Merge pull request #8619

e71c8bf wallet: background sync with just the view key (j-berman)

10 files changed, 1634 insertions, 89 deletions

diff --git a/src/wallet/api/wallet.cpp b/src/wallet/api/wallet.cpp
index a6dabddb6..40481fc4b 100644
--- a/src/wallet/api/wallet.cpp
+++ b/src/wallet/api/wallet.cpp
@@ -58,6 +58,40 @@ using namespace cryptonote;
 #undef MONERO_DEFAULT_LOG_CATEGORY
 #define MONERO_DEFAULT_LOG_CATEGORY "WalletAPI"
 
+#define LOCK_REFRESH() \
+    bool refresh_enabled = m_refreshEnabled; \
+    m_refreshEnabled = false; \
+    m_wallet->stop(); \
+    m_refreshCV.notify_one(); \
+    boost::mutex::scoped_lock lock(m_refreshMutex); \
+    boost::mutex::scoped_lock lock2(m_refreshMutex2); \
+    epee::misc_utils::auto_scope_leave_caller scope_exit_handler = epee::misc_utils::create_scope_leave_handler([&](){ \
+        /* m_refreshMutex's still locked here */ \
+        if (refresh_enabled) \
+            startRefresh(); \
+    })
+
+#define PRE_VALIDATE_BACKGROUND_SYNC() \
+  do \
+  { \
+    clearStatus(); \
+    if (m_wallet->key_on_device()) \
+    { \
+        setStatusError(tr("HW wallet cannot use background sync")); \
+        return false; \
+    } \
+    if (m_wallet->watch_only()) \
+    { \
+        setStatusError(tr("View only wallet cannot use background sync")); \
+        return false; \
+    } \
+    if (m_wallet->get_multisig_status().multisig_is_active) \
+    { \
+        setStatusError(tr("Multisig wallet cannot use background sync")); \
+        return false; \
+    } \
+  } while (0)
+
 namespace Monero {
 
 namespace {
@@ -766,6 +800,8 @@ bool WalletImpl::close(bool store)
 
 std::string WalletImpl::seed(const std::string& seed_offset) const
 {
+    if (checkBackgroundSync("cannot get seed"))
+        return std::string();
     epee::wipeable_string seed;
     if (m_wallet)
         m_wallet->get_seed(seed, seed_offset);
@@ -779,6 +815,8 @@ std::string WalletImpl::getSeedLanguage() const
 
 void WalletImpl::setSeedLanguage(const std::string &arg)
 {
+    if (checkBackgroundSync("cannot set seed language"))
+        return;
     m_wallet->set_seed_language(arg);
 }
 
@@ -802,6 +840,8 @@ void WalletImpl::statusWithErrorString(int& status, std::string& errorString) co
 
 bool WalletImpl::setPassword(const std::string &password)
 {
+    if (checkBackgroundSync("cannot change password"))
+        return false;
     clearStatus();
     try {
         m_wallet->change_password(m_wallet->get_wallet_file(), m_password, password);
@@ -931,6 +971,8 @@ bool WalletImpl::init(const std::string &daemon_address, uint64_t upper_transact
 
 void WalletImpl::setRefreshFromBlockHeight(uint64_t refresh_from_block_height)
 {
+    if (checkBackgroundSync("cannot change refresh height"))
+        return;
     m_wallet->set_refresh_from_block_height(refresh_from_block_height);
 }
 
@@ -1039,6 +1081,8 @@ void WalletImpl::refreshAsync()
 
 bool WalletImpl::rescanBlockchain()
 {
+    if (checkBackgroundSync("cannot rescan blockchain"))
+        return false;
     clearStatus();
     m_refreshShouldRescan = true;
     doRefresh();
@@ -1047,6 +1091,8 @@ bool WalletImpl::rescanBlockchain()
 
 void WalletImpl::rescanBlockchainAsync()
 {
+    if (checkBackgroundSync("cannot rescan blockchain"))
+        return;
     m_refreshShouldRescan = true;
     refreshAsync();
 }
@@ -1070,7 +1116,7 @@ int WalletImpl::autoRefreshInterval() const
 UnsignedTransaction *WalletImpl::loadUnsignedTx(const std::string &unsigned_filename) {
   clearStatus();
   UnsignedTransactionImpl * transaction = new UnsignedTransactionImpl(*this);
-  if (!m_wallet->load_unsigned_tx(unsigned_filename, transaction->m_unsigned_tx_set)){
+  if (checkBackgroundSync("cannot load tx") || !m_wallet->load_unsigned_tx(unsigned_filename, transaction->m_unsigned_tx_set)){
     setStatusError(tr("Failed to load unsigned transactions"));
     transaction->m_status = UnsignedTransaction::Status::Status_Error;
     transaction->m_errorString = errorString();
@@ -1090,6 +1136,8 @@ UnsignedTransaction *WalletImpl::loadUnsignedTx(const std::string &unsigned_file
 
 bool WalletImpl::submitTransaction(const string &fileName) {
   clearStatus();
+  if (checkBackgroundSync("cannot submit tx"))
+    return false;
   std::unique_ptr<PendingTransactionImpl> transaction(new PendingTransactionImpl(*this));
 
   bool r = m_wallet->load_tx(fileName, transaction->m_pending_tx);
@@ -1113,6 +1161,8 @@ bool WalletImpl::exportKeyImages(const string &filename, bool all)
     setStatusError(tr("Wallet is view only"));
     return false;
   }
+  if (checkBackgroundSync("cannot export key images"))
+    return false;
   
   try
   {
@@ -1133,6 +1183,8 @@ bool WalletImpl::exportKeyImages(const string &filename, bool all)
 
 bool WalletImpl::importKeyImages(const string &filename)
 {
+  if (checkBackgroundSync("cannot import key images"))
+    return false;
   if (!trustedDaemon()) {
     setStatusError(tr("Key images can only be imported with a trusted daemon"));
     return false;
@@ -1156,6 +1208,8 @@ bool WalletImpl::importKeyImages(const string &filename)
 
 bool WalletImpl::exportOutputs(const string &filename, bool all)
 {
+    if (checkBackgroundSync("cannot export outputs"))
+        return false;
     if (m_wallet->key_on_device())
     {
         setStatusError(string(tr("Not supported on HW wallets.")) + filename);
@@ -1186,6 +1240,8 @@ bool WalletImpl::exportOutputs(const string &filename, bool all)
 
 bool WalletImpl::importOutputs(const string &filename)
 {
+    if (checkBackgroundSync("cannot import outputs"))
+        return false;
     if (m_wallet->key_on_device())
     {
         setStatusError(string(tr("Not supported on HW wallets.")) + filename);
@@ -1218,6 +1274,8 @@ bool WalletImpl::importOutputs(const string &filename)
 
 bool WalletImpl::scanTransactions(const std::vector<std::string> &txids)
 {
+    if (checkBackgroundSync("cannot scan transactions"))
+        return false;
     if (txids.empty())
     {
         setStatusError(string(tr("Failed to scan transactions: no transaction ids provided.")));
@@ -1256,8 +1314,86 @@ bool WalletImpl::scanTransactions(const std::vector<std::string> &txids)
     return true;
 }
 
+bool WalletImpl::setupBackgroundSync(const Wallet::BackgroundSyncType background_sync_type, const std::string &wallet_password, const optional<std::string> &background_cache_password)
+{
+    try
+    {
+        PRE_VALIDATE_BACKGROUND_SYNC();
+
+        tools::wallet2::BackgroundSyncType bgs_type;
+        switch (background_sync_type)
+        {
+            case Wallet::BackgroundSync_Off: bgs_type = tools::wallet2::BackgroundSyncOff; break;
+            case Wallet::BackgroundSync_ReusePassword: bgs_type = tools::wallet2::BackgroundSyncReusePassword; break;
+            case Wallet::BackgroundSync_CustomPassword: bgs_type = tools::wallet2::BackgroundSyncCustomPassword; break;
+            default: setStatusError(tr("Unknown background sync type")); return false;
+        }
+
+        boost::optional<epee::wipeable_string> bgc_password = background_cache_password
+            ? boost::optional<epee::wipeable_string>(*background_cache_password)
+            : boost::none;
+
+        LOCK_REFRESH();
+        m_wallet->setup_background_sync(bgs_type, wallet_password, bgc_password);
+    }
+    catch (const std::exception &e)
+    {
+        LOG_ERROR("Failed to setup background sync: " << e.what());
+        setStatusError(string(tr("Failed to setup background sync: ")) + e.what());
+        return false;
+    }
+    return true;
+}
+
+Wallet::BackgroundSyncType WalletImpl::getBackgroundSyncType() const
+{
+    switch (m_wallet->background_sync_type())
+    {
+        case tools::wallet2::BackgroundSyncOff: return Wallet::BackgroundSync_Off;
+        case tools::wallet2::BackgroundSyncReusePassword: return Wallet::BackgroundSync_ReusePassword;
+        case tools::wallet2::BackgroundSyncCustomPassword: return Wallet::BackgroundSync_CustomPassword;
+        default: setStatusError(tr("Unknown background sync type")); return Wallet::BackgroundSync_Off;
+    }
+}
+
+bool WalletImpl::startBackgroundSync()
+{
+    try
+    {
+        PRE_VALIDATE_BACKGROUND_SYNC();
+        LOCK_REFRESH();
+        m_wallet->start_background_sync();
+    }
+    catch (const std::exception &e)
+    {
+        LOG_ERROR("Failed to start background sync: " << e.what());
+        setStatusError(string(tr("Failed to start background sync: ")) + e.what());
+        return false;
+    }
+    return true;
+}
+
+bool WalletImpl::stopBackgroundSync(const std::string &wallet_password)
+{
+    try
+    {
+        PRE_VALIDATE_BACKGROUND_SYNC();
+        LOCK_REFRESH();
+        m_wallet->stop_background_sync(epee::wipeable_string(wallet_password));
+    }
+    catch (const std::exception &e)
+    {
+        LOG_ERROR("Failed to stop background sync: " << e.what());
+        setStatusError(string(tr("Failed to stop background sync: ")) + e.what());
+        return false;
+    }
+    return true;
+}
+
 void WalletImpl::addSubaddressAccount(const std::string& label)
 {
+    if (checkBackgroundSync("cannot add account"))
+        return;
     m_wallet->add_subaddress_account(label);
 }
 size_t WalletImpl::numSubaddressAccounts() const
@@ -1270,10 +1406,14 @@ size_t WalletImpl::numSubaddresses(uint32_t accountIndex) const
 }
 void WalletImpl::addSubaddress(uint32_t accountIndex, const std::string& label)
 {
+    if (checkBackgroundSync("cannot add subbaddress"))
+        return;
     m_wallet->add_subaddress(accountIndex, label);
 }
 std::string WalletImpl::getSubaddressLabel(uint32_t accountIndex, uint32_t addressIndex) const
 {
+    if (checkBackgroundSync("cannot get subbaddress label"))
+        return "";
     try
     {
         return m_wallet->get_subaddress_label({accountIndex, addressIndex});
@@ -1287,6 +1427,8 @@ std::string WalletImpl::getSubaddressLabel(uint32_t accountIndex, uint32_t addre
 }
 void WalletImpl::setSubaddressLabel(uint32_t accountIndex, uint32_t addressIndex, const std::string &label)
 {
+    if (checkBackgroundSync("cannot set subbaddress label"))
+        return;
     try
     {
         return m_wallet->set_subaddress_label({accountIndex, addressIndex}, label);
@@ -1300,6 +1442,9 @@ void WalletImpl::setSubaddressLabel(uint32_t accountIndex, uint32_t addressIndex
 
 MultisigState WalletImpl::multisig() const {
     MultisigState state;
+    if (checkBackgroundSync("cannot use multisig"))
+        return state;
+
     const multisig::multisig_account_status ms_status{m_wallet->get_multisig_status()};
 
     state.isMultisig = ms_status.multisig_is_active;
@@ -1312,6 +1457,8 @@ MultisigState WalletImpl::multisig() const {
 }
 
 string WalletImpl::getMultisigInfo() const {
+    if (checkBackgroundSync("cannot use multisig"))
+        return string();
     try {
         clearStatus();
         return m_wallet->get_multisig_first_kex_msg();
@@ -1324,6 +1471,8 @@ string WalletImpl::getMultisigInfo() const {
 }
 
 string WalletImpl::makeMultisig(const vector<string>& info, const uint32_t threshold) {
+    if (checkBackgroundSync("cannot make multisig"))
+        return string();
     try {
         clearStatus();
 
@@ -1479,6 +1628,9 @@ PendingTransaction *WalletImpl::createTransactionMultDest(const std::vector<stri
     PendingTransactionImpl * transaction = new PendingTransactionImpl(*this);
 
     do {
+        if (checkBackgroundSync("cannot create transactions"))
+            break;
+
         std::vector<uint8_t> extra;
         std::string extra_nonce;
         vector<cryptonote::tx_destination_entry> dsts;
@@ -1645,6 +1797,9 @@ PendingTransaction *WalletImpl::createSweepUnmixableTransaction()
     PendingTransactionImpl * transaction = new PendingTransactionImpl(*this);
 
     do {
+        if (checkBackgroundSync("cannot sweep"))
+            break;
+
         try {
             transaction->m_pending_tx = m_wallet->create_unmixable_sweep_transactions();
             pendingTxPostProcess(transaction);
@@ -1778,11 +1933,15 @@ uint32_t WalletImpl::defaultMixin() const
 
 void WalletImpl::setDefaultMixin(uint32_t arg)
 {
+    if (checkBackgroundSync("cannot set default mixin"))
+        return;
     m_wallet->default_mixin(arg);
 }
 
 bool WalletImpl::setCacheAttribute(const std::string &key, const std::string &val)
 {
+    if (checkBackgroundSync("cannot set cache attribute"))
+        return false;
     m_wallet->set_attribute(key, val);
     return true;
 }
@@ -1796,6 +1955,8 @@ std::string WalletImpl::getCacheAttribute(const std::string &key) const
 
 bool WalletImpl::setUserNote(const std::string &txid, const std::string &note)
 {
+    if (checkBackgroundSync("cannot set user note"))
+        return false;
     cryptonote::blobdata txid_data;
     if(!epee::string_tools::parse_hexstr_to_binbuff(txid, txid_data) || txid_data.size() != sizeof(crypto::hash))
       return false;
@@ -1807,6 +1968,8 @@ bool WalletImpl::setUserNote(const std::string &txid, const std::string &note)
 
 std::string WalletImpl::getUserNote(const std::string &txid) const
 {
+    if (checkBackgroundSync("cannot get user note"))
+        return "";
     cryptonote::blobdata txid_data;
     if(!epee::string_tools::parse_hexstr_to_binbuff(txid, txid_data) || txid_data.size() != sizeof(crypto::hash))
       return "";
@@ -1817,6 +1980,9 @@ std::string WalletImpl::getUserNote(const std::string &txid) const
 
 std::string WalletImpl::getTxKey(const std::string &txid_str) const
 {
+    if (checkBackgroundSync("cannot get tx key"))
+        return "";
+
     crypto::hash txid;
     if(!epee::string_tools::hex_to_pod(txid_str, txid))
     {
@@ -1901,6 +2067,9 @@ bool WalletImpl::checkTxKey(const std::string &txid_str, std::string tx_key_str,
 
 std::string WalletImpl::getTxProof(const std::string &txid_str, const std::string &address_str, const std::string &message) const
 {
+    if (checkBackgroundSync("cannot get tx proof"))
+        return "";
+
     crypto::hash txid;
     if (!epee::string_tools::hex_to_pod(txid_str, txid))
     {
@@ -1957,6 +2126,9 @@ bool WalletImpl::checkTxProof(const std::string &txid_str, const std::string &ad
 }
 
 std::string WalletImpl::getSpendProof(const std::string &txid_str, const std::string &message) const {
+    if (checkBackgroundSync("cannot get spend proof"))
+        return "";
+
     crypto::hash txid;
     if(!epee::string_tools::hex_to_pod(txid_str, txid))
     {
@@ -1999,6 +2171,9 @@ bool WalletImpl::checkSpendProof(const std::string &txid_str, const std::string
 }
 
 std::string WalletImpl::getReserveProof(bool all, uint32_t account_index, uint64_t amount, const std::string &message) const {
+    if (checkBackgroundSync("cannot get reserve proof"))
+        return "";
+
     try
     {
         clearStatus();
@@ -2045,6 +2220,9 @@ bool WalletImpl::checkReserveProof(const std::string &address, const std::string
 
 std::string WalletImpl::signMessage(const std::string &message, const std::string &address)
 {
+    if (checkBackgroundSync("cannot sign message"))
+        return "";
+
     if (address.empty()) {
         return m_wallet->sign(message, tools::wallet2::sign_with_spend_key);
     }
@@ -2171,6 +2349,16 @@ bool WalletImpl::isDeterministic() const
     return m_wallet->is_deterministic();
 }
 
+bool WalletImpl::isBackgroundSyncing() const
+{
+    return m_wallet->is_background_syncing();
+}
+
+bool WalletImpl::isBackgroundWallet() const
+{
+    return m_wallet->is_background_wallet();
+}
+
 void WalletImpl::clearStatus() const
 {
     boost::lock_guard<boost::mutex> l(m_statusMutex);
@@ -2239,9 +2427,7 @@ void WalletImpl::doRefresh()
             if(rescan)
                 m_wallet->rescan_blockchain(false);
             m_wallet->refresh(trustedDaemon());
-            if (!m_synchronized) {
-                m_synchronized = true;
-            }
+            m_synchronized = m_wallet->is_synced();
             // assuming if we have empty history, it wasn't initialized yet
             // for further history changes client need to update history in
             // "on_money_received" and "on_money_sent" callbacks
@@ -2344,6 +2530,24 @@ bool WalletImpl::doInit(const string &daemon_address, const std::string &proxy_a
     return true;
 }
 
