Keep vanetza-idf in obu-firmware, so a plain clone builds the firmware

obu-firmware builds against the vanetza-idf C-ITS library, which until now
came from the colleague's microbu-esp32c5 tree beside the repository and was
not tracked here, so a clone of this repository could not build the firmware
it ships. The library alone is now part of obu-firmware, as
obu-firmware/external/vanetza-idf: their external/vanetza-idf at commit
cf4b99f, unchanged (9775 files; see its PROVENANCE.md). CMake takes it from
there by default; -DVANETZA_IDF_DIR still points the build elsewhere.

The rest of the colleague's tree (their own VAM firmware, PKI tooling,
station-link Python tools, the V2X2MAP bridge) stays out of this repository
and gitignored; nothing is pushed to their repository. NOTES.md, docs/06,
TODO.md and the pcap verifier's usage line point at the new location.

obu-firmware/external/vanetza-idf/vanetza/security/hmac.cpp

@@ -0,0 +1,58 @@
+#include <vanetza/security/hmac.hpp>
+#include <algorithm>
+#include <array>
+
+#if defined VANETZA_WITH_OPENSSL
+#include <openssl/hmac.h>
+#endif
+
+#if defined VANETZA_WITH_CRYPTOPP
+#include <cryptopp/hmac.h>
+#include <cryptopp/sha.h>
+#endif
+
+namespace vanetza
+{
+namespace security
+{
+
+#if defined VANETZA_WITH_OPENSSL
+KeyTag create_hmac_tag_openssl(const ByteBuffer& data, const HmacKey& hmacKey)
+{
+    KeyTag keyTag;
+    std::array<std::uint8_t, EVP_MAX_MD_SIZE> tag;
+    unsigned int len = 0;
+    HMAC(EVP_sha256(), hmacKey.data(), static_cast<int>(hmacKey.size()),
+         data.data(), data.size(), tag.data(), &len);
+    std::copy_n(tag.data(), keyTag.size(), keyTag.data());
+    return keyTag;
+}
+#endif
+
+#if defined VANETZA_WITH_CRYPTOPP
+KeyTag create_hmac_tag_cryptopp(const ByteBuffer& data, const HmacKey& hmacKey)
+{
+    KeyTag keyTag;
+    std::array<std::uint8_t, 32> tag;
+    CryptoPP::HMAC<CryptoPP::SHA256> mac(hmacKey.data(), hmacKey.size());
+    mac.Update(data.data(), data.size());
+    mac.Final(tag.data());
+    std::copy_n(tag.data(), keyTag.size(), keyTag.data());
+    return keyTag;
+}
+#endif
+
+KeyTag create_hmac_tag(const ByteBuffer& data, const HmacKey& hmacKey)
+{
+#if defined VANETZA_WITH_OPENSSL
+    return create_hmac_tag_openssl(data, hmacKey);
+#elif defined VANETZA_WITH_CRYPTOPP
+    return create_hmac_tag_cryptopp(data, hmacKey);
+#else
+#   warning "no HMAC implementation available"
+    return KeyTag {};
+#endif
+}
+
+} // namespace security
+} // namespace vanetza