When BIP70 is disabled, get PaymentRequest merchant using string search

The merchant name is stored in the X.509 certificate embedded in a
PaymentRequest. Use some string searching to locate it so that it
can be shown to the user in the transaction details when BIP70 support
was not configured.

src/qt/transactiondesc.cpp

@@ -49,6 +49,36 @@ QString TransactionDesc::FormatTxStatus(const interfaces::WalletTx& wtx, const i
     }
 }
 
+#ifndef ENABLE_BIP70
+// Takes an encoded PaymentRequest as a string and tries to find the Common Name of the X.509 certificate
+// used to sign the PaymentRequest.
+bool GetPaymentRequestMerchant(const std::string& pr, QString& merchant)
+{
+    // Search for the supported pki type strings
+    if (pr.find(std::string({0x12, 0x0b}) + "x509+sha256") != std::string::npos || pr.find(std::string({0x12, 0x09}) + "x509+sha1") != std::string::npos) {
+        // We want the common name of the Subject of the cert. This should be the second occurrence
+        // of the bytes 0x0603550403. The first occurrence of those is the common name of the issuer.
+        // After those bytes will be either 0x13 or 0x0C, then length, then either the ascii or utf8
+        // string with the common name which is the merchant name
+        size_t cn_pos = pr.find({0x06, 0x03, 0x55, 0x04, 0x03});
+        if (cn_pos != std::string::npos) {
+            cn_pos = pr.find({0x06, 0x03, 0x55, 0x04, 0x03}, cn_pos + 5);
+            if (cn_pos != std::string::npos) {
+                cn_pos += 5;
+                if (pr[cn_pos] == 0x13 || pr[cn_pos] == 0x0c) {
+                    cn_pos++; // Consume the type
+                    int str_len = pr[cn_pos];
+                    cn_pos++; // Consume the string length
+                    merchant = QString::fromUtf8(pr.data() + cn_pos, str_len);
+                    return true;
+                }
+            }
+        }
+    }
+    return false;
+}
+#endif
+
 QString TransactionDesc::toHTML(interfaces::Node& node, interfaces::Wallet& wallet, TransactionRecord *rec, int unit)
 {
     int numBlocks;
@@ -255,26 +285,34 @@ QString TransactionDesc::toHTML(interfaces::Node& node, interfaces::Wallet& wall
     strHTML += "<b>" + tr("Output index") + ":</b> " + QString::number(rec->getOutputIndex()) + "<br>";
 
     // Message from normal bitcoin:URI (bitcoin:123...?message=example)
-    for (const std::pair<std::string, std::string>& r : orderForm)
+    for (const std::pair<std::string, std::string>& r : orderForm) {
         if (r.first == "Message")
             strHTML += "<br><b>" + tr("Message") + ":</b><br>" + GUIUtil::HtmlEscape(r.second, true) + "<br>";
 
-#ifdef ENABLE_BIP70
-    //
-    // PaymentRequest info:
-    //
-    for (const std::pair<std::string, std::string>& r : orderForm)
-    {
+        //
+        // PaymentRequest info:
+        //
         if (r.first == "PaymentRequest")
         {
+            QString merchant;
+#ifdef ENABLE_BIP70
             PaymentRequestPlus req;
             req.parse(QByteArray::fromRawData(r.second.data(), r.second.size()));
-            QString merchant;
-            if (req.getMerchant(PaymentServer::getCertStore(), merchant))
+            if (!req.getMerchant(PaymentServer::getCertStore(), merchant)) {
+                merchant.clear();
+            }
+#else
+            if (!GetPaymentRequestMerchant(r.second, merchant)) {
+                merchant.clear();
+            } else {
+                merchant += tr(" (Certificate was not verified)");
+            }
+#endif
+            if (!merchant.isNull()) {
                 strHTML += "<b>" + tr("Merchant") + ":</b> " + GUIUtil::HtmlEscape(merchant) + "<br>";
+            }
         }
     }
-#endif
 
     if (wtx.is_coinbase)
     {