+bool WalletImpl::checkBackgroundSync(const std::string &message) const
+{
+    clearStatus();
+    if (m_wallet->is_background_wallet())
+    {
+        LOG_ERROR("Background wallets " + message);
+        setStatusError(tr("Background wallets ") + message);
+        return true;
+    }
+    if (m_wallet->is_background_syncing())
+    {
+        LOG_ERROR(message + " while background syncing");
+        setStatusError(message + tr(" while background syncing. Stop background syncing first."));
+        return true;
+    }
+    return false;
+}
+
 bool WalletImpl::parse_uri(const std::string &uri, std::string &address, std::string &payment_id, uint64_t &amount, std::string &tx_description, std::string &recipient_name, std::vector<std::string> &unknown_parameters, std::string &error)
 {
     return m_wallet->parse_uri(uri, address, payment_id, amount, tx_description, recipient_name, unknown_parameters, error);
@@ -2362,6 +2566,8 @@ std::string WalletImpl::getDefaultDataDir() const
 bool WalletImpl::rescanSpent()
 {
   clearStatus();
+  if (checkBackgroundSync("cannot rescan spent"))
+    return false;
   if (!trustedDaemon()) {
     setStatusError(tr("Rescan spent can only be used with a trusted daemon"));
     return false;
diff --git a/src/wallet/api/wallet.h b/src/wallet/api/wallet.h
index 7ca126dd7..1901efbca 100644
--- a/src/wallet/api/wallet.h
+++ b/src/wallet/api/wallet.h
@@ -173,6 +173,13 @@ public:
     bool importOutputs(const std::string &filename) override;
     bool scanTransactions(const std::vector<std::string> &txids) override;
 
+    bool setupBackgroundSync(const BackgroundSyncType background_sync_type, const std::string &wallet_password, const optional<std::string> &background_cache_password = optional<std::string>()) override;
+    BackgroundSyncType getBackgroundSyncType() const override;
+    bool startBackgroundSync() override;
+    bool stopBackgroundSync(const std::string &wallet_password) override;
+    bool isBackgroundSyncing() const override;
+    bool isBackgroundWallet() const override;
+
     virtual void disposeTransaction(PendingTransaction * t) override;
     virtual uint64_t estimateTransactionFee(const std::vector<std::pair<std::string, uint64_t>> &destinations,
                                             PendingTransaction::Priority priority) const override;
@@ -239,6 +246,7 @@ private:
     bool isNewWallet() const;
     void pendingTxPostProcess(PendingTransactionImpl * pending);
     bool doInit(const std::string &daemon_address, const std::string &proxy_address, uint64_t upper_transaction_size_limit = 0, bool ssl = false);
+    bool checkBackgroundSync(const std::string &message) const;
 
 private:
     friend class PendingTransactionImpl;
@@ -254,6 +262,10 @@ private:
     mutable boost::mutex m_statusMutex;
     mutable int m_status;
     mutable std::string m_errorString;
+    // TODO: harden password handling in the wallet API, see relevant discussion
+    // https://github.com/monero-project/monero-gui/issues/1537
+    // https://github.com/feather-wallet/feather/issues/72#issuecomment-1405602142
+    // https://github.com/monero-project/monero/pull/8619#issuecomment-1632951461
     std::string m_password;
     std::unique_ptr<TransactionHistoryImpl> m_history;
     std::unique_ptr<Wallet2CallbackImpl> m_wallet2Callback;
diff --git a/src/wallet/api/wallet2_api.h b/src/wallet/api/wallet2_api.h
index bd641fc0e..4eaf80e70 100644
--- a/src/wallet/api/wallet2_api.h
+++ b/src/wallet/api/wallet2_api.h
@@ -446,6 +446,12 @@ struct Wallet
         ConnectionStatus_WrongVersion
     };
 
+    enum BackgroundSyncType {
+        BackgroundSync_Off = 0,
+        BackgroundSync_ReusePassword = 1,
+        BackgroundSync_CustomPassword = 2
+    };
+
     virtual ~Wallet() = 0;
     virtual std::string seed(const std::string& seed_offset = "") const = 0;
     virtual std::string getSeedLanguage() const = 0;
@@ -946,6 +952,42 @@ struct Wallet
      */
     virtual bool scanTransactions(const std::vector<std::string> &txids) = 0;
 
+    /*!
+     * \brief setupBackgroundSync       - setup background sync mode with just a view key
+     * \param background_sync_type      - the mode the wallet background syncs in
+     * \param wallet_password
+     * \param background_cache_password - custom password to encrypt background cache, only needed for custom password background sync type
+     * \return                          - true on success
+     */
+    virtual bool setupBackgroundSync(const BackgroundSyncType background_sync_type, const std::string &wallet_password, const optional<std::string> &background_cache_password) = 0;
+
+    /*!
+     * \brief getBackgroundSyncType     - get mode the wallet background syncs in
+     * \return                          - the type, or off if type is unknown
+     */
+    virtual BackgroundSyncType getBackgroundSyncType() const = 0;
+
+    /**
+     * @brief startBackgroundSync - sync the chain in the background with just view key
+     */
+    virtual bool startBackgroundSync() = 0;
+
+    /**
+     * @brief stopBackgroundSync  - bring back spend key and process background synced txs
+     * \param wallet_password
+     */
+    virtual bool stopBackgroundSync(const std::string &wallet_password) = 0;
+
+    /**
+     * @brief isBackgroundSyncing - returns true if the wallet is background syncing
+     */
+    virtual bool isBackgroundSyncing() const = 0;
+
+    /**
+     * @brief isBackgroundWallet - returns true if the wallet is a background wallet
+     */
+    virtual bool isBackgroundWallet() const = 0;
+
     virtual TransactionHistory * history() = 0;
     virtual AddressBook * addressBook() = 0;
     virtual Subaddress * subaddress() = 0;
diff --git a/src/wallet/wallet2.cpp b/src/wallet/wallet2.cpp
index 951e8cb6a..f5d4583c3 100644
--- a/src/wallet/wallet2.cpp
+++ b/src/wallet/wallet2.cpp
@@ -156,6 +156,8 @@ static const std::string MULTISIG_SIGNATURE_MAGIC = "SigMultisigPkV1";
 
 static const std::string ASCII_OUTPUT_MAGIC = "MoneroAsciiDataV1";
 
+static const std::string BACKGROUND_WALLET_SUFFIX = ".background";
+
 boost::mutex tools::wallet2::default_daemon_address_lock;
 std::string tools::wallet2::default_daemon_address = "";
 
@@ -1008,14 +1010,14 @@ uint64_t num_priv_multisig_keys_post_setup(uint64_t threshold, uint64_t total)
  * @param keys_data_key the chacha key that encrypts wallet keys files
  * @return crypto::chacha_key the chacha key that encrypts the wallet cache files
  */
-crypto::chacha_key derive_cache_key(const crypto::chacha_key& keys_data_key)
+crypto::chacha_key derive_cache_key(const crypto::chacha_key& keys_data_key, const unsigned char domain_separator)
 {
   static_assert(HASH_SIZE == sizeof(crypto::chacha_key), "Mismatched sizes of hash and chacha key");
 
   crypto::chacha_key cache_key;
   epee::mlocked<tools::scrubbed_arr<char, HASH_SIZE+1>> cache_key_data;
   memcpy(cache_key_data.data(), &keys_data_key, HASH_SIZE);
-  cache_key_data[HASH_SIZE] = config::HASH_KEY_WALLET_CACHE;
+  cache_key_data[HASH_SIZE] = domain_separator;
   cn_fast_hash(cache_key_data.data(), HASH_SIZE+1, (crypto::hash&) cache_key);
 
   return cache_key;
@@ -1103,7 +1105,7 @@ wallet_keys_unlocker::wallet_keys_unlocker(wallet2 &w, const boost::optional<too
   boost::lock_guard<boost::mutex> lock(lockers_lock);
   if (lockers++ > 0)
     locked = false;
-  if (!locked || w.is_unattended() || w.ask_password() != tools::wallet2::AskPasswordToDecrypt || w.watch_only())
+  if (!locked || w.is_unattended() || w.ask_password() != tools::wallet2::AskPasswordToDecrypt || w.watch_only() || w.is_background_syncing())
   {
     locked = false;
     return;
@@ -1219,6 +1221,11 @@ wallet2::wallet2(network_type nettype, uint64_t kdf_rounds, bool unattended, std
   m_ignore_outputs_above(MONEY_SUPPLY),
   m_ignore_outputs_below(0),
   m_track_uses(false),
+  m_is_background_wallet(false),
+  m_background_sync_type(BackgroundSyncOff),
+  m_background_syncing(false),
+  m_processing_background_cache(false),
+  m_custom_background_key(boost::none),
   m_show_wallet_name_when_locked(false),
   m_inactivity_lock_timeout(DEFAULT_INACTIVITY_LOCK_TIMEOUT),
   m_setup_background_mining(BackgroundMiningMaybe),
@@ -1854,6 +1861,9 @@ bool has_nonrequested_tx_at_height_or_above_requested(uint64_t height, const std
 //----------------------------------------------------------------------------------------------------
 void wallet2::scan_tx(const std::unordered_set<crypto::hash> &txids)
 {
+  THROW_WALLET_EXCEPTION_IF(m_background_syncing || m_is_background_wallet, error::wallet_internal_error,
+    "cannot scan tx from background wallet");
+
   // Get the transactions from daemon in batches sorted lowest height to highest
   tx_entry_data txs_to_scan = get_tx_entries(txids);
   if (txs_to_scan.tx_entries.empty())
@@ -2161,11 +2171,11 @@ void wallet2::scan_output(const cryptonote::transaction &tx, bool miner_tx, cons
   THROW_WALLET_EXCEPTION_IF(i >= tx.vout.size(), error::wallet_internal_error, "Invalid vout index");
 
   // if keys are encrypted, ask for password
-  if (m_ask_password == AskPasswordToDecrypt && !m_unattended && !m_watch_only && !m_multisig_rescan_k)
+  if (m_ask_password == AskPasswordToDecrypt && !m_unattended && !m_watch_only && !m_multisig_rescan_k && !m_background_syncing)
   {
     static critical_section password_lock;
     CRITICAL_REGION_LOCAL(password_lock);
-    if (!m_encrypt_keys_after_refresh)
+    if (!m_encrypt_keys_after_refresh && !m_processing_background_cache)
     {
       boost::optional<epee::wipeable_string> pwd = m_callback->on_get_password(pool ? "output found in pool" : "output received");
       THROW_WALLET_EXCEPTION_IF(!pwd, error::password_needed, tr("Password is needed to compute key image for incoming monero"));
@@ -2177,7 +2187,7 @@ void wallet2::scan_output(const cryptonote::transaction &tx, bool miner_tx, cons
   crypto::public_key output_public_key;
   THROW_WALLET_EXCEPTION_IF(!get_output_public_key(tx.vout[i], output_public_key), error::wallet_internal_error, "Failed to get output public key");
 
-  if (m_multisig)
+  if (m_multisig || m_background_syncing/*no spend key*/)
   {
     tx_scan_info.in_ephemeral.pub = output_public_key;
     tx_scan_info.in_ephemeral.sec = crypto::null_skey;
@@ -2434,6 +2444,22 @@ void wallet2::process_new_transaction(const crypto::hash &txid, const cryptonote
         THROW_WALLET_EXCEPTION_IF(tx.vout.size() != o_indices.size(), error::wallet_internal_error,
             "transactions outputs size=" + std::to_string(tx.vout.size()) +
             " not match with daemon response size=" + std::to_string(o_indices.size()));
+
+        // we're going to re-process this receive when background sync is disabled
+        if (m_background_syncing && m_background_sync_data.txs.find(txid) == m_background_sync_data.txs.end())
+        {
+          size_t bgs_idx = m_background_sync_data.txs.size();
+          background_synced_tx_t bgs_tx = {
+            .index_in_background_sync_data = bgs_idx,
+            .tx                            = tx,
+            .output_indices                = o_indices,
+            .height                        = height,
+            .block_timestamp               = ts,
+            .double_spend_seen             = double_spend_seen
+          };
+          LOG_PRINT_L2("Adding received tx " << txid << " to background sync data (idx=" << bgs_idx << ")");
+          m_background_sync_data.txs.insert({txid, std::move(bgs_tx)});
+        }
       }
 
       for(size_t o: outs)
@@ -2459,7 +2485,7 @@ void wallet2::process_new_transaction(const crypto::hash &txid, const cryptonote
 	    td.m_tx = (const cryptonote::transaction_prefix&)tx;
 	    td.m_txid = txid;
             td.m_key_image = tx_scan_info[o].ki;
-            td.m_key_image_known = !m_watch_only && !m_multisig;
+            td.m_key_image_known = !m_watch_only && !m_multisig && !m_background_syncing;
             if (!td.m_key_image_known)
             {
               // we might have cold signed, and have a mapping to key images
@@ -2649,10 +2675,25 @@ void wallet2::process_new_transaction(const crypto::hash &txid, const cryptonote
         set_spent(it->second, height);
         if (!ignore_callbacks && 0 != m_callback)
           m_callback->on_money_spent(height, txid, tx, amount, tx, td.m_subaddr_index);
+
+        if (m_background_syncing && m_background_sync_data.txs.find(txid) == m_background_sync_data.txs.end())
+        {
+          size_t bgs_idx = m_background_sync_data.txs.size();
+          background_synced_tx_t bgs_tx = {
+            .index_in_background_sync_data = bgs_idx,
+            .tx                            = tx,
+            .output_indices                = o_indices,
+            .height                        = height,
+            .block_timestamp               = ts,
+            .double_spend_seen             = double_spend_seen
+          };
+          LOG_PRINT_L2("Adding spent tx " << txid << " to background sync data (idx=" << bgs_idx << ")");
+          m_background_sync_data.txs.insert({txid, std::move(bgs_tx)});
+        }
       }
     }
 
-    if (!pool && m_track_uses)
+    if (!pool && (m_track_uses || (m_background_syncing && it == m_key_images.end())))
     {
       PERF_TIMER(track_uses);
       const uint64_t amount = in_to_key.amount;
@@ -2666,7 +2707,27 @@ void wallet2::process_new_transaction(const crypto::hash &txid, const cryptonote
           {
             size_t idx = i->second;
             THROW_WALLET_EXCEPTION_IF(idx >= m_transfers.size(), error::wallet_internal_error, "Output tracker cache index out of range");
-            m_transfers[idx].m_uses.push_back(std::make_pair(height, txid));
+
+            if (m_track_uses)
+              m_transfers[idx].m_uses.push_back(std::make_pair(height, txid));
+
+            // We'll re-process all txs which *might* be spends when we disable
+            // background sync and retrieve the spend key. We don't know if an
+            // output is a spend in this tx if we don't know its key image.
+            if (m_background_syncing && !m_transfers[idx].m_key_image_known && m_background_sync_data.txs.find(txid) == m_background_sync_data.txs.end())
+            {
+              size_t bgs_idx = m_background_sync_data.txs.size();
+              background_synced_tx_t bgs_tx = {
+                .index_in_background_sync_data = bgs_idx,
+                .tx                            = tx,
+                .output_indices                = o_indices,
+                .height                        = height,
+                .block_timestamp               = ts,
+                .double_spend_seen             = double_spend_seen
+              };
+              LOG_PRINT_L2("Adding plausible spent tx " << txid << " to background sync data (idx=" << bgs_idx << ")");
+              m_background_sync_data.txs.insert({txid, std::move(bgs_tx)});
+            }
           }
         }
       }
@@ -2676,7 +2737,24 @@ void wallet2::process_new_transaction(const crypto::hash &txid, const cryptonote
           continue;
         for (uint64_t offset: offsets)
           if (offset == td.m_global_output_index)
-            td.m_uses.push_back(std::make_pair(height, txid));
+          {
+            if (m_track_uses)
+              td.m_uses.push_back(std::make_pair(height, txid));
+            if (m_background_syncing && !td.m_key_image_known && m_background_sync_data.txs.find(txid) == m_background_sync_data.txs.end())
+            {
+              size_t bgs_idx = m_background_sync_data.txs.size();
+              background_synced_tx_t bgs_tx = {
+                .index_in_background_sync_data = bgs_idx,
+                .tx                            = tx,
+                .output_indices                = o_indices,
+                .height                        = height,
+                .block_timestamp               = ts,
+                .double_spend_seen             = double_spend_seen
+              };
+              LOG_PRINT_L2("Adding plausible spent tx " << txid << " to background sync data (idx=" << bgs_idx << ")");
+              m_background_sync_data.txs.insert({txid, std::move(bgs_tx)});
+            }
+          }
       }
     }
   }
@@ -3049,8 +3127,8 @@ void wallet2::pull_blocks(bool first, bool try_incremental, uint64_t start_heigh
   req.start_height = start_height;
   req.no_miner_tx = m_refresh_type == RefreshNoCoinbase;
 
-  req.requested_info = first ? COMMAND_RPC_GET_BLOCKS_FAST::BLOCKS_AND_POOL : COMMAND_RPC_GET_BLOCKS_FAST::BLOCKS_ONLY;
-  if (try_incremental)
+  req.requested_info = (first && !m_background_syncing) ? COMMAND_RPC_GET_BLOCKS_FAST::BLOCKS_AND_POOL : COMMAND_RPC_GET_BLOCKS_FAST::BLOCKS_ONLY;
+  if (try_incremental && !m_background_syncing)
     req.pool_info_since = m_pool_info_query_time;
 
   {
@@ -3073,7 +3151,7 @@ void wallet2::pull_blocks(bool first, bool try_incremental, uint64_t start_heigh
       << ", height " << blocks_start_height + blocks.size() << ", node height " << res.current_height
       << ", pool info " << static_cast<unsigned int>(res.pool_info_extent));
 
