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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,90 @@
+#pragma once
+
+#include "keys.hpp"
+#include "sha.hpp"
+#include <vanetza/common/byte_buffer.hpp>
+#include <boost/optional/optional.hpp>
+#include <memory>
+
+namespace vanetza
+{
+namespace pki
+{
+
+class SecurityModule
+{
+public:
+    class EciesContext
+    {
+    public:
+        virtual ~EciesContext() = default;
+        virtual PublicKey ephemeral_public_key() const = 0;
+        virtual PublicKey recipient_public_key() const = 0;
+        virtual ByteBuffer shared_secret() const = 0;
+        virtual ByteBuffer encrypted_key() const = 0;
+        virtual ByteBuffer authentication_tag() const = 0;
+        virtual ByteBuffer nonce() const = 0;
+        virtual void nonce(const ByteBuffer&) = 0;
+
+        virtual ByteBuffer encrypt(const ByteBuffer& plaintext) = 0;
+        virtual ByteBuffer decrypt(const std::uint8_t* buffer, std::size_t length) = 0;
+    };
+
+    virtual ~SecurityModule() = default;
+    virtual Sha256Hash calculate_sha256_hash(const std::uint8_t* buffer, std::size_t length) = 0;
+    virtual Sha384Hash calculate_sha384_hash(const std::uint8_t* buffer, std::size_t length) = 0;
+    virtual bool verify(const Sha256Hash&, const Signature&, const PublicKey&) = 0;
+    virtual bool verify(const Sha384Hash&, const Signature&, const PublicKey&) = 0;
+
+    virtual boost::optional<Signature> sign(const ByteBuffer& digest, const PublicKey&) = 0;
+
+    // True if the private key matching `key` is available for signing.
+    virtual bool can_sign(const PublicKey& key);
+
+    virtual PublicKey create_key(KeyType) = 0;
+    virtual bool discard_key(const PublicKey&) = 0;
+
+    virtual ByteBuffer generate_nonce(std::size_t length) = 0;
+    virtual std::unique_ptr<EciesContext> create_ecies_context(const PublicKey& receiver, const Sha256Hash& info) = 0;
+    virtual ByteBuffer calculate_hmac_sha256(const ByteBuffer& key, const ByteBuffer& data) = 0;
+};
+
+class ScopedKeyPair
+{
+public:
+    ScopedKeyPair(SecurityModule& sm, KeyType type) : m_security(&sm), m_public_key(sm.create_key(type))
+    {
+    }
+
+    // no copy
+    ScopedKeyPair(const ScopedKeyPair&) = delete;
+    ScopedKeyPair& operator=(const ScopedKeyPair&) = delete;
+    // no move
+    ScopedKeyPair(ScopedKeyPair&&) = delete;
+    ScopedKeyPair& operator=(ScopedKeyPair&&) = delete;
+
+    ~ScopedKeyPair()
+    {
+        if (m_security) {
+            m_security->discard_key(m_public_key);
+        }
+    }
+
+    const PublicKey& public_key() const
+    {
+        return m_public_key;
+    }
+
+    void commit()
+    {
+        // will no longer discard created key
+        m_security = nullptr;
+    }
+
+private:
+    SecurityModule* m_security = nullptr;
+    PublicKey m_public_key;
+};
+
+} // namespace pki
+} // namespace vanetza