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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,110 @@
+#include <vanetza/security/v2/certificate_cache.hpp>
+#include <chrono>
+
+namespace vanetza
+{
+namespace security
+{
+namespace v2
+{
+
+CertificateCache::CertificateCache(const Runtime& rt) : m_runtime(rt)
+{
+}
+
+void CertificateCache::insert(const Certificate& certificate)
+{
+    const HashedId8 id = calculate_hash(certificate);
+
+    // this may drop expired entries and extend some's lifetime
+    std::list<Certificate> certs = lookup(id, certificate.subject_info.subject_type);
+
+    // TODO: implement equality comparison for Certificate
+    if (certs.size()) {
+        const auto binary_insert = convert_for_signing(certificate);
+        for (auto& cert : certs) {
+            const auto binary_found = convert_for_signing(cert);
+            if (binary_insert == binary_found) {
+                return;
+            }
+        }
+    }
+
+    Clock::duration lifetime = Clock::duration::zero();
+    if (certificate.subject_info.subject_type == SubjectType::Authorization_Ticket) {
+        // section 7.1 in ETSI TS 103 097 v1.2.1
+        // there must be a CAM with the authorization ticket every one second
+        // we choose two seconds here to account for one missed message
+        lifetime = std::chrono::seconds(2);
+    } else if (certificate.subject_info.subject_type == SubjectType::Authorization_Authority) {
+        // section 7.1 in ETSI TS 103 097 v1.2.1
+        // chains are only sent upon request, there will probably only be a few authoritation authorities in use
+        // one hour is an arbitrarily choosen cache period for now
+        lifetime = std::chrono::seconds(3600);
+    }
+
+    if (lifetime > Clock::duration::zero()) {
+        CachedCertificate entry;
+        entry.certificate = certificate;
+        map_type::iterator stored = m_certificates.emplace(id, entry);
+        heap_type::handle_type& handle = stored->second.handle;
+        handle = m_expiries.push(Expiry { m_runtime.now() + lifetime, stored });
+    }
+}
+
+std::list<Certificate> CertificateCache::lookup(const HashedId8& id, SubjectType type)
+{
+    drop_expired();
+
+    using iterator = std::multimap<HashedId8, CachedCertificate>::iterator;
+    std::pair<iterator, iterator> range = m_certificates.equal_range(id);
+
+    std::list<Certificate> matches;
+    for (auto item = range.first; item != range.second; ++item) {
+        const Certificate& cert = item->second.certificate;
+
+        auto subject_type = cert.subject_info.subject_type;
+        if (subject_type != type) {
+            continue;
+        }
+
+        matches.push_back(cert);
+
+        // renew cached certificate
+        if (subject_type == SubjectType::Authorization_Ticket) {
+            refresh(item->second.handle, std::chrono::seconds(2));
+        } else if (subject_type == SubjectType::Authorization_Authority) {
+            refresh(item->second.handle, std::chrono::seconds(3600));
+        }
+    }
+
+    return matches;
+}
+
+void CertificateCache::drop_expired()
+{
+    while (!m_expiries.empty() && is_expired(m_expiries.top())) {
+        m_certificates.erase(m_expiries.top().certificate);
+        m_expiries.pop();
+    }
+}
+
+bool CertificateCache::is_expired(const Expiry& expiry) const
+{
+    return m_runtime.now() > expiry;
+}
+
+void CertificateCache::refresh(heap_type::handle_type& handle, Clock::duration lifetime)
+{
+    static_cast<Clock::time_point&>(*handle) = m_runtime.now() + lifetime;
+    m_expiries.update(handle);
+}
+
+CertificateCache::Expiry::Expiry(Clock::time_point expiry, map_type::iterator it) :
+    Clock::time_point(expiry), certificate(it)
+{
+}
+
+} // namespace v2
+} // namespace security
+} // namespace vanetza