-  if (first)
+  if (first && !m_background_syncing)
   {
     if (res.pool_info_extent != COMMAND_RPC_GET_BLOCKS_FAST::NONE)
     {
@@ -3587,6 +3665,9 @@ void wallet2::process_unconfirmed_transfer(bool incremental, const crypto::hash
 // incremental update anymore, because with that we might miss some txs altogether.
 void wallet2::update_pool_state(std::vector<std::tuple<cryptonote::transaction, crypto::hash, bool>> &process_txs, bool refreshed, bool try_incremental)
 {
+  process_txs.clear();
+  if (m_background_syncing)
+    return;
   bool updated = false;
   if (m_pool_info_query_time != 0 && try_incremental)
   {
@@ -4121,6 +4202,8 @@ void wallet2::refresh(bool trusted_daemon, uint64_t start_height, uint64_t & blo
   }
 
   m_first_refresh_done = true;
+  if (m_background_syncing || m_is_background_wallet)
+    m_background_sync_data.first_refresh_done = true;
 
   LOG_PRINT_L1("Refresh done, blocks received: " << blocks_fetched << ", balance (all accounts): " << print_money(balance_all(false)) << ", unlocked: " << print_money(unlocked_balance_all(false)));
 }
@@ -4203,6 +4286,14 @@ wallet2::detached_blockchain_data wallet2::detach_blockchain(uint64_t height, st
       td.m_uses.pop_back();
   }
 
+  for (auto it = m_background_sync_data.txs.begin(); it != m_background_sync_data.txs.end(); )
+  {
+    if(height <= it->second.height)
+      it = m_background_sync_data.txs.erase(it);
+    else
+      ++it;
+  }
+
   if (output_tracker_cache)
     output_tracker_cache->clear();
 
@@ -4277,8 +4368,12 @@ void wallet2::handle_reorg(uint64_t height, std::map<std::pair<uint64_t, uint64_
   //               C
   THROW_WALLET_EXCEPTION_IF(height < m_blockchain.offset() && m_blockchain.size() > m_blockchain.offset(),
       error::wallet_internal_error, "Daemon claims reorg below last checkpoint");
+
   detached_blockchain_data dbd = detach_blockchain(height, output_tracker_cache);
 
+  if (m_background_syncing && height < m_background_sync_data.start_height)
+    m_background_sync_data.start_height = height;
+
   if (m_callback)
     m_callback->on_reorg(height, dbd.detached_blockchain.size(), dbd.detached_tx_hashes.size());
 }
@@ -4288,6 +4383,7 @@ bool wallet2::deinit()
   if(m_is_initialized) {
     m_is_initialized = false;
     unlock_keys_file();
+    unlock_background_keys_file();
     m_account.deinit();
   }
   return true;
@@ -4314,6 +4410,7 @@ bool wallet2::clear()
   m_device_last_key_image_sync = 0;
   m_pool_info_query_time = 0;
   m_skip_to_height = 0;
+  m_background_sync_data = background_sync_data_t{};
   return true;
 }
 //----------------------------------------------------------------------------------------------------
@@ -4332,13 +4429,30 @@ void wallet2::clear_soft(bool keep_key_images)
   m_scanned_pool_txs[1].clear();
   m_pool_info_query_time = 0;
   m_skip_to_height = 0;
+  m_background_sync_data = background_sync_data_t{};
 
   cryptonote::block b;
   generate_genesis(b);
   m_blockchain.push_back(get_block_hash(b));
   m_last_block_reward = cryptonote::get_outs_money_amount(b.miner_tx);
 }
-
+//----------------------------------------------------------------------------------------------------
+void wallet2::clear_user_data()
+{
+  for (auto i = m_confirmed_txs.begin(); i != m_confirmed_txs.end(); ++i)
+    i->second.m_dests.clear();
+  for (auto i = m_unconfirmed_txs.begin(); i != m_unconfirmed_txs.end(); ++i)
+    i->second.m_dests.clear();
+  for (auto i = m_transfers.begin(); i != m_transfers.end(); ++i)
+    i->m_frozen = false;
+  m_tx_keys.clear();
+  m_tx_notes.clear();
+  m_address_book.clear();
+  m_subaddress_labels.clear();
+  m_attributes.clear();
+  m_account_tags = std::pair<std::map<std::string, std::string>, std::vector<std::string>>();
+}
+//----------------------------------------------------------------------------------------------------
 /*!
  * \brief Stores wallet information to wallet file.
  * \param  keys_file_name Name of wallet file
@@ -4350,16 +4464,35 @@ bool wallet2::store_keys(const std::string& keys_file_name, const epee::wipeable
 {
   boost::optional<wallet2::keys_file_data> keys_file_data = get_keys_file_data(password, watch_only);
   CHECK_AND_ASSERT_MES(keys_file_data != boost::none, false, "failed to generate wallet keys data");
-
+  return store_keys_file_data(keys_file_name, keys_file_data.get());
+}
+//----------------------------------------------------------------------------------------------------
+bool wallet2::store_keys(const std::string& keys_file_name, const crypto::chacha_key& key, bool watch_only, bool background_keys_file)
+{
+  boost::optional<wallet2::keys_file_data> keys_file_data = get_keys_file_data(key, watch_only, background_keys_file);
+  CHECK_AND_ASSERT_MES(keys_file_data != boost::none, false, "failed to generate wallet keys data");
+  return store_keys_file_data(keys_file_name, keys_file_data.get(), background_keys_file);
+}
+//----------------------------------------------------------------------------------------------------
+bool wallet2::store_keys_file_data(const std::string& keys_file_name, wallet2::keys_file_data &keys_file_data, bool background_keys_file)
+{
   std::string tmp_file_name = keys_file_name + ".new";
   std::string buf;
-  bool r = ::serialization::dump_binary(keys_file_data.get(), buf);
+  bool r = ::serialization::dump_binary(keys_file_data, buf);
   r = r && save_to_file(tmp_file_name, buf);
   CHECK_AND_ASSERT_MES(r, false, "failed to generate wallet keys file " << tmp_file_name);
 
-  unlock_keys_file();
+  if (!background_keys_file)
+    unlock_keys_file();
+  else
+    unlock_background_keys_file();
+
   std::error_code e = tools::replace_file(tmp_file_name, keys_file_name);
-  lock_keys_file();
+
+  if (!background_keys_file)
+    lock_keys_file();
+  else
+    lock_background_keys_file(keys_file_name);
 
   if (e) {
     boost::filesystem::remove(tmp_file_name);
@@ -4372,25 +4505,26 @@ bool wallet2::store_keys(const std::string& keys_file_name, const epee::wipeable
 //----------------------------------------------------------------------------------------------------
 boost::optional<wallet2::keys_file_data> wallet2::get_keys_file_data(const epee::wipeable_string& password, bool watch_only)
 {
+  crypto::chacha_key key;
+  crypto::generate_chacha_key(password.data(), password.size(), key, m_kdf_rounds);
+  verify_password_with_cached_key(key);
+  return get_keys_file_data(key, watch_only);
+}
+//----------------------------------------------------------------------------------------------------
+boost::optional<wallet2::keys_file_data> wallet2::get_keys_file_data(const crypto::chacha_key& key, bool watch_only, bool background_keys_file)
+{
   epee::byte_slice account_data;
   std::string multisig_signers;
   std::string multisig_derivations;
   cryptonote::account_base account = m_account;
 
-  crypto::chacha_key key;
-  crypto::generate_chacha_key(password.data(), password.size(), key, m_kdf_rounds);
-
-  // We use m_cache_key as a deterministic test to see if given key corresponds to original password
-  const crypto::chacha_key cache_key = derive_cache_key(key);
-  THROW_WALLET_EXCEPTION_IF(cache_key != m_cache_key, error::invalid_password);
-
   if (m_ask_password == AskPasswordToDecrypt && !m_unattended && !m_watch_only)
   {
     account.encrypt_viewkey(key);
     account.decrypt_keys(key);
   }
 
-  if (watch_only)
+  if (watch_only || background_keys_file)
     account.forget_spend_key();
 
   account.encrypt_keys(key);
@@ -4525,6 +4659,9 @@ boost::optional<wallet2::keys_file_data> wallet2::get_keys_file_data(const epee:
   value2.SetInt(m_track_uses ? 1 : 0);
   json.AddMember("track_uses", value2, json.GetAllocator());
 
+  value2.SetInt(m_background_sync_type);
+  json.AddMember("background_sync_type", value2, json.GetAllocator());
+
   value2.SetInt(m_show_wallet_name_when_locked ? 1 : 0);
   json.AddMember("show_wallet_name_when_locked", value2, json.GetAllocator());
 
@@ -4585,6 +4722,12 @@ boost::optional<wallet2::keys_file_data> wallet2::get_keys_file_data(const epee:
   value2.SetInt(m_enable_multisig ? 1 : 0);
   json.AddMember("enable_multisig", value2, json.GetAllocator());
 
+  if (m_background_sync_type == BackgroundSyncCustomPassword && !background_keys_file && m_custom_background_key)
+  {
+    value.SetString(reinterpret_cast<const char*>(m_custom_background_key.get().data()), m_custom_background_key.get().size());
+    json.AddMember("custom_background_key", value, json.GetAllocator());
+  }
+
   // Serialize the JSON object
   rapidjson::StringBuffer buffer;
   rapidjson::Writer<rapidjson::StringBuffer> writer(buffer);
@@ -4611,13 +4754,81 @@ void wallet2::setup_keys(const epee::wipeable_string &password)
     m_account.decrypt_viewkey(key);
   }
 
-  m_cache_key = derive_cache_key(key);
+  m_cache_key = derive_cache_key(key, config::HASH_KEY_WALLET_CACHE);
 
   get_ringdb_key();
 }
 //----------------------------------------------------------------------------------------------------
+void validate_background_cache_password_usage(const tools::wallet2::BackgroundSyncType background_sync_type, const boost::optional<epee::wipeable_string> &background_cache_password, const bool multisig, const bool watch_only, const bool key_on_device)
+{
+  THROW_WALLET_EXCEPTION_IF(multisig || watch_only || key_on_device, error::wallet_internal_error, multisig
+      ? "Background sync not implemented for multisig wallets" : watch_only
+      ? "Background sync not implemented for view only wallets"
+      : "Background sync not implemented for HW wallets");
+
+  switch (background_sync_type)
+  {
+    case tools::wallet2::BackgroundSyncOff:
+    {
+      THROW_WALLET_EXCEPTION(error::wallet_internal_error, "background sync is not enabled");
+      break;
+    }
+    case tools::wallet2::BackgroundSyncReusePassword:
+    {
+      THROW_WALLET_EXCEPTION_IF(background_cache_password, error::wallet_internal_error,
+          "unexpected custom background cache password");
+      break;
+    }
+    case tools::wallet2::BackgroundSyncCustomPassword:
+    {
+      THROW_WALLET_EXCEPTION_IF(!background_cache_password, error::wallet_internal_error,
+          "expected custom background cache password");
+      break;
+    }
+    default: THROW_WALLET_EXCEPTION(error::wallet_internal_error, "unknown background sync type");
+  }
+}
+//----------------------------------------------------------------------------------------------------
+void get_custom_background_key(const epee::wipeable_string &password, crypto::chacha_key &custom_background_key, const uint64_t kdf_rounds)
+{
+  crypto::chacha_key key;
+  crypto::generate_chacha_key(password.data(), password.size(), key, kdf_rounds);
+  custom_background_key = derive_cache_key(key, config::HASH_KEY_BACKGROUND_KEYS_FILE);
+}
+//----------------------------------------------------------------------------------------------------
+const crypto::chacha_key wallet2::get_cache_key()
+{
+  if (m_background_sync_type == BackgroundSyncCustomPassword && m_background_syncing)
+  {
+    THROW_WALLET_EXCEPTION_IF(!m_custom_background_key, error::wallet_internal_error, "Custom background key not set");
+    // Domain separate keys used to encrypt background keys file and cache
+    return derive_cache_key(m_custom_background_key.get(), config::HASH_KEY_BACKGROUND_CACHE);
+  }
+  else
+  {
+    return m_cache_key;
+  }
+}
+//----------------------------------------------------------------------------------------------------
+void wallet2::verify_password_with_cached_key(const epee::wipeable_string &password)
+{
+  crypto::chacha_key key;
+  crypto::generate_chacha_key(password.data(), password.size(), key, m_kdf_rounds);
+  verify_password_with_cached_key(key);
+}
+//----------------------------------------------------------------------------------------------------
+void wallet2::verify_password_with_cached_key(const crypto::chacha_key &key)
+{
+  // We use m_cache_key as a deterministic test to see if given key corresponds to original password
+  const crypto::chacha_key cache_key = derive_cache_key(key, config::HASH_KEY_WALLET_CACHE);
+  THROW_WALLET_EXCEPTION_IF(cache_key != m_cache_key, error::invalid_password);
+}
+//----------------------------------------------------------------------------------------------------
 void wallet2::change_password(const std::string &filename, const epee::wipeable_string &original_password, const epee::wipeable_string &new_password)
 {
+  THROW_WALLET_EXCEPTION_IF(m_background_syncing || m_is_background_wallet, error::wallet_internal_error,
+      "cannot change password from background wallet");
+
   if (m_ask_password == AskPasswordToDecrypt && !m_unattended && !m_watch_only)
     decrypt_keys(original_password);
   setup_keys(new_password);
@@ -4676,8 +4887,24 @@ bool wallet2::load_keys_buf(const std::string& keys_buf, const epee::wipeable_st
   std::string account_data;
   account_data.resize(keys_file_data.account_data.size());
   crypto::chacha20(keys_file_data.account_data.data(), keys_file_data.account_data.size(), key, keys_file_data.iv, &account_data[0]);
-  if (json.Parse(account_data.c_str()).HasParseError() || !json.IsObject())
+  const bool try_v0_format = json.Parse(account_data.c_str()).HasParseError() || !json.IsObject();
+  if (try_v0_format)
     crypto::chacha8(keys_file_data.account_data.data(), keys_file_data.account_data.size(), key, keys_file_data.iv, &account_data[0]);
+
+  // Check if it's a background keys file if both of the above formats fail
+  {
+    m_is_background_wallet = false;
+    m_background_syncing = false;
+    cryptonote::account_base account_data_check;
+    if (try_v0_format && !epee::serialization::load_t_from_binary(account_data_check, account_data))
+    {
+      get_custom_background_key(password, key, m_kdf_rounds);
+      crypto::chacha20(keys_file_data.account_data.data(), keys_file_data.account_data.size(), key, keys_file_data.iv, &account_data[0]);
+      m_is_background_wallet = !json.Parse(account_data.c_str()).HasParseError() && json.IsObject();
+      m_background_syncing = m_is_background_wallet; // start a background wallet background syncing
+    }
+  }
+
   // The contents should be JSON if the wallet follows the new format.
   if (json.Parse(account_data.c_str()).HasParseError())
   {
@@ -4714,6 +4941,7 @@ bool wallet2::load_keys_buf(const std::string& keys_buf, const epee::wipeable_st
     m_ignore_outputs_above = MONEY_SUPPLY;
     m_ignore_outputs_below = 0;
     m_track_uses = false;
+    m_background_sync_type = BackgroundSyncOff;
     m_show_wallet_name_when_locked = false;
     m_inactivity_lock_timeout = DEFAULT_INACTIVITY_LOCK_TIMEOUT;
     m_setup_background_mining = BackgroundMiningMaybe;
@@ -4728,6 +4956,7 @@ bool wallet2::load_keys_buf(const std::string& keys_buf, const epee::wipeable_st
     encrypted_secret_keys = false;
     m_enable_multisig = false;
     m_allow_mismatched_daemon_version = false;
+    m_custom_background_key = boost::none;
   }
   else if(json.IsObject())
   {
@@ -4955,6 +5184,39 @@ bool wallet2::load_keys_buf(const std::string& keys_buf, const epee::wipeable_st
 
