189   }
 190 }
 191
 192 std::string FakeSecureMessageDelegate::GetPrivateKeyForPublicKey(
 193     const std::string& public_key) {
 194   return kPrivateKeyPrefix + public_key;
 195 }
 196
 197 }  // namespace proximity_auth