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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,72 @@
+#include "signed_builder.hpp"
+#include "asn1.hpp"
+#include "certificate.hpp"
+#include "hashed_id8.hpp"
+#include "mock_credential_storage.hpp"
+#include "openssl_security_module.hpp"
+#include "stub_certificate.hpp"
+#include "validation.hpp"
+#include <vanetza/asn1/security/SignedData.h>
+#include <vanetza/common/byte_buffer.hpp>
+#include <gtest/gtest.h>
+#include <cstring>
+#include <memory>
+
+namespace vanetza
+{
+namespace pki
+{
+
+class SignedBuilderTest : public ::testing::Test
+{
+protected:
+    SignedBuilderTest() : m_credentials(std::make_shared<MockCredentialStorage>()), m_security(m_credentials)
+    {
+    }
+
+    std::shared_ptr<MockCredentialStorage> m_credentials;
+    OpenSslSecurityModule m_security;
+};
+
+// create_external_signed produces an EtsiTs103097Data-SignedExternalPayload
+// envelope whose SignedData:
+//   - tbsData.payload.extDataHash is set (not data)
+//   - extDataHash carries SHA-256(external_payload) when hash_algo is SHA256
+//   - signer = digest(cert) when a cert is provided
+//   - signature verifies against the cert's verification key
+TEST_F(SignedBuilderTest, external_signed_carries_payload_hash_and_verifies)
+{
+    PublicKey ec_key = m_security.create_key(KeyType::NistP256);
+    Certificate ec_cert = build_stub_certificate(ec_key);
+    ByteBuffer payload { 0xDE, 0xAD, 0xBE, 0xEF, 0xCA, 0xFE };
+
+    SignedData envelope = create_external_signed(payload, m_security, ec_key, HashAlgorithm::SHA256, &ec_cert);
+    ASSERT_EQ(Vanetza_Security_Ieee1609Dot2Content_PR_signedData, envelope->content->present);
+    Vanetza_Security_SignedData_t* sd = envelope->content->choice.signedData;
+    ASSERT_NE(nullptr, sd);
+
+    // payload.extDataHash present, payload.data absent
+    ASSERT_NE(nullptr, sd->tbsData);
+    ASSERT_NE(nullptr, sd->tbsData->payload);
+    ASSERT_NE(nullptr, sd->tbsData->payload->extDataHash);
+    EXPECT_EQ(nullptr, sd->tbsData->payload->data);
+    ASSERT_EQ(Vanetza_Security_HashedData_PR_sha256HashedData, sd->tbsData->payload->extDataHash->present);
+
+    // hash equals SHA-256(payload)
+    Sha256Hash expected = m_security.calculate_sha256_hash(payload.data(), payload.size());
+    const auto& hash_oct = sd->tbsData->payload->extDataHash->choice.sha256HashedData;
+    ASSERT_EQ(32, hash_oct.size);
+    EXPECT_EQ(0, std::memcmp(hash_oct.buf, expected.octets.data(), 32));
+
+    // signer = digest, matches HashedId8(ec_cert)
+    ASSERT_EQ(Vanetza_Security_SignerIdentifier_PR_digest, sd->signer.present);
+    HashedId8 ec_hid8 = ec_cert.calculate_hashed_id8(m_security);
+    EXPECT_TRUE(sd->signer.choice.digest == ec_hid8.octets);
+
+    // signature verifies against ec_cert's verification key
+    Sha256Hash digest = calculate_digest<Sha256Hash>(m_security, *sd->tbsData, &ec_cert.raw());
+    EXPECT_TRUE(m_security.verify(digest, make_signature(sd->signature), ec_key));
+}
+
+} // namespace pki
+} // namespace vanetza