http://github.com/google/tink: Tink is a small cryptographic library that provides a safe, simple, agile and fast way to accomplish some common cryptographic tasks. 
// 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.aead;

import com.google.crypto.tink.Aead;
import com.google.crypto.tink.KeyTemplate;
import com.google.crypto.tink.KeyTypeManager;
import com.google.crypto.tink.Registry;
import com.google.crypto.tink.proto.AesGcmKey;
import com.google.crypto.tink.proto.AesGcmKeyFormat;
import com.google.crypto.tink.proto.KeyData.KeyMaterialType;
import com.google.crypto.tink.subtle.AesGcmJce;
import com.google.crypto.tink.subtle.Random;
import com.google.crypto.tink.subtle.Validators;
import com.google.protobuf.ByteString;
import com.google.protobuf.ExtensionRegistryLite;
import com.google.protobuf.InvalidProtocolBufferException;
import java.io.IOException;
import java.io.InputStream;
import java.security.GeneralSecurityException;


This key manager generates new AesGcmKey keys and produces new instances of 
AesGcmJce. 
/**
 * This key manager generates new {@code AesGcmKey} keys and produces new instances of {@code
 * AesGcmJce}.
 */
public final class AesGcmKeyManager extends KeyTypeManager<AesGcmKey> {
  AesGcmKeyManager() {
    super(
        AesGcmKey.class,
        new PrimitiveFactory<Aead, AesGcmKey>(Aead.class) {
          @Override
          public Aead getPrimitive(AesGcmKey key) throws GeneralSecurityException {
            return new AesGcmJce(key.getKeyValue().toByteArray());
          }
        });
  }

  @Override
  public String getKeyType() {
    return "type.googleapis.com/google.crypto.tink.AesGcmKey";
  }

  @Override
  public int getVersion() {
    return 0;
  }

  @Override
  public KeyMaterialType keyMaterialType() {
    return KeyMaterialType.SYMMETRIC;
  }

  @Override
  public void validateKey(AesGcmKey key) throws GeneralSecurityException {
    Validators.validateVersion(key.getVersion(), getVersion());
    Validators.validateAesKeySize(key.getKeyValue().size());
  }

  @Override
  public AesGcmKey parseKey(ByteString byteString) throws InvalidProtocolBufferException {
    return AesGcmKey.parseFrom(byteString, ExtensionRegistryLite.getEmptyRegistry());
  }

  @Override
  public KeyFactory<AesGcmKeyFormat, AesGcmKey> keyFactory() {
    return new KeyFactory<AesGcmKeyFormat, AesGcmKey>(AesGcmKeyFormat.class) {
      @Override
      public void validateKeyFormat(AesGcmKeyFormat format) throws GeneralSecurityException {
        Validators.validateAesKeySize(format.getKeySize());
      }

      @Override
      public AesGcmKeyFormat parseKeyFormat(ByteString byteString)
          throws InvalidProtocolBufferException {
        return AesGcmKeyFormat.parseFrom(byteString, ExtensionRegistryLite.getEmptyRegistry());
      }

      @Override
      public AesGcmKey createKey(AesGcmKeyFormat format) throws GeneralSecurityException {
        return AesGcmKey.newBuilder()
            .setKeyValue(ByteString.copyFrom(Random.randBytes(format.getKeySize())))
            .setVersion(getVersion())
            .build();
      }

      @Override
      public AesGcmKey deriveKey(AesGcmKeyFormat format, InputStream inputStream)
          throws GeneralSecurityException {
        Validators.validateVersion(format.getVersion(), getVersion());

        byte[] pseudorandomness = new byte[format.getKeySize()];
        try {
          int read = inputStream.read(pseudorandomness);
          if (read != format.getKeySize()) {
            throw new GeneralSecurityException("Not enough pseudorandomness given");
          }
          return AesGcmKey.newBuilder()
              .setKeyValue(ByteString.copyFrom(pseudorandomness))
              .setVersion(getVersion())
              .build();
        } catch (IOException e) {
          throw new GeneralSecurityException("Reading pseudorandomness failed", e);
        }
      }
    };
  }

  public static void register(boolean newKeyAllowed) throws GeneralSecurityException {
    Registry.registerKeyManager(new AesGcmKeyManager(), newKeyAllowed);
  }

