GUI: Add a warning prompt when sending to an already-used address

src/qt/sendcoinsdialog.cpp

@@ -482,6 +482,77 @@ void SendCoinsDialog::sendButtonClicked([[maybe_unused]] bool checked)
     if (!PrepareSendText(question_string, informative_text, detailed_text)) return;
     assert(m_current_transaction);
 
+    bool have_warning = false;
+    for (int i = 0; i < ui->entries->count(); ++i) {
+        SendCoinsEntry *entry = qobject_cast<SendCoinsEntry*>(ui->entries->itemAt(i)->widget());
+        if (entry && entry->hasPaytoWarning()) {
+            have_warning = true;
+            break;
+        }
+    }
+    if (have_warning) {
+        auto recipients = m_current_transaction->getRecipients();
+        struct prior_usage_info_t {
+            CAmount total_amount{0};
+            int num_txs{0};
+            qint64 tx_time_oldest;
+            qint64 tx_time_newest;
+        };
+        QMap<QString, prior_usage_info_t> prior_usage_info;
+        {
+            QStringList addresses;
+            for (const auto& recipient : recipients) {
+                addresses.append(recipient.address);
+            }
+            model->findAddressUsage(addresses, [&prior_usage_info](const QString& address, const interfaces::WalletTx& wtx, uint32_t output_index){
+                auto& info = prior_usage_info[address];
+                info.total_amount += wtx.tx->vout[output_index].nValue;
+                ++info.num_txs;
+                if (info.num_txs == 1 || wtx.time < info.tx_time_oldest) {
+                    info.tx_time_oldest = wtx.time;
+                }
+                if (info.num_txs == 1 || wtx.time > info.tx_time_newest) {
+                    info.tx_time_newest = wtx.time;
+                }
+            });
+        }
+
+        QString reuse_question, reuse_details;
+        if (recipients.size() > 1) {
+            reuse_question = tr("You've already paid some of these addresses.");
+        } else {
+            reuse_question = tr("You've already paid this address.");
+        }
+
+        for (const auto& rcp : recipients) {
+            if (!prior_usage_info.contains(rcp.address)) continue;
+            if (!reuse_details.isEmpty()) reuse_details.append("\n\n");
+            const auto& rcp_prior_usage_info = prior_usage_info.value(rcp.address);
+            const QString label_and_address = rcp.label.isEmpty() ? rcp.address : (QString("'") + rcp.label + "' (" + rcp.address + ")");
+            if (rcp_prior_usage_info.num_txs == 1) {
+                //: %1 is an amount (eg, "1 BTC"); %2 is a Bitcoin address and its label; %3 is a date (eg, "2019-05-08")
+                reuse_details.append(tr("Sent %1 to %2 on %3").arg(BitcoinUnits::formatWithUnit(model->getOptionsModel()->getDisplayUnit(), rcp_prior_usage_info.total_amount), label_and_address, GUIUtil::dateStr(rcp_prior_usage_info.tx_time_newest)));
+            } else {
+                //: %1 is an amount (eg, "1 BTC"); %2 is a Bitcoin address and its label; %3 is the number of transactions; %4 and %5 are dates (eg, "2019-05-08"), earlier first
+                reuse_details.append(tr("Sent %1 to %2 across %3 transactions from %4 through %5").arg(BitcoinUnits::formatWithUnit(model->getOptionsModel()->getDisplayUnit(), rcp_prior_usage_info.total_amount), label_and_address, QString::number(rcp_prior_usage_info.num_txs), GUIUtil::dateStr(rcp_prior_usage_info.tx_time_oldest), GUIUtil::dateStr(rcp_prior_usage_info.tx_time_newest)));
+            }
+        }
+
+        reuse_question.append("<br /><br /><span style='font-size:10pt;'>");
+        reuse_question.append(tr("Bitcoin addresses are intended to only be used once, for a single payment. Sending to the same address again will harm the recipient's security, as well as the privacy of all Bitcoin users!"));
+        reuse_question.append("</span>");
+
+        SendConfirmationDialog confirmation_dialog(tr("Already paid"), reuse_question, "", reuse_details, ADDRESS_REUSE_OVERRIDE_DELAY, /*enable_send=*/true, /*always_show_unsigned=*/false, this);
+        confirmation_dialog.setIcon(QMessageBox::Warning);
+        confirmation_dialog.confirmButtonText = tr("Override");
+        confirmation_dialog.m_yes_button = QMessageBox::Ignore;
+        confirmation_dialog.m_cancel_button = QMessageBox::Ok;
+        if (static_cast<QMessageBox::StandardButton>(confirmation_dialog.exec()) == QMessageBox::Cancel) {
+            fNewRecipientAllowed = true;
+            return;
+        }
+    }
+
     const QString confirmation = tr("Confirm send coins");
     const bool enable_send{!model->wallet().privateKeysDisabled() || model->wallet().hasExternalSigner()};
     const bool always_show_unsigned{model->getOptionsModel()->getEnablePSBTControls()};

src/qt/sendcoinsdialog.h

@@ -123,6 +123,7 @@ Q_SIGNALS:
 
