diff --git a/proto/asn1-pdu/.gitignore b/proto/asn1-pdu/.gitignore
new file mode 100644
index 0000000..2c439eb
--- /dev/null
+++ b/proto/asn1-pdu/.gitignore
@@ -0,0 +1,7 @@
+*.py
+*.pyc
+__pycache__
+
+*.pb.h
+*.pb.cc
+*.o
diff --git a/proto/asn1-pdu/Makefile b/proto/asn1-pdu/Makefile
new file mode 100644
index 0000000..e531e9f
--- /dev/null
+++ b/proto/asn1-pdu/Makefile
@@ -0,0 +1,14 @@
+LPM_DIR=../../../libprotobuf-mutator
+PROTOBUF_DIR=$(LPM_DIR)/build/external.protobuf
+FLAGS=-O2 -Wall -Wextra -D_FORTIFY_SOURCE=2 -fstack-protector-strong -fstack-clash-protection -fPIE -fPIC -Wl,-z,relro -Wl,-z,now -Wl,-z,noexecstack -Wl,-z,separate-code
+
+all: proto cpp
+
+proto: *.proto
+	protoc *.proto --python_out=. --cpp_out=.
+
+cpp: *.h *.cc
+	$(CXX) -c $(FLAGS) *.cc -I$(LPM_DIR) -I$(PROTOBUF_DIR)/include
+
+clean:
+	$(RM) *.o
diff --git a/proto/tests/.gitignore b/proto/tests/.gitignore
new file mode 100644
index 0000000..f167ab7
--- /dev/null
+++ b/proto/tests/.gitignore
@@ -0,0 +1,5 @@
+*.py
+*.pyc
+*.pem
+*.der
+*.proto
diff --git a/proto/tests/Makefile b/proto/tests/Makefile
new file mode 100644
index 0000000..4a65059
--- /dev/null
+++ b/proto/tests/Makefile
@@ -0,0 +1,16 @@
+LPM_DIR=../../../libprotobuf-mutator
+PROTOBUF_DIR=$(LPM_DIR)/build/external.protobuf
+ASN1_PROTOBUF_DIR=../asn1-pdu
+FLAGS=-O2 -Wall -Wextra -D_FORTIFY_SOURCE=2 -fstack-protector-strong -fstack-clash-protection -fPIE -fPIC -Wl,-z,relro -Wl,-z,now -Wl,-z,noexecstack -Wl,-z,separate-code
+
+all: protobuf tests
+
+protobuf:
+	$(MAKE) -C $(ASN1_PROTOBUF_DIR) all
+
+tests: protobuf_to_der.cpp
+	$(CXX) $(FLAGS) -o protobuf_to_der.o protobuf_to_der.cpp $(ASN1_PROTOBUF_DIR)/*.o -I$(PROTOBUF_DIR)/include -I$(LPM_DIR) -I$(ASN1_PROTOBUF_DIR) $(LPM_DIR)/build/src/libfuzzer/libprotobuf-mutator-libfuzzer.a $(LPM_DIR)/build/src/libprotobuf-mutator.a $(PROTOBUF_DIR)/lib/libprotobuf.a -lcrypto
+
+clean:
+	$(RM) *.o
+	$(MAKE) -C $(ASN1_PROTOBUF_DIR) clean
diff --git a/proto/tests/README.md b/proto/tests/README.md
new file mode 100644
index 0000000..6f112ea
--- /dev/null
+++ b/proto/tests/README.md
@@ -0,0 +1,56 @@
+# Proto tests
+
+## Setup
+
+Clone and build `libprotobuf-mutator`:
+
+```sh
+$ git clone https://github.com/google/libprotobuf-mutator.git libprotobuf-mutator
+$ cd libprotobuf-mutator
+$ mkdir build && cd build
+$ cmake .. -GNinja -DLIB_PROTO_MUTATOR_DOWNLOAD_PROTOBUF=ON -DLIB_PROTO_MUTATOR_TESTING=OFF -DCMAKE_BUILD_TYPE=Release
+$ ninja
+```
+
+Clone `google-fuzzing` and build test files:
+
+```
+$ git clone https://github.com/google/fuzzing.git fuzzing
+$ cd fuzzing/proto/tests
+$ make
+```
+
+## Run
+
+Generate python files:
+
+```sh
+$ protoc ../asn1-pdu/*.proto --python_out=../asn1-pdu
+```
+
+Generate a test certificate and convert it to the DER format:
+
+```sh
+$ openssl req -nodes -new -x509 -keyout key.pem -out cert.pem
+$ openssl x509 -in cert.pem -out cert.der -outform DER
+```
+
+Convert it to protobuf:
+
+```sh
+$ ./x509_to_protobuf.py cert.pem cert.proto
+```
+
+Convert the protobuf back to DER:
+
+```sh
+$ ./protobuf_to_der.o cert.proto cert-2.der
+```
+