  
Returns:a KeyTemplate that generates new instances of AES-GCM with the following parameters: 
    
On Android KitKat (API level 19), the Aead instance generated by this key template does not support associated data. It might not work at all in older versions.
/**
   * @return a {@link KeyTemplate} that generates new instances of AES-GCM with the following
   *     parameters:
   *     <ul>
   *       <li>Key size: 16 bytes
   *       <li>Prefix type: {@link KeyTemplate.OutputPrefixType#TINK}
   *     </ul>
   *     <p>On Android KitKat (API level 19), the {@link com.google.crypto.tink.Aead} instance
   *     generated by this key template does not support associated data. It might not work at all
   *     in older versions.
   */
  public static final KeyTemplate aes128GcmTemplate() {
    return createKeyTemplate(16, KeyTemplate.OutputPrefixType.TINK);
  }

  
Returns:a KeyTemplate that generates new instances of AES-GCM with the following parameters: 
    
Keys generated from this template should create ciphertexts compatible with other
    libraries.
    
On Android KitKat (API level 19), the Aead instance generated by this key template does not support associated data. It might not work at all in older versions.
/**
   * @return a {@link KeyTemplate} that generates new instances of AES-GCM with the following
   *     parameters:
   *     <ul>
   *       <li>Key size: 16 bytes
   *       <li>Prefix type: {@link KeyTemplate.OutputPrefixType#RAW} (no prefix)
   *     </ul>
   *     <p>Keys generated from this template should create ciphertexts compatible with other
   *     libraries.
   *     <p>On Android KitKat (API level 19), the {@link com.google.crypto.tink.Aead} instance
   *     generated by this key template does not support associated data. It might not work at all
   *     in older versions.
   */
  public static final KeyTemplate rawAes128GcmTemplate() {
    return createKeyTemplate(16, KeyTemplate.OutputPrefixType.RAW);
  }

  
Returns:a KeyTemplate that generates new instances of AES-GCM with the following parameters: 
    
On Android KitKat (API level 19), the Aead instance generated by this key template does not support associated data. It might not work at all in older versions.
/**
   * @return a {@link KeyTemplate} that generates new instances of AES-GCM with the following
   *     parameters:
   *     <ul>
   *       <li>Key size: 32 bytes
   *       <li>Prefix type: {@link KeyTemplate.OutputPrefixType#TINK}
   *     </ul>
   *     <p>On Android KitKat (API level 19), the {@link com.google.crypto.tink.Aead} instance
   *     generated by this key template does not support associated data. It might not work at all
   *     in older versions.
   */
  public static final KeyTemplate aes256GcmTemplate() {
    return createKeyTemplate(32, KeyTemplate.OutputPrefixType.TINK);
  }

  
Returns:a KeyTemplate that generates new instances of AES-GCM with the following parameters: 
    
Keys generated from this template should create ciphertexts compatible with other
    libraries.
    
On Android KitKat (API level 19), the Aead instance generated by this key template does not support associated data. It might not work at all in older versions.
/**
   * @return a {@link KeyTemplate} that generates new instances of AES-GCM with the following
   *     parameters:
   *     <ul>
   *       <li>Key size: 32 bytes
   *       <li>Prefix type: {@link KeyTemplate.OutputPrefixType#RAW} (no prefix)
   *     </ul>
   *     <p>Keys generated from this template should create ciphertexts compatible with other
   *     libraries.
   *     <p>On Android KitKat (API level 19), the {@link com.google.crypto.tink.Aead} instance
   *     generated by this key template does not support associated data. It might not work at all
   *     in older versions.
   */
  public static final KeyTemplate rawAes256GcmTemplate() {
    return createKeyTemplate(32, KeyTemplate.OutputPrefixType.RAW);
  }

  
Returns:a KeyTemplate containing a AesGcmKeyFormat with some specified parameters.
/**
   * @return a {@link KeyTemplate} containing a {@link AesGcmKeyFormat} with some specified
   *     parameters.
   */
  private static KeyTemplate createKeyTemplate(
      int keySize, KeyTemplate.OutputPrefixType prefixType) {
    AesGcmKeyFormat format = AesGcmKeyFormat.newBuilder().setKeySize(keySize).build();
    return KeyTemplate.create(
        new AesGcmKeyManager().getKeyType(), format.toByteArray(), prefixType);
  }
}