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FLASHING.md says how to take a newer version of the colleague's tree (copy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/v2/secured_message.cpp

@@ -0,0 +1,138 @@
+#include <vanetza/common/byte_buffer.hpp>
+#include <vanetza/common/byte_buffer_sink.hpp>
+#include <vanetza/security/exception.hpp>
+#include <vanetza/security/v2/serialization.hpp>
+#include <vanetza/security/v2/secured_message.hpp>
+#include <boost/iostreams/stream.hpp>
+
+namespace vanetza
+{
+namespace security
+{
+namespace v2
+{
+
+HeaderField* SecuredMessage::header_field(HeaderFieldType type)
+{
+    HeaderField* match = nullptr;
+    for (auto& field : header_fields) {
+        if (get_type(field) == type) {
+            match = &field;
+            break;
+        }
+    }
+    return match;
+}
+
+const HeaderField* SecuredMessage::header_field(HeaderFieldType type) const
+{
+    const HeaderField* match = nullptr;
+    for (auto& field : header_fields) {
+        if (get_type(field) == type) {
+            match = &field;
+            break;
+        }
+    }
+    return match;
+}
+
+TrailerField* SecuredMessage::trailer_field(TrailerFieldType type)
+{
+    TrailerField* match = nullptr;
+    for (auto& field : trailer_fields) {
+        if (get_type(field) == type) {
+            match = &field;
+            break;
+        }
+    }
+    return match;
+}
+
+const TrailerField* SecuredMessage::trailer_field(TrailerFieldType type) const
+{
+    const TrailerField* match = nullptr;
+    for (auto& field : trailer_fields) {
+        if (get_type(field) == type) {
+            match = &field;
+            break;
+        }
+    }
+    return match;
+}
+
+size_t get_size(const SecuredMessage& message)
+{
+    size_t size = sizeof(uint8_t); // protocol version
+    size += get_size(message.header_fields);
+    size += length_coding_size(get_size(message.header_fields));
+    size += get_size(message.trailer_fields);
+    size += length_coding_size(get_size(message.trailer_fields));
+    size += get_size(message.payload);
+    return size;
+}
+
+void serialize(OutputArchive& ar, const SecuredMessage& message)
+{
+    const uint8_t protocol_version = message.protocol_version();
+    ar << protocol_version;
+    serialize(ar, message.header_fields);
+    serialize(ar, message.payload);
+    serialize(ar, message.trailer_fields);
+}
+
+size_t deserialize(InputArchive& ar, SecuredMessage& message)
+{
+    uint8_t protocol_version = 0;
+    ar >> protocol_version;
+    size_t length = sizeof(protocol_version);
+    if (protocol_version == 2) {
+        const size_t hdr_length = deserialize(ar, message.header_fields);
+        length += hdr_length + length_coding_size(hdr_length);
+        length += deserialize(ar, message.payload);
+        const size_t trlr_length = deserialize(ar, message.trailer_fields);
+        length += trlr_length + length_coding_size(trlr_length);
+    } else {
+        throw deserialization_error("Unsupported SecuredMessage protocol version");
+    }
+    return length;
+}
+
+ByteBuffer convert_for_signing(const SecuredMessage& message, const std::list<TrailerField>& trailer_fields)
+{
+    ByteBuffer buf;
+    byte_buffer_sink sink(buf);
+
+    boost::iostreams::stream_buffer<byte_buffer_sink> stream(sink);
+    OutputArchive ar(stream);
+
+    const uint8_t protocol_version = message.protocol_version();
+    ar << protocol_version;
+    serialize(ar, message.header_fields);
+    serialize(ar, message.payload);
+
+    // Encode the total length, all trailer fields before the signature and the type of the signature
+    // (see TS 103 097 v1.2.1, section 5.6)
+    serialize_length(ar, get_size(trailer_fields));
+    for (auto& elem : trailer_fields) {
+        TrailerFieldType type = get_type(elem);
+        if (type == TrailerFieldType::Signature) {
+            serialize(ar, type);
+            break; // exclude fields after signature
+        } else {
+            serialize(ar, elem);
+        }
+    }
+
+    stream.close();
+    return buf;
+}
+
+ItsAid get_its_aid(const SecuredMessage& msg)
+{
+    const IntX* raw = msg.header_field<HeaderFieldType::Its_Aid>();
+    return raw ? raw->get() : 0;
+}
+
+} // namespace v2
+} // namespace security
+} // namespace vanetza