     GET_FIELD_FROM_JSON_RETURN_ON_ERROR(json, enable_multisig, int, Int, false, false);
     m_enable_multisig = field_enable_multisig;
+
+    GET_FIELD_FROM_JSON_RETURN_ON_ERROR(json, background_sync_type, BackgroundSyncType, Int, false, BackgroundSyncOff);
+    m_background_sync_type = field_background_sync_type;
+
+    // Load encryption key used to encrypt background cache
+    crypto::chacha_key custom_background_key;
+    m_custom_background_key = boost::none;
+    if (m_background_sync_type == BackgroundSyncCustomPassword && !m_is_background_wallet)
+    {
+      if (!json.HasMember("custom_background_key"))
+      {
+        LOG_ERROR("Field custom_background_key not found in JSON");
+        return false;
+      }
+      else if (!json["custom_background_key"].IsString())
+      {
+        LOG_ERROR("Field custom_background_key found in JSON, but not String");
+        return false;
+      }
+      else if (json["custom_background_key"].GetStringLength() != sizeof(crypto::chacha_key))
+      {
+        LOG_ERROR("Field custom_background_key found in JSON, but not correct length");
+        return false;
+      }
+      const char *field_custom_background_key = json["custom_background_key"].GetString();
+      memcpy(custom_background_key.data(), field_custom_background_key, sizeof(crypto::chacha_key));
+      m_custom_background_key = boost::optional<crypto::chacha_key>(custom_background_key);
+      LOG_PRINT_L1("Loaded custom background key derived from custom password");
+    }
+    else if (json.HasMember("custom_background_key"))
+    {
+      LOG_ERROR("Unexpected field custom_background_key found in JSON");
+    }
   }
   else
   {
@@ -5018,12 +5280,17 @@ bool wallet2::load_keys_buf(const std::string& keys_buf, const epee::wipeable_st
   const cryptonote::account_keys& keys = m_account.get_keys();
   hw::device &hwdev = m_account.get_device();
   r = r && hwdev.verify_keys(keys.m_view_secret_key,  keys.m_account_address.m_view_public_key);
-  if (!m_watch_only && !m_multisig && hwdev.device_protocol() != hw::device::PROTOCOL_COLD)
+  if (!m_watch_only && !m_multisig && hwdev.device_protocol() != hw::device::PROTOCOL_COLD && !m_is_background_wallet)
     r = r && hwdev.verify_keys(keys.m_spend_secret_key, keys.m_account_address.m_spend_public_key);
   THROW_WALLET_EXCEPTION_IF(!r, error::wallet_files_doesnt_correspond, m_keys_file, m_wallet_file);
 
   if (r)
-    setup_keys(password);
+  {
+    if (!m_is_background_wallet)
+      setup_keys(password);
+    else
+      m_custom_background_key = boost::optional<crypto::chacha_key>(key);
+  }
 
   return true;
 }
@@ -5038,11 +5305,12 @@ bool wallet2::load_keys_buf(const std::string& keys_buf, const epee::wipeable_st
  * can be used prior to rewriting wallet keys file, to ensure user has entered the correct password
  *
  */
-bool wallet2::verify_password(const epee::wipeable_string& password)
+bool wallet2::verify_password(const epee::wipeable_string& password, crypto::secret_key &spend_key_out)
 {
   // this temporary unlocking is necessary for Windows (otherwise the file couldn't be loaded).
   unlock_keys_file();
-  bool r = verify_password(m_keys_file, password, m_account.get_device().device_protocol() == hw::device::PROTOCOL_COLD || m_watch_only || m_multisig, m_account.get_device(), m_kdf_rounds);
+  const bool no_spend_key = m_account.get_device().device_protocol() == hw::device::PROTOCOL_COLD || m_watch_only || m_multisig || m_is_background_wallet;
+  bool r = verify_password(m_keys_file, password, no_spend_key, m_account.get_device(), m_kdf_rounds, spend_key_out);
   lock_keys_file();
   return r;
 }
@@ -5060,7 +5328,7 @@ bool wallet2::verify_password(const epee::wipeable_string& password)
  * can be used prior to rewriting wallet keys file, to ensure user has entered the correct password
  *
  */
-bool wallet2::verify_password(const std::string& keys_file_name, const epee::wipeable_string& password, bool no_spend_key, hw::device &hwdev, uint64_t kdf_rounds)
+bool wallet2::verify_password(const std::string& keys_file_name, const epee::wipeable_string& password, bool no_spend_key, hw::device &hwdev, uint64_t kdf_rounds, crypto::secret_key &spend_key_out)
 {
   rapidjson::Document json;
   wallet2::keys_file_data keys_file_data;
@@ -5077,9 +5345,22 @@ bool wallet2::verify_password(const std::string& keys_file_name, const epee::wip
   std::string account_data;
   account_data.resize(keys_file_data.account_data.size());
   crypto::chacha20(keys_file_data.account_data.data(), keys_file_data.account_data.size(), key, keys_file_data.iv, &account_data[0]);
-  if (json.Parse(account_data.c_str()).HasParseError() || !json.IsObject())
+  const bool try_v0_format = json.Parse(account_data.c_str()).HasParseError() || !json.IsObject();
+  if (try_v0_format)
     crypto::chacha8(keys_file_data.account_data.data(), keys_file_data.account_data.size(), key, keys_file_data.iv, &account_data[0]);
 
+  // Check if it's a background keys file if both of the above formats fail
+  {
+    cryptonote::account_base account_data_check;
+    if (try_v0_format && !epee::serialization::load_t_from_binary(account_data_check, account_data))
+    {
+      get_custom_background_key(password, key, kdf_rounds);
+      crypto::chacha20(keys_file_data.account_data.data(), keys_file_data.account_data.size(), key, keys_file_data.iv, &account_data[0]);
+      const bool is_background_wallet = json.Parse(account_data.c_str()).HasParseError() && json.IsObject();
+      no_spend_key = no_spend_key || is_background_wallet;
+    }
+  }
+
   // The contents should be JSON if the wallet follows the new format.
   if (json.Parse(account_data.c_str()).HasParseError())
   {
@@ -5104,6 +5385,7 @@ bool wallet2::verify_password(const std::string& keys_file_name, const epee::wip
   r = r && hwdev.verify_keys(keys.m_view_secret_key,  keys.m_account_address.m_view_public_key);
   if(!no_spend_key)
     r = r && hwdev.verify_keys(keys.m_spend_secret_key, keys.m_account_address.m_spend_public_key);
+  spend_key_out = (!no_spend_key && r) ? keys.m_spend_secret_key : crypto::null_skey;
   return r;
 }
 
@@ -5115,9 +5397,7 @@ void wallet2::encrypt_keys(const crypto::chacha_key &key)
 
 void wallet2::decrypt_keys(const crypto::chacha_key &key)
 {
-  // We use m_cache_key as a deterministic test to see if given key corresponds to original password
-  const crypto::chacha_key cache_key = derive_cache_key(key);
-  THROW_WALLET_EXCEPTION_IF(cache_key != m_cache_key, error::invalid_password);
+  verify_password_with_cached_key(key);
 
   m_account.encrypt_viewkey(key);
   m_account.decrypt_keys(key);
@@ -5868,11 +6148,30 @@ void wallet2::rewrite(const std::string& wallet_name, const epee::wipeable_strin
 {
   if (wallet_name.empty())
     return;
+  THROW_WALLET_EXCEPTION_IF(m_background_syncing || m_is_background_wallet, error::wallet_internal_error,
+    "cannot change wallet settings from background wallet");
   prepare_file_names(wallet_name);
   boost::system::error_code ignored_ec;
   THROW_WALLET_EXCEPTION_IF(!boost::filesystem::exists(m_keys_file, ignored_ec), error::file_not_found, m_keys_file);
   bool r = store_keys(m_keys_file, password, m_watch_only);
   THROW_WALLET_EXCEPTION_IF(!r, error::file_save_error, m_keys_file);
+
+  // Update the background keys file when we rewrite the main wallet keys file
+  if (m_background_sync_type == BackgroundSyncCustomPassword && m_custom_background_key)
+  {
+    const std::string background_keys_filename = make_background_keys_file_name(wallet_name);
+    if (!lock_background_keys_file(background_keys_filename))
+    {
+      LOG_ERROR("Background keys file " << background_keys_filename << " is opened by another wallet program and cannot be rewritten");
+      return; // not fatal, background keys file will just have different wallet settings
+    }
+    store_background_keys(m_custom_background_key.get());
+    store_background_cache(m_custom_background_key.get(), true/*do_reset_background_sync_data*/);
+  }
+  else if (m_background_sync_type == BackgroundSyncReusePassword)
+  {
+    reset_background_sync_data(m_background_sync_data);
+  }
 }
 /*!
  * \brief Writes to a file named based on the normal wallet (doesn't generate key, assumes it's already there)
@@ -5906,6 +6205,16 @@ bool wallet2::wallet_valid_path_format(const std::string& file_path)
   return !file_path.empty();
 }
 //----------------------------------------------------------------------------------------------------
+std::string wallet2::make_background_wallet_file_name(const std::string &wallet_file)
+{
+  return wallet_file + BACKGROUND_WALLET_SUFFIX;
+}
+//----------------------------------------------------------------------------------------------------
+std::string wallet2::make_background_keys_file_name(const std::string &wallet_file)
+{
+  return make_background_wallet_file_name(wallet_file) + ".keys";
+}
+//----------------------------------------------------------------------------------------------------
 bool wallet2::parse_long_payment_id(const std::string& payment_id_str, crypto::hash& payment_id)
 {
   cryptonote::blobdata payment_id_data;
@@ -6131,10 +6440,78 @@ void wallet2::load(const std::string& wallet_, const epee::wipeable_string& pass
     THROW_WALLET_EXCEPTION_IF(true, error::file_read_error, "failed to load keys from buffer");
   }
 
-  wallet_keys_unlocker unlocker(*this, m_ask_password == AskPasswordToDecrypt && !m_unattended && !m_watch_only, password);
+  wallet_keys_unlocker unlocker(*this, m_ask_password == AskPasswordToDecrypt && !m_unattended && !m_watch_only && !m_is_background_wallet, password);
 
   //keys loaded ok!
   //try to load wallet cache. but even if we failed, it is not big problem
+  load_wallet_cache(use_fs, cache_buf);
+
+  // Wallets used to wipe, but not erase, old unused multisig key info, which lead to huge memory leaks.
+  // Here we erase these multisig keys if they're zero'd out to free up space.
+  for (auto &td : m_transfers)
+  {
+    auto mk_it = td.m_multisig_k.begin();
+    while (mk_it != td.m_multisig_k.end())
+    {
+      if (*mk_it == rct::zero())
+        mk_it = td.m_multisig_k.erase(mk_it);
+      else
+        ++mk_it;
+    }
+  }
+
+  cryptonote::block genesis;
+  generate_genesis(genesis);
+  crypto::hash genesis_hash = get_block_hash(genesis);
+
+  if (m_blockchain.empty())
+  {
+    m_blockchain.push_back(genesis_hash);
+    m_last_block_reward = cryptonote::get_outs_money_amount(genesis.miner_tx);
+  }
+  else
+  {
+    check_genesis(genesis_hash);
+  }
+
+  trim_hashchain();
+
+  if (get_num_subaddress_accounts() == 0)
+    add_subaddress_account(tr("Primary account"));
+
+  try
+  {
+    find_and_save_rings(false);
+  }
+  catch (const std::exception &e)
+  {
+    MERROR("Failed to save rings, will try again next time");
+  }
+
+  try
+  {
+    if (use_fs)
+      m_message_store.read_from_file(get_multisig_wallet_state(), m_mms_file, m_load_deprecated_formats);
+  }
+  catch (const std::exception &e)
+  {
+    MERROR("Failed to initialize MMS, it will be unusable");
+  }
+
+  try
+  {
+    if (use_fs)
+      process_background_cache_on_open();
+  }
+  catch (const std::exception &e)
+  {
+    MERROR("Failed to process background cache on open: " << e.what());
+  }
+}
+//----------------------------------------------------------------------------------------------------
+void wallet2::load_wallet_cache(const bool use_fs, const std::string& cache_buf)
+{
+  boost::system::error_code e;
   bool cache_missing = use_fs ? (!boost::filesystem::exists(m_wallet_file, e) || e) : cache_buf.empty();
   if (cache_missing)
   {
@@ -6148,7 +6525,7 @@ void wallet2::load(const std::string& wallet_, const epee::wipeable_string& pass
     bool r = true;
     if (use_fs)
     {
-      load_from_file(m_wallet_file, cache_file_buf, std::numeric_limits<size_t>::max());
+      r = load_from_file(m_wallet_file, cache_file_buf, std::numeric_limits<size_t>::max());
       THROW_WALLET_EXCEPTION_IF(!r, error::file_read_error, m_wallet_file);
     }
 
@@ -6161,7 +6538,7 @@ void wallet2::load(const std::string& wallet_, const epee::wipeable_string& pass
       THROW_WALLET_EXCEPTION_IF(!r, error::wallet_internal_error, "internal error: failed to deserialize \"" + m_wallet_file + '\"');
       std::string cache_data;
       cache_data.resize(cache_file_data.cache_data.size());
-      crypto::chacha20(cache_file_data.cache_data.data(), cache_file_data.cache_data.size(), m_cache_key, cache_file_data.iv, &cache_data[0]);
+      crypto::chacha20(cache_file_data.cache_data.data(), cache_file_data.cache_data.size(), get_cache_key(), cache_file_data.iv, &cache_data[0]);
 
       try {
         bool loaded = false;
@@ -6251,57 +6628,76 @@ void wallet2::load(const std::string& wallet_, const epee::wipeable_string& pass
       m_account_public_address.m_view_public_key  != m_account.get_keys().m_account_address.m_view_public_key,
       error::wallet_files_doesnt_correspond, m_keys_file, m_wallet_file);
   }
+}
+//----------------------------------------------------------------------------------------------------
+void wallet2::process_background_cache_on_open()
+{
+  if (m_wallet_file.empty())
+    return;
+  if (m_background_syncing || m_is_background_wallet)
+    return;
+  if (m_background_sync_type == BackgroundSyncOff)
+    return;
 
-  // Wallets used to wipe, but not erase, old unused multisig key info, which lead to huge memory leaks.
-  // Here we erase these multisig keys if they're zero'd out to free up space.
-  for (auto &td : m_transfers)
+  if (m_background_sync_type == BackgroundSyncReusePassword)
   {
-    auto mk_it = td.m_multisig_k.begin();
-    while (mk_it != td.m_multisig_k.end())
+    const background_sync_data_t background_sync_data = m_background_sync_data;
+    const hashchain blockchain = m_blockchain;
+    process_background_cache(background_sync_data, blockchain, m_last_block_reward);
+
+    // Reset the background cache after processing
+    reset_background_sync_data(m_background_sync_data);
+  }
+  else if (m_background_sync_type == BackgroundSyncCustomPassword)
+  {
+    // If the background wallet files don't exist, recreate them
+    const std::string background_keys_file = make_background_keys_file_name(m_wallet_file);
+    const std::string background_wallet_file = make_background_wallet_file_name(m_wallet_file);
+    const bool background_keys_file_exists = boost::filesystem::exists(background_keys_file);
+    const bool background_wallet_exists = boost::filesystem::exists(background_wallet_file);
+
+    THROW_WALLET_EXCEPTION_IF(!lock_background_keys_file(background_keys_file), error::background_wallet_already_open, background_wallet_file);
+    THROW_WALLET_EXCEPTION_IF(!m_custom_background_key, error::wallet_internal_error, "Custom background key not set");
+
+    if (!background_keys_file_exists)
     {
-      if (*mk_it == rct::zero())
-        mk_it = td.m_multisig_k.erase(mk_it);
-      else
-        ++mk_it;
+      MDEBUG("Background keys file not found, restoring");
+      store_background_keys(m_custom_background_key.get());
     }
-  }
 
-  cryptonote::block genesis;
-  generate_genesis(genesis);
-  crypto::hash genesis_hash = get_block_hash(genesis);
+    if (!background_wallet_exists)
+    {
+      MDEBUG("Background cache not found, restoring");
+      store_background_cache(m_custom_background_key.get(), true/*do_reset_background_sync_data*/);
+      return;
+    }
 
-  if (m_blockchain.empty())
-  {
-    m_blockchain.push_back(genesis_hash);
-    m_last_block_reward = cryptonote::get_outs_money_amount(genesis.miner_tx);
-  }
-  else
-  {
-    check_genesis(genesis_hash);
-  }
+    MDEBUG("Loading background cache");
 
-  trim_hashchain();
+    // Set up a minimal background wallet2 instance
+    std::unique_ptr<wallet2> background_w2(new wallet2(m_nettype));
+    background_w2->m_is_background_wallet = true;
+    background_w2->m_background_syncing = true;
+    background_w2->m_background_sync_type = m_background_sync_type;
+    background_w2->m_custom_background_key = m_custom_background_key;
 
-  if (get_num_subaddress_accounts() == 0)
-    add_subaddress_account(tr("Primary account"));
+    cryptonote::account_base account = m_account;
+    account.forget_spend_key();
+    background_w2->m_account = account;
 
-  try
-  {
-    find_and_save_rings(false);
-  }
-  catch (const std::exception &e)
-  {
-    MERROR("Failed to save rings, will try again next time");
-  }
-  
-  try
-  {
-    if (use_fs)
-      m_message_store.read_from_file(get_multisig_wallet_state(), m_mms_file, m_load_deprecated_formats);
+    // Load background cache from file
+    background_w2->clear();
+    background_w2->prepare_file_names(background_wallet_file);
+    background_w2->load_wallet_cache(true/*use_fs*/);
+
+    process_background_cache(background_w2->m_background_sync_data, background_w2->m_blockchain, background_w2->m_last_block_reward);
+
+    // Reset the background cache after processing
+    store_background_cache(m_custom_background_key.get(), true/*do_reset_background_sync_data*/);
   }
-  catch (const std::exception &e)
+  else
   {
-    MERROR("Failed to initialize MMS, it will be unusable");
+    THROW_WALLET_EXCEPTION(error::wallet_internal_error, "unknown background sync type");
   }
 }
 //----------------------------------------------------------------------------------------------------
@@ -6383,6 +6779,8 @@ void wallet2::store_to(const std::string &path, const epee::wipeable_string &pas
     same_file = canonical_old_path == canonical_new_path;
   }
 
+  THROW_WALLET_EXCEPTION_IF(m_is_background_wallet && !same_file, error::wallet_internal_error,
+    "Cannot save background wallet files to a different location");
 
   if (!same_file)
   {
@@ -6399,6 +6797,21 @@ void wallet2::store_to(const std::string &path, const epee::wipeable_string &pas
       }
     }
   }
+  else if (m_background_sync_type == BackgroundSyncCustomPassword && m_background_syncing && !m_is_background_wallet)
+  {
+    // We're background syncing, so store the wallet cache as a background cache
+    // keeping the background sync data
+    try
+    {
+      THROW_WALLET_EXCEPTION_IF(!m_custom_background_key, error::wallet_internal_error, "Custom background key not set");
+      store_background_cache(m_custom_background_key.get(), false/*do_reset_background_sync_data*/);
+    }
+    catch (const std::exception &e)
+    {
+      MERROR("Failed to store background cache while background syncing: " << e.what());
+    }
+    return;
+  }
 
