Flow 23 --- RSA-2048 → ML-KEM-768 Immediate Rotation

/*
 * Copyright 2025 ANKATech Solutions Inc
 *
 * 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.
 *
 * SPDX-License-Identifier: Apache-2.0
 */
package co.ankatech.ankasecure.sdk.examples;

import co.ankatech.ankasecure.sdk.AuthenticatedSdk;
import co.ankatech.ankasecure.sdk.exception.AnkaSecureSdkException;
import co.ankatech.ankasecure.sdk.model.EncryptResult;
import co.ankatech.ankasecure.sdk.model.GenerateKeySpec;
import co.ankatech.ankasecure.sdk.model.KeyGenerationSummarySpec;

import java.nio.file.Files;
import java.util.List;
import java.util.Objects;
import java.util.Properties;

import static co.ankatech.ankasecure.sdk.examples.ExampleUtil.*;
import static java.nio.charset.StandardCharsets.UTF_8;

/**
 * Scenario 23 — Key Rotation with Capability Reduction (RSA-2048 → ML-KEM-768).
 *
 * <p>This scenario demonstrates handling capability reduction when rotating from
 * RSA (which supports encrypt/decrypt/sign/verify) to ML-KEM (which only
 * supports encrypt/decrypt). The rotation requires explicit acknowledgment
 * of the loss of signing capabilities.</p>
 *
 * @author ANKATech Solutions Inc.
 * @since 3.0.0
 * @see ExampleUtil
 * @see AuthenticatedSdk
 */
public final class ExampleScenario23 {

    private static final String RSA_KID =
            "scenario23_rsa_" + System.currentTimeMillis();

    /** No instantiation &mdash; this class only exposes {@link #main(String[])}. */
    private ExampleScenario23() { }

    /**
     * Runs the key rotation with capability reduction scenario.
     *
     * <p>Loads CLI properties, authenticates against ANKASecure&copy;,
     * and delegates to the scenario logic. On any unrecoverable error
     * the JVM terminates via
     * {@link ExampleUtil#fatal(String, Throwable)}.</p>
     *
     * @param args command-line arguments (ignored)
     */
    public static void main(String[] args) {
        System.out.println("===== SCENARIO 23 START =====");
        System.out.println("""
                Purpose :
                  * Provision an RSA-2048 key and rotate it to ML-KEM-768.
                  * Demonstrate capability reduction handling (losing sign/verify).
                  * Encrypt with original KID and observe successor selection.
                Steps   :
                  1) Generate RSA-2048 key (supports: encrypt, decrypt, sign, verify)
                  2) Rotate to ML-KEM-768 successor (supports: encrypt, decrypt only)
                  3) Encrypt plaintext using rotated key
                --------------------------------------------------------------""");

        try {
            Files.createDirectories(TEMP_DIR);

            Properties props = loadProperties();
            AuthenticatedSdk sdk = authenticate(props);

            runScenario(sdk);

        } catch (Exception ex) {
            fatal("Scenario 23 failed", ex);
        }

        System.out.println("===== SCENARIO 23 END =====");
    }

    /* ====================================================================== */
    private static void runScenario(AuthenticatedSdk sdk)
            throws AnkaSecureSdkException {

        /* 1 – RSA-2048 key ------------------------------------------------ */
        sdk.generateKey(new GenerateKeySpec()
                .setKid(RSA_KID)
                .setKty("RSA")
                .setAlg("RSA-2048")
                .setKeyOps(List.of("encrypt", "decrypt")));
        System.out.println("[1] RSA-2048 key generated   -> kid = " + RSA_KID);

        /* 2 – rotate to ML-KEM-768 ------------------------------------- */
        String kemKid = "scenario23_mlkem_" + System.currentTimeMillis();
        KeyGenerationSummarySpec successor = sdk.rotateKey(
                RSA_KID,
                new GenerateKeySpec()
                        .setKid(kemKid)
                        .setKty("ML-KEM")
                        .setAlg("ML-KEM-768")
                        .setAcknowledgeCapabilityReduction(true));
                        // RSA supports sign/verify, ML-KEM does not - acknowledging reduction

        Objects.requireNonNull(successor, "Rotation must return successor metadata");
        System.out.println("[2] Successor created        -> kid = " + successor.getKid());
        System.out.println("    Capability reduction acknowledged: sign/verify → (removed)");

        /* 3 – encrypt with original KID -------------------------------- */
        byte[] plaintext = "Post-rotation encryption demo".getBytes(UTF_8);
        EncryptResult enc = sdk.encrypt(RSA_KID, plaintext);

        System.out.println("[3] Encryption metadata");
        System.out.println("    Key requested  : " + nullSafe(enc.getKeyRequested()));
        System.out.println("    Key used       : " + nullSafe(enc.getActualKeyUsed()));
        System.out.println("    Algorithm used : " + nullSafe(enc.getAlgorithmUsed()));
    }
}