--- a/nbproject/configurations.xml	Sat Jul 17 18:13:27 2021 +0200
+++ b/nbproject/configurations.xml	Sat Jul 17 18:58:22 2021 +0200
@@ -52,6 +52,7 @@
           <in>GenericASN1ContentHandler.h</in>
           <in>ProxyVector.h</in>
           <in>TransactionalBuffer.h</in>
+          <in>ValidatingASN1ContentHandler.h</in>
           <in>XMLContentHandler.h</in>
         </df>
         <in>XMLDocumentConstructor.h</in>
@@ -138,6 +139,11 @@
       </item>
       <item path="src/lib/TransactionalBuffer.h" ex="false" tool="3" flavor2="0">
       </item>
+      <item path="src/lib/ValidatingASN1ContentHandler.h"
+            ex="false"
+            tool="3"
+            flavor2="0">
+      </item>
       <item path="src/lib/XMLContentHandler.h" ex="false" tool="3" flavor2="0">
       </item>
       <item path="src/relpipe-in-xmltable.cpp" ex="false" tool="1" flavor2="0">
@@ -205,6 +211,11 @@
       </item>
       <item path="src/lib/TransactionalBuffer.h" ex="false" tool="3" flavor2="0">
       </item>
+      <item path="src/lib/ValidatingASN1ContentHandler.h"
+            ex="false"
+            tool="3"
+            flavor2="0">
+      </item>
       <item path="src/lib/XMLContentHandler.h" ex="false" tool="3" flavor2="0">
       </item>
       <item path="src/relpipe-in-xmltable.cpp" ex="false" tool="1" flavor2="0">

--- a/src/lib/BasicASN1Reader.h	Sat Jul 17 18:13:27 2021 +0200
+++ b/src/lib/BasicASN1Reader.h	Sat Jul 17 18:58:22 2021 +0200
@@ -23,6 +23,7 @@
 #include <regex>
 
 #include "ASN1Reader.h"
+#include "ValidatingASN1ContentHandler.h"
 
 namespace relpipe {
 namespace in {
@@ -284,6 +285,8 @@
 
 	bool isValidBER(const std::string& input) {
 		BasicASN1Reader encapsulatedReader;
+		std::shared_ptr<ValidatingASN1ContentHandler> validatingHandler = std::make_shared<ValidatingASN1ContentHandler>();
+		encapsulatedReader.addHandler(validatingHandler);
 		try {
 			encapsulatedReader.write((const uint8_t*) input.c_str(), input.size());
 			encapsulatedReader.close();

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/lib/ValidatingASN1ContentHandler.h	Sat Jul 17 18:58:22 2021 +0200
@@ -0,0 +1,102 @@
+/**
+ * Relational pipes
+ * Copyright © 2021 František Kučera (Frantovo.cz, GlobalCode.info)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, version 3 of the License.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+#pragma once
+
+#include <string>
+#include <stdexcept>
+
+#include "ASN1ContentHandler.h"
+
+namespace relpipe {
+namespace in {
+namespace asn1 {
+namespace lib {
+
+/**
+ * Does not generate any output but enforces certain rules (related to stream and collection start/end).
+ * When invalid behavior occurs, an exception is thrown.
+ * 
+ */
+class ValidatingASN1ContentHandler : public ASN1ContentHandler {
+private:
+	bool streamOpened = false;
+	bool streamClosed = false;
+	size_t collectionLevel = 0;
+public:
+
+	virtual void writeStreamStart() {
+		if (streamOpened) throw std::logic_error("ValidatingASN1ContentHandler: tried to open the stream twice"); // TODO: better exception
+		if (streamClosed) throw std::logic_error("ValidatingASN1ContentHandler: tried to open the stream that was already closed"); // TODO: better exception
+		streamOpened = true;
+	}
+
+	virtual void writeStreamEnd() {
+		if (streamClosed) throw std::logic_error("ValidatingASN1ContentHandler: tried to close the stream twice"); // TODO: better exception
+		if (!streamOpened) throw std::logic_error("ValidatingASN1ContentHandler: tried to close a stream that was already closed or never opened"); // TODO: better exception
+		if (collectionLevel != 0) throw std::logic_error("ValidatingASN1ContentHandler: not all opened collections was closed – remaining: " + std::to_string(collectionLevel)); // TODO: better exception
+		streamClosed = true;
+	}
+
+	virtual void writeCollectionStart(const Header& header) {
+		if (!streamOpened) throw std::logic_error("ValidatingASN1ContentHandler: tried to open a collection while the stream was not opened"); // TODO: better exception
+		if (streamClosed) throw std::logic_error("ValidatingASN1ContentHandler: tried to open a collection while the stream was already closed"); // TODO: better exception
+		collectionLevel++;
+	}
+
+	virtual void writeCollectionEnd() {
+		if (streamClosed) throw std::logic_error("ValidatingASN1ContentHandler: tried to close a collection while the stream was already closed"); // TODO: better exception
+		if (collectionLevel == 0) throw std::logic_error("ValidatingASN1ContentHandler: tried to close a collection while none was opened"); // TODO: better exception
+		collectionLevel--;
+	}
+
+	virtual void finalCheck() {
+		if (!streamOpened) throw std::logic_error("ValidatingASN1ContentHandler: the stream was not opened at all"); // TODO: better exception
+		if (!streamClosed) throw std::logic_error("ValidatingASN1ContentHandler: the stream was opened but not closed"); // TODO: better exception
+	}
+
+	// These events are intentionally ignored:
+
+	void writeBitString(const Header& header, std::vector<bool> value) override {
+	}
+
+	void writeBoolean(const Header& header, bool value) override {
+	}
+
+	void writeDateTime(const Header& header, DateTime value) override {
+	}
+
+	void writeInteger(const Header& header, Integer value) override {
+	}
+
+	void writeNull(const Header& header) override {
+	}
+
+	void writeOID(const Header& header, ObjectIdentifier value) override {
+	}
+
+	void writeOctetString(const Header& header, std::string value) override {
+	}
+
+	void writeTextString(const Header& header, std::string value) override {
+	}
+
+};
+
+}
+}
+}
+}