   // get wallet cache data
   boost::optional<wallet2::cache_file_data> cache_file_data = get_cache_file_data();
@@ -6492,6 +6905,22 @@ void wallet2::store_to(const std::string &path, const epee::wipeable_string &pas
     // store should only exist if the MMS is really active
     m_message_store.write_to_file(get_multisig_wallet_state(), m_mms_file);
   }
+
+  if (m_background_sync_type == BackgroundSyncCustomPassword && !m_background_syncing && !m_is_background_wallet)
+  {
+    // Update the background wallet cache when we store the main wallet cache
+    // Note: if background syncing when this is called, it means the background
+    // wallet is open and was already stored above
+    try
+    {
+      THROW_WALLET_EXCEPTION_IF(!m_custom_background_key, error::wallet_internal_error, "Custom background key not set");
+      store_background_cache(m_custom_background_key.get(), true/*do_reset_background_sync_data*/);
+    }
+    catch (const std::exception &e)
+    {
+      MERROR("Failed to update background cache: " << e.what());
+    }
+  }
 }
 //----------------------------------------------------------------------------------------------------
 boost::optional<wallet2::cache_file_data> wallet2::get_cache_file_data()
@@ -6509,7 +6938,7 @@ boost::optional<wallet2::cache_file_data> wallet2::get_cache_file_data()
     std::string cipher;
     cipher.resize(cache_file_data.get().cache_data.size());
     cache_file_data.get().iv = crypto::rand<crypto::chacha_iv>();
-    crypto::chacha20(cache_file_data.get().cache_data.data(), cache_file_data.get().cache_data.size(), m_cache_key, cache_file_data.get().iv, &cipher[0]);
+    crypto::chacha20(cache_file_data.get().cache_data.data(), cache_file_data.get().cache_data.size(), get_cache_key(), cache_file_data.get().iv, &cipher[0]);
     cache_file_data.get().cache_data = cipher;
     return cache_file_data;
   }
@@ -8560,6 +8989,34 @@ bool wallet2::is_keys_file_locked() const
   return m_keys_file_locker->locked();
 }
 
+bool wallet2::lock_background_keys_file(const std::string &background_keys_file)
+{
+  if (background_keys_file.empty() || !boost::filesystem::exists(background_keys_file))
+    return true;
+  if (m_background_keys_file_locker && m_background_keys_file_locker->locked())
+    return true;
+  m_background_keys_file_locker.reset(new tools::file_locker(background_keys_file));
+  return m_background_keys_file_locker->locked();
+}
+
+bool wallet2::unlock_background_keys_file()
+{
+  if (!m_background_keys_file_locker)
+  {
+    MDEBUG("background keys file locker is not set");
+    return false;
+  }
+  m_background_keys_file_locker.reset();
+  return true;
+}
+
+bool wallet2::is_background_keys_file_locked() const
+{
+  if (!m_background_keys_file_locker)
+    return false;
+  return m_background_keys_file_locker->locked();
+}
+
 bool wallet2::tx_add_fake_output(std::vector<std::vector<tools::wallet2::get_outs_entry>> &outs, uint64_t global_index, const crypto::public_key& output_public_key, const rct::key& mask, uint64_t real_index, bool unlocked, std::unordered_set<crypto::public_key> &valid_public_keys_cache) const
 {
   if (!unlocked) // don't add locked outs
@@ -13241,6 +13698,413 @@ bool wallet2::import_key_images(signed_tx_set & signed_tx, size_t offset, bool o
   return import_key_images(signed_tx.key_images, offset, only_selected_transfers ? boost::make_optional(selected_transfers) : boost::none);
 }
 
+/*
+  In background sync mode, we use just the view key when the wallet is scanning
+  to identify all txs where:
+
+    1. We received an output.
+    2. We spent an output.
+    3. We *may* have spent a received output but we didn't know for sure because
+       the spend key was not loaded while background sync was enabled.
+
+  When the user is ready to use the spend key again, we call this function to
+  process all those background synced transactions with the spend key loaded,
+  so that we can properly generate key images for the transactions which we
+  we were not able to do so for while background sync was enabled. This allows
+  us to determine *all* receives and spends the user completed while the wallet
+  had background sync enabled. Once this function completes, we can continue
+  scanning from where the background sync left off.
+
+  Txs of type 3 (txs which we *may* have spent received output(s)) are txs where
+  1+ rings contain an output that the user received and the wallet does not know
+  the associated key image for that output. We don't know if the user spent in
+  this type of tx or not. This function will generate key images for all outputs
+  we don't know key images for, and then check if those outputs were spent in
+  the txs of type 3.
+
+  By storing this type of "plausible spend tx" when scanning in background sync
+  mode, we avoid the need to query the daemon with key images when background
+  sync mode is disabled to see if those key images were spent. This would
+  reveal key images to 3rd party nodes for users who don't run their own.
+  Although this is not a perfect solution to avoid revealing key images to a 3rd
+  party node (since tx submission trivially reveals key images to a node), it's
+  probably better than revealing *unused* key images to a 3rd party node, which
+  would enable the 3rd party to deduce that a tx is spending an output at least
+  X old when the key image is included in the chain.
+*/
+void wallet2::process_background_cache(const background_sync_data_t &background_sync_data, const hashchain &background_synced_chain, uint64_t last_block_reward)
+{
+  // We expect the spend key to be in a decrypted state while
+  // m_processing_background_cache is true
+  m_processing_background_cache = true;
+  auto done_processing = epee::misc_utils::create_scope_leave_handler([&, this]() {
+    m_processing_background_cache = false;
+  });
+
+  if (m_background_syncing || m_multisig || m_watch_only || key_on_device())
+    return;
+
+  if (!background_sync_data.first_refresh_done)
+  {
+    MDEBUG("Skipping processing background cache, background cache has not synced yet");
+    return;
+  }
+
+  // Skip processing if wallet cache is synced higher than background cache
+  const uint64_t current_height = m_blockchain.size();
+  const uint64_t background_height = background_synced_chain.size();
+  MDEBUG("Background cache height " << background_height << " , wallet height " << current_height);
+  if (current_height > background_height)
+  {
+    MWARNING("Skipping processing background cache, synced height is higher than background cache");
+    return;
+  }
+
+  if (m_refresh_from_block_height  < background_sync_data.wallet_refresh_from_block_height ||
+      m_subaddress_lookahead_major > background_sync_data.subaddress_lookahead_major ||
+      m_subaddress_lookahead_minor > background_sync_data.subaddress_lookahead_minor ||
+      m_refresh_type               < background_sync_data.wallet_refresh_type)
+  {
+    MWARNING("Skipping processing background cache, background wallet sync settings did not match main wallet's");
+    MDEBUG("Wallet settings: " <<
+      ", m_refresh_from_block_height: "  << m_refresh_from_block_height  << " vs " << background_sync_data.wallet_refresh_from_block_height <<
+      ", m_subaddress_lookahead_major: " << m_subaddress_lookahead_major << " vs " << background_sync_data.subaddress_lookahead_major <<
+      ", m_subaddress_lookahead_minor: " << m_subaddress_lookahead_minor << " vs " << background_sync_data.subaddress_lookahead_minor <<
+      ", m_refresh_type: "               << m_refresh_type               << " vs " << background_sync_data.wallet_refresh_type);
+    return;
+  }
+
+  // Sort background synced txs in the order they appeared in the cache so that
+  // we process them in the order they appeared in the chain. Thus if tx2 spends
+  // from tx1, we will know because tx1 is processed before tx2.
+  std::vector<std::pair<crypto::hash, background_synced_tx_t>> sorted_bgs_cache(background_sync_data.txs.begin(), background_sync_data.txs.end());
+  std::sort(sorted_bgs_cache.begin(), sorted_bgs_cache.end(),
+    [](const std::pair<crypto::hash, background_synced_tx_t>& l, const std::pair<crypto::hash, background_synced_tx_t>& r)
+      {
+        uint64_t left_index = l.second.index_in_background_sync_data;
+        uint64_t right_index = r.second.index_in_background_sync_data;
+        THROW_WALLET_EXCEPTION_IF(
+            (left_index < right_index && l.second.height > r.second.height) ||
+            (left_index > right_index && l.second.height < r.second.height),
+            error::wallet_internal_error, "Unexpected background sync data order");
+        return left_index < right_index;
+      });
+
+  // All txs in the background cache should have height >= sync start height,
+  // but not fatal if not
+  if (!sorted_bgs_cache.empty() && sorted_bgs_cache[0].second.height < background_sync_data.start_height)
+    MWARNING("First tx in background cache has height (" << sorted_bgs_cache[0].second.height << ") lower than sync start height (" << background_sync_data.start_height << ")");
+
+  // We want to process all background synced txs in order to make sure
+  // the wallet state updates correctly. First we remove all txs from the wallet
+  // from before the background sync start height, then re-process them in
+  // chronological order. The background cache should contain a superset of
+  // *all* the wallet's txs from after the background sync start height.
+  MDEBUG("Processing " << background_sync_data.txs.size() << " background synced txs starting from height " << background_sync_data.start_height);
+  detached_blockchain_data dbd = detach_blockchain(background_sync_data.start_height);
+
+  for (const auto &bgs_tx : sorted_bgs_cache)
+  {
+    MDEBUG("Processing background synced tx " << bgs_tx.first);
+
+    process_new_transaction(bgs_tx.first, bgs_tx.second.tx, bgs_tx.second.output_indices, bgs_tx.second.height, 0, bgs_tx.second.block_timestamp,
+        cryptonote::is_coinbase(bgs_tx.second.tx), false/*pool*/, bgs_tx.second.double_spend_seen, {}, {}, true/*ignore_callbacks*/);
+
+    // Re-set destination addresses if they were previously set
+    if (m_confirmed_txs.find(bgs_tx.first) != m_confirmed_txs.end() &&
+        dbd.detached_confirmed_txs_dests.find(bgs_tx.first) != dbd.detached_confirmed_txs_dests.end())
+    {
+      m_confirmed_txs[bgs_tx.first].m_dests = std::move(dbd.detached_confirmed_txs_dests[bgs_tx.first]);
+    }
+  }
+
+  m_blockchain = background_synced_chain;
+  m_last_block_reward = last_block_reward;
+
+  MDEBUG("Finished processing background sync data");
+}
+//----------------------------------------------------------------------------------------------------
+void wallet2::reset_background_sync_data(background_sync_data_t &background_sync_data)
+{
+  background_sync_data.first_refresh_done = false;
+  background_sync_data.start_height = get_blockchain_current_height();
+  background_sync_data.txs.clear();
+
+  background_sync_data.wallet_refresh_from_block_height = m_refresh_from_block_height;
+  background_sync_data.subaddress_lookahead_major = m_subaddress_lookahead_major;
+  background_sync_data.subaddress_lookahead_minor = m_subaddress_lookahead_minor;
+  background_sync_data.wallet_refresh_type = m_refresh_type;
+}
+//----------------------------------------------------------------------------------------------------
+void wallet2::store_background_cache(const crypto::chacha_key &custom_background_key, const bool do_reset_background_sync_data)
+{
+  MDEBUG("Storing background cache (do_reset_background_sync_data=" << do_reset_background_sync_data << ")");
+
+  THROW_WALLET_EXCEPTION_IF(m_background_sync_type != BackgroundSyncCustomPassword, error::wallet_internal_error,
+      "Can only write a background cache when using a custom background password");
+  THROW_WALLET_EXCEPTION_IF(m_wallet_file.empty(), error::wallet_internal_error,
+      "No wallet file known, can't store background cache");
+
+  std::unique_ptr<wallet2> background_w2(new wallet2(m_nettype));
+  background_w2->prepare_file_names(make_background_wallet_file_name(m_wallet_file));
+
+  // Make sure background wallet is opened by this wallet
+  THROW_WALLET_EXCEPTION_IF(!lock_background_keys_file(background_w2->m_keys_file),
+      error::background_wallet_already_open, background_w2->m_wallet_file);
+
+  // Load a background wallet2 instance using this wallet2 instance
+  std::string this_wallet2;
+  bool r = ::serialization::dump_binary(*this, this_wallet2);
+  THROW_WALLET_EXCEPTION_IF(!r, error::wallet_internal_error, "Failed to serialize wallet cache");
+
+  background_w2->clear();
+  r = ::serialization::parse_binary(this_wallet2, *background_w2);
+  THROW_WALLET_EXCEPTION_IF(!r, error::wallet_internal_error, "Failed to deserialize wallet cache");
+
+  // Clear sensitive data from background cache not needed to sync
+  background_w2->clear_user_data();
+
+  background_w2->m_is_background_wallet = true;
+  if (do_reset_background_sync_data)
+    reset_background_sync_data(background_w2->m_background_sync_data);
+  else
+    background_w2->m_background_sync_data = m_background_sync_data;
+  background_w2->m_background_syncing = true;
+
+  background_w2->m_custom_background_key = boost::optional<crypto::chacha_key>(custom_background_key);
+  background_w2->m_background_sync_type = m_background_sync_type;
+  background_w2->store();
+
+  MDEBUG("Background cache stored (" << background_w2->m_transfers.size() << " transfers, " << background_w2->m_background_sync_data.txs.size() << " background synced txs)");
+}
+//----------------------------------------------------------------------------------------------------
+void wallet2::store_background_keys(const crypto::chacha_key &custom_background_key)
+{
+  MDEBUG("Storing background keys");
+
+  THROW_WALLET_EXCEPTION_IF(m_wallet_file.empty(), error::wallet_internal_error,
+      "No wallet file known, can't store background keys");
+
+  const std::string background_keys_file = make_background_keys_file_name(m_wallet_file);
+  bool r = store_keys(background_keys_file, custom_background_key, false/*watch_only*/, true/*background_keys_file*/);
+  THROW_WALLET_EXCEPTION_IF(!r, error::file_save_error, background_keys_file);
+  THROW_WALLET_EXCEPTION_IF(!is_background_keys_file_locked(), error::wallet_internal_error, background_keys_file + "\" should be locked");
+
+  // GUI uses the address file to differentiate non-mainnet wallets in the UI
+  const std::string background_address_file = make_background_wallet_file_name(m_wallet_file) + ".address.txt";
+  if (m_nettype != MAINNET && !boost::filesystem::exists(background_address_file))
+  {
+    r = save_to_file(background_address_file, m_account.get_public_address_str(m_nettype), true);
+    if (!r) MERROR("String with address text not saved");
+  }
+
+  MDEBUG("Background keys stored");
+}
+//----------------------------------------------------------------------------------------------------
+void wallet2::write_background_sync_wallet(const epee::wipeable_string &wallet_password, const epee::wipeable_string &background_cache_password)
+{
+  MDEBUG("Storing background sync wallet");
+  THROW_WALLET_EXCEPTION_IF(m_background_sync_type != BackgroundSyncCustomPassword, error::wallet_internal_error,
+      "Can only write a background sync wallet when using a custom background password");
+  THROW_WALLET_EXCEPTION_IF(m_background_syncing || m_is_background_wallet, error::wallet_internal_error,
+      "Can't write background sync wallet from an existing background cache");
+  THROW_WALLET_EXCEPTION_IF(wallet_password == background_cache_password,
+      error::background_custom_password_same_as_wallet_password);
+
+  // Set the background encryption key
+  crypto::chacha_key custom_background_key;
+  get_custom_background_key(background_cache_password, custom_background_key, m_kdf_rounds);
+
+  // Keep the background encryption key in memory so the main wallet can update
+  // the background cache when it stores the main wallet cache
+  m_custom_background_key = boost::optional<crypto::chacha_key>(custom_background_key);
+
+  if (m_wallet_file.empty() || m_keys_file.empty())
+    return;
+
+  // Save background keys file, then background cache, then update main wallet settings
+  store_background_keys(custom_background_key);
+  store_background_cache(custom_background_key, true/*do_reset_background_sync_data*/);
+  bool r = store_keys(m_keys_file, wallet_password, false/*watch_only*/);
+  THROW_WALLET_EXCEPTION_IF(!r, error::file_save_error, m_keys_file);
+
+  MDEBUG("Background sync wallet saved successfully");
+}
+//----------------------------------------------------------------------------------------------------
+void wallet2::setup_background_sync(BackgroundSyncType background_sync_type, const epee::wipeable_string &wallet_password, const boost::optional<epee::wipeable_string> &background_cache_password)
+{
+  MDEBUG("Setting background sync to type " << background_sync_type);
+  THROW_WALLET_EXCEPTION_IF(m_background_syncing || m_is_background_wallet, error::wallet_internal_error,
+      "Can't set background sync type from an existing background cache");
+  verify_password_with_cached_key(wallet_password);
+
+  if (background_sync_type != BackgroundSyncOff)
+    validate_background_cache_password_usage(background_sync_type, background_cache_password, m_multisig, m_watch_only, key_on_device());
+
+  THROW_WALLET_EXCEPTION_IF(background_sync_type == BackgroundSyncCustomPassword && wallet_password == background_cache_password,
+      error::background_custom_password_same_as_wallet_password);
+
+  if (m_background_sync_type == background_sync_type && background_sync_type != BackgroundSyncCustomPassword)
+    return; // No need to make any changes
+
+  if (!m_wallet_file.empty())
+  {
+    // Delete existing background files if they already exist
+    const std::string old_background_wallet_file = make_background_wallet_file_name(m_wallet_file);
+    const std::string old_background_keys_file = make_background_keys_file_name(m_wallet_file);
+    const std::string old_background_address_file = old_background_wallet_file + ".address.txt";
+
+    // Make sure no other program is using the background wallet
+    THROW_WALLET_EXCEPTION_IF(!lock_background_keys_file(old_background_keys_file),
+        error::background_wallet_already_open, old_background_wallet_file);
+
+    if (boost::filesystem::exists(old_background_wallet_file))
+      if (!boost::filesystem::remove(old_background_wallet_file))
+        LOG_ERROR("Error deleting background wallet file: " << old_background_wallet_file);
+
+    if (boost::filesystem::exists(old_background_keys_file))
+      if (!boost::filesystem::remove(old_background_keys_file))
+        LOG_ERROR("Error deleting background keys file: " << old_background_keys_file);
+
+    if (boost::filesystem::exists(old_background_address_file))
+      if (!boost::filesystem::remove(old_background_address_file))
+        LOG_ERROR("Error deleting background address file: " << old_background_address_file);
+  }
+
+  m_background_sync_type = background_sync_type;
+  m_custom_background_key = boost::none;
+
+  // Write the new files
+  switch (background_sync_type)
+  {
+    case BackgroundSyncOff:
+    case BackgroundSyncReusePassword: rewrite(m_wallet_file, wallet_password); break;
+    case BackgroundSyncCustomPassword: write_background_sync_wallet(wallet_password, background_cache_password.get()); break;
+    default: THROW_WALLET_EXCEPTION(error::wallet_internal_error, "unknown background sync type");
+  }
+
+  MDEBUG("Done setting background sync type");
+}
+//----------------------------------------------------------------------------------------------------
+/*
+  When background syncing, the wallet scans using just the view key, without
+  keeping the spend key in decrypted state. When a user returns to the wallet
+  and decrypts the spend key, the wallet processes the background synced txs,
+  then the wallet picks up scanning normally right where the background sync
+  left off.
+*/
+void wallet2::start_background_sync()
+{
+  THROW_WALLET_EXCEPTION_IF(m_background_sync_type == BackgroundSyncOff, error::wallet_internal_error,
+      "must setup background sync first before using background sync");
+  THROW_WALLET_EXCEPTION_IF(m_is_background_wallet, error::wallet_internal_error,
+      "Can't start background syncing from a background wallet (it is always background syncing)");
+
+  MDEBUG("Starting background sync");
+
+  if (m_background_syncing)
+  {
+    MDEBUG("Already background syncing");
+    return;
+  }
+
+  if (m_background_sync_type == BackgroundSyncCustomPassword && !m_wallet_file.empty())
+  {
+    // Save the current state of the wallet cache. Only necessary when using a
+    // custom background password which uses distinct background wallet to sync.
+    // When reusing wallet password to sync we reuse the main wallet cache.
+    store();
+
+    // Wipe user data from the background wallet cache not needed to sync.
+    // Only wipe user data from background cache if wallet cache is stored
+    // on disk; otherwise we could lose the data.
+    clear_user_data();
+
+    // Wipe m_cache_key since it can be used to decrypt main wallet cache
+    m_cache_key.scrub();
+  }
+
+  reset_background_sync_data(m_background_sync_data);
+  m_background_syncing = true;
+
+  // Wipe the spend key from memory
+  m_account.forget_spend_key();
+
+  MDEBUG("Background sync started at height " << m_background_sync_data.start_height);
+}
+//----------------------------------------------------------------------------------------------------
+void wallet2::stop_background_sync(const epee::wipeable_string &wallet_password, const crypto::secret_key &spend_secret_key)
+{
+  MDEBUG("Stopping background sync");
+
+  // Verify provided password and spend secret key. If no spend secret key is
+  // provided, recover it from the wallet keys file
+  crypto::secret_key recovered_spend_key = crypto::null_skey;
+  if (!m_wallet_file.empty())
+  {
+    THROW_WALLET_EXCEPTION_IF(!verify_password(wallet_password, recovered_spend_key), error::invalid_password);
+  }
+  else
+  {
+    verify_password_with_cached_key(wallet_password);
+  }
+
+  if (spend_secret_key != crypto::null_skey)
+  {
+    THROW_WALLET_EXCEPTION_IF(!m_wallet_file.empty() && spend_secret_key != recovered_spend_key,
+        error::invalid_spend_key);
+    MDEBUG("Setting spend secret key with the provided key");
+    recovered_spend_key = spend_secret_key;
+  }
+
+  // Verify private spend key derives to wallet's public spend key
+  const auto verify_spend_key = [this](crypto::secret_key &recovered_spend_key) -> bool
+  {
+    crypto::public_key spend_public_key;
+    return recovered_spend_key != crypto::null_skey &&
+        crypto::secret_key_to_public_key(recovered_spend_key, spend_public_key) &&
+        m_account.get_keys().m_account_address.m_spend_public_key == spend_public_key;
+  };
+  THROW_WALLET_EXCEPTION_IF(!verify_spend_key(recovered_spend_key), error::invalid_spend_key);
+
+  THROW_WALLET_EXCEPTION_IF(m_background_sync_type == BackgroundSyncOff, error::wallet_internal_error,
+      "must setup background sync first before using background sync");
+  THROW_WALLET_EXCEPTION_IF(m_is_background_wallet, error::wallet_internal_error,
+      "Can't stop background syncing from a background wallet");
+
+  if (!m_background_syncing)
+    return;
+
+  // Copy background cache, we're about to overwrite it
+  const background_sync_data_t background_sync_data = m_background_sync_data;
+  const hashchain background_synced_chain = m_blockchain;
+  const uint64_t last_block_reward = m_last_block_reward;
+
+  if (m_background_sync_type == BackgroundSyncCustomPassword && !m_wallet_file.empty())
+  {
+    // Reload the wallet from disk
+    load(m_wallet_file, wallet_password);
+    THROW_WALLET_EXCEPTION_IF(!verify_spend_key(recovered_spend_key), error::invalid_spend_key);
+  }
+  m_background_syncing = false;
+
+  // Set the plaintext spend key
+  m_account.set_spend_key(recovered_spend_key);
+
+  // Encrypt the spend key when done if needed
+  epee::misc_utils::auto_scope_leave_caller keys_reencryptor;
+  if (m_ask_password == AskPasswordToDecrypt && !m_unattended && !m_watch_only)
+    keys_reencryptor = epee::misc_utils::create_scope_leave_handler([&, this]{encrypt_keys(wallet_password);});
+
+  // Now we can use the decrypted spend key to process background cache
+  process_background_cache(background_sync_data, background_synced_chain, last_block_reward);
+
+  // Reset the background cache after processing
+  reset_background_sync_data(m_background_sync_data);
+
+  MDEBUG("Background sync stopped");
+}
+//----------------------------------------------------------------------------------------------------
 wallet2::payment_container wallet2::export_payments() const
 {
   payment_container payments;
diff --git a/src/wallet/wallet2.h b/src/wallet/wallet2.h
index d06dc678b..34833bed6 100644
--- a/src/wallet/wallet2.h
+++ b/src/wallet/wallet2.h
@@ -250,6 +250,20 @@ private:
       BackgroundMiningNo = 2,
     };
 
