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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,85 @@
+#include "pem.hpp"
+#include "openssl.hpp"
+#include <openssl/core_names.h>
+#include <openssl/pem.h>
+
+namespace vanetza
+{
+namespace pki
+{
+
+namespace
+{
+
+boost::optional<PrivateKey> parse_pem_private_key(const void* data, std::size_t size)
+{
+    OpenSslPointer<BIO> bio { BIO_new(BIO_s_mem()) };
+    int rc = BIO_write(bio.raw(), data, size);
+    if (rc < 0) {
+        throw security::openssl::Exception();
+    }
+
+    EVP_PKEY* evp_key_raw = PEM_read_bio_PrivateKey(bio.raw(), nullptr, nullptr, nullptr);
+    if (evp_key_raw == nullptr) {
+        return boost::none;
+    } else if (EVP_PKEY_get_base_id(evp_key_raw) != EVP_PKEY_EC) {
+        return boost::none;
+    }
+    OpenSslPointer<EVP_PKEY> evp_key { evp_key_raw }; /*< safe deallocation*/
+
+    BIGNUM* bn_priv_raw = nullptr;
+    EVP_PKEY_get_bn_param(evp_key.raw(), OSSL_PKEY_PARAM_PRIV_KEY, &bn_priv_raw);
+    OpenSslPointer<BIGNUM> bn_priv { bn_priv_raw }; /*< safe deallocation */
+
+    char group[80];
+    if (!EVP_PKEY_get_group_name(evp_key.raw(), group, sizeof(group), nullptr)) {
+        return boost::none;
+    }
+
+    PrivateKey priv_key;
+    priv_key.type = openssl_key_type_from_group_name(group);
+    priv_key.key.resize(BN_num_bytes(bn_priv_raw));
+    BN_bn2bin(bn_priv_raw, priv_key.key.data());
+
+    return priv_key;
+}
+
+} // namespace
+
+boost::optional<PrivateKey> parse_pem_private_key(const std::string& pem)
+{
+    return parse_pem_private_key(pem.data(), pem.size());
+}
+
+boost::optional<PrivateKey> parse_pem_private_key(const ByteBuffer& pem)
+{
+    return parse_pem_private_key(pem.data(), pem.size());
+}
+
+std::string make_pem(const PrivateKey& priv)
+{
+    OpenSslPointer<EC_KEY> ec_key = make_ec_key(priv);
+    OpenSslPointer<EVP_PKEY> evp_key { EVP_PKEY_new() };
+    openssl_result(EVP_PKEY_set1_EC_KEY(evp_key.raw(), ec_key.raw()), "EVP_PKEY_set1_EC_KEY");
+
+    // unencrypted PKCS#8 PrivateKeyInfo, read back by parse_pem_private_key
+    OpenSslPointer<BIO> bio { BIO_new(BIO_s_mem()) };
+    openssl_result(PEM_write_bio_PrivateKey(bio.raw(), evp_key.raw(), nullptr, nullptr, 0, nullptr, nullptr),
+        "PEM_write_bio_PrivateKey");
+
+    char* data = nullptr;
+    long len = BIO_get_mem_data(bio.raw(), &data);
+    return std::string(data, len);
+}
+
+void write_pem_private_key(const PrivateKey& priv, const std::filesystem::path& path)
+{
+    std::string pem = make_pem(priv);
+    auto bio = make_owner_only_bio(path);
+    if (BIO_write(bio.raw(), pem.data(), pem.size()) != static_cast<int>(pem.size())) {
+        throw OpenSslException(ERR_get_error(), "writing PEM to key file failed");
+    }
+}
+
+} // namespace pki
+} // namespace vanetza