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package zeroecho.core.alg.ecdh;

import java.security.GeneralSecurityException;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.spec.ECGenParameterSpec;

import zeroecho.core.alg.ecdsa.EcdsaPrivateKeyBuilder;
import zeroecho.core.alg.ecdsa.EcdsaPublicKeyBuilder;
import zeroecho.core.spi.AsymmetricKeyPairGenerator;

/**
 * <h2>ECDH Key Pair Generator</h2>
 *
 * Implementation of {@link zeroecho.core.spi.AsymmetricKeyPairGenerator} for
 * elliptic curve Diffie-Hellman (ECDH) key pairs.
 *
 * <p>
 * This builder generates fresh EC key pairs suitable for ECDH key agreement. It
 * accepts an {@link EcdhCurveSpec} that defines the named curve to use (e.g.,
 * {@code P-256}, {@code P-384}, {@code P-521}).
 * </p>
 *
 * <h2>Usage</h2> <pre>{@code
 * // Generate a key pair on P-256
 * KeyPair kp = new EcdhKeyGenBuilder().generateKeyPair(EcdhCurveSpec.P256);
 * }</pre>
 *
 * <p>
 * Import of existing keys is not supported by this builder. Instead, use
 * {@link EcdsaPublicKeyBuilder} and {@link EcdsaPrivateKeyBuilder} together
 * with {@code EcdsaPublicKeySpec} or {@code EcdsaPrivateKeySpec} when importing
 * encoded elliptic curve keys.
 * </p>
 *
 * @since 1.0
 */
public final class EcdhKeyGenBuilder implements AsymmetricKeyPairGenerator<EcdhCurveSpec> {
    /**
     * Generates a new elliptic curve key pair for use in ECDH key agreement.
     *
     * <p>
     * The curve is determined from the provided {@link EcdhCurveSpec}. Internally,
     * a standard JCA {@link KeyPairGenerator} for {@code "EC"} is initialized with
     * the corresponding {@link ECGenParameterSpec}.
     * </p>
     *
     * @param spec curve specification selecting the named curve
     * @return a freshly generated {@link KeyPair} suitable for ECDH
     * @throws GeneralSecurityException if the curve is not supported by the
     *                                  underlying JCA provider or if key generation
     *                                  fails
     */
    @Override
    public KeyPair generateKeyPair(EcdhCurveSpec spec) throws GeneralSecurityException {
        KeyPairGenerator kpg = KeyPairGenerator.getInstance("EC");
        kpg.initialize(new ECGenParameterSpec(spec.curveName()));
        return kpg.generateKeyPair();
    }
}