 
 #define SEND_CONFIRM_DELAY   3
+#define ADDRESS_REUSE_OVERRIDE_DELAY   10
 
 class SendConfirmationDialog : public QMessageBox
 {
@@ -130,6 +131,7 @@ class SendConfirmationDialog : public QMessageBox
 
 public:
     bool m_delete_on_close{false};
+    QString confirmButtonText{tr("Send")};
     QMessageBox::StandardButton m_yes_button{QMessageBox::Yes};
     QMessageBox::StandardButton m_cancel_button{QMessageBox::Cancel};
 
@@ -147,7 +149,6 @@ private:
     QAbstractButton *m_psbt_button;
     QTimer countDownTimer;
     int secDelay;
-    QString confirmButtonText{tr("Send")};
     bool m_enable_save;
     bool m_enable_send;
     QString m_psbt_button_text{tr("Create Unsigned")};

src/qt/sendcoinsentry.cpp

@@ -150,6 +150,11 @@ bool SendCoinsEntry::validate(interfaces::Node& node)
     return retval;
 }
 
+bool SendCoinsEntry::hasPaytoWarning() const
+{
+    return ui->payTo->hasWarning();
+}
+
 SendCoinsRecipient SendCoinsEntry::getValue()
 {
     recipient.address = ui->payTo->text();

src/qt/sendcoinsentry.h

@@ -33,6 +33,7 @@ public:
 
     void setModel(WalletModel *model);
     bool validate(interfaces::Node& node);
+    bool hasPaytoWarning() const;
     SendCoinsRecipient getValue();
 
     /** Return whether the entry is still empty and unedited */

src/qt/test/wallettests.cpp

@@ -58,34 +58,46 @@ using wallet::WalletRescanReserver;
 
 namespace
 {
-//! Press "Yes" or "Cancel" buttons in modal send confirmation dialog.
-void ConfirmSend(QString* text = nullptr, QMessageBox::StandardButton confirm_type = QMessageBox::Yes)
+void ConfirmSendAttempt(QString* text, QMessageBox::StandardButton confirm_type)
 {
-    QTimer::singleShot(0, [text, confirm_type]() {
         for (QWidget* widget : QApplication::topLevelWidgets()) {
             if (widget->inherits("SendConfirmationDialog")) {
                 SendConfirmationDialog* dialog = qobject_cast<SendConfirmationDialog*>(widget);
                 if (text) *text = dialog->text();
                 QAbstractButton* button = dialog->button(confirm_type);
-                if (!button) {
-                    const QMessageBox::ButtonRole confirm_role = [confirm_type](){
+                const QMessageBox::ButtonRole confirm_role = [confirm_type, button](){
+                    if (button) return QMessageBox::InvalidRole;
                     switch (confirm_type) {
                         case QMessageBox::Yes: return QMessageBox::YesRole;
                         case QMessageBox::Cancel: return QMessageBox::NoRole;
-                            default: assert(0);
+                        default: return QMessageBox::InvalidRole;
                     }
                 }();
                 for (QAbstractButton* maybe_button : dialog->buttons()) {
                     if (dialog->buttonRole(maybe_button) == confirm_role) {
                         button = maybe_button;
+                    } else if (maybe_button->text().startsWith("Override")) {
+                        button = maybe_button;
                         break;
                     }
                 }
-                }
                 button->setEnabled(true);
                 button->click();
+                if (!button->text().startsWith("Override")) return;
             }
         }
+
+    // Try again
+    QTimer::singleShot(0, [text, confirm_type]{
+        ConfirmSendAttempt(text, confirm_type);
+    });
+}
+
+//! Press "Yes" or "Cancel" buttons in modal send confirmation dialog.
+void ConfirmSend(QString* text = nullptr, QMessageBox::StandardButton confirm_type = QMessageBox::Yes)
+{
+    QTimer::singleShot(0, [text, confirm_type]{
+        ConfirmSendAttempt(text, confirm_type);
     });
 }