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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,126 @@
+#ifndef BACKEND_HPP_ZMRDTY2O
+#define BACKEND_HPP_ZMRDTY2O
+
+#include <vanetza/common/byte_buffer.hpp>
+#include <vanetza/common/factory.hpp>
+#include <vanetza/security/ecdsa256.hpp>
+#include <vanetza/security/hash_algorithm.hpp>
+#include <vanetza/security/private_key.hpp>
+#include <vanetza/security/public_key.hpp>
+#include <vanetza/security/signature.hpp>
+#include <boost/optional/optional.hpp>
+#include <memory>
+#include <string>
+
+namespace vanetza
+{
+namespace security
+{
+
+/**
+ * Interface to cryptographic features
+ */
+class Backend
+{
+public:
+    /**
+     * \brief calculate signature for given data and private key
+     *
+     * \param private_key Secret private key
+     * \param data buffer with plaintext data
+     * \return calculated signature
+     */
+    virtual EcdsaSignature sign_data(const ecdsa256::PrivateKey& private_key, const ByteBuffer& data) = 0;
+
+    /**
+     * \brief calculate signature for given digest and private key
+     * 
+     * \param private_key secret private key
+     * \param digest hash value of data
+     * \return calculated signature
+     */
+    virtual Signature sign_digest(const PrivateKey&, const ByteBuffer& digest) = 0;
+
+    /**
+     * \brief try to verify data using public key and signature
+     *
+     * \param public_key Public key
+     * \param data plaintext
+     * \param sig signature of data
+     * \return true if the data could be verified
+     */
+    virtual bool verify_data(const ecdsa256::PublicKey& public_key, const ByteBuffer& data, const EcdsaSignature& sig) = 0;
+
+    /**
+     * \brief try to verify digest using public key and signature
+     * 
+     * \param public_key public key
+     * \param digest hash value of data
+     * \param sig signature of data
+     * \return true if data could be verified
+     */
+    virtual bool verify_digest(const PublicKey& public_key, const ByteBuffer& digest, const Signature& sig) = 0;
+
+    /**
+     * \brief decompress a possibly compressed elliptic curve point
+     *
+     * \param ecc_point elliptic curve point
+     * \return uncompressed point
+     */
+    virtual boost::optional<Uncompressed> decompress_point(const EccPoint& ecc_point) = 0;
+
+    /**
+     * \brief calculate hash value of data
+     * 
+     * \param algo hash algorithm
+     * \param data buffer with data
+     * \return buffer containing calculated hash value
+     */
+    virtual ByteBuffer calculate_hash(HashAlgorithm algo, const ByteBuffer& data) = 0;
+
+    /**
+     * \brief generate a private key and the corresponding public key
+     * \return generated key pair
+     */
+    virtual ecdsa256::KeyPair generate_key_pair() = 0;
+
+    virtual ~Backend() = default;
+};
+
+/**
+ * \brief get factory containing builtin backend implementations
+ *
+ * Included set of backends depends on CMake build configuration.
+ * At least the "Null" backend is always included.
+ * \return factory
+ */
+const Factory<Backend>& builtin_backends();
+
+/**
+ * \brief create a backend instance
+ *
+ * A backend named "default" is guaranteed not to return a nullptr.
+ * However, it might be a dummy backend.
+ *
+ * \param name identifying name of backend implementation
+ * \param factory build backend registered by name from this factory
+ * \return backend instance (if available) or nullptr
+ */
+std::unique_ptr<Backend> create_backend(const std::string& name, const Factory<Backend>& = builtin_backends());
+
+/**
+ * \brief create a backend instance
+ * 
+ * Will never return a nullptr but throw an exception if backend is unavailable.
+ * 
+ * \param name identifying name of backend implementation
+ * \throws runtime_error if backend is unavailable
+ * \return backend instance
+ */
+std::unique_ptr<Backend> create_backend_or_throw(const std::string& name);
+
+} // namespace security
+} // namespace vanetza
+
+#endif /* BACKEND_HPP_ZMRDTY2O */
+