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FLASHING.md says how to take a newer version of the colleague's tree (copy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/backend_cryptopp.hpp

@@ -0,0 +1,76 @@
+#ifndef BACKEND_CRYPTOPP_HPP_JQWA9MLZ
+#define BACKEND_CRYPTOPP_HPP_JQWA9MLZ
+
+#include <vanetza/security/backend.hpp>
+#include <cryptopp/eccrypto.h>
+#include <cryptopp/osrng.h>
+#include <cryptopp/sha.h>
+
+namespace vanetza
+{
+namespace security
+{
+
+class BackendCryptoPP : public Backend
+{
+public:
+    using Ecdsa256 = CryptoPP::ECDSA<CryptoPP::ECP, CryptoPP::SHA256>;
+    using Ecdsa384 = CryptoPP::ECDSA<CryptoPP::ECP, CryptoPP::SHA384>;
+
+    static constexpr auto backend_name = "CryptoPP";
+
+    BackendCryptoPP();
+
+    /// \see Backend::sign_data
+    EcdsaSignature sign_data(const ecdsa256::PrivateKey& private_key, const ByteBuffer& data_buffer) override;
+
+    /// \see Backend::sign_digest
+    Signature sign_digest(const PrivateKey&, const ByteBuffer& digest) override;
+
+    /// \see Backend::verify_data
+    bool verify_data(const ecdsa256::PublicKey& public_key, const ByteBuffer& data, const EcdsaSignature& sig) override;
+
+    /// \see Backend::verify_digest
+    bool verify_digest(const PublicKey&, const ByteBuffer& digest, const Signature&) override;
+
+    /// \see Backend::decompress_point
+    boost::optional<Uncompressed> decompress_point(const EccPoint& ecc_point) override;
+
+    /// \see Backend::calculate_hash
+    ByteBuffer calculate_hash(HashAlgorithm, const ByteBuffer&) override;
+
+    /// \see Backend::generate_key_pair
+    ecdsa256::KeyPair generate_key_pair() override;
+
+private:
+    /// internal sign method using crypto++ private key
+    EcdsaSignature sign_data(const Ecdsa256::PrivateKey& key, const ByteBuffer& data);
+
+    /// internal verify method using crypto++ public key
+    bool verify_data(const Ecdsa256::PublicKey& key, const ByteBuffer& data, const ByteBuffer& sig);
+
+    /// create private key
+    Ecdsa256::PrivateKey generate_private_key();
+
+    /// derive public key from private key
+    Ecdsa256::PublicKey generate_public_key(const Ecdsa256::PrivateKey&);
+
+    /// adapt generic public key to internal structure
+    Ecdsa256::PublicKey internal_public_key(const ecdsa256::PublicKey&);
+
+    /// adapt generic private key to internal structure
+    Ecdsa256::PrivateKey internal_private_key(const ecdsa256::PrivateKey&);
+
+    CryptoPP::AutoSeededRandomPool m_prng;
+};
+
+namespace cryptopp
+{
+/// Derive the public key from a private key (pub = priv * G), on the key's own curve.
+PublicKey derive_public_key(const PrivateKey& private_key);
+} // namespace cryptopp
+
+} // namespace security
+} // namespace vanetza
+
+#endif /* BACKEND_CRYPTOPP_HPP_JQWA9MLZ */