+    enum BackgroundSyncType {
+      BackgroundSyncOff = 0,
+      BackgroundSyncReusePassword = 1,
+      BackgroundSyncCustomPassword = 2,
+    };
+
+    static BackgroundSyncType background_sync_type_from_str(const std::string &background_sync_type_str)
+    {
+      if (background_sync_type_str == "off") return BackgroundSyncOff;
+      if (background_sync_type_str == "reuse-wallet-password") return BackgroundSyncReusePassword;
+      if (background_sync_type_str == "custom-background-password") return BackgroundSyncCustomPassword;
+      throw std::logic_error("Unknown background sync type");
+    };
+
     enum ExportFormat {
       Binary = 0,
       Ascii,
@@ -276,7 +290,12 @@ private:
     //! Just parses variables.
     static std::unique_ptr<wallet2> make_dummy(const boost::program_options::variables_map& vm, bool unattended, const std::function<boost::optional<password_container>(const char *, bool)> &password_prompter);
 
-    static bool verify_password(const std::string& keys_file_name, const epee::wipeable_string& password, bool no_spend_key, hw::device &hwdev, uint64_t kdf_rounds);
+    static bool verify_password(const std::string& keys_file_name, const epee::wipeable_string& password, bool no_spend_key, hw::device &hwdev, uint64_t kdf_rounds)
+    {
+      crypto::secret_key spend_key = crypto::null_skey;
+      return verify_password(keys_file_name, password, no_spend_key, hwdev, kdf_rounds, spend_key);
+    };
+    static bool verify_password(const std::string& keys_file_name, const epee::wipeable_string& password, bool no_spend_key, hw::device &hwdev, uint64_t kdf_rounds, crypto::secret_key &spend_key_out);
     static bool query_device(hw::device::device_type& device_type, const std::string& keys_file_name, const epee::wipeable_string& password, uint64_t kdf_rounds = 1);
 
     wallet2(cryptonote::network_type nettype = cryptonote::MAINNET, uint64_t kdf_rounds = 1, bool unattended = false, std::unique_ptr<epee::net_utils::http::http_client_factory> http_client_factory = std::unique_ptr<epee::net_utils::http::http_client_factory>(new net::http::client_factory()));
@@ -786,6 +805,54 @@ private:
       END_SERIALIZE()
     };
 
+    struct background_synced_tx_t
+    {
+      uint64_t index_in_background_sync_data;
+      cryptonote::transaction tx;
+      std::vector<uint64_t> output_indices;
+      uint64_t height;
+      uint64_t block_timestamp;
+      bool double_spend_seen;
+
+      BEGIN_SERIALIZE_OBJECT()
+        VERSION_FIELD(0)
+        VARINT_FIELD(index_in_background_sync_data)
+
+        // prune tx; don't need to keep signature data
+        if (!tx.serialize_base(ar))
+          return false;
+
+        FIELD(output_indices)
+        VARINT_FIELD(height)
+        VARINT_FIELD(block_timestamp)
+        FIELD(double_spend_seen)
+      END_SERIALIZE()
+    };
+
+    struct background_sync_data_t
+    {
+      bool first_refresh_done = false;
+      uint64_t start_height = 0;
+      std::unordered_map<crypto::hash, background_synced_tx_t> txs;
+
+      // Relevant wallet settings
+      uint64_t wallet_refresh_from_block_height;
+      size_t subaddress_lookahead_major;
+      size_t subaddress_lookahead_minor;
+      RefreshType wallet_refresh_type;
+
+      BEGIN_SERIALIZE_OBJECT()
+        VERSION_FIELD(0)
+        FIELD(first_refresh_done)
+        FIELD(start_height)
+        FIELD(txs)
+        FIELD(wallet_refresh_from_block_height)
+        VARINT_FIELD(subaddress_lookahead_major)
+        VARINT_FIELD(subaddress_lookahead_minor)
+        VARINT_FIELD(wallet_refresh_type)
+      END_SERIALIZE()
+    };
+
     typedef std::tuple<uint64_t, crypto::public_key, rct::key> get_outs_entry;
 
     struct parsed_block
@@ -998,7 +1065,8 @@ private:
     /*!
      * \brief verifies given password is correct for default wallet keys file
      */
-    bool verify_password(const epee::wipeable_string& password);
+    bool verify_password(const epee::wipeable_string& password) {crypto::secret_key key = crypto::null_skey; return verify_password(password, key);};
+    bool verify_password(const epee::wipeable_string& password, crypto::secret_key &spend_key_out);
     cryptonote::account_base& get_account(){return m_account;}
     const cryptonote::account_base& get_account()const{return m_account;}
 
@@ -1085,6 +1153,7 @@ private:
 
     cryptonote::network_type nettype() const { return m_nettype; }
     bool watch_only() const { return m_watch_only; }
+    bool is_background_wallet() const { return m_is_background_wallet; }
     multisig::multisig_account_status get_multisig_status() const;
     bool has_multisig_partial_key_images() const;
     bool has_unknown_key_images() const;
@@ -1294,11 +1363,17 @@ private:
         return;
       }
       a & m_has_ever_refreshed_from_node;
+      if(ver < 31)
+      {
+        m_background_sync_data = background_sync_data_t{};
+        return;
+      }
+      a & m_background_sync_data;
     }
 
     BEGIN_SERIALIZE_OBJECT()
       MAGIC_FIELD("monero wallet cache")
-      VERSION_FIELD(1)
+      VERSION_FIELD(2)
       FIELD(m_blockchain)
       FIELD(m_transfers)
       FIELD(m_account_public_address)
@@ -1331,6 +1406,12 @@ private:
         return true;
       }
       FIELD(m_has_ever_refreshed_from_node)
+      if (version < 2)
+      {
+        m_background_sync_data = background_sync_data_t{};
+        return true;
+      }
+      FIELD(m_background_sync_data)
     END_SERIALIZE()
 
     /*!
@@ -1346,6 +1427,8 @@ private:
      * \return                Whether path is valid format
      */
     static bool wallet_valid_path_format(const std::string& file_path);
+    static std::string make_background_wallet_file_name(const std::string &wallet_file);
+    static std::string make_background_keys_file_name(const std::string &wallet_file);
     static bool parse_long_payment_id(const std::string& payment_id_str, crypto::hash& payment_id);
     static bool parse_short_payment_id(const std::string& payment_id_str, crypto::hash8& payment_id);
     static bool parse_payment_id(const std::string& payment_id_str, crypto::hash& payment_id);
@@ -1392,6 +1475,9 @@ private:
     void ignore_outputs_below(uint64_t value) { m_ignore_outputs_below = value; }
     bool track_uses() const { return m_track_uses; }
     void track_uses(bool value) { m_track_uses = value; }
+    BackgroundSyncType background_sync_type() const { return m_background_sync_type; }
+    void setup_background_sync(BackgroundSyncType background_sync_type, const epee::wipeable_string &wallet_password, const boost::optional<epee::wipeable_string> &background_cache_password);
+    bool is_background_syncing() const { return m_background_syncing; }
     bool show_wallet_name_when_locked() const { return m_show_wallet_name_when_locked; }
     void show_wallet_name_when_locked(bool value) { m_show_wallet_name_when_locked = value; }
     BackgroundMiningSetupType setup_background_mining() const { return m_setup_background_mining; }
@@ -1666,6 +1752,9 @@ private:
     uint64_t get_bytes_sent() const;
     uint64_t get_bytes_received() const;
 
+    void start_background_sync();
+    void stop_background_sync(const epee::wipeable_string &wallet_password, const crypto::secret_key &spend_secret_key = crypto::null_skey);
+
     // MMS -------------------------------------------------------------------------------------------------
     mms::message_store& get_message_store() { return m_message_store; };
     const mms::message_store& get_message_store() const { return m_message_store; };
@@ -1698,6 +1787,9 @@ private:
      * \return                Whether it was successful.
      */
     bool store_keys(const std::string& keys_file_name, const epee::wipeable_string& password, bool watch_only = false);
+    bool store_keys(const std::string& keys_file_name, const crypto::chacha_key& key, bool watch_only = false, bool background_keys_file = false);
+    boost::optional<wallet2::keys_file_data> get_keys_file_data(const crypto::chacha_key& key, bool watch_only = false, bool background_keys_file = false);
+    bool store_keys_file_data(const std::string& keys_file_name, wallet2::keys_file_data &keys_file_data, bool background_keys_file = false);
     /*!
      * \brief Load wallet keys information from wallet file.
      * \param keys_file_name Name of wallet file
@@ -1711,6 +1803,7 @@ private:
      */
     bool load_keys_buf(const std::string& keys_buf, const epee::wipeable_string& password);
     bool load_keys_buf(const std::string& keys_buf, const epee::wipeable_string& password, boost::optional<crypto::chacha_key>& keys_to_encrypt);
+    void load_wallet_cache(const bool use_fs, const std::string& cache_buf = "");
     void process_new_transaction(const crypto::hash &txid, const cryptonote::transaction& tx, const std::vector<uint64_t> &o_indices, uint64_t height, uint8_t block_version, uint64_t ts, bool miner_tx, bool pool, bool double_spend_seen, const tx_cache_data &tx_cache_data, std::map<std::pair<uint64_t, uint64_t>, size_t> *output_tracker_cache = NULL, bool ignore_callbacks = false);
     bool should_skip_block(const cryptonote::block &b, uint64_t height) const;
     void process_new_blockchain_entry(const cryptonote::block& b, const cryptonote::block_complete_entry& bche, const parsed_block &parsed_block, const crypto::hash& bl_id, uint64_t height, const std::vector<tx_cache_data> &tx_cache_data, size_t tx_cache_data_offset, std::map<std::pair<uint64_t, uint64_t>, size_t> *output_tracker_cache = NULL);
@@ -1719,6 +1812,15 @@ private:
     void get_short_chain_history(std::list<crypto::hash>& ids, uint64_t granularity = 1) const;
     bool clear();
     void clear_soft(bool keep_key_images=false);
+    /*
+     * clear_user_data clears data created by the user, which is mostly data
+     * that a view key cannot identify on chain. This function was initially
+     * added to ensure that a "background" wallet (a wallet that syncs with just
+     * a view key hot in memory) does not have any sensitive data loaded that it
+     * does not need in order to sync. Future devs should take care to ensure
+     * that this function deletes data that is not useful for background syncing
+     */
+    void clear_user_data();
     void pull_blocks(bool first, bool try_incremental, uint64_t start_height, uint64_t& blocks_start_height, const std::list<crypto::hash> &short_chain_history, std::vector<cryptonote::block_complete_entry> &blocks, std::vector<cryptonote::COMMAND_RPC_GET_BLOCKS_FAST::block_output_indices> &o_indices, uint64_t &current_height, std::vector<std::tuple<cryptonote::transaction, crypto::hash, bool>>& process_pool_txs);
     void pull_hashes(uint64_t start_height, uint64_t& blocks_start_height, const std::list<crypto::hash> &short_chain_history, std::vector<crypto::hash> &hashes);
     void fast_refresh(uint64_t stop_height, uint64_t &blocks_start_height, std::list<crypto::hash> &short_chain_history, bool force = false);
@@ -1770,10 +1872,23 @@ private:
     bool get_ring(const crypto::chacha_key &key, const crypto::key_image &key_image, std::vector<uint64_t> &outs);
     crypto::chacha_key get_ringdb_key();
     void setup_keys(const epee::wipeable_string &password);
+    const crypto::chacha_key get_cache_key();
+    void verify_password_with_cached_key(const epee::wipeable_string &password);
+    void verify_password_with_cached_key(const crypto::chacha_key &key);
     size_t get_transfer_details(const crypto::key_image &ki) const;
     tx_entry_data get_tx_entries(const std::unordered_set<crypto::hash> &txids);
     void sort_scan_tx_entries(std::vector<process_tx_entry_t> &unsorted_tx_entries);
     void process_scan_txs(const tx_entry_data &txs_to_scan, const tx_entry_data &txs_to_reprocess, const std::unordered_set<crypto::hash> &tx_hashes_to_reprocess, detached_blockchain_data &dbd);
+    void write_background_sync_wallet(const epee::wipeable_string &wallet_password, const epee::wipeable_string &background_cache_password);
+    void process_background_cache_on_open();
+    void process_background_cache(const background_sync_data_t &background_sync_data, const hashchain &background_chain, uint64_t last_block_reward);
+    void reset_background_sync_data(background_sync_data_t &background_sync_data);
+    void store_background_cache(const crypto::chacha_key &custom_background_key, const bool do_reset_background_sync_data = true);
+    void store_background_keys(const crypto::chacha_key &custom_background_key);
+
+    bool lock_background_keys_file(const std::string &background_keys_file);
+    bool unlock_background_keys_file();
+    bool is_background_keys_file_locked() const;
 
