security/keymint/support/remote_prov_utils.cpp

/*
 * Copyright (c) 2019, The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include <iterator>
#include <tuple>

#include <android-base/properties.h>
#include <cppbor.h>
#include <json/json.h>
#include <openssl/base64.h>
#include <openssl/rand.h>
#include <remote_prov/remote_prov_utils.h>

namespace aidl::android::hardware::security::keymint::remote_prov {

bytevec kTestMacKey(32 /* count */, 0 /* byte value */);

bytevec randomBytes(size_t numBytes) {
    bytevec retval(numBytes);
    RAND_bytes(retval.data(), numBytes);
    return retval;
}

ErrMsgOr<EekChain> generateEekChain(size_t length, const bytevec& eekId) {
    if (length < 2) {
        return "EEK chain must contain at least 2 certs.";
    }

    auto eekChain = cppbor::Array();

    bytevec prev_priv_key;
    for (size_t i = 0; i < length - 1; ++i) {
        bytevec pub_key(ED25519_PUBLIC_KEY_LEN);
        bytevec priv_key(ED25519_PRIVATE_KEY_LEN);

        ED25519_keypair(pub_key.data(), priv_key.data());

        // The first signing key is self-signed.
        if (prev_priv_key.empty()) prev_priv_key = priv_key;

        auto coseSign1 = constructCoseSign1(prev_priv_key,
                                            cppbor::Map() /* payload CoseKey */
                                                    .add(CoseKey::KEY_TYPE, OCTET_KEY_PAIR)
                                                    .add(CoseKey::ALGORITHM, EDDSA)
                                                    .add(CoseKey::CURVE, ED25519)
                                                    .add(CoseKey::PUBKEY_X, pub_key)
                                                    .canonicalize()
                                                    .encode(),
                                            {} /* AAD */);
        if (!coseSign1) return coseSign1.moveMessage();
        eekChain.add(coseSign1.moveValue());

        prev_priv_key = priv_key;
    }

    bytevec pub_key(X25519_PUBLIC_VALUE_LEN);
    bytevec priv_key(X25519_PRIVATE_KEY_LEN);
    X25519_keypair(pub_key.data(), priv_key.data());

    auto coseSign1 = constructCoseSign1(prev_priv_key,
                                        cppbor::Map() /* payload CoseKey */
                                                .add(CoseKey::KEY_TYPE, OCTET_KEY_PAIR)
                                                .add(CoseKey::KEY_ID, eekId)
                                                .add(CoseKey::ALGORITHM, ECDH_ES_HKDF_256)
                                                .add(CoseKey::CURVE, cppcose::X25519)
                                                .add(CoseKey::PUBKEY_X, pub_key)
                                                .canonicalize()
                                                .encode(),
                                        {} /* AAD */);
    if (!coseSign1) return coseSign1.moveMessage();
    eekChain.add(coseSign1.moveValue());

    return EekChain{eekChain.encode(), pub_key, priv_key};
}

bytevec getProdEekChain() {
    bytevec prodEek;
    prodEek.reserve(1 + sizeof(kCoseEncodedRootCert) + sizeof(kCoseEncodedGeekCert));

    // In CBOR encoding, 0x82 indicates an array of two items
    prodEek.push_back(0x82);
    prodEek.insert(prodEek.end(), std::begin(kCoseEncodedRootCert), std::end(kCoseEncodedRootCert));
    prodEek.insert(prodEek.end(), std::begin(kCoseEncodedGeekCert), std::end(kCoseEncodedGeekCert));

    return prodEek;
}

ErrMsgOr<bytevec> verifyAndParseCoseSign1Cwt(const cppbor::Array* coseSign1,
                                             const bytevec& signingCoseKey, const bytevec& aad) {
    if (!coseSign1 || coseSign1->size() != kCoseSign1EntryCount) {
        return "Invalid COSE_Sign1";
    }

    const cppbor::Bstr* protectedParams = coseSign1->get(kCoseSign1ProtectedParams)->asBstr();
    const cppbor::Map* unprotectedParams = coseSign1->get(kCoseSign1UnprotectedParams)->asMap();
    const cppbor::Bstr* payload = coseSign1->get(kCoseSign1Payload)->asBstr();
    const cppbor::Bstr* signature = coseSign1->get(kCoseSign1Signature)->asBstr();

    if (!protectedParams || !unprotectedParams || !payload || !signature) {
        return "Invalid COSE_Sign1";
    }

    auto [parsedProtParams, _, errMsg] = cppbor::parse(protectedParams);
    if (!parsedProtParams) {
        return errMsg + " when parsing protected params.";
    }
    if (!parsedProtParams->asMap()) {
        return "Protected params must be a map";
    }

    auto& algorithm = parsedProtParams->asMap()->get(ALGORITHM);
    if (!algorithm || !algorithm->asInt() || algorithm->asInt()->value() != EDDSA) {
        return "Unsupported signature algorithm";
    }

