Keep the colleague's microbu-esp32c5 tree in this repository

obu-firmware builds against vanetza-idf from microbu-esp32c5/external, but
that tree was gitignored, so a clone of this repository could not build the
firmware it ships. It is now committed here as ordinary files in its own
folder, microbu-esp32c5/: the colleague's commit cf4b99f plus the V2X2MAP
bridge's signature verification (--trust) used on the bench. Nothing is
fetched from or pushed to the colleague's repository; this repository and
its remotes carry everything. The folder's own .gitignore keeps build output,
downloaded components and private key material out, as it did there; the
committed file set is identical to that repository's tracked files.

The ESP32-C5 is still flashed from obu-firmware/, which only takes
vanetza-idf from microbu-esp32c5/, so the two stay separate folders.
FLASHING.md says how to take a newer version of the colleague's tree (copy
it over the folder, rebuild, test, commit).

microbu-esp32c5/external/vanetza-idf/vanetza/security/pqc/fndsa512_oqs.cpp

@@ -0,0 +1,99 @@
+#include <vanetza/security/pqc/fndsa512.hpp>
+#include <oqs/oqs.h>
+#include <memory>
+#include <stdexcept>
+
+namespace vanetza
+{
+namespace security
+{
+namespace pqc
+{
+
+namespace
+{
+
+using OqsSignature = std::unique_ptr<OQS_SIG, decltype(&OQS_SIG_free)>;
+
+OqsSignature create_signature_context()
+{
+    OqsSignature context { OQS_SIG_new(OQS_SIG_alg_falcon_padded_512), OQS_SIG_free };
+    if (!context) {
+        throw std::runtime_error("liboqs does not provide Falcon-padded-512");
+    }
+
+    if (context->length_public_key != fndsa512_public_key_size ||
+            context->length_secret_key != fndsa512_private_key_size ||
+            context->length_signature != fndsa512_signature_size) {
+        throw std::runtime_error("liboqs Falcon-padded-512 parameters do not match the ASN.1 profile");
+    }
+
+    return context;
+}
+
+class OqsBackend final : public Backend
+{
+public:
+    KeyPair generate_key_pair() override
+    {
+        auto context = create_signature_context();
+        KeyPair key_pair;
+        key_pair.public_key.bytes.resize(fndsa512_public_key_size);
+        key_pair.private_key.bytes.resize(fndsa512_private_key_size);
+
+        const auto result = OQS_SIG_keypair(
+            context.get(), key_pair.public_key.bytes.data(), key_pair.private_key.bytes.data());
+        if (result != OQS_SUCCESS) {
+            throw std::runtime_error("liboqs failed to generate an FN-DSA-512 key pair");
+        }
+
+        return key_pair;
+    }
+
+    Signature sign(const PrivateKey& private_key, const ByteBuffer& data) override
+    {
+        if (private_key.bytes.size() != fndsa512_private_key_size) {
+            throw std::invalid_argument("FN-DSA-512 private key has an invalid size");
+        }
+
+        auto context = create_signature_context();
+        Signature signature;
+        signature.bytes.resize(fndsa512_signature_size);
+        std::size_t signature_size = 0;
+        const auto result = OQS_SIG_sign(
+            context.get(), signature.bytes.data(), &signature_size,
+            data.data(), data.size(), private_key.bytes.data());
+        if (result != OQS_SUCCESS) {
+            throw std::runtime_error("liboqs failed to create an FN-DSA-512 signature");
+        }
+        if (signature_size != fndsa512_signature_size) {
+            throw std::runtime_error("liboqs returned a non-padded FN-DSA-512 signature");
+        }
+
+        return signature;
+    }
+
+    bool verify(const PublicKey& public_key, const ByteBuffer& data, const Signature& signature) override
+    {
+        if (public_key.bytes.size() != fndsa512_public_key_size ||
+                signature.bytes.size() != fndsa512_signature_size) {
+            return false;
+        }
+
+        auto context = create_signature_context();
+        return OQS_SIG_verify(
+            context.get(), data.data(), data.size(), signature.bytes.data(),
+            signature.bytes.size(), public_key.bytes.data()) == OQS_SUCCESS;
+    }
+};
+
+} // namespace
+
+std::unique_ptr<Backend> create_fndsa512_backend()
+{
+    return std::unique_ptr<Backend> { new OqsBackend() };
+}
+
+} // namespace pqc
+} // namespace security
+} // namespace vanetza