     void register_devices();
     hw::device& lookup_device(const std::string & device_descriptor);
@@ -1885,6 +2000,8 @@ private:
     uint64_t m_ignore_outputs_above;
     uint64_t m_ignore_outputs_below;
     bool m_track_uses;
+    bool m_is_background_wallet;
+    BackgroundSyncType m_background_sync_type;
     bool m_show_wallet_name_when_locked;
     uint32_t m_inactivity_lock_timeout;
     BackgroundMiningSetupType m_setup_background_mining;
@@ -1912,6 +2029,7 @@ private:
 
     uint64_t m_last_block_reward;
     std::unique_ptr<tools::file_locker> m_keys_file_locker;
+    std::unique_ptr<tools::file_locker> m_background_keys_file_locker;
     
     mms::message_store m_message_store;
     bool m_original_keys_available;
@@ -1919,6 +2037,7 @@ private:
     crypto::secret_key m_original_view_secret_key;
 
     crypto::chacha_key m_cache_key;
+    boost::optional<crypto::chacha_key> m_custom_background_key = boost::none;
     std::shared_ptr<wallet_keys_unlocker> m_encrypt_keys_after_refresh;
 
     bool m_unattended;
@@ -1934,9 +2053,13 @@ private:
 
     static boost::mutex default_daemon_address_lock;
     static std::string default_daemon_address;
+
+    bool m_background_syncing;
+    bool m_processing_background_cache;
+    background_sync_data_t m_background_sync_data;
   };
 }
-BOOST_CLASS_VERSION(tools::wallet2, 30)
+BOOST_CLASS_VERSION(tools::wallet2, 31)
 BOOST_CLASS_VERSION(tools::wallet2::transfer_details, 12)
 BOOST_CLASS_VERSION(tools::wallet2::multisig_info, 1)
 BOOST_CLASS_VERSION(tools::wallet2::multisig_info::LR, 0)
@@ -1952,6 +2075,8 @@ BOOST_CLASS_VERSION(tools::wallet2::signed_tx_set, 1)
 BOOST_CLASS_VERSION(tools::wallet2::tx_construction_data, 4)
 BOOST_CLASS_VERSION(tools::wallet2::pending_tx, 3)
 BOOST_CLASS_VERSION(tools::wallet2::multisig_sig, 1)
+BOOST_CLASS_VERSION(tools::wallet2::background_synced_tx_t, 0)
+BOOST_CLASS_VERSION(tools::wallet2::background_sync_data_t, 0)
 
 namespace boost
 {
@@ -2450,6 +2575,29 @@ namespace boost
         return;
       a & x.multisig_sigs;
     }
+
+    template <class Archive>
+    inline void serialize(Archive& a, tools::wallet2::background_synced_tx_t &x, const boost::serialization::version_type ver)
+    {
+      a & x.index_in_background_sync_data;
+      a & x.tx;
+      a & x.output_indices;
+      a & x.height;
+      a & x.block_timestamp;
+      a & x.double_spend_seen;
+    }
+
+    template <class Archive>
+    inline void serialize(Archive& a, tools::wallet2::background_sync_data_t &x, const boost::serialization::version_type ver)
+    {
+      a & x.first_refresh_done;
+      a & x.start_height;
+      a & x.txs;
+      a & x.wallet_refresh_from_block_height;
+      a & x.subaddress_lookahead_major;
+      a & x.subaddress_lookahead_minor;
+      a & x.wallet_refresh_type;
+    }
   }
 }
 
diff --git a/src/wallet/wallet_errors.h b/src/wallet/wallet_errors.h
index 2bbfc3167..0315a7102 100644
--- a/src/wallet/wallet_errors.h
+++ b/src/wallet/wallet_errors.h
@@ -63,6 +63,7 @@ namespace tools
     //       invalid_password
     //       invalid_priority
     //       invalid_multisig_seed
+    //       invalid_spend_key
     //       refresh_error *
     //         acc_outs_lookup_error
     //         block_parse_error
@@ -97,6 +98,9 @@ namespace tools
     //       wallet_files_doesnt_correspond
     //       scan_tx_error *
     //         wont_reprocess_recent_txs_via_untrusted_daemon
+    //       background_sync_error *
+    //         background_wallet_already_open
+    //         background_custom_password_same_as_wallet_password
     //
     // * - class with protected ctor
 
@@ -304,6 +308,16 @@ namespace tools
       std::string to_string() const { return wallet_logic_error::to_string(); }
     };
 
+    struct invalid_spend_key : public wallet_logic_error
+    {
+      explicit invalid_spend_key(std::string&& loc)
+        : wallet_logic_error(std::move(loc), "invalid spend key")
+      {
+      }
+
+      std::string to_string() const { return wallet_logic_error::to_string(); }
+    };
+
     //----------------------------------------------------------------------------------------------------
     struct invalid_pregenerated_random : public wallet_logic_error
     {
@@ -948,6 +962,31 @@ namespace tools
       }
     };
     //----------------------------------------------------------------------------------------------------
+    struct background_sync_error : public wallet_logic_error
+    {
+    protected:
+      explicit background_sync_error(std::string&& loc, const std::string& message)
+        : wallet_logic_error(std::move(loc), message)
+      {
+      }
+    };
+    //----------------------------------------------------------------------------------------------------
+    struct background_wallet_already_open : public background_sync_error
+    {
+      explicit background_wallet_already_open(std::string&& loc, const std::string& background_wallet_file)
+        : background_sync_error(std::move(loc), "background wallet " + background_wallet_file + " is already opened by another wallet program")
+      {
+      }
+    };
+    //----------------------------------------------------------------------------------------------------
+    struct background_custom_password_same_as_wallet_password : public background_sync_error
+    {
+      explicit background_custom_password_same_as_wallet_password(std::string&& loc)
+        : background_sync_error(std::move(loc), "custom background password must be different than wallet password")
+      {
+      }
+    };
+    //----------------------------------------------------------------------------------------------------
 
 #if !defined(_MSC_VER)
 
diff --git a/src/wallet/wallet_rpc_server.cpp b/src/wallet/wallet_rpc_server.cpp
index 86bfadeae..f998c88b7 100644
--- a/src/wallet/wallet_rpc_server.cpp
+++ b/src/wallet/wallet_rpc_server.cpp
@@ -73,6 +73,54 @@ using namespace epee;
     } \
   } while(0)
 
+#define CHECK_IF_BACKGROUND_SYNCING() \
+  do \
+  { \
+    if (!m_wallet) { return not_open(er); } \
+    if (m_wallet->is_background_wallet()) \
+    { \
+      er.code = WALLET_RPC_ERROR_CODE_IS_BACKGROUND_WALLET; \
+      er.message = "This command is disabled for background wallets."; \
+      return false; \
+    } \
+    if (m_wallet->is_background_syncing()) \
+    { \
+      er.code = WALLET_RPC_ERROR_CODE_IS_BACKGROUND_SYNCING; \
+      er.message = "This command is disabled while background syncing. Stop background syncing to use this command."; \
+      return false; \
+    } \
+  } while(0)
+
+#define PRE_VALIDATE_BACKGROUND_SYNC() \
+  do \
+  { \
+    if (!m_wallet) { return not_open(er); } \
+    if (m_restricted) \
+    { \
+      er.code = WALLET_RPC_ERROR_CODE_DENIED; \
+      er.message = "Command unavailable in restricted mode."; \
+      return false; \
+    } \
+    if (m_wallet->key_on_device()) \
+    { \
+      er.code = WALLET_RPC_ERROR_CODE_UNKNOWN_ERROR; \
+      er.message = "Command not supported by HW wallet"; \
+      return false; \
+    } \
+    if (m_wallet->get_multisig_status().multisig_is_active) \
+    { \
+      er.code = WALLET_RPC_ERROR_CODE_UNKNOWN_ERROR; \
+      er.message = "Multisig wallet cannot enable background sync"; \
+      return false; \
+    } \
+    if (m_wallet->watch_only()) \
+    { \
+      er.code = WALLET_RPC_ERROR_CODE_WATCH_ONLY; \
+      er.message = "Watch-only wallet cannot enable background sync"; \
+      return false; \
+    } \
+  } while (0)
+
 namespace
 {
   const command_line::arg_descriptor<std::string, true> arg_rpc_bind_port = {"rpc-bind-port", "Sets bind port for server"};
@@ -291,6 +339,9 @@ namespace tools
   {
     if (!m_wallet)
       return;
+    // Background mining can be toggled from the main wallet
+    if (m_wallet->is_background_wallet() || m_wallet->is_background_syncing())
+      return;
 
     tools::wallet2::BackgroundMiningSetupType setup = m_wallet->setup_background_mining();
     if (setup == tools::wallet2::BackgroundMiningNo)
@@ -581,6 +632,7 @@ namespace tools
   bool wallet_rpc_server::on_create_address(const wallet_rpc::COMMAND_RPC_CREATE_ADDRESS::request& req, wallet_rpc::COMMAND_RPC_CREATE_ADDRESS::response& res, epee::json_rpc::error& er, const connection_context *ctx)
   {
     if (!m_wallet) return not_open(er);
+    CHECK_IF_BACKGROUND_SYNCING();
     try
     {
       if (req.count < 1 || req.count > 65536) {
@@ -618,6 +670,7 @@ namespace tools
   bool wallet_rpc_server::on_label_address(const wallet_rpc::COMMAND_RPC_LABEL_ADDRESS::request& req, wallet_rpc::COMMAND_RPC_LABEL_ADDRESS::response& res, epee::json_rpc::error& er, const connection_context *ctx)
   {
     if (!m_wallet) return not_open(er);
+    CHECK_IF_BACKGROUND_SYNCING();
     try
     {
       m_wallet->set_subaddress_label(req.index, req.label);
@@ -680,6 +733,7 @@ namespace tools
   bool wallet_rpc_server::on_create_account(const wallet_rpc::COMMAND_RPC_CREATE_ACCOUNT::request& req, wallet_rpc::COMMAND_RPC_CREATE_ACCOUNT::response& res, epee::json_rpc::error& er, const connection_context *ctx)
   {
     if (!m_wallet) return not_open(er);
+    CHECK_IF_BACKGROUND_SYNCING();
     try
     {
       m_wallet->add_subaddress_account(req.label);
@@ -697,6 +751,7 @@ namespace tools
   bool wallet_rpc_server::on_label_account(const wallet_rpc::COMMAND_RPC_LABEL_ACCOUNT::request& req, wallet_rpc::COMMAND_RPC_LABEL_ACCOUNT::response& res, epee::json_rpc::error& er, const connection_context *ctx)
   {
     if (!m_wallet) return not_open(er);
+    CHECK_IF_BACKGROUND_SYNCING();
     try
     {
       m_wallet->set_subaddress_label({req.account_index, 0}, req.label);
@@ -712,6 +767,7 @@ namespace tools
   bool wallet_rpc_server::on_get_account_tags(const wallet_rpc::COMMAND_RPC_GET_ACCOUNT_TAGS::request& req, wallet_rpc::COMMAND_RPC_GET_ACCOUNT_TAGS::response& res, epee::json_rpc::error& er, const connection_context *ctx)
   {
     if (!m_wallet) return not_open(er);
+    CHECK_IF_BACKGROUND_SYNCING();
     const std::pair<std::map<std::string, std::string>, std::vector<std::string>> account_tags = m_wallet->get_account_tags();
     for (const std::pair<const std::string, std::string>& p : account_tags.first)
     {
@@ -731,6 +787,7 @@ namespace tools
   bool wallet_rpc_server::on_tag_accounts(const wallet_rpc::COMMAND_RPC_TAG_ACCOUNTS::request& req, wallet_rpc::COMMAND_RPC_TAG_ACCOUNTS::response& res, epee::json_rpc::error& er, const connection_context *ctx)
   {
     if (!m_wallet) return not_open(er);
+    CHECK_IF_BACKGROUND_SYNCING();
     try
     {
       m_wallet->set_account_tag(req.accounts, req.tag);
@@ -746,6 +803,7 @@ namespace tools
   bool wallet_rpc_server::on_untag_accounts(const wallet_rpc::COMMAND_RPC_UNTAG_ACCOUNTS::request& req, wallet_rpc::COMMAND_RPC_UNTAG_ACCOUNTS::response& res, epee::json_rpc::error& er, const connection_context *ctx)
   {
     if (!m_wallet) return not_open(er);
+    CHECK_IF_BACKGROUND_SYNCING();
     try
     {
       m_wallet->set_account_tag(req.accounts, "");
@@ -761,6 +819,7 @@ namespace tools
   bool wallet_rpc_server::on_set_account_tag_description(const wallet_rpc::COMMAND_RPC_SET_ACCOUNT_TAG_DESCRIPTION::request& req, wallet_rpc::COMMAND_RPC_SET_ACCOUNT_TAG_DESCRIPTION::response& res, epee::json_rpc::error& er, const connection_context *ctx)
   {
     if (!m_wallet) return not_open(er);
+    CHECK_IF_BACKGROUND_SYNCING();
     try
     {
       m_wallet->set_account_tag_description(req.tag, req.description);
@@ -791,6 +850,7 @@ namespace tools
   bool wallet_rpc_server::on_freeze(const wallet_rpc::COMMAND_RPC_FREEZE::request& req, wallet_rpc::COMMAND_RPC_FREEZE::response& res, epee::json_rpc::error& er, const connection_context *ctx)
   {
     if (!m_wallet) return not_open(er);
+    CHECK_IF_BACKGROUND_SYNCING();
     try
     {
       if (req.key_image.empty())
@@ -819,6 +879,7 @@ namespace tools
   bool wallet_rpc_server::on_thaw(const wallet_rpc::COMMAND_RPC_THAW::request& req, wallet_rpc::COMMAND_RPC_THAW::response& res, epee::json_rpc::error& er, const connection_context *ctx)
   {
     if (!m_wallet) return not_open(er);
+    CHECK_IF_BACKGROUND_SYNCING();
     try
     {
       if (req.key_image.empty())
@@ -847,6 +908,7 @@ namespace tools
   bool wallet_rpc_server::on_frozen(const wallet_rpc::COMMAND_RPC_FROZEN::request& req, wallet_rpc::COMMAND_RPC_FROZEN::response& res, epee::json_rpc::error& er, const connection_context *ctx)
   {
     if (!m_wallet) return not_open(er);
+    CHECK_IF_BACKGROUND_SYNCING();
     try
     {
       if (req.key_image.empty())
@@ -874,6 +936,8 @@ namespace tools
   //------------------------------------------------------------------------------------------------------------------------------
   bool wallet_rpc_server::validate_transfer(const std::list<wallet_rpc::transfer_destination>& destinations, const std::string& payment_id, std::vector<cryptonote::tx_destination_entry>& dsts, std::vector<uint8_t>& extra, bool at_least_one_destination, epee::json_rpc::error& er)
   {
+    CHECK_IF_BACKGROUND_SYNCING();
+
     crypto::hash8 integrated_payment_id = crypto::null_hash8;
     std::string extra_nonce;
     for (auto it = destinations.begin(); it != destinations.end(); it++)
@@ -1203,6 +1267,7 @@ namespace tools
     }
 
     CHECK_MULTISIG_ENABLED();
+    CHECK_IF_BACKGROUND_SYNCING();
 
     cryptonote::blobdata blob;
     if (!epee::string_tools::parse_hexstr_to_binbuff(req.unsigned_txset, blob))
@@ -1284,6 +1349,7 @@ namespace tools
       er.message = "command not supported by watch-only wallet";
       return false;
     }
+    CHECK_IF_BACKGROUND_SYNCING();
     if(req.unsigned_txset.empty() && req.multisig_txset.empty())
     {
       er.code = WALLET_RPC_ERROR_CODE_UNKNOWN_ERROR;
@@ -1553,6 +1619,7 @@ namespace tools
     }
 
