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package zeroecho.core.alg.chacha;

import zeroecho.core.annotation.Describable;
import zeroecho.core.spec.AlgorithmKeySpec;

/**
 * <h2>ChaCha20 key generation specification</h2>
 *
 * Immutable record describing the key size for ChaCha20 key generation. Only
 * 256-bit keys are permitted by the ChaCha20 design.
 *
 * <h3>Validation</h3>
 * <ul>
 * <li>The constructor enforces {@code keySizeBits == 256}.</li>
 * <li>Any other size results in an {@link IllegalArgumentException}.</li>
 * </ul>
 *
 * <h3>Factory method</h3>
 * <ul>
 * <li>{@link #chacha256()} provides a convenient way to obtain a standard
 * 256-bit spec.</li>
 * </ul>
 *
 * <h3>Usage example</h3> <pre>{@code
 * ChaChaKeyGenSpec spec = ChaChaKeyGenSpec.chacha256();
 * SecretKey key = cryptoAlgorithm.generateSecret(spec);
 * }</pre>
 *
 * @param keySizeBits key size in bits (must be 256)
 * @since 1.0
 */
public record ChaChaKeyGenSpec(int keySizeBits) implements AlgorithmKeySpec, Describable {
    /**
     * Constructs a new ChaCha20 key generation spec.
     *
     * @param keySizeBits must be 256; otherwise an exception is thrown
     * @throws IllegalArgumentException if {@code keySizeBits != 256}
     */
    public ChaChaKeyGenSpec {
        if (keySizeBits != 256) { // NOPMD
            throw new IllegalArgumentException("ChaCha20 keySizeBits must be 256");
        }
    }

    /**
     * Returns the standard 256-bit key generation spec.
     *
     * @return a new {@code ChaChaKeyGenSpec} with size 256
     */
    public static ChaChaKeyGenSpec chacha256() {
        return new ChaChaKeyGenSpec(256);
    }

    /**
     * Returns a short human-readable description of this spec.
     *
     * @return the string {@code "256bits"}
     */
    @Override
    public String description() {
        return "256bits";
    }
}
