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 * Copyright (C) 2026, Leo Galambos
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package zeroecho.core.alg.sphincsplus;

import java.security.GeneralSecurityException;
import java.security.KeyFactory;
import java.security.PrivateKey;
import java.security.spec.PKCS8EncodedKeySpec;
import java.util.Arrays;

import zeroecho.core.spi.PrivateKeyImporter;

/**
 * Builder for importing SPHINCS+ private keys from encoded specifications.
 *
 * <p>
 * {@code SphincsPlusPrivateKeyBuilder} integrates with the JCA
 * {@link KeyFactory} to parse PKCS#8-encoded SPHINCS+ private keys. Unlike
 * {@link SphincsPlusKeyGenBuilder}, this builder does not generate new key
 * pairs but focuses solely on importing private key material.
 * </p>
 *
 * <p>
 * The exact supported operation is
 * {@link #importPrivate(SphincsPlusPrivateKeySpec)}. Other key operations are
 * registered through their own exact interfaces.
 * </p>
 *
 * <h2>Example</h2> <pre>{@code
 * // Assuming bytes contain a PKCS#8-encoded SPHINCS+ private key:
 * SphincsPlusPrivateKeySpec spec =
 *     new SphincsPlusPrivateKeySpec("BCPQC", encodedBytes);
 *
 * PrivateKey priv =
 *     new SphincsPlusPrivateKeyBuilder().importPrivate(spec);
 * }</pre>
 *
 * @since 1.0
 */
public final class SphincsPlusPrivateKeyBuilder implements PrivateKeyImporter<SphincsPlusPrivateKeySpec> {

    /**
     * Imports a SPHINCS+ private key from PKCS#8 encoding.
     *
     * <p>
     * The method uses {@link KeyFactory} with algorithm {@code "SPHINCSPlus"} and
     * the provider specified in the {@code spec}. If no provider name is given, the
     * default provider resolution is used.
     * </p>
     *
     * @param spec private key specification containing PKCS#8-encoded key material
     *             and optional provider name
     * @return the imported SPHINCS+ {@link PrivateKey}
     * @throws NullPointerException     if {@code spec} or its encoded bytes are
     *                                  null
     * @throws GeneralSecurityException if the key factory cannot be initialized or
     *                                  the encoded key is invalid
     */
    @Override
    public PrivateKey importPrivate(SphincsPlusPrivateKeySpec spec) throws GeneralSecurityException {
        KeyFactory kf = (spec.providerName() == null) ? KeyFactory.getInstance("SPHINCSPlus")
                : KeyFactory.getInstance("SPHINCSPlus", spec.providerName());
        byte[] encoded = spec.encoded();
        try {
            return kf.generatePrivate(new PKCS8EncodedKeySpec(encoded));
        } finally {
            Arrays.fill(encoded, (byte) 0);
        }
    }
}
