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Tink is a small cryptographic library that provides a safe, simple, agile and fast way to accomplish some common cryptographic tasks.

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// Copyright 2017 Google 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 com.google.crypto.tink.subtle;

import java.security.GeneralSecurityException;
import java.security.interfaces.ECPrivateKey;
import java.security.interfaces.ECPublicKey;

/**
 * HKDF-based KEM (key encapsulation mechanism) for ECIES recipient.
 *
 * @since 1.0.0
 */
public final class EciesHkdfRecipientKem {
  private ECPrivateKey recipientPrivateKey;

  public EciesHkdfRecipientKem(final ECPrivateKey recipientPrivateKey) {
    this.recipientPrivateKey = recipientPrivateKey;
  }

  public byte[] generateKey(
      byte[] kemBytes,
      String hmacAlgo,
      final byte[] hkdfSalt,
      final byte[] hkdfInfo,
      int keySizeInBytes,
      EllipticCurves.PointFormatType pointFormat)
      throws GeneralSecurityException {
    ECPublicKey ephemeralPublicKey = EllipticCurves.getEcPublicKey(
        recipientPrivateKey.getParams(), pointFormat, kemBytes);
    byte[] sharedSecret = EllipticCurves.computeSharedSecret(
        recipientPrivateKey, ephemeralPublicKey);
    return Hkdf.computeEciesHkdfSymmetricKey(
        kemBytes, sharedSecret, hmacAlgo, hkdfSalt, hkdfInfo, keySizeInBytes);
  }
}