    // TODO(jbires): Handle CWTs as the CoseSign1 payload in a less hacky way. Since the CWT payload
    //               is extremely remote provisioning specific, probably just make a separate
    //               function there.
    auto [parsedPayload, __, payloadErrMsg] = cppbor::parse(payload);
    if (!parsedPayload) return payloadErrMsg + " when parsing key";
    if (!parsedPayload->asMap()) return "CWT must be a map";
    auto serializedKey = parsedPayload->asMap()->get(-4670552)->clone();
    if (!serializedKey || !serializedKey->asBstr()) return "Could not find key entry";

    bool selfSigned = signingCoseKey.empty();
    auto key =
            CoseKey::parseEd25519(selfSigned ? serializedKey->asBstr()->value() : signingCoseKey);
    if (!key) return "Bad signing key: " + key.moveMessage();

    bytevec signatureInput =
            cppbor::Array().add("Signature1").add(*protectedParams).add(aad).add(*payload).encode();

    if (!ED25519_verify(signatureInput.data(), signatureInput.size(), signature->value().data(),
                        key->getBstrValue(CoseKey::PUBKEY_X)->data())) {
        return "Signature verification failed";
    }

    return serializedKey->asBstr()->value();
}

ErrMsgOr<std::vector<BccEntryData>> validateBcc(const cppbor::Array* bcc) {
    if (!bcc || bcc->size() == 0) return "Invalid BCC";

    std::vector<BccEntryData> result;

    bytevec prevKey;
    // TODO(jbires): Actually process the pubKey at the start of the new bcc entry
    for (size_t i = 1; i < bcc->size(); ++i) {
        const cppbor::Array* entry = bcc->get(i)->asArray();
        if (!entry || entry->size() != kCoseSign1EntryCount) {
            return "Invalid BCC entry " + std::to_string(i) + ": " + prettyPrint(entry);
        }
        auto payload = verifyAndParseCoseSign1Cwt(entry, std::move(prevKey), bytevec{} /* AAD */);
        if (!payload) {
            return "Failed to verify entry " + std::to_string(i) + ": " + payload.moveMessage();
        }

        auto& certProtParms = entry->get(kCoseSign1ProtectedParams);
        if (!certProtParms || !certProtParms->asBstr()) return "Invalid prot params";
        auto [parsedProtParms, _, errMsg] = cppbor::parse(certProtParms->asBstr()->value());
        if (!parsedProtParms || !parsedProtParms->asMap()) return "Invalid prot params";

        result.push_back(BccEntryData{*payload});

        // This entry's public key is the signing key for the next entry.
        prevKey = payload.moveValue();
    }

    return result;
}

JsonOutput jsonEncodeCsrWithBuild(const cppbor::Array& csr) {
    const std::string kFingerprintProp = "ro.build.fingerprint";

    if (!::android::base::WaitForPropertyCreation(kFingerprintProp)) {
        return JsonOutput::Error("Unable to read build fingerprint");
    }

    bytevec csrCbor = csr.encode();
    size_t base64Length;
    int rc = EVP_EncodedLength(&base64Length, csrCbor.size());
    if (!rc) {
        return JsonOutput::Error("Error getting base64 length. Size overflow?");
    }

    std::vector<char> base64(base64Length);
    rc = EVP_EncodeBlock(reinterpret_cast<uint8_t*>(base64.data()), csrCbor.data(), csrCbor.size());
    ++rc;  // Account for NUL, which BoringSSL does not for some reason.
    if (rc != base64Length) {
        return JsonOutput::Error("Error writing base64. Expected " + std::to_string(base64Length) +
                                 " bytes to be written, but " + std::to_string(rc) +
                                 " bytes were actually written.");
    }

    Json::Value json(Json::objectValue);
    json["build_fingerprint"] = ::android::base::GetProperty(kFingerprintProp, /*default=*/"");
    json["csr"] = base64.data();  // Boring writes a NUL-terminated c-string

    Json::StreamWriterBuilder factory;
    factory["indentation"] = "";  // disable pretty formatting
    return JsonOutput::Ok(Json::writeString(factory, json));
}

}  // namespace aidl::android::hardware::security::keymint::remote_prov