+Make sure OpenSSL can properly parse the certificate and compare it to the
+initial one:
+
+```
+$ openssl x509 -inform DER -in cert-2.der -noout -text
+$ openssl x509 -inform DER -in cert.der -noout -text
+```
diff --git a/proto/tests/protobuf_to_der.cpp b/proto/tests/protobuf_to_der.cpp
new file mode 100644
index 0000000..8ba782a
--- /dev/null
+++ b/proto/tests/protobuf_to_der.cpp
@@ -0,0 +1,62 @@
+#include <fstream>
+#include <openssl/evp.h>
+#include "x509_certificate.pb.h"
+
+#include "asn1_pdu_to_der.h"
+#include "x509_certificate_to_der.h"
+
+std::string base64_encode(const unsigned char *buf, int len) {
+    const auto expected_size = 4 * ((len + 2) / 3);
+    auto output = reinterpret_cast<char *>(calloc(expected_size + 1, 1));
+    const auto actual_size = EVP_EncodeBlock(reinterpret_cast<unsigned char *>(output), buf, len);
+    if (expected_size != actual_size) {
+        std::cerr << "Wrong base64 output length: expected " << expected_size << " but got " << actual_size << ".\n";
+    }
+    return output;
+}
+
+std::string readFile(const char* path) {
+    printf("Reading file at %s\n", path);
+    std::ifstream file(path, std::ios::in | std::ios::binary);
+    std::string str = std::string((std::istreambuf_iterator<char>(file)), std::istreambuf_iterator<char>());
+    printf("Read %ld bytes\n\n", str.length());
+    return str;
+}
+
+void writeFile(const char* path, std::vector<uint8_t> data) {
+    printf("Writing %ld bytes to %s\n", data.size(), path);
+    std::ofstream file(path, std::ios::out | std::ios::binary);
+    std::copy(data.begin(), data.end(), std::ostreambuf_iterator<char>(file));
+}
+
+int main(int argc, char **argv) {
+    if (argc != 3) {
+        printf("Usage: ./protobuf_to_der.o <in file> <out file>\n");
+        return 1;
+    }
+
+    std::string protobuf = readFile(argv[1]);
+    std::string b64Protobuf = base64_encode((unsigned char *)protobuf.c_str(), protobuf.length());
+    printf("Protobuf: %s\n\n", b64Protobuf.c_str());
+
+    x509_certificate::X509Certificate input;
+    bool parse_ok = input.ParseFromArray(protobuf.c_str(), protobuf.length());
+    if (!parse_ok) {
+        printf("ParseFromArray failed!\n");
+        return 1;
+    }
+
+    std::string serialized;
+    input.SerializeToString(&serialized);
+    std::string b64Serialized = base64_encode((unsigned char *)serialized.c_str(), serialized.length());
+    printf("Re-serialized protobuf: %s\n\n", b64Serialized.c_str());
+
+    std::vector<uint8_t> asn1 = x509_certificate::X509CertificateToDER(input);
+
+    std::string b64asn1 = base64_encode(asn1.data(), asn1.size());
+    printf("ASN.1: %s\n", b64asn1.c_str());
+
+    writeFile(argv[2], asn1);
+
+    return 0;
+}
diff --git a/proto/tests/protobuf_to_der.o b/proto/tests/protobuf_to_der.o
new file mode 100755
index 0000000..c905c92
Binary files /dev/null and b/proto/tests/protobuf_to_der.o differ
diff --git a/proto/tests/requirements.txt b/proto/tests/requirements.txt
new file mode 100644
index 0000000..d0c4975
--- /dev/null
+++ b/proto/tests/requirements.txt
@@ -0,0 +1,2 @@
+asn1crypto
+protobuf