Flow 16 --- Detached-JWS Stream Verification

/*
 * 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.
 */

package co.ankatech.ankasecure.sdk.examples;

import co.ankatech.ankasecure.sdk.AuthenticatedSdk;
import co.ankatech.ankasecure.sdk.exception.AnkaSecureSdkException;
import co.ankatech.ankasecure.sdk.exception.SdkErrorCode;
import co.ankatech.ankasecure.sdk.model.SignResult;
import co.ankatech.ankasecure.sdk.model.VerifySignatureResult;
import co.ankatech.ankasecure.sdk.util.FileIO;

import java.nio.file.Files;
import java.nio.file.Path;
import java.util.Properties;
import java.util.concurrent.TimeUnit;

import static co.ankatech.ankasecure.sdk.examples.ExampleUtil.*;

/**
 * Scenario 16 — XMSS Streaming Detached JWS Verification.
 *
 * <p>End-to-end, file-oriented demonstration of streaming detached JWS
 * verification with a pre-provisioned XMSS key (the SDK is data-plane-only —
 * it does not create keys).</p>
 *
 * <p><strong>Every run spends one signature of a finite key.</strong> XMSS is a stateful
 * hash-based scheme: {@code XMSS-SHA2_10_256} holds 2<sup>10</sup>&nbsp;=&nbsp;1024 one-time
 * signatures and the playground key is shared by everyone running the examples. When they are all
 * used, the sign step is refused with
 * {@link SdkErrorCode#STATEFUL_KEY_EXHAUSTED}; the scenario then names the key and stops, because
 * the only remedy is rotating it, which is a control-plane operation this data-plane SDK does not
 * offer. A {@link SdkErrorCode#STATEFUL_KEY_BUSY} refusal (another run is reserving the key's next
 * signature at the same moment) is retried once after the server's {@code Retry-After}.</p>
 *
 * @see ExampleUtil
 * @see ExamplePlaygroundKeys
 * @see AuthenticatedSdk
 */
public final class ExampleScenario16 {

    private static final Path LARGE_PAYLOAD = TEMP_DIR.resolve("scenario16_payload.bin");
    private static final Path DETACHED_SIG  = TEMP_DIR.resolve("scenario16_payload.sig");

    /** No instantiation &mdash; this class only exposes {@link #main(String[])}. */
    private ExampleScenario16() { }

    /**
     * Runs the XMSS streaming detached JWS verification 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 16 START =====");
        System.out.println("""
                Purpose :
                  * Resolve a pre-provisioned XMSS sign/verify key.
                  * Create and sign a 1 MB payload as a detached JWS.
                  * Verify the detached signature in streaming mode.
                Steps   :
                  1) Resolve pre-provisioned XMSS key
                  2) Create dummy payload
                  3) Sign payload (detached JWS)
                  4) Verify signature stream
                --------------------------------------------------------------""");

        try {
            prepareWorkingDir();

            Properties            props      = loadProperties();
            AuthenticatedSdk      sdk        = authenticate(props);
            ExamplePlaygroundKeys playground = ExamplePlaygroundKeys.from(props, sdk);

            runScenario(sdk, playground);

        } catch (Exception ex) {
            fatal("Scenario 16 failed", ex);
        }

        System.out.println("===== SCENARIO 16 END =====");
    }

    /* ====================================================================== */
    private static void runScenario(AuthenticatedSdk sdk,
                                    ExamplePlaygroundKeys playground) throws Exception {

        /* 1 – resolve pre-provisioned XMSS key (data-plane SDK: no key creation) */
        final String signKid = playground.kidForAlgorithm("XMSS-SHA2_10_256");
        System.out.println("[1] Using pre-provisioned key -> kid = " + signKid);

        /* 2 – 1 MB dummy payload ------------------------------------ */
        byte[] blob = new byte[1 * 1024 * 1024];               // zero-filled buffer
        FileIO.writeBytes(LARGE_PAYLOAD, blob);
        System.out.println("[2] Payload created         -> " + LARGE_PAYLOAD);

        /* 3 – detached-JWS streaming sign ----------------------------- */
        SignResult signMeta = signHonouringStatefulRefusals(sdk, signKid);
        System.out.println("[3] Detached JWS written    -> " + DETACHED_SIG);
        printSignMeta(signMeta);

        /* 4 – detached-JWS streaming verify --------------------------- */
        VerifySignatureResult verifyMeta =
                sdk.verifySignatureStream(LARGE_PAYLOAD, DETACHED_SIG);
        System.out.println("[4] Signature valid?        -> " + verifyMeta.isValid());
        printVerifyMeta(verifyMeta);
    }

    /**
     * Signs the payload, handling the two refusals a stateful key adds.
     *
     * <p>{@link SdkErrorCode#STATEFUL_KEY_BUSY} is transient: wait the server's
     * {@code Retry-After} (one second when absent) and try once more.
     * {@link SdkErrorCode#STATEFUL_KEY_EXHAUSTED} is permanent for this key: print what to do and
     * rethrow, so {@link ExampleUtil#fatal(String, Throwable)} reports the run as failed.</p>
     *
     * @param sdk     the authenticated SDK
     * @param signKid the XMSS key to sign with
     * @return the signing result
     * @throws Exception when the sign fails for any other reason, or the key is exhausted
     */
    private static SignResult signHonouringStatefulRefusals(AuthenticatedSdk sdk, String signKid)
            throws Exception {
        try {
            return sdk.signFileStream(signKid, LARGE_PAYLOAD, DETACHED_SIG);
        } catch (AnkaSecureSdkException refused) {
            if (refused.getErrorCode() == SdkErrorCode.STATEFUL_KEY_BUSY) {
                // The SDK bounds the hint at AnkaSecureSdkException.MAX_RETRY_AFTER_SECONDS (one
                // hour); sleeping in SECONDS keeps the wait free of any millisecond arithmetic.
                Long seconds = refused.getRetryAfterSeconds();
                long wait = (seconds == null) ? 1L : seconds;
                System.out.println("[3] Key " + signKid + " is busy reserving its next signature;"
                        + " retrying in " + wait + " s");
                TimeUnit.SECONDS.sleep(wait);
                return sdk.signFileStream(signKid, LARGE_PAYLOAD, DETACHED_SIG);
            }
            if (refused.getErrorCode() == SdkErrorCode.STATEFUL_KEY_EXHAUSTED) {
                System.out.println("[3] Key " + signKid + " has used all of its one-time signatures."
                        + " Rotate it with the Key Management SDK (or its CLI, ankasecure-km);"
                        + " the signatures it already produced still verify.");
            }
            throw refused;
        }
    }
}