// 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;

import java.security.GeneralSecurityException;

/**
 * Interface for hybrid decryption.
 *
 * <p>Hybrid Encryption combines the efficiency of symmetric encryption with the convenience of
 * public-key encryption: to encrypt a message a fresh symmetric key is generated and used to
 * encrypt the actual plaintext data, while the recipient’s public key is used to encrypt the
 * symmetric key only, and the final ciphertext consists of the symmetric ciphertext and the
 * encrypted symmetric key.
 *
 * <h3>WARNING</h3>
 *
 * <p>Hybrid Encryption does not provide authenticity, that is the recipient of an encrypted message
 * does not know the identity of the sender. Similar to general public-key encryption schemes the
 * security goal of Hybrid Encryption is to provide privacy only. In other words, Hybrid Encryption
 * is secure if and only if the recipient can accept anonymous messages or can rely on other
 * mechanisms to authenticate the sender.
 *
 * <h3>Security guarantees</h3>
 *
 * <p>The functionality of Hybrid Encryption is represented as a pair of primitives (interfaces):
 * {@link HybridEncrypt} for encryption of data, and {@link HybridDecrypt} for decryption.
 * Implementations of these interfaces are secure against adaptive chosen ciphertext attacks. In
 * addition to {@code plaintext} the encryption takes an extra parameter {@code contextInfo}, which
 * usually is public data implicit from the context, but should be bound to the resulting
 * ciphertext, i.e. the ciphertext allows for checking the integrity of {@code contextInfo} (but
 * there are no guarantees wrt. the secrecy or authenticity of {@code contextInfo}).
 *
 * <p>{@code contextInfo} can be empty or null, but to ensure the correct decryption of a ciphertext
 * the same value must be provided for the decryption operation as was used during encryption (cf.
 * {@link HybridEncrypt}).
 *
 * <p>A concrete instantiation of this interface can implement the binding of {@code contextInfo} to
 * the ciphertext in various ways, for example:
 *
 * <ul>
 *   <li>use {@code contextInfo} as "associated data"-input for the employed AEAD symmetric
 *       encryption (cf. https://tools.ietf.org/html/rfc5116).
 *   <li>use {@code contextInfo} as "CtxInfo"-input for HKDF (if the implementation uses HKDF as key
 *       derivation function, cf. https://tools.ietf.org/html/rfc5869).
 * </ul>
 *
 * @since 1.0.0
 */
public interface HybridDecrypt {
  /**
   * Decryption operation: decrypts {@code ciphertext} verifying the integrity of {@code
   * contextInfo}.
   *
   * @return resulting plaintext
   */
  byte[] decrypt(final byte[] ciphertext, final byte[] contextInfo) throws GeneralSecurityException;
}