     CHECK_MULTISIG_ENABLED();
+    CHECK_IF_BACKGROUND_SYNCING();
 
     try
     {
@@ -2115,6 +2182,7 @@ namespace tools
             er.message = "The wallet is watch-only. Cannot retrieve seed.";
             return false;
           }
+          CHECK_IF_BACKGROUND_SYNCING();
           if (!m_wallet->is_deterministic())
           {
             er.code = WALLET_RPC_ERROR_CODE_NON_DETERMINISTIC;
@@ -2143,6 +2211,7 @@ namespace tools
             er.message = "The wallet is watch-only. Cannot retrieve spend key.";
             return false;
           }
+          CHECK_IF_BACKGROUND_SYNCING();
           epee::wipeable_string key = epee::to_hex::wipeable_string(m_wallet->get_account().get_keys().m_spend_secret_key);
           res.key = std::string(key.data(), key.size());
       }
@@ -2164,6 +2233,7 @@ namespace tools
       er.message = "Command unavailable in restricted mode.";
       return false;
     }
+    CHECK_IF_BACKGROUND_SYNCING();
 
     try
     {
@@ -2177,6 +2247,79 @@ namespace tools
     return true;
   }
   //------------------------------------------------------------------------------------------------------------------------------
+  bool wallet_rpc_server::on_setup_background_sync(const wallet_rpc::COMMAND_RPC_SETUP_BACKGROUND_SYNC::request& req, wallet_rpc::COMMAND_RPC_SETUP_BACKGROUND_SYNC::response& res, epee::json_rpc::error& er, const connection_context *ctx)
+  {
+    try
+    {
+      PRE_VALIDATE_BACKGROUND_SYNC();
+      const tools::wallet2::BackgroundSyncType background_sync_type = tools::wallet2::background_sync_type_from_str(req.background_sync_type);
+      boost::optional<epee::wipeable_string> background_cache_password = boost::none;
+      if (background_sync_type == tools::wallet2::BackgroundSyncCustomPassword)
+        background_cache_password = boost::optional<epee::wipeable_string>(req.background_cache_password);
+      m_wallet->setup_background_sync(background_sync_type, req.wallet_password, background_cache_password);
+    }
+    catch (...)
+    {
+      handle_rpc_exception(std::current_exception(), er, WALLET_RPC_ERROR_CODE_UNKNOWN_ERROR);
+      return false;
+    }
+    return true;
+  }
+  //------------------------------------------------------------------------------------------------------------------------------
+  bool wallet_rpc_server::on_start_background_sync(const wallet_rpc::COMMAND_RPC_START_BACKGROUND_SYNC::request& req, wallet_rpc::COMMAND_RPC_START_BACKGROUND_SYNC::response& res, epee::json_rpc::error& er, const connection_context *ctx)
+  {
+    try
+    {
+      PRE_VALIDATE_BACKGROUND_SYNC();
+      m_wallet->start_background_sync();
+    }
+    catch (...)
+    {
+      handle_rpc_exception(std::current_exception(), er, WALLET_RPC_ERROR_CODE_UNKNOWN_ERROR);
+      return false;
+    }
+    return true;
+  }
+  //------------------------------------------------------------------------------------------------------------------------------
+  bool wallet_rpc_server::on_stop_background_sync(const wallet_rpc::COMMAND_RPC_STOP_BACKGROUND_SYNC::request& req, wallet_rpc::COMMAND_RPC_STOP_BACKGROUND_SYNC::response& res, epee::json_rpc::error& er, const connection_context *ctx)
+  {
+    try
+    {
+      PRE_VALIDATE_BACKGROUND_SYNC();
+      crypto::secret_key spend_secret_key = crypto::null_skey;
+
+      // Load the spend key from seed if seed is provided
+      if (!req.seed.empty())
+      {
+        crypto::secret_key recovery_key;
+        std::string language;
+
+        if (!crypto::ElectrumWords::words_to_bytes(req.seed, recovery_key, language))
+        {
+          er.code = WALLET_RPC_ERROR_CODE_UNKNOWN_ERROR;
+          er.message = "Electrum-style word list failed verification";
+          return false;
+        }
+
+        if (!req.seed_offset.empty())
+          recovery_key = cryptonote::decrypt_key(recovery_key, req.seed_offset);
+
+        // generate spend key
+        cryptonote::account_base account;
+        account.generate(recovery_key, true, false);
+        spend_secret_key = account.get_keys().m_spend_secret_key;
+      }
+
+      m_wallet->stop_background_sync(req.wallet_password, spend_secret_key);
+    }
+    catch (...)
+    {
+      handle_rpc_exception(std::current_exception(), er, WALLET_RPC_ERROR_CODE_UNKNOWN_ERROR);
+      return false;
+    }
+    return true;
+  }
+  //------------------------------------------------------------------------------------------------------------------------------
   bool wallet_rpc_server::on_sign(const wallet_rpc::COMMAND_RPC_SIGN::request& req, wallet_rpc::COMMAND_RPC_SIGN::response& res, epee::json_rpc::error& er, const connection_context *ctx)
   {
     if (!m_wallet) return not_open(er);
@@ -2186,6 +2329,7 @@ namespace tools
       er.message = "Command unavailable in restricted mode.";
       return false;
     }
+    CHECK_IF_BACKGROUND_SYNCING();
 
     tools::wallet2::message_signature_type_t signature_type = tools::wallet2::sign_with_spend_key;
     if (req.signature_type == "spend" || req.signature_type == "")
@@ -2278,6 +2422,7 @@ namespace tools
       er.message = "Command unavailable in restricted mode.";
       return false;
     }
+    CHECK_IF_BACKGROUND_SYNCING();
 
     if (req.txids.size() != req.notes.size())
     {
@@ -2350,6 +2495,7 @@ namespace tools
       er.message = "Command unavailable in restricted mode.";
       return false;
     }
+    CHECK_IF_BACKGROUND_SYNCING();
 
     m_wallet->set_attribute(req.key, req.value);
 
@@ -2377,6 +2523,7 @@ namespace tools
   bool wallet_rpc_server::on_get_tx_key(const wallet_rpc::COMMAND_RPC_GET_TX_KEY::request& req, wallet_rpc::COMMAND_RPC_GET_TX_KEY::response& res, epee::json_rpc::error& er, const connection_context *ctx)
   {
     if (!m_wallet) return not_open(er);
+    CHECK_IF_BACKGROUND_SYNCING();
 
     crypto::hash txid;
     if (!epee::string_tools::hex_to_pod(req.txid, txid))
@@ -2468,6 +2615,7 @@ namespace tools
   bool wallet_rpc_server::on_get_tx_proof(const wallet_rpc::COMMAND_RPC_GET_TX_PROOF::request& req, wallet_rpc::COMMAND_RPC_GET_TX_PROOF::response& res, epee::json_rpc::error& er, const connection_context *ctx)
   {
     if (!m_wallet) return not_open(er);
+    CHECK_IF_BACKGROUND_SYNCING();
 
     crypto::hash txid;
     if (!epee::string_tools::hex_to_pod(req.txid, txid))
@@ -2584,6 +2732,7 @@ namespace tools
   bool wallet_rpc_server::on_get_reserve_proof(const wallet_rpc::COMMAND_RPC_GET_RESERVE_PROOF::request& req, wallet_rpc::COMMAND_RPC_GET_RESERVE_PROOF::response& res, epee::json_rpc::error& er, const connection_context *ctx)
   {
     if (!m_wallet) return not_open(er);
+    CHECK_IF_BACKGROUND_SYNCING();
 
     boost::optional<std::pair<uint32_t, uint64_t>> account_minreserve;
     if (!req.all)
@@ -2826,6 +2975,7 @@ namespace tools
       er.message = "command not supported by HW wallet";
       return false;
     }
+    CHECK_IF_BACKGROUND_SYNCING();
 
     try
     {
@@ -2855,6 +3005,7 @@ namespace tools
       er.message = "command not supported by HW wallet";
       return false;
     }
+    CHECK_IF_BACKGROUND_SYNCING();
 
     cryptonote::blobdata blob;
     if (!epee::string_tools::parse_hexstr_to_binbuff(req.outputs_data_hex, blob))
@@ -2880,6 +3031,7 @@ namespace tools
   bool wallet_rpc_server::on_export_key_images(const wallet_rpc::COMMAND_RPC_EXPORT_KEY_IMAGES::request& req, wallet_rpc::COMMAND_RPC_EXPORT_KEY_IMAGES::response& res, epee::json_rpc::error& er, const connection_context *ctx)
   {
     if (!m_wallet) return not_open(er);
+    CHECK_IF_BACKGROUND_SYNCING();
     try
     {
       std::pair<uint64_t, std::vector<std::pair<crypto::key_image, crypto::signature>>> ski = m_wallet->export_key_images(req.all);
@@ -2916,6 +3068,7 @@ namespace tools
       er.message = "This command requires a trusted daemon.";
       return false;
     }
+    CHECK_IF_BACKGROUND_SYNCING();
     try
     {
       std::vector<std::pair<crypto::key_image, crypto::signature>> ski;
@@ -2984,6 +3137,7 @@ namespace tools
   bool wallet_rpc_server::on_get_address_book(const wallet_rpc::COMMAND_RPC_GET_ADDRESS_BOOK_ENTRY::request& req, wallet_rpc::COMMAND_RPC_GET_ADDRESS_BOOK_ENTRY::response& res, epee::json_rpc::error& er, const connection_context *ctx)
   {
     if (!m_wallet) return not_open(er);
+    CHECK_IF_BACKGROUND_SYNCING();
     const auto ab = m_wallet->get_address_book();
     if (req.entries.empty())
     {
@@ -3029,6 +3183,7 @@ namespace tools
       er.message = "Command unavailable in restricted mode.";
       return false;
     }
+    CHECK_IF_BACKGROUND_SYNCING();
 
     cryptonote::address_parse_info info;
     er.message = "";
@@ -3071,6 +3226,7 @@ namespace tools
       er.message = "Command unavailable in restricted mode.";
       return false;
     }
+    CHECK_IF_BACKGROUND_SYNCING();
 
     const auto ab = m_wallet->get_address_book();
     if (req.index >= ab.size())
@@ -3133,6 +3289,7 @@ namespace tools
       er.message = "Command unavailable in restricted mode.";
       return false;
     }
+    CHECK_IF_BACKGROUND_SYNCING();
 
     const auto ab = m_wallet->get_address_book();
     if (req.index >= ab.size())
@@ -3203,6 +3360,7 @@ namespace tools
           er.message = "Command unavailable in restricted mode.";
           return false;
       }
+      CHECK_IF_BACKGROUND_SYNCING();
 
       std::unordered_set<crypto::hash> txids;
       std::list<std::string>::const_iterator i = req.txids.begin();
@@ -3242,6 +3400,7 @@ namespace tools
       er.message = "Command unavailable in restricted mode.";
       return false;
     }
+    CHECK_IF_BACKGROUND_SYNCING();
     try
     {
       m_wallet->rescan_spent();
@@ -3506,6 +3665,7 @@ namespace tools
       er.message = "Command unavailable in restricted mode.";
       return false;
     }
+    CHECK_IF_BACKGROUND_SYNCING();
     if (m_wallet->verify_password(req.old_password))
     {
       try
@@ -4039,6 +4199,7 @@ namespace tools
       er.message = "wallet is watch-only and cannot be made multisig";
       return false;
     }
+    CHECK_IF_BACKGROUND_SYNCING();
 
     res.multisig_info = m_wallet->get_multisig_first_kex_msg();
     return true;
@@ -4066,6 +4227,7 @@ namespace tools
       er.message = "wallet is watch-only and cannot be made multisig";
       return false;
     }
+    CHECK_IF_BACKGROUND_SYNCING();
 
     try
     {
diff --git a/src/wallet/wallet_rpc_server.h b/src/wallet/wallet_rpc_server.h
index bc025d138..954316b52 100644
--- a/src/wallet/wallet_rpc_server.h
+++ b/src/wallet/wallet_rpc_server.h
@@ -160,6 +160,9 @@ namespace tools
         MAP_JON_RPC_WE("set_log_categories", on_set_log_categories, wallet_rpc::COMMAND_RPC_SET_LOG_CATEGORIES)
         MAP_JON_RPC_WE("estimate_tx_size_and_weight", on_estimate_tx_size_and_weight, wallet_rpc::COMMAND_RPC_ESTIMATE_TX_SIZE_AND_WEIGHT)
         MAP_JON_RPC_WE("get_version",        on_get_version,        wallet_rpc::COMMAND_RPC_GET_VERSION)
+        MAP_JON_RPC_WE("setup_background_sync", on_setup_background_sync, wallet_rpc::COMMAND_RPC_SETUP_BACKGROUND_SYNC)
+        MAP_JON_RPC_WE("start_background_sync", on_start_background_sync, wallet_rpc::COMMAND_RPC_START_BACKGROUND_SYNC)
+        MAP_JON_RPC_WE("stop_background_sync", on_stop_background_sync, wallet_rpc::COMMAND_RPC_STOP_BACKGROUND_SYNC)
       END_JSON_RPC_MAP()
     END_URI_MAP2()
 
@@ -252,6 +255,9 @@ namespace tools
       bool on_set_log_categories(const wallet_rpc::COMMAND_RPC_SET_LOG_CATEGORIES::request& req, wallet_rpc::COMMAND_RPC_SET_LOG_CATEGORIES::response& res, epee::json_rpc::error& er, const connection_context *ctx = NULL);
       bool on_estimate_tx_size_and_weight(const wallet_rpc::COMMAND_RPC_ESTIMATE_TX_SIZE_AND_WEIGHT::request& req, wallet_rpc::COMMAND_RPC_ESTIMATE_TX_SIZE_AND_WEIGHT::response& res, epee::json_rpc::error& er, const connection_context *ctx = NULL);
       bool on_get_version(const wallet_rpc::COMMAND_RPC_GET_VERSION::request& req, wallet_rpc::COMMAND_RPC_GET_VERSION::response& res, epee::json_rpc::error& er, const connection_context *ctx = NULL);
+      bool on_setup_background_sync(const wallet_rpc::COMMAND_RPC_SETUP_BACKGROUND_SYNC::request& req, wallet_rpc::COMMAND_RPC_SETUP_BACKGROUND_SYNC::response& res, epee::json_rpc::error& er, const connection_context *ctx = NULL);
+      bool on_start_background_sync(const wallet_rpc::COMMAND_RPC_START_BACKGROUND_SYNC::request& req, wallet_rpc::COMMAND_RPC_START_BACKGROUND_SYNC::response& res, epee::json_rpc::error& er, const connection_context *ctx = NULL);
+      bool on_stop_background_sync(const wallet_rpc::COMMAND_RPC_STOP_BACKGROUND_SYNC::request& req, wallet_rpc::COMMAND_RPC_STOP_BACKGROUND_SYNC::response& res, epee::json_rpc::error& er, const connection_context *ctx = NULL);
 
       //json rpc v2
       bool on_query_key(const wallet_rpc::COMMAND_RPC_QUERY_KEY::request& req, wallet_rpc::COMMAND_RPC_QUERY_KEY::response& res, epee::json_rpc::error& er, const connection_context *ctx = NULL);
diff --git a/src/wallet/wallet_rpc_server_commands_defs.h b/src/wallet/wallet_rpc_server_commands_defs.h
index acf24c87b..816004ccb 100644
--- a/src/wallet/wallet_rpc_server_commands_defs.h
+++ b/src/wallet/wallet_rpc_server_commands_defs.h
@@ -2729,5 +2729,69 @@ namespace wallet_rpc
     typedef epee::misc_utils::struct_init<response_t> response;
   };
 
+  struct COMMAND_RPC_SETUP_BACKGROUND_SYNC
+  {
+    struct request_t
+    {
+      std::string background_sync_type;
+      std::string wallet_password;
+      std::string background_cache_password;
+
+      BEGIN_KV_SERIALIZE_MAP()
+        KV_SERIALIZE(background_sync_type)
+        KV_SERIALIZE(wallet_password)
+        KV_SERIALIZE_OPT(background_cache_password, (std::string)"")
+      END_KV_SERIALIZE_MAP()
+    };
+    typedef epee::misc_utils::struct_init<request_t> request;
+
+    struct response_t
+    {
+      BEGIN_KV_SERIALIZE_MAP()
+      END_KV_SERIALIZE_MAP()
+    };
+    typedef epee::misc_utils::struct_init<response_t> response;
+  };
+
+  struct COMMAND_RPC_START_BACKGROUND_SYNC
+  {
+    struct request_t
+    {
+      BEGIN_KV_SERIALIZE_MAP()
+      END_KV_SERIALIZE_MAP()
+    };
+    typedef epee::misc_utils::struct_init<request_t> request;
+
+    struct response_t
+    {
+      BEGIN_KV_SERIALIZE_MAP()
+      END_KV_SERIALIZE_MAP()
+    };
+    typedef epee::misc_utils::struct_init<response_t> response;
+  };
+
+  struct COMMAND_RPC_STOP_BACKGROUND_SYNC
+  {
+    struct request_t
+    {
+      std::string wallet_password;
+      std::string seed;
+      std::string seed_offset;
+
+      BEGIN_KV_SERIALIZE_MAP()
+        KV_SERIALIZE(wallet_password)
+        KV_SERIALIZE_OPT(seed, (std::string)"")
+        KV_SERIALIZE_OPT(seed_offset, (std::string)"")
+      END_KV_SERIALIZE_MAP()
+    };
+    typedef epee::misc_utils::struct_init<request_t> request;
+
+    struct response_t
+    {
+      BEGIN_KV_SERIALIZE_MAP()
+      END_KV_SERIALIZE_MAP()
+    };
+    typedef epee::misc_utils::struct_init<response_t> response;
+  };
 }
 }
diff --git a/src/wallet/wallet_rpc_server_error_codes.h b/src/wallet/wallet_rpc_server_error_codes.h
index ca2de0cc6..8b0797b41 100644
--- a/src/wallet/wallet_rpc_server_error_codes.h
+++ b/src/wallet/wallet_rpc_server_error_codes.h
@@ -81,3 +81,5 @@
 #define WALLET_RPC_ERROR_CODE_DISABLED               -48
 #define WALLET_RPC_ERROR_CODE_PROXY_ALREADY_DEFINED  -49
 #define WALLET_RPC_ERROR_CODE_NONZERO_UNLOCK_TIME    -50
+#define WALLET_RPC_ERROR_CODE_IS_BACKGROUND_WALLET   -51
+#define WALLET_RPC_ERROR_CODE_IS_BACKGROUND